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<chapter id="security">
  <chapterinfo>
    <authorgroup>
      <author>
	<firstname>Matthew</firstname>
	<surname>Dillon</surname>
	<contrib>Much of this chapter has been taken from the
	security(7) manual page by </contrib>
      </author>
    </authorgroup>
  </chapterinfo>

  <title>Security</title>
  <indexterm><primary>security</primary></indexterm>

  <sect1>
    <title>Synopsis</title>

    <para>This chapter will provide a basic introduction to system security
      concepts, some general good rules of thumb, and some advanced topics
      under FreeBSD.  A lot of the topics covered here can be applied
      to system and Internet security in general as well.  The Internet
      is no longer a <quote>friendly</quote> place in which everyone
      wants to be your kind neighbor.  Securing your system is imperative
      to protect your data, intellectual property, time, and much more
      from the hands of hackers and the like.</para>

    <para>FreeBSD provides an array of utilities and mechanisms to
      ensure the integrity and security of your system and
      network.</para>

    <para>After reading this chapter, you will know:</para>

    <itemizedlist>
      <listitem>
	<para>Basic system security concepts, in respect to FreeBSD.</para>
      </listitem>

      <listitem>
	<para>About the various crypt mechanisms available in FreeBSD,
	  such as DES and MD5.</para>
      </listitem>

      <listitem>
	<para>How to setup S/Key, an alternative, one-time password
	  authentication system.</para>
      </listitem>

      <listitem>
	<para>How to setup Kerberos, another alternative
	  authentication system.</para>
      </listitem>

      <listitem>
	<para>How to create firewalls using IPFW.</para>
      </listitem>

      <listitem>
	<para>How to configure IPSec.</para>
      </listitem>
     
      <listitem>
	<para>How to configure and use OpenSSH, FreeBSD's SSH
	  implementation.</para>
      </listitem>
    </itemizedlist>

    <para>Before reading this chapter, you should:</para>

    <itemizedlist>
      <listitem>
	<para>Understand basic FreeBSD and Internet concepts.</para>
      </listitem>
    </itemizedlist>

  </sect1>

  <sect1 id="security-intro">
    <title>Introduction</title>

    <para>Security is a function that begins and ends with the system
      administrator.  While all BSD Unix multi-user systems have some
      inherent security, the job of building and maintaining additional
      security mechanisms to keep those users <quote>honest</quote> is
      probably one of the single largest undertakings of the sysadmin.
      Machines are only as secure as you make them, and security concerns
      are ever competing with the human necessity for convenience.  Unix
      systems, in general, are capable of running a huge number of
      simultaneous processes and many of these processes operate as
      servers &ndash; meaning that external entities can connect and talk
      to them.  As yesterday's mini-computers and mainframes become
      today's desktops, and as computers become networked and
      internetworked, security becomes an even bigger issue.</para>

    <para>Security is best implemented through a layered
      <quote>onion</quote> approach.  In a nutshell, what you want to do is
      to create as many layers of security as are convenient and then
      carefully monitor the system for intrusions.  You do not want to
      overbuild your security or you will interfere with the detection
      side, and detection is one of the single most important aspects of
      any security mechanism.  For example, it makes little sense to set
      the <literal>schg</literal> flags (see &man.chflags.1;) on every 
      system binary because
      while this may temporarily protect the binaries, it prevents an
      attacker who has broken in from making an easily detectable change
      that may result in your security mechanisms not detecting the attacker
      at all.</para>

    <para>System security also pertains to dealing with various forms of
      attack, including attacks that attempt to crash, or otherwise make a
      system unusable, but do not attempt to compromise the
      <username>root</username> account (<quote>break root</quote>).
      Security concerns
      can be split up into several categories:</para>

    <orderedlist>
      <listitem>
	<para>Denial of service attacks.</para>
      </listitem>

      <listitem>
	<para>User account compromises.</para>
      </listitem>

      <listitem>
	<para>Root compromise through accessible servers.</para>
      </listitem>

      <listitem>
	<para>Root compromise via user accounts.</para>
      </listitem>

      <listitem>
	<para>Backdoor creation.</para>
      </listitem>
    </orderedlist>

    <indexterm>
      <primary>DoS attacks</primary>
      <see>Denial of Service (DoS)</see>
    </indexterm>
    <indexterm>
      <primary>security</primary>
      <secondary>DoS attacks</secondary>
      <see>Denial of Service (DoS)</see>
    </indexterm>
    <indexterm><primary>Denial of Service (DoS)</primary></indexterm>

    <para>A denial of service attack is an action that deprives the
      machine of needed resources.  Typically, DoS attacks are
      brute-force mechanisms that attempt to crash or otherwise make a
      machine unusable by overwhelming its servers or network stack.  Some
      DoS attacks try to take advantage of bugs in the networking
      stack to crash a machine with a single packet.  The latter can only
      be fixed by applying a bug fix to the kernel.  Attacks on servers
      can often be fixed by properly specifying options to limit the load
      the servers incur on the system under adverse conditions.
      Brute-force network attacks are harder to deal with.  A
      spoofed-packet attack, for example, is nearly impossible to stop,
      short of cutting your system off from the Internet.  It may not be
      able to take your machine down, but it can saturate your
      Internet connection.</para>

    <indexterm>
      <primary>security</primary>
      <secondary>account compromises</secondary>
    </indexterm>

    <para>A user account compromise is even more common than a DoS
      attack.  Many sysadmins still run standard 
      <application>telnetd</application>, <application>rlogind</application>,
      <application>rshd</application>,
      and <application>ftpd</application> servers on their machines.
      These servers, by default, do
      not operate over encrypted connections.  The result is that if you
      have any moderate-sized user base, one or more of your users logging
      into your system from a remote location (which is the most common
      and convenient way to login to a system) will have his or her
      password sniffed.  The attentive system admin will analyze his
      remote access logs looking for suspicious source addresses even for
      successful logins.</para>

    <para>One must always assume that once an attacker has access to a
      user account, the attacker can break <username>root</username>.
      However, the reality is that in a well secured and maintained system,
      access to a user account does not necessarily give the attacker
      access to <username>root</username>.  The distinction is important
      because without access to <username>root</username> the attacker
      cannot generally hide his tracks and may, at best, be able to do
      nothing more than mess with the user's files, or crash the machine.
      User account compromises are very common because users tend not to
      take the precautions that sysadmins take.</para>

    <indexterm>
      <primary>security</primary>
      <secondary>backdoors</secondary>
    </indexterm>

    <para>System administrators must keep in mind that there are
      potentially many ways to break <username>root</username> on a machine.
      The attacker may know the <username>root</username> password,
      the attacker may find a bug in a root-run server and be able
      to break <username>root</username> over a network
      connection to that server, or the attacker may know of a bug in
      a suid-root program that allows the attacker to break
      <username>root</username> once he has broken into a user's account.
      If an attacker has found a way to break <username>root</username>
      on a machine, the attacker may not have a need
      to install a backdoor.  Many of the <username>root</username> holes
      found and closed to date involve a considerable amount of work
      by the attacker to cleanup after himself, so most attackers install
      backdoors.  A backdoor provides the attacker with a way to easily
      regain <username>root</username> access to the system, but it
      also gives the smart system administrator a convenient way
      to detect the intrusion.
      Making it impossible for an attacker to install a backdoor may
      actually be detrimental to your security, because it will not
      close off the hole the attacker found to break in the first
      place.</para>


    <para>Security remedies should always be implemented with a
      multi-layered <quote>onion peel</quote> approach and can be
      categorized as follows:</para>

    <orderedlist>
      <listitem>
	<para>Securing <username>root</username> and staff accounts.</para>
      </listitem>

      <listitem>
	<para>Securing <username>root</username> &ndash; root-run servers
	  and suid/sgid binaries.</para>
      </listitem>

      <listitem>
	<para>Securing user accounts.</para>
      </listitem>

      <listitem>
	<para>Securing the password file.</para>
      </listitem>

      <listitem>
	<para>Securing the kernel core, raw devices, and
	  filesystems.</para>
      </listitem>

      <listitem>
	<para>Quick detection of inappropriate changes made to the
	  system.</para>
      </listitem>

      <listitem>
	<para>Paranoia.</para>
      </listitem>
    </orderedlist>

    <para>The next section of this chapter will cover the above bullet
      items in greater depth.</para>
  </sect1>

  <indexterm>
    <primary>security</primary>
    <secondary>securing</secondary>
  </indexterm>

  <sect1 id="securing-freebsd">
    <title>Securing FreeBSD</title>

    <note>
      <title>Command vs. Protocol</title>
      <para>Throughout this document, we will use
       <application>bold</application> text to refer to a command or
       application. This is used for instances such as ssh, since it is
       a protocol as well as command.</para>
    </note>

    <para>The sections that follow will cover the methods of securing your
      FreeBSD system that were mentioned in the <link
        linkend="security-intro">last section</link> of this chapter.</para>

    <sect2 id="securing-root-and-staff">
      <title>Securing the <username>root</username> Account and
	Staff Accounts</title>
      <indexterm>
        <primary><command>su</command></primary>
      </indexterm>

      <para>First off, do not bother securing staff accounts if you have
	not secured the <username>root</username> account.
	Most systems have a password assigned to the <username>root</username>
	account.  The first thing you do is assume
	that the password is <emphasis>always</emphasis> compromised.
	This does not mean that you should remove the password.  The
	password is almost always necessary for console access to the
	machine.  What it does mean is that you should not make it
	possible to use the password outside of the console or possibly
	even with the &man.su.1; command.  For example, make sure that
	your pty's are specified as being insecure in the
	<filename>/etc/ttys</filename> file so that direct
	<username>root</username> logins
	via <command>telnet</command> or <command>rlogin</command> are
	disallowed.  If using other login services such as
        <application>sshd</application>, make sure that direct
	<username>root</username> logins are disabled there as well.
	You can do this by editing
        your <filename>/etc/ssh/sshd_config</filename> file, and making
        sure that <literal>PermitRootLogin</literal> is set to
        <literal>NO</literal>.  Consider every access method &ndash;
        services such as FTP often fall through the cracks.
	Direct <username>root</username> logins should only be allowed
	via the system console.</para>
      <indexterm>
        <primary><groupname>wheel</groupname></primary>
      </indexterm>

      <para>Of course, as a sysadmin you have to be able to get to
	<username>root</username>, so we open up a few holes.
	But we make sure these holes require additional password
	verification to operate.  One way to make <username>root</username>
	accessible is to add appropriate staff accounts to the
	<groupname>wheel</groupname> group (in
	<filename>/etc/group</filename>). The staff members placed in the
	<groupname>wheel</groupname> group are allowed to
	<command>su</command> to <username>root</username>.
	You should never give staff
	members native wheel access by putting them in the
	<groupname>wheel</groupname> group in their password entry.  Staff
	accounts should be placed in a <groupname>staff</groupname> group, and
	then added to the <groupname>wheel</groupname> group via the
	<filename>/etc/group</filename> file.  Only those staff members
	who actually need to have <username>root</username> access
	should be placed in the
	<groupname>wheel</groupname> group.  It is also possible, when using
	an authentication method such as Kerberos, to use Kerberos'
	<filename>.k5login</filename> file in the <username>root</username>
	account to allow a &man.ksu.1; to <username>root</username>
	without having to place anyone at all in the
	<groupname>wheel</groupname> group.  This may be the better solution
	since the <groupname>wheel</groupname> mechanism still allows an
	intruder to break <username>root</username> if the intruder
	has gotten hold of your
	password file and can break into a staff account.  While having
	the <groupname>wheel</groupname> mechanism is better than having
	nothing at all, it is not necessarily the safest option.</para>

      <para>An indirect way to secure staff accounts, and ultimately
        <username>root</username> access is to use an alternative
	login access method and
        do what is known as <quote>starring</quote> out the encrypted
        password for the staff accounts. Using the &man.vipw.8;
        command, one can replace each instance of an encrypted password
        with a single <quote><literal>*</literal></quote> character.
	This command will update the <filename>/etc/master.passwd</filename>
	file and user/password database to disable password-authenticated
        logins.</para>

      <para>A staff account entry such as:</para>

      <programlisting>foobar:R9DT/Fa1/LV9U:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh</programlisting>

      <para>Should be changed to this:</para>

      <programlisting>foobar:*:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh</programlisting>

      <para>This change will prevent normal logins from occurring,
        since the encrypted password will never match
        <quote><literal>*</literal></quote>.  With this done,
	staff members must use
        another mechanism to authenticate themselves such as
        &man.kerberos.1; or &man.ssh.1; using a public/private key
        pair.  When using something like Kerberos, one generally must
        secure the machines which run the Kerberos servers and your
        desktop workstation.  When using a public/private key pair
        with ssh, one must generally secure
        the machine used to login <emphasis>from</emphasis> (typically
        one's workstation).  An additional layer of protection can be
        added to the key pair by password protecting the key pair when
        creating it with &man.ssh-keygen.1;. Being able to
        <quote>star</quote> out the passwords for staff accounts also
        guarantees that staff members can only login through secure
        access methods that you have setup.  This forces all staff
        members to use secure, encrypted connections for all of their
        sessions, which closes an important hole used by many
        intruders: sniffing the network from an unrelated,
        less secure machine.</para>

      <para>The more indirect security mechanisms also assume that you are
	logging in from a more restrictive server to a less restrictive
	server.  For example, if your main box is running all sorts of
	servers, your workstation should not be running any.  In order for
	your workstation to be reasonably secure you should run as few
	servers as possible, up to and including no servers at all, and
	you should run a password-protected screen blanker.  Of course,
	given physical access to a workstation an attacker can break any
	sort of security you put on it.  This is definitely a problem that
	you should consider, but you should also consider the fact that the
	vast majority of break-ins occur remotely, over a network, from
	people who do not have physical access to your workstation or
	servers.</para>
      <indexterm><primary>Kerberos</primary></indexterm>

      <para>Using something like Kerberos also gives you the ability to
	disable or change the password for a staff account in one place,
	and have it immediately effect all the machines on which the staff
	member may have an account.  If a staff member's account gets
	compromised, the ability to instantly change his password on all
	machines should not be underrated.  With discrete passwords,
	changing a password on N machines can be a mess.  You can also
	impose re-passwording restrictions with Kerberos:  not only can a
	Kerberos ticket be made to timeout after a while, but the Kerberos
	system can require that the user choose a new password after a
	certain period of time (say, once a month).</para>
    </sect2>

    <sect2>
      <title>Securing Root-run Servers and SUID/SGID Binaries</title>

      <indexterm>
        <primary><command>ntalk</command></primary>
      </indexterm>
      <indexterm>
        <primary><command>comsat</command></primary>
      </indexterm>
      <indexterm>
        <primary><command>finger</command></primary>
      </indexterm>
      <indexterm>
        <primary>sandboxes</primary>
      </indexterm>
      <indexterm>
        <primary><application>sshd</application></primary>
      </indexterm>
      <indexterm>
        <primary><application>telnetd</application></primary>
      </indexterm>
      <indexterm>
        <primary><application>rshd</application></primary>
      </indexterm>
      <indexterm>
        <primary><application>rlogind</application></primary>
      </indexterm>

      <para>The prudent sysadmin only runs the servers he needs to, no
	more, no less.  Be aware that third party servers are often the
	most bug-prone.  For example, running an old version of
	<application>imapd</application> or
	<application>popper</application> is like giving a universal
	<username>root</username> ticket out to the entire world.
	Never run a server that you have not checked out carefully.
	Many servers do not need to be run as <username>root</username>.
	For example, the <application>ntalk</application>,
	<application>comsat</application>, and
	<application>finger</application> daemons can be run in special
	user <firstterm>sandboxes</firstterm>.  A sandbox is not perfect,
	unless you go through a large amount of trouble, but the onion
	approach to security still stands: If someone is able to break
	in through a server running in a sandbox, they still have to
	break out of the sandbox.  The more layers the attacker must
	break through, the lower the likelihood of his success.  Root
	holes have historically been found in virtually every server
	ever run as <username>root</username>, including basic system servers.
	If you are running a machine through which people only login via
	<application>sshd</application> and never login via
	<application>telnetd</application> or
	<application>rshd</application> or
	<application>rlogind</application>, then turn off those
	services!</para>
      
      <para>FreeBSD now defaults to running
	<application>ntalkd</application>,
	<application>comsat</application>, and
	<application>finger</application> in a sandbox.  Another program
	which may be a candidate for running in a sandbox is &man.named.8;.
	<filename>/etc/defaults/rc.conf</filename> includes the arguments
	necessary to run <application>named</application> in a sandbox in a
	commented-out form.  Depending on whether you are installing a new
	system or upgrading an existing system, the special user accounts
	used by these sandboxes may not be installed.  The prudent
	sysadmin would research and implement sandboxes for servers
	whenever possible.</para>
      <indexterm>
        <primary><application>sendmail</application></primary>
      </indexterm>

      <para>There are a number of other servers that typically do not run
	in sandboxes: <application>sendmail</application>,
	<application>popper</application>,
	<application>imapd</application>, <application>ftpd</application>,
	and others.  There are alternatives to some of these, but
	installing them may require more work than you are willing to
	perform (the convenience factor strikes again). You may have to
	run these servers as <username>root</username> and rely on other
	mechanisms to detect break-ins that might occur through them.</para>

      <para>The other big potential <username>root</username> holes in a
	system are the
	suid-root and sgid binaries installed on the system.  Most of
	these binaries, such as <application>rlogin</application>, reside
	in <filename>/bin</filename>, <filename>/sbin</filename>,
	<filename>/usr/bin</filename>, or <filename>/usr/sbin</filename>.
	While nothing is 100% safe, the system-default suid and sgid
	binaries can be considered reasonably safe.  Still,
	<username>root</username> holes are occasionally found in these
	binaries.  A <username>root</username> hole was found in
	<literal>Xlib</literal> in 1998 that made
	<application>xterm</application> (which is typically suid)
	vulnerable.  It is better to be safe than sorry and the prudent
	sysadmin will restrict suid binaries, that only staff should run,
	to a special group that only staff can access, and get rid of
	(<command>chmod 000</command>) any suid binaries that nobody uses.
	A server with no display generally does not need an
	<application>xterm</application> binary.  Sgid binaries can be
	almost as dangerous.  If an intruder can break an sgid-kmem binary,
	the intruder might be able to read <filename>/dev/kmem</filename>
	and thus read the encrypted password file, potentially compromising
	any passworded account.  Alternatively an intruder who breaks
	group <literal>kmem</literal> can monitor keystrokes sent through
	pty's, including pty's used by users who login through secure
	methods.  An intruder that breaks the <groupname>tty</groupname>
	group can write to
	almost any user's tty.  If a user is running a terminal program or
	emulator with a keyboard-simulation feature, the intruder can
	potentially generate a data stream that causes the user's terminal
	to echo a command, which is then run as that user.</para>
    </sect2>

    <sect2 id="secure-users">
      <title>Securing User Accounts</title>

      <para>User accounts are usually the most difficult to secure.  While
	you can impose Draconian access restrictions on your staff and
	<quote>star</quote> out their passwords, you may not be able to
	do so with any general user accounts you might have.  If you do
	have sufficient control, then you may win out and be able to secure
	the user accounts properly.  If not, you simply have to be more
	vigilant in your monitoring of those accounts.  Use of
	ssh and Kerberos for user accounts is
	more problematic, due to the extra administration and technical
	support required, but still a very good solution compared to a
	crypted password file.</para>
    </sect2>

    <sect2>
      <title>Securing the Password File</title>

      <para>The only sure fire way is to <literal>*</literal> out as many
	passwords as you can and use ssh or
	Kerberos for access to those accounts.  Even though the encrypted
	password file (<filename>/etc/spwd.db</filename>) can only be read
	by <username>root</username>, it may be possible for an intruder
	to obtain read access to that file even if the attacker cannot
	obtain root-write access.</para>

      <para>Your security scripts should always check for and report
	changes to the password file (see the <link
	  linkend="security-integrity">Checking file integrity</link> section
	below).</para>
    </sect2>

    <sect2>
      <title>Securing the Kernel Core, Raw Devices, and
	Filesystems</title>

      <para>If an attacker breaks <username>root</username> he can do
        just about anything, but
	there are certain conveniences.  For example, most modern kernels
	have a packet sniffing device driver built in.  Under FreeBSD it
	is called the <devicename>bpf</devicename> device.  An intruder
	will commonly attempt to run a packet sniffer on a compromised
	machine.  You do not need to give the intruder the capability and
	most systems do not have the need for the
	<devicename>bpf</devicename> device compiled in.</para>

      <indexterm>
        <primary><command>sysctl</command></primary>
      </indexterm>
      <para>But even if you turn off the <devicename>bpf</devicename>
	device, you still have
	<devicename>/dev/mem</devicename> and 
	<devicename>/dev/kmem</devicename>
	to worry about.  For that matter, the intruder can still write to
	raw disk devices.  Also, there is another kernel feature called
	the module loader, &man.kldload.8;.  An enterprising intruder can
	use a KLD module to install his own <devicename>bpf</devicename>
	device, or other sniffing
	device, on a running kernel.  To avoid these problems you have to
	run the kernel at a higher secure level, at least securelevel 1.
	The securelevel can be set with a <command>sysctl</command> on
	the <varname>kern.securelevel</varname> variable.  Once you have
	set the securelevel to 1, write access to raw devices will be
	denied and special <command>chflags</command> flags,
	such as <literal>schg</literal>,
	will be enforced.  You must also ensure that the
	<literal>schg</literal> flag is set on critical startup binaries,
	directories, and script files &ndash; everything that gets run up
	to the point where the securelevel is set.  This might be overdoing
	it, and upgrading the system is much more difficult when you
	operate at a higher secure level.  You may compromise and run the
	system at a higher secure level but not set the
	<literal>schg</literal> flag for every system file and directory
	under the sun.  Another possibility is to simply mount
	<filename>/</filename> and <filename>/usr</filename> read-only.
	It should be noted that being too Draconian in what you attempt to
	protect may prevent the all-important detection of an
	intrusion.</para>
    </sect2>

    <sect2 id="security-integrity">
      <title>Checking File Integrity: Binaries, Configuration Files,
	Etc.</title>

      <para>When it comes right down to it, you can only protect your core
	system configuration and control files so much before the
	convenience factor rears its ugly head.  For example, using
	<command>chflags</command> to set the <literal>schg</literal> bit
	on most of the files in <filename>/</filename> and
	<filename>/usr</filename> is probably counterproductive, because
	while it may protect the files, it also closes a detection window.
	The last layer of your security onion is perhaps the most
	important &ndash; detection.  The rest of your security is pretty
	much useless (or, worse, presents you with a false sense of
	safety) if you cannot detect potential incursions.  Half the job
	of the onion is to slow down the attacker, rather than stop him, in
	order to give the detection side of the equation a chance to catch
	him in the act.</para>

      <para>The best way to detect an incursion is to look for modified,
	missing, or unexpected files.  The best way to look for modified
	files is from another (often centralized) limited-access system.
	Writing your security scripts on the extra-secure limited-access
	system makes them mostly invisible to potential attackers, and this
	is important.  In order to take maximum advantage you generally
	have to give the limited-access box significant access to the
	other machines in the business, usually either by doing a
	read-only NFS export of the other machines to the limited-access
	box, or by setting up ssh key-pairs to
	allow the limited-access box to ssh to
	the other machines.  Except for its network traffic, NFS is the
	least visible method &ndash; allowing you to monitor the
	filesystems on each client box virtually undetected.  If your
	limited-access server is connected to the client boxes through a
	switch, the NFS method is often the better choice.  If your
	limited-access server is connected to the client boxes through a
	hub, or through several layers of routing, the NFS method may be
	too insecure (network-wise) and using
	ssh may be the better choice even with
	the audit-trail tracks that ssh
	lays.</para>

      <para>Once you give a limited-access box, at least read access to the
	client systems it is supposed to monitor, you must write scripts
	to do the actual monitoring.  Given an NFS mount, you can write
	scripts out of simple system utilities such as &man.find.1; and
	&man.md5.1;.  It is best to physically md5 the client-box files
	at least once a day, and to test control files such as those
	found in <filename>/etc</filename> and
	<filename>/usr/local/etc</filename> even more often.  When
	mismatches are found, relative to the base md5 information the
	limited-access machine knows is valid, it should scream at a
	sysadmin to go check it out.  A good security script will also
	check for inappropriate suid binaries and for new or deleted files
	on system partitions such as <filename>/</filename> and
	<filename>/usr</filename>.</para>

      <para>When using ssh rather than NFS,
	writing the security script is much more difficult.   You
	essentially have to <command>scp</command> the scripts to the client 
	box in order to
	run them, making them visible, and for safety you also need to
	<command>scp</command> the binaries (such as find) that those
	scripts use.  The <application>ssh</application> client on the
	client box may already be compromised.  All in all, using
	ssh may be necessary when running over
	insecure links, but it is also a lot harder to deal with.</para>

      <para>A good security script will also check for changes to user and
	staff members access configuration files:
	<filename>.rhosts</filename>, <filename>.shosts</filename>,
	<filename>.ssh/authorized_keys</filename> and so forth&hellip;
	files that might fall outside the purview of the
	<literal>MD5</literal> check.</para>

      <para>If you have a huge amount of user disk space, it may take too
	long to run through every file on those partitions.  In this case,
	setting mount flags to disallow suid binaries and devices on those
	partitions is a good idea.  The <literal>nodev</literal> and
	<literal>nosuid</literal> options (see &man.mount.8;) are what you
	want to look into.  You should probably scan them anyway, at least
	once a week, since the object of this layer is to detect a break-in
	whether or not the break-in is effective.</para>

      <para>Process accounting (see &man.accton.8;) is a relatively
	low-overhead feature of the operating system which might help
	as a post-break-in evaluation mechanism.  It is especially
	useful in tracking down how an intruder has actually broken into
	a system, assuming the file is still intact after the break-in
	occurs.</para>

      <para>Finally, security scripts should process the log files, and the
	logs themselves should be generated in as secure a manner as
	possible &ndash; remote syslog can be very useful.  An intruder
	tries to cover his tracks, and log files are critical to the
	sysadmin trying to track down the time and method of the initial
	break-in.  One way to keep a permanent record of the log files is
	to run the system console to a serial port and collect the
	information on a continuing basis through a secure machine
	monitoring the consoles.</para>
    </sect2>

    <sect2>
      <title>Paranoia</title>

      <para>A little paranoia never hurts.  As a rule, a sysadmin can add
	any number of security features, as long as they do not effect
	convenience, and can add security features that
	<emphasis>do</emphasis> effect convenience with some added thought.
	Even more importantly, a security administrator should mix it up a
	bit &ndash; if you use recommendations such as those given by this
	document verbatim, you give away your methodologies to the
	prospective attacker who also has access to this document.</para>
    </sect2>

    <sect2>
      <title>Denial of Service Attacks</title>
      <indexterm><primary>Denial of Service (DoS)</primary></indexterm>

      <para>This section covers Denial of Service attacks.  A DoS attack
	is typically a packet attack.  While there is not much you can do
	about modern spoofed packet attacks that saturate your network,
	you can generally limit the damage by ensuring that the attacks
	cannot take down your servers.</para>

      <orderedlist>
	<listitem>
	  <para>Limiting server forks.</para>
	</listitem>

	<listitem>
	  <para>Limiting springboard attacks (ICMP response attacks, ping
	    broadcast, etc.).</para>
	</listitem>

	<listitem>
	  <para>Kernel Route Cache.</para>
	</listitem>
      </orderedlist>

      <para>A common DoS attack is against a forking server that attempts
	to cause the server to eat processes, file descriptors, and memory,
	until the machine dies.  <application>inetd</application>
	(see &man.inetd.8;) has several
	options to limit this sort of attack.  It should be noted that
	while it is possible to prevent a machine from going down, it is
	not generally possible to prevent a service from being disrupted
	by the attack.  Read the <application>inetd</application> manual
	page carefully and pay
	specific attention to the <option>-c</option>, <option>-C</option>,
	and <option>-R</option> options.  Note that spoofed-IP attacks
	will circumvent the <option>-C</option> option to 
	<application>inetd</application>, so
	typically a combination of options must be used.  Some standalone
	servers have self-fork-limitation parameters.</para>

      <para><application>Sendmail</application> has its
	<option>-OMaxDaemonChildren</option> option, which tends to work
	much better than trying to use sendmail's load limiting options
	due to the load lag.  You should specify a
	<literal>MaxDaemonChildren</literal> parameter, when you start
	<application>sendmail</application>, high enough to handle your
	expected load, but not so high that the computer cannot handle that
	number of <application>sendmails</application> without falling on
	its face.  It is also prudent to run sendmail in queued mode
	(<option>-ODeliveryMode=queued</option>) and to run the daemon
	(<command>sendmail -bd</command>) separate from the queue-runs
	(<command>sendmail -q15m</command>). If you still want real-time
	delivery you can run the queue at a much lower interval, such as
	<option>-q1m</option>, but be sure to specify a reasonable
	<literal>MaxDaemonChildren</literal> option for
	<emphasis>that</emphasis> sendmail to prevent cascade failures.</para>

      <para><application>Syslogd</application> can be attacked directly
	and it is strongly recommended that you use the <option>-s</option>
	option whenever possible, and the <option>-a</option> option
	otherwise.</para>

      <para>You should also be fairly careful with connect-back services
	such as <application>tcpwrapper</application>'s reverse-identd,
	which can be attacked directly.  You generally do not want to use
	the reverse-ident feature of
	<application>tcpwrappers</application> for this reason.</para>

      <para>It is a very good idea to protect internal services from
	external access by firewalling them off at your border routers.
	The idea here is to prevent saturation attacks from outside your
	LAN, not so much to protect internal services from network-based
	<username>root</username> compromise.
	Always configure an exclusive firewall, i.e.,
	<quote>firewall everything <emphasis>except</emphasis> ports A, B,
	C, D, and M-Z</quote>.  This way you can firewall off all of your
	low ports except for certain specific services such as
	<application>named</application> (if you are primary for a zone),
	<application>ntalkd</application>,
	<application>sendmail</application>, and other Internet-accessible
	services.  If you try to configure the firewall the other way
	&ndash; as an inclusive or permissive firewall, there is a good
	chance that you will forget to <quote>close</quote> a couple of
	services, or that you will add a new internal service and forget
	to update the firewall.  You can still open up the high-numbered
	port range on the firewall, to allow permissive-like operation,
	without compromising your low ports.  Also take note that FreeBSD
	allows you to control the range of port numbers used for dynamic
	binding, via the various <varname>net.inet.ip.portrange</varname>
	<command>sysctl</command>'s (<command>sysctl -a | fgrep
	portrange</command>), which can also ease the complexity of your
	firewall's configuration.  For example, you might use a normal
	first/last range of 4000 to 5000, and a hiport range of 49152 to
	65535, then block off everything under 4000 in your firewall
	(except for certain specific Internet-accessible ports, of
	course).</para>

      <indexterm><primary>ICMP_BANDLIM</primary></indexterm>

      <para>Another common DoS attack is called a springboard attack
	&ndash; to attack a server in a manner that causes the server to
	generate responses which overloads the server, the local
	network, or some other machine.  The most common attack of this
	nature is the <emphasis>ICMP ping broadcast attack</emphasis>.
	The attacker spoofs ping packets sent to your LAN's broadcast
	address with the source IP address set to the actual machine they
	wish to attack.  If your border routers are not configured to
	stomp on ping's to broadcast addresses, your LAN winds up
	generating sufficient responses to the spoofed source address to
	saturate the victim, especially when the attacker uses the same
	trick on several dozen broadcast addresses over several dozen
	different networks at once.  Broadcast attacks of over a hundred
	and twenty megabits have been measured.  A second common
	springboard attack is against the ICMP error reporting system.
	By constructing packets that generate ICMP error responses, an
	attacker can saturate a server's incoming network and cause the
	server to saturate its outgoing network with ICMP responses.  This
	type of attack can also crash the server by running it out of
	mbuf's, especially if the server cannot drain the ICMP responses
	it generates fast enough.  The FreeBSD kernel has a new kernel
	compile option called <option>ICMP_BANDLIM</option>
	which limits the effectiveness
	of these sorts of attacks.  The last major class of springboard
	attacks is related to certain internal 
	<application>inetd</application> services such as the
	udp echo service.  An attacker simply spoofs a UDP packet with the
	source address being server A's echo port, and the destination
	address being server B's echo port, where server A and B are both
	on your LAN.  The two servers then bounce this one packet back and
	forth between each other.  The attacker can overload both servers
	and their LANs simply by injecting a few packets in this manner.
	Similar problems exist with the internal 
	<application>chargen</application> port.  A
	competent sysadmin will turn off all of these inetd-internal test
	services.</para>

      <para>Spoofed packet attacks may also be used to overload the kernel
	route cache.  Refer to the <varname>net.inet.ip.rtexpire</varname>,
	<varname>rtminexpire</varname>, and <varname>rtmaxcache</varname>
	<command>sysctl</command> parameters.  A spoofed packet attack
	that uses a random source IP will cause the kernel to generate a
	temporary cached route in the route table, viewable with
	<command>netstat -rna | fgrep W3</command>.  These routes
	typically timeout in 1600 seconds or so.  If the kernel detects
	that the cached route table has gotten too big it will dynamically
	reduce the <varname>rtexpire</varname> but will never decrease it 
	to less than <varname>rtminexpire</varname>.  There are two 
	problems:</para>
	
      <orderedlist>
	<listitem>
	  <para>The kernel does not react quickly enough when a lightly
	    loaded server is suddenly attacked.</para>
	</listitem>
	
	<listitem>
	  <para>The <varname>rtminexpire</varname> is not low enough for
	    the kernel to survive a sustained attack.</para>
	</listitem>
      </orderedlist>
      
      <para>If your servers are connected to the Internet via a T3 or
        better, it may be prudent to manually override both
	<varname>rtexpire</varname> and <varname>rtminexpire</varname>
	via &man.sysctl.8;.  Never set either parameter to zero (unless
	you want to crash the machine).  Setting both
	parameters to 2 seconds should be sufficient to protect the route
	table from attack.</para>
    </sect2>

    <sect2>
      <title>Access Issues with Kerberos and SSH</title>
      <indexterm><primary><command>ssh</command></primary></indexterm>
      <indexterm><primary>Kerberos</primary></indexterm>

      <para>There are a few issues with both Kerberos and
	ssh that need to be addressed if
	you intend to use them.  Kerberos V is an excellent
	authentication protocol, but there are bugs in the kerberized
	<application>telnet</application> and
	<application>rlogin</application> applications that make them
	unsuitable for dealing with binary streams.  Also, by default
	Kerberos does not encrypt a session unless you use the
	<option>-x</option> option.  <application>ssh</application>
	encrypts everything by default.</para>

      <para>ssh works quite well in every
	respect except that it forwards encryption keys by default.  What
	this means is that if you have a secure workstation holding keys
	that give you access to the rest of the system, and you
	ssh to an insecure machine, your keys
	becomes exposed.  The actual keys themselves are not exposed, but
	ssh installs a forwarding port for the
	duration of your login, and if an attacker has broken
	<username>root</username> on the
	insecure machine he can utilize that port to use your keys to gain
	access to any other machine that your keys unlock.</para>

      <para>We recommend that you use ssh in
	combination with Kerberos whenever possible for staff logins.
	<application>ssh</application> can be compiled with Kerberos
	support.  This reduces your reliance on potentially exposable
	ssh keys while at the same time
	protecting passwords via Kerberos.  ssh
	keys should only be used for automated tasks from secure machines
	(something that Kerberos is unsuited to do).  We also recommend that
	you either turn off key-forwarding in the
	ssh configuration, or that you make use
	of the <literal>from=IP/DOMAIN</literal> option that
	ssh allows in its
	<filename>authorized_keys</filename> file to make the key only
	usable to entities logging in from specific machines.</para>
    </sect2>
  </sect1>

  <sect1 id="crypt">
    <sect1info>
      <authorgroup>
	<author>
	  <firstname>Bill</firstname>
	  <surname>Swingle</surname>
	  <contrib>Parts rewritten and updated by </contrib>
	</author>
      </authorgroup>
      <!-- 21 Mar 2000 -->
    </sect1info>

    <title>DES, MD5, and Crypt</title>
    <indexterm>
      <primary>security</primary>
      <secondary>crypt</secondary>
    </indexterm>

    <indexterm><primary>crypt</primary></indexterm>
    <indexterm><primary>DES</primary></indexterm>
    <indexterm><primary>MD5</primary></indexterm>

    <para>Every user on a Unix system has a password associated with
      their account.  It seems obvious that these passwords need to be
      known only to the user and the actual operating system.  In
      order to keep these passwords secret, they are encrypted with
      what is known as a <quote>one-way hash</quote>, that is, they can
      only be easily encrypted but not decrypted.  In other words, what
      we told you a moment ago was obvious is not even true:  the
      operating system itself does not <emphasis>really</emphasis> know
      the password.  It only knows the <emphasis>encrypted</emphasis>
      form of the password.  The only way to get the
      <quote>plain-text</quote> password is by a brute force search of the
      space of possible passwords.</para>

    <para>Unfortunately the only secure way to encrypt passwords when
      Unix came into being was based on DES, the Data Encryption
      Standard.  This was not such a problem for users resident in
      the US, but since the source code for DES could not be exported
      outside the US, FreeBSD had to find a way to both comply with
      US law and retain compatibility with all the other Unix
      variants that still used DES.</para>

    <para>The solution was to divide up the encryption libraries 
      so that US users could install the DES libraries and use
      DES but international users still had an encryption method
      that could be exported abroad.  This is how FreeBSD came to
      use MD5 as its default encryption method.  MD5 is believed to
      be more secure than DES, so installing DES is offered primarily
      for compatibility reasons.</para>

    <sect2>
      <title>Recognizing Your Crypt Mechanism</title>

      <para>Before FreeBSD 4.4 <filename>libcrypt.a</filename> was a
	symbolic link pointing to the library which was used for
	encryption.  FreeBSD 4.4 changed <filename>libcrypt.a</filename> to
	provide a configurable password authentication hash library.
	Currently the library supports DES, MD5 and Blowfish hash
	functions.  By default FreeBSD uses MD5 to encrypt
	passwords.</para>

      <para>It is pretty easy to identify which encryption method 
	FreeBSD is set up to use.  Examining the encrypted passwords in
	the <filename>/etc/master.passwd</filename> file is one way.
	Passwords encrypted with the MD5 hash are longer than those
	encrypted with the DES hash and also begin with the characters
	<literal>&dollar;1&dollar;</literal>.  Passwords starting with
	<literal>&dollar;2&dollar;</literal> are encrypted with the
	Blowfish hash function. DES password strings do not
	have any particular identifying characteristics, but they are
	shorter than MD5 passwords, and are coded in a 64-character
	alphabet which does not include the <literal>&dollar;</literal>
	character, so a relatively short string which does not begin with
	a dollar sign is very likely a DES password.</para>

      <para>The password format used for new passwords is controlled
	by the <quote>passwd_format</quote> login capability in
	<filename>/etc/login.conf</filename>, which takes values of
	<quote>des</quote> or <quote>md5</quote> or
	<quote>blf</quote>.  See the &man.login.conf.5; manual page
	for more information about login capabilities.</para>

    </sect2>
  </sect1>

  <sect1 id="skey">
    <title>S/Key</title>
    <indexterm><primary>S/Key</primary></indexterm>
    <indexterm>
      <primary>security</primary>
      <secondary>S/Key</secondary>
    </indexterm>

    <para>S/Key is a one-time password scheme based on a one-way hash
      function.  FreeBSD uses the MD4 hash for compatibility but other
      systems have used MD5 and DES-MAC.  S/Key has been part of the
      FreeBSD base system since version 1.1.5 and is also used on a
      growing number of other operating systems.  S/Key is a registered
      trademark of Bell Communications Research, Inc.</para>

    <para>From version 5.0 of FreeBSD, S/Key has been replaced with
      the functionally equivalent OPIE (Onetime Passwords In
      Everything). OPIE uses the MD5 hash by default.</para>

    <para>There are three different sorts of passwords which we will talk
      about in the discussion below.  The first is your usual Unix-style or
      Kerberos password; we will call this a <quote>Unix password</quote>.
      The second sort is the one-time password which is generated by the
      S/Key <command>key</command> program or the OPIE
      <command>opiekey</command> program and accepted by the
      <command>keyinit</command> or <command>opiepasswd</command> programs
      and the login prompt; we will
      call this a <quote>one-time password</quote>.  The final sort of
      password is the secret password which you give to the
      <command>key</command>/<command>opiekey</command> programs (and
      sometimes the
      <command>keyinit</command>/<command>opiepasswd</command> programs)
      which it uses to generate
      one-time passwords; we will call it a <quote>secret password</quote>
      or just unqualified <quote>password</quote>.</para>

    <para>The secret password does not have anything to do with your Unix
      password; they can be the same but this is not recommended.  S/Key
      and OPIE secret passwords are not limited to 8 characters like Unix
      passwords, they can be as long as you like.  Passwords of six or
      seven word long phrases are fairly common.  For the most part, the
      S/Key or OPIE system operates completely independently of the Unix
      password system.</para>

    <para>Besides the password, there are two other pieces of data that
      are important to S/Key and OPIE.  One is what is known as the
      <quote>seed</quote> or <quote>key</quote>, consisting of two letters
      and five digits.  The other is what is called the <quote>iteration
      count</quote>, a number between 1 and 100.  S/Key creates the
      one-time password by concatenating the seed and the secret password,
      then applying the MD4/MD5 hash as many times as specified by the
      iteration count and turning the result into six short English words.
      These six English words are your one-time password.  The
      authentication system (primarily PAM) keeps
      track of the last one-time password used, and the user is
      authenticated if the hash of the user-provided password is equal to
      the previous password.  Because a one-way hash is used it is
      impossible to generate future one-time passwords if a successfully
      used password is captured; the iteration count is decremented after
      each successful login to keep the user and the login program in
      sync.  When the iteration count gets down to 1, S/Key and OPIE must be
      reinitialized.</para>

    <para>There are three programs involved in each system
      which we will discuss below. The <command>key</command> and
      <command>opiekey</command> programs accept an iteration
      count, a seed, and a secret password, and generate a one-time
      password or or a consecutive list of one-time passwords. The
      <command>keyinit</command> and <command>opiepasswd</command>
      programs are used to initialize S/Key and OPIE respectively,
      and to change passwords, iteration counts, or seeds; they
      take either a secret passphrase, or an iteration count,
      seed, and one-time password. The <command>keyinfo</command>
      and <command>opieinfo</command> programs examine the
      relevant credentials files (<filename>/etc/skeykeys</filename> or
      <filename>/etc/opiekeys</filename>) and print out the invoking user's
      current iteration count and seed.</para>

    <para>There are four different sorts of operations we will cover.  The
      first is using <command>keyinit</command> or
      <command>opiepasswd</command> over a secure connection to set up
      one-time-passwords for the first time,  or to change your password
      or seed.  The second operation is using <command>keyinit</command>
      or <command>opiepasswd</command> over an insecure connection, in
      conjunction with <command>key</command> or <command>opiekey</command>
      over a secure connection,  to do the same.  The third is using
      <command>key</command>/<command>opiekey</command> to log in over
      an insecure connection.  The fourth is using <command>key</command>
      or <command>opiekey</command> to generate a number of keys which
      can be written down or printed out to carry with you when going to
      some location without secure connections to anywhere.</para>

    <sect2>
      <title>Secure Connection Initialization</title>

      <para>To initialize S/Key for the first time, change your password,
	or change your seed while logged in over a secure connection
	(e.g., on the console of a machine or via ssh), use the
	<command>keyinit</command> command without any parameters while
	logged in as yourself:</para>

      <screen>&prompt.user; <userinput>keyinit</userinput>
Adding unfurl:
Reminder - Only use this method if you are directly connected.
If you are using telnet or rlogin exit with no password and use keyinit -s.
Enter secret password: 
Again secret password: 

ID unfurl s/key is 99 to17757
DEFY CLUB PRO NASH LACE SOFT</screen>

      <para>For OPIE, <command>opiepasswd</command> is used instead:</para>

      <screen>&prompt.user; <userinput>opiepasswd -c</userinput>
[grimreaper] ~ $ opiepasswd -f -c
Adding unfurl:
Only use this method from the console; NEVER from remote. If you are using
telnet, xterm, or a dial-in, type ^C now or exit with no password.
Then run opiepasswd without the -c parameter.
Using MD5 to compute responses.
Enter new secret pass phrase:
Again new secret pass phrase:
ID unfurl OTP key is 499 to4268
MOS MALL GOAT ARM AVID COED
</screen>

      <para>At the <prompt>Enter new secret pass phrase:</prompt> or
        <prompt>Enter secret password:</prompt> prompts, you
	should enter a password or phrase.  Remember, this is not the
	password that you will use to login with, this is used to generate
	your one-time login keys.  The <quote>ID</quote> line gives the
	parameters of your particular instance; your login name, the
        iteration count, and seed.  When logging in the system
	will remember these parameters and present them back to you so you
	do not have to remember them.  The last line gives the particular
	one-time password which corresponds to those parameters and your
	secret password; if you were to re-login immediately, this
	one-time password is the one you would use.</para>
    </sect2>

    <sect2>
      <title>Insecure Connection Initialization</title>
      
      <para>To initialize or change your secret password over an
	insecure connection, you will need to already have a secure
	connection to some place where you can run <command>key</command>
        or <command>opiekey</command>; this might be in the form of a
	desk accessory on a Macintosh, or a shell prompt on a machine you
	trust.  You will also need to make up an iteration count (100 is
	probably a good value), and you may make up your own seed or use a
	randomly-generated one.  Over on the insecure connection (to the
	machine you are initializing), use the <command>keyinit
	-s</command> command:</para>

      <screen>&prompt.user; <userinput>keyinit -s</userinput>
Updating unfurl:
Old key: to17758
Reminder you need the 6 English words from the key command.
Enter sequence count from 1 to 9999: <userinput>100</userinput>
Enter new key [default to17759]: 
s/key 100 to 17759
s/key access password:
s/key access password:<userinput>CURE MIKE BANE HIM RACY GORE</userinput>
</screen>

      <para>For OPIE, you need to use <command>opiepasswd</command>:</para>

      <screen>&prompt.user; <userinput>opiepasswd</userinput>

Updating unfurl:
You need the response from an OTP generator.
Old secret pass phrase:
        otp-md5 498 to4268 ext
        Response: GAME GAG WELT OUT DOWN CHAT
New secret pass phrase:
        otp-md5 499 to4269
        Response: LINE PAP MILK NELL BUOY TROY

ID mark OTP key is 499 gr4269
LINE PAP MILK NELL BUOY TROY
</screen>

      <para>To accept the default seed (which the
	<command>keyinit</command> program confusingly calls a
	<literal>key</literal>), press <keycap>Return</keycap>.
	Then before entering an
	access password, move over to your secure connection or S/Key desk
	accessory, and give it the same parameters:</para>

      <screen>&prompt.user; <userinput>key 100 to17759</userinput>
Reminder - Do not use this program while logged in via telnet or rlogin.
Enter secret password: <userinput>&lt;secret password&gt;</userinput>
CURE MIKE BANE HIM RACY GORE</screen>

      <para>Or for OPIE:</para>

      <screen>&prompt.user; <userinput>opiekey 498 to4268</userinput>
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT
</screen>

      <para>Now switch back over to the insecure connection, and copy the
	one-time password generated over to the relevant program.</para>
    </sect2>

    <sect2>
      <title>Generating a Single one-time Password</title>

      <para>Once you have initialized S/Key or OPIE, when you login you will be 
	presented with a prompt like this:</para>

<screen>&prompt.user; <userinput>telnet example.com</userinput>
Trying 10.0.0.1...
Connected to example.com
Escape character is '^]'.

FreeBSD/i386 (example.com) (ttypa)

login: <userinput>&lt;username&gt;</userinput>
s/key 97 fw13894
Password: </screen>

      <para>Or for OPIE:</para>

<screen>&prompt.user; <userinput>telnet example.com</userinput>
Trying 10.0.0.1...
Connected to example.com
Escape character is '^]'.

FreeBSD/i386 (example.com) (ttypa)

login: <userinput>&lt;username&gt;</userinput>
otp-md5 498 gr4269 ext
Password: </screen>

      <para>As a side note, the S/Key and OPIE prompts have a useful feature
	(not shown here): if you press <keycap>Return</keycap>
	at the password prompt, the
	prompter will turn echo on, so you can see what you are
	typing.  This can be extremely useful if you are attempting to
	type in a password by hand, such as from a printout.</para>

      <indexterm><primary>MS-DOS</primary></indexterm>
      <indexterm><primary>Windows</primary></indexterm>
      <indexterm><primary>MacOS</primary></indexterm>

      <para>At this point you need to generate your one-time password to
	answer this login prompt.  This must be done on a trusted system
	that you can run <command>key</command> or
        <command>opiekey</command> on.  (There are versions of these for DOS,
	Windows and MacOS as well.) They need both the iteration count and
	the seed as command line options.  You can cut-and-paste these
        right from the login prompt on the machine that you are logging
        in to.</para>

      <para>On the trusted system:</para>

      <screen>&prompt.user; <userinput>key 97 fw13894</userinput>
Reminder - Do not use this program while logged in via telnet or rlogin.
Enter secret password: 
WELD LIP ACTS ENDS ME HAAG</screen>

      <para>For OPIE:</para>

      <screen>&prompt.user; <userinput>opiekey 498 to4268</userinput>
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT</screen>

      <para>Now that you have your one-time password you can continue
	logging in:</para>

      <screen>login: <userinput>&lt;username&gt;</userinput>
s/key 97 fw13894
Password: <userinput>&lt;return to enable echo&gt;</userinput>
s/key 97 fw13894
Password [echo on]: WELD LIP ACTS ENDS ME HAAG
Last login: Tue Mar 21 11:56:41 from 10.0.0.2 ... </screen>

    </sect2>

    <sect2>
      <title>Generating Multiple one-time Passwords</title>

      <para>Sometimes you have to go places where you do not have
	access to a trusted machine or secure connection.  In this case,
	it is possible to use the <command>key</command> command to
	generate a number of one-time passwords before hand to be printed
	out and taken with you.  For example:</para>

      <screen>&prompt.user; <userinput>key -n 5 30 zz99999</userinput>
Reminder - Do not use this program while logged in via telnet or rlogin.
Enter secret password: <userinput>&lt;secret password&gt;</userinput>
26: SODA RUDE LEA LIND BUDD SILT 
27: JILT SPY DUTY GLOW COWL ROT  
28: THEM OW COLA RUNT BONG SCOT  
29: COT MASH BARR BRIM NAN FLAG  
30: CAN KNEE CAST NAME FOLK BILK</screen>

      <para>The <option>-n 5</option> requests five keys in sequence, the
	<option>30</option> specifies what the last iteration number
	should be.  Note that these are printed out in
	<emphasis>reverse</emphasis> order of eventual use.  If you are
	really paranoid, you might want to write the results down by hand;
	otherwise you can cut-and-paste into <command>lpr</command>.  Note
	that each line shows both the iteration count and the one-time
	password; you may still find it handy to scratch off passwords as
	you use them.</para>
    </sect2>

    <sect2>
      <title>Restricting Use of Unix Passwords</title>

      <para>Restrictions can be placed on the use of Unix passwords based
	on the host name, user name, terminal port, or IP address of a
	login session.  These restrictions can be found in the
	configuration file <filename>/etc/skey.access</filename>.  The
	&man.skey.access.5; manual page has more information on the complete
	format of the file and also details some security cautions to be
	aware of before depending on this file for security.</para>

      <para>If there is no <filename>/etc/skey.access</filename> file
	(this is the FreeBSD default), then all users will be allowed to
	use Unix passwords.  If the file exists, however, then all users
	will be required to use S/Key unless explicitly permitted to do
	otherwise by configuration statements in the
	<filename>skey.access</filename> file.  In all cases, Unix
	passwords are permitted on the console.</para>

      <para>Here is a sample configuration file which illustrates the
	three most common sorts of configuration statements:</para>

      <programlisting>permit internet 192.168.0.0 255.255.0.0
permit user fnord
permit port ttyd0</programlisting>

      <para>The first line (<literal>permit internet</literal>) allows
	users whose IP source address (which is vulnerable to spoofing)
	matches the specified value and mask, to use Unix passwords.  This
	should not be considered a security mechanism, but rather, a means
	to remind authorized users that they are using an insecure network
	and need to use S/Key for authentication.</para>

      <para>The second line (<literal>permit user</literal>) allows the
	specified username, in this case <username>fnord</username>, to use
	Unix passwords at any time.  Generally speaking, this should only
	be used for people who are either unable to use the
	<command>key</command> program, like those with dumb terminals, or
	those who are uneducable.</para>

      <para>The third line (<literal>permit port</literal>) allows all
	users logging in on the specified terminal line to use Unix
	passwords; this would be used for dial-ups.</para>
    </sect2>
  </sect1>

  <sect1 id="kerberos">
    <sect1info>
      <authorgroup>
	<author>
	  <firstname>Mark</firstname>
	  <surname>Murray</surname>
	  <contrib>Contributed by </contrib>
	</author>
      </authorgroup>
      <authorgroup>
	<author>
	  <firstname>Mark</firstname>
	  <surname>Dapoz</surname>
	  <contrib>Based on a contribution by </contrib>
	</author>
      </authorgroup>
    </sect1info>

    <title>Kerberos</title>
    <indexterm><primary>Kerberos</primary></indexterm>
    
    <para>Kerberos is a network add-on system/protocol that allows users to
      authenticate themselves through the services of a secure server.
      Services such as remote login, remote copy, secure inter-system file
      copying and other high-risk tasks are made considerably safer and more
      controllable.</para>

    <para>The following instructions can be used as a guide on how to set up
      Kerberos as distributed for FreeBSD.  However, you should refer to the
      relevant manual pages for a complete description.</para>

    <sect2>
      <title>Installing Kerberos</title>

      <indexterm><primary>MIT</primary></indexterm>
      <indexterm>
	<primary>Kerberos</primary>
	<secondary>installing</secondary>
      </indexterm>
      <para>Kerberos is an optional component of FreeBSD.  The easiest
        way to install this software is by selecting the 'krb4' or
        'krb5' distribution in <application>sysinstall</application>
        during the initial installation of FreeBSD.  This will install
        the 'eBones' (KerberosIV) or 'Heimdal' (Kerberos5)
        implementation of Kerberos.  These implementations are
        included because they are developed outside the USA/Canada and
        were thus available to system owners outside those countries
        during the era of restrictive export controls on cryptographic
        code from the USA.</para>

      <para>Alternatively, the MIT implementation of Kerberos is
        available from the ports collection as
        <filename role="package">security/krb5</filename>.</para>
    </sect2>

    <sect2>
      <title>Creating the Initial Database</title>
      
      <para>This is done on the Kerberos server only.  First make sure that
	you do not have any old Kerberos databases around.  You should change
	to the directory <filename>/etc/kerberosIV</filename> and check that
	only the following files are present:</para>
	  
      <screen>&prompt.root; <userinput>cd /etc/kerberosIV</userinput>
&prompt.root; <userinput>ls</userinput>
README		krb.conf        krb.realms</screen>
	  
      <para>If any additional files (such as <filename>principal.*</filename>
	or <filename>master_key</filename>) exist, then use the
	<command>kdb_destroy</command> command to destroy the old Kerberos
	database, or if Kerberos is not running, simply delete the extra
	files.</para>
	  
      <para>You should now edit the <filename>krb.conf</filename> and
	<filename>krb.realms</filename> files to define your Kerberos realm.
	In this case the realm will be <filename>EXAMPLE.COM</filename> and the
	server is <hostid role="fqdn">grunt.example.com</hostid>.  We edit
	or create the <filename>krb.conf</filename> file:</para>
	  
      <screen>&prompt.root; <userinput>cat krb.conf</userinput>
EXAMPLE.COM
EXAMPLE.COM grunt.example.com admin server
CS.BERKELEY.EDU okeeffe.berkeley.edu
ATHENA.MIT.EDU kerberos.mit.edu
ATHENA.MIT.EDU kerberos-1.mit.edu
ATHENA.MIT.EDU kerberos-2.mit.edu
ATHENA.MIT.EDU kerberos-3.mit.edu
LCS.MIT.EDU kerberos.lcs.mit.edu
TELECOM.MIT.EDU bitsy.mit.edu
ARC.NASA.GOV trident.arc.nasa.gov</screen>
	  
      <para>In this case, the other realms do not need to be there.  They are
	here as an example of how a machine may be made aware of multiple
	realms.  You may wish to not include them for simplicity.</para>
	  
      <para>The first line names the realm in which this system works.  The
	other lines contain realm/host entries.  The first item on a line is a
	realm, and the second is a host in that realm that is acting as a
	<quote>key distribution center</quote>.  The words <literal>admin
	  server</literal> following a host's name means that host also
	provides an administrative database server.  For further explanation
	of these terms, please consult the Kerberos manual pages.</para>
	  
      <para>Now we have to add <hostid role="fqdn">grunt.example.com</hostid>
	to the <filename>EXAMPLE.COM</filename> realm and also add an entry to
	put all hosts in the <hostid role="domainname">.example.com</hostid>
	domain in the <filename>EXAMPLE.COM</filename> realm.  The
	<filename>krb.realms</filename> file would be updated as
	follows:</para>
	  
      <screen>&prompt.root; <userinput>cat krb.realms</userinput>
grunt.example.com EXAMPLE.COM
.example.com EXAMPLE.COM
.berkeley.edu CS.BERKELEY.EDU
.MIT.EDU ATHENA.MIT.EDU
.mit.edu ATHENA.MIT.EDU</screen>
	  
      <para>Again, the other realms do not need to be there.  They are here as
	an example of how a machine may be made aware of multiple realms.  You
	may wish to remove them to simplify things.</para>
	  
      <para>The first line puts the <emphasis>specific</emphasis> system into
	the named realm.  The rest of the lines show how to default systems of
	a particular subdomain to a named realm.</para>
	  
      <para>Now we are ready to create the database.  This only needs to run
	on the Kerberos server (or Key Distribution Center).  Issue the
	<command>kdb_init</command> command to do this:</para>
	  
      <screen>&prompt.root; <userinput>kdb_init</userinput>
<prompt>Realm name [default  ATHENA.MIT.EDU ]:</prompt> <userinput>EXAMPLE.COM</userinput>
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.
		
<prompt>Enter Kerberos master key:</prompt> </screen>
	  
      <para>Now we have to save the key so that servers on the local machine
	can pick it up.  Use the <command>kstash</command> command to do
	this.</para>
	  
      <screen>&prompt.root; <userinput>kstash</userinput>
	      
<prompt>Enter Kerberos master key:</prompt>

Current Kerberos master key version is 1.

Master key entered. BEWARE!</screen>
	  
      <para>This saves the encrypted master password in
	<filename>/etc/kerberosIV/master_key</filename>.</para>
    </sect2>
    
    <sect2>
      <title>Making It All Run</title>
	  
      <para>Two principals need to be added to the database for
	<emphasis>each</emphasis> system that will be secured with Kerberos.
	Their names are <literal>kpasswd</literal> and <literal>rcmd</literal>
	These two principals are made for each system, with the instance being
	the name of the individual system.</para>
	  
      <para>These daemons, <command>kpasswd</command> and
	<command>rcmd</command> allow other systems to change Kerberos
	passwords and run commands like <command>rcp</command>,
	<command>rlogin</command> and <command>rsh</command>.</para>
	  
      <para>Now let us add these entries:</para>
	    
      <screen>&prompt.root; <userinput>kdb_edit</userinput>
Opening database...

<prompt>Enter Kerberos master key:</prompt>

Current Kerberos master key version is 1.

Master key entered.  BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.

<prompt>Principal name:</prompt> <userinput>passwd</userinput>
<prompt>Instance:</prompt> <userinput>grunt</userinput>

&lt;Not found&gt;, <prompt>Create [y] ?</prompt> <userinput>y</userinput>

Principal: passwd, Instance: grunt, kdc_key_ver: 1
<prompt>New Password:</prompt>                    &lt;---- enter RANDOM here
Verifying password

<prompt>New Password:</prompt> &lt;---- enter RANDOM here

<prompt>Random password [y] ?</prompt> <userinput>y</userinput>

Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt>
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt> <userinput>rcmd</userinput>
<prompt>Instance:</prompt> <userinput>grunt</userinput>

&lt;Not found&gt;, <prompt>Create [y] ?</prompt>

Principal: rcmd, Instance: grunt, kdc_key_ver: 1
<prompt>New Password:</prompt>		&lt;---- enter RANDOM here
Verifying password

<prompt>New Password:</prompt>           &lt;---- enter RANDOM here

<prompt>Random password [y] ?</prompt>

Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt>
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt>         &lt;---- null entry here will cause an exit</screen>
    </sect2>

    <sect2>
      <title>Creating the Server File</title>
      
      <para>We now have to extract all the instances which define the
	services on each machine.  For this we use the
	<command>ext_srvtab</command> command.  This will create a file
	which must be copied or moved <emphasis>by secure
	  means</emphasis> to each Kerberos client's
	<filename>/etc/kerberosIV</filename> directory.  This file must
	be present on each server and client, and is crucial to the
	operation of Kerberos.</para>
	  
	  
      <screen>&prompt.root; <userinput>ext_srvtab grunt</userinput>
<prompt>Enter Kerberos master key:</prompt>
		
Current Kerberos master key version is 1.

Master key entered. BEWARE!
Generating 'grunt-new-srvtab'....</screen>
	  
      <para>Now, this command only generates a temporary file which must be
	renamed to <filename>srvtab</filename> so that all the servers can pick
	it up.  Use the <command>mv</command> command to move it into place on
	the original system:</para>
	  
      <screen>&prompt.root; <userinput>mv grunt-new-srvtab srvtab</userinput></screen>
	  
      <para>If the file is for a client system, and the network is not deemed
	safe, then copy the
	<filename><replaceable>client</replaceable>-new-srvtab</filename> to
	removable media and transport it by secure physical means.  Be sure to
	rename it to <filename>srvtab</filename> in the client's
	<filename>/etc/kerberosIV</filename> directory, and make sure it is
	mode 600:</para>
	  
      <screen>&prompt.root; <userinput>mv grumble-new-srvtab srvtab</userinput>
&prompt.root; <userinput>chmod 600 srvtab</userinput></screen>
    </sect2>
    
    <sect2>
      <title>Populating the Database</title>
      
      <para>We now have to add some user entries into the database.  First
	let us create an entry for the user <username>jane</username>.  Use the
	<command>kdb_edit</command> command to do this:</para>
	  
      <screen>&prompt.root; <userinput>kdb_edit</userinput>
Opening database...

<prompt>Enter Kerberos master key:</prompt>

Current Kerberos master key version is 1.

Master key entered.  BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.

<prompt>Principal name:</prompt> <userinput>jane</userinput>
<prompt>Instance:</prompt>

&lt;Not found&gt;, <prompt>Create [y] ?</prompt> <userinput>y</userinput>

Principal: jane, Instance: , kdc_key_ver: 1
<prompt>New Password:</prompt>                &lt;---- enter a secure password here
Verifying password

<prompt>New Password:</prompt>                &lt;---- re-enter the password here
Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt>
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt>		   &lt;---- null entry here will cause an exit</screen>
    </sect2>

    <sect2>
      <title>Testing It All Out</title>
      
      <para>First we have to start the Kerberos daemons.  NOTE that if you
	have correctly edited your <filename>/etc/rc.conf</filename> then this
	will happen automatically when you reboot.  This is only necessary on
	the Kerberos server.  Kerberos clients will automagically get what
	they need from the <filename>/etc/kerberosIV</filename>
	directory.</para>
	  
      <screen>&prompt.root; <userinput>kerberos &amp;</userinput>
Kerberos server starting
Sleep forever on error
Log file is /var/log/kerberos.log
Current Kerberos master key version is 1.

Master key entered. BEWARE!

Current Kerberos master key version is 1
Local realm: EXAMPLE.COM
&prompt.root; <userinput>kadmind -n &amp;</userinput>
KADM Server KADM0.0A initializing
Please do not use 'kill -9' to kill this job, use a
regular kill instead

Current Kerberos master key version is 1.

Master key entered.  BEWARE!</screen>
	  
      <para>Now we can try using the <command>kinit</command> command to get a
	ticket for the id <username>jane</username> that we created
	above:</para>
	  
      <screen>&prompt.user; <userinput>kinit jane</userinput>
MIT Project Athena (grunt.example.com)
Kerberos Initialization for "jane"
<prompt>Password:</prompt> </screen>
	  
      <para>Try listing the tokens using <command>klist</command> to see if we
	really have them:</para>
	  
      <screen>&prompt.user; <userinput>klist</userinput>
Ticket file:    /tmp/tkt245
Principal:      jane@EXAMPLE.COM

  Issued           Expires          Principal
Apr 30 11:23:22  Apr 30 19:23:22  krbtgt.EXAMPLE.COM@EXAMPLE.COM</screen>
	  
      <para>Now try changing the password using <command>passwd</command> to
	check if the <application>kpasswd</application> daemon can get 
	authorization to the Kerberos database:</para>
	  
      <screen>&prompt.user; <userinput>passwd</userinput>
realm EXAMPLE.COM
<prompt>Old password for jane:</prompt>
<prompt>New Password for jane:</prompt>
Verifying password
<prompt>New Password for jane:</prompt>
Password changed.</screen>
    </sect2>

    <sect2>
      <title>Adding <command>su</command> Privileges</title>
      
      <para>Kerberos allows us to give <emphasis>each</emphasis> user
	who needs <username>root</username> privileges their own
	<emphasis>separate</emphasis> <command>su</command> password.
	We could now add an id which is authorized to
	<command>su</command> to <username>root</username>.  This is
	controlled by having an instance of <username>root</username>
	associated with a principal.  Using <command>kdb_edit</command>
	we can create the entry <literal>jane.root</literal> in the
	Kerberos database:</para>
	  
      <screen>&prompt.root; <userinput>kdb_edit</userinput>
Opening database...

<prompt>Enter Kerberos master key:</prompt>

Current Kerberos master key version is 1.

Master key entered.  BEWARE!
Previous or default values are in [brackets] ,
enter return to leave the same, or new value.

<prompt>Principal name:</prompt> <userinput>jane</userinput>
<prompt>Instance:</prompt> <userinput>root</userinput>

&lt;Not found&gt;, Create [y] ? y

Principal: jane, Instance: root, kdc_key_ver: 1
<prompt>New Password:</prompt>                    &lt;---- enter a SECURE password here
Verifying password

<prompt>New Password:</prompt>    	 	 &lt;---- re-enter the password here

Principal's new key version = 1
<prompt>Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ?</prompt>
<prompt>Max ticket lifetime (*5 minutes) [ 255 ] ?</prompt> <userinput>12</userinput> &lt;--- Keep this short!
<prompt>Attributes [ 0 ] ?</prompt>
Edit O.K.
<prompt>Principal name:</prompt>		         &lt;---- null entry here will cause an exit</screen>
	  
      <para>Now try getting tokens for it to make sure it works:</para>
      
      <screen>&prompt.root; <userinput>kinit jane.root</userinput>
MIT Project Athena (grunt.example.com)
Kerberos Initialization for "jane.root"
<prompt>Password:</prompt></screen>
	  
      <para>Now we need to add the user to <username>root</username>'s
	  <filename>.klogin</filename> file:</para>
	  
      <screen>&prompt.root; <userinput>cat /root/.klogin</userinput>
jane.root@EXAMPLE.COM</screen>
	  
      <para>Now try doing the <command>su</command>:</para>
	  
      <screen>&prompt.user; <prompt>su</prompt>
<prompt>Password:</prompt></screen>
	  
      <para>and take a look at what tokens we have:</para>
	  
      <screen>&prompt.root; klist
Ticket file:	/tmp/tkt_root_245
Principal:      jane.root@EXAMPLE.COM

  Issued           Expires          Principal
May  2 20:43:12  May  3 04:43:12  krbtgt.EXAMPLE.COM@EXAMPLE.COM</screen>
    </sect2>

    <sect2>
      <title>Using Other Commands</title>
      
      <para>In an earlier example, we created a principal called
	<literal>jane</literal> with an instance <literal>root</literal>.
	This was based on a user with the same name as the principal, and this
	is a Kerberos default; that a
	<literal>&lt;principal&gt;.&lt;instance&gt;</literal> of the form
	<literal>&lt;username&gt;.</literal><username>root</username> will allow
	that <literal>&lt;username&gt;</literal> to <command>su</command> to
	<username>root</username> if the necessary entries are in the
	<filename>.klogin</filename> file in <username>root</username>'s
	home directory:</para>
	  
      <screen>&prompt.root; <userinput>cat /root/.klogin</userinput>
jane.root@EXAMPLE.COM</screen>
      
      <para>Likewise, if a user has in their own home directory lines of the
	form:</para>
      
      <screen>&prompt.user; <userinput>cat ~/.klogin</userinput>
jane@EXAMPLE.COM
jack@EXAMPLE.COM</screen>
	  
      <para>This allows anyone in the <filename>EXAMPLE.COM</filename> realm
	who has authenticated themselves to <username>jane</username> or
	<username>jack</username> (via <command>kinit</command>, see above)
	access to <command>rlogin</command> to <username>jane</username>'s
	account or files on this system (<hostid>grunt</hostid>) via
	<command>rlogin</command>, <command>rsh</command> or
	<command>rcp</command>.</para>
	  
      <para>For example, <username>jane</username> now logs into another system using
	Kerberos:</para>
	  
	    <screen>&prompt.user; <userinput>kinit</userinput>
MIT Project Athena (grunt.example.com)
<prompt>Password:</prompt>
&prompt.user; <userinput>rlogin grunt</userinput>
Last login: Mon May  1 21:14:47 from grumble
Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
        The Regents of the University of California.   All rights reserved.

FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995</screen>
	  
      <para>Or Jack logs into Jane's account on the same machine
	(<username>jane</username> having
	set up the <filename>.klogin</filename> file as above, and the person
	in charge of Kerberos having set up principal
	<emphasis>jack</emphasis> with a null instance:</para>
	  
      <screen>&prompt.user; <userinput>kinit</userinput>
&prompt.user; <userinput>rlogin grunt -l jane</userinput>
MIT Project Athena (grunt.example.com)
<prompt>Password:</prompt>
Last login: Mon May  1 21:16:55 from grumble
Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
        The Regents of the University of California.   All rights reserved.
FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995</screen>
    </sect2>
  </sect1>
  
  <sect1 id="firewalls">
    <sect1info>
      <authorgroup>
	<author>
	  <firstname>Gary</firstname>
	  <surname>Palmer</surname>
	  <contrib>Contributed by </contrib>
	</author>
	<author>
	  <firstname>Alex</firstname>
	  <surname>Nash</surname>
	</author>
      </authorgroup>
    </sect1info>

    <title>Firewalls</title>
    <indexterm><primary>firewall</primary></indexterm>
    <indexterm>
      <primary>security</primary>
      <secondary>firewalls</secondary>
    </indexterm>

    <para>Firewalls are an area of increasing interest for people who are
      connected to the Internet, and are even finding applications on private
      networks to provide enhanced security.  This section will hopefully
      explain what firewalls are, how to use them, and how to use the
      facilities provided in the FreeBSD kernel to implement them.</para>

    <note>
      <para>People often think that having a firewall between your
	internal network and the <quote>Big Bad Internet</quote> will solve all
	your security problems.  It may help, but a poorly setup firewall
	system is more of a security risk than not having one at all.  A
	firewall can add another layer of security to your systems, but it
	cannot stop a really determined cracker from penetrating your internal
	network.  If you let internal security lapse because you believe your
	firewall to be impenetrable, you have just made the crackers job that
	much easier.</para>
    </note>

    <sect2>
      <title>What Is a Firewall?</title>
      
      <para>There are currently two distinct types of firewalls in common use
	on the Internet today.  The first type is more properly called a
	<emphasis>packet filtering router</emphasis>, where the kernel on a
	multi-homed machine chooses whether to forward or block packets based
	on a set of rules.  The second type, known as a <emphasis>proxy
	  server</emphasis>, relies on daemons to provide authentication and to
	forward packets, possibly on a multi-homed machine which has kernel
	packet forwarding disabled.</para>
	  
      <para>Sometimes sites combine the two types of firewalls, so that only a
	certain machine (known as a <emphasis>bastion host</emphasis>) is
	allowed to send packets through a packet filtering router onto an
	internal network.  Proxy services are run on the bastion host, which
	are generally more secure than normal authentication
	mechanisms.</para>
	  
      <para>FreeBSD comes with a kernel packet filter (known as
	IPFW), which is what the rest of this
	section will concentrate on.  Proxy servers can be built on FreeBSD
	from third party software, but there is such a variety of proxy
	servers available that it would be impossible to cover them in this
	section.</para>
	  
      <sect3 id="firewalls-packet-filters">
	<title>Packet Filtering Routers</title>

	<para>A router is a machine which forwards packets between two or more
	  networks.  A packet filtering router has an extra piece of code in
	  its kernel which compares each packet to a list of rules before
	  deciding if it should be forwarded or not.  Most modern IP routing
	  software has packet filtering code within it that defaults to
	  forwarding all packets.  To enable the filters, you need to define a
	  set of rules for the filtering code so it can decide if the
	  packet should be allowed to pass or not.</para>
	    
	<para>To decide whether a packet should be passed on, the code looks
	  through its set of rules for a rule which matches the contents of
	  this packets headers.  Once a match is found, the rule action is
	  obeyed.  The rule action could be to drop the packet, to forward the
	  packet, or even to send an ICMP message back to the originator.
	  Only the first match counts, as the rules are searched in order.
	  Hence, the list of rules can be referred to as a <quote>rule
	  chain</quote>.</para>
	    
	<para>The packet matching criteria varies depending on the software
	  used, but typically you can specify rules which depend on the source
	  IP address of the packet, the destination IP address, the source
	  port number, the destination port number (for protocols which
	  support ports), or even the packet type (UDP, TCP, ICMP,
	  etc).</para>
      </sect3>
	  
      <sect3 id="firewalls-proxy-servers">
	<title>Proxy Servers</title>
	    
	<para>Proxy servers are machines which have had the normal system
	  daemons (<application>telnetd</application>, 
	  <application>ftpd</application>, etc) replaced with special servers.
	  These
	  servers are called <emphasis>proxy servers</emphasis> as they
	  normally only allow onward connections to be made.  This enables you
	  to run (for example) a proxy telnet server on your firewall host,
	  and people can telnet in to your firewall from the outside, go
	  through some authentication mechanism, and then gain access to the
	  internal network (alternatively, proxy servers can be used for
	  signals coming from the internal network and heading out).</para>
	    
	<para>Proxy servers are normally more secure than normal servers, and
	  often have a wider variety of authentication mechanisms available,
	  including <quote>one-shot</quote> password systems so that even if
	  someone manages to discover what password you used, they will not be
	  able to use it to gain access to your systems as the password
	  instantly expires.  As they do not actually give users access to the
	  host machine, it becomes a lot more difficult for someone to install
	  backdoors around your security system.</para>
	    
	<para>Proxy servers often have ways of restricting access further, so
	  that only certain hosts can gain access to the servers, and often
	  they can be set up so that you can limit which users can talk to
	  which destination machine.  Again, what facilities are available
	  depends largely on what proxy software you choose.</para>
      </sect3>
    </sect2>

    <sect2>
      <title>What Does IPFW Allow Me to Do?</title>
      <indexterm><primary><command>ipfw</command></primary></indexterm>
      
      <para>IPFW, the software supplied with
	FreeBSD, is a packet filtering and accounting system which resides in
	the kernel, and has a user-land control utility,
	    &man.ipfw.8;.  Together, they allow you to define and query the
	rules currently used by the kernel in its routing decisions.</para>
	  
      <para>There are two related parts to IPFW.
	The firewall section allows you to perform packet filtering.  There is
	also an IP accounting section which allows you to track usage of your
	router, based on similar rules to the firewall section.  This allows
	you to see (for example) how much traffic your router is getting from
	a certain machine, or how much WWW (World Wide Web) traffic it is
	forwarding.</para>
	  
      <para>As a result of the way that IPFW is
	designed, you can use IPFW on non-router
	machines to perform packet filtering on incoming and outgoing
	connections.  This is a special case of the more general use of
	IPFW, and the same commands and techniques
	should be used in this situation.</para>
    </sect2>

    <sect2>
      <title>Enabling IPFW on FreeBSD</title>
      <indexterm>
        <primary><command>ipfw</command></primary>
        <secondary>enabling</secondary>
      </indexterm>
      
      <para>As the main part of the IPFW system
	lives in the kernel, you will need to add one or more options to your
	kernel configuration file, depending on what facilities you want, and
	recompile your kernel.  See "Reconfiguring your Kernel" (<xref
	linkend="kernelconfig">)
	for more details on how to recompile your
	kernel.</para>
      
      <para>There are currently three kernel configuration options relevant to
	IPFW:</para>
	  
      <variablelist>
	<varlistentry>
	  <term><literal>options IPFIREWALL</literal></term>

	  <listitem>
	    <para>Compiles into the kernel the code for packet
	      filtering.</para>
	  </listitem>
	</varlistentry>
	      
	<varlistentry>
	  <term><literal>options IPFIREWALL_VERBOSE</literal></term>
		
	  <listitem>
	    <para>Enables code to allow logging of packets through
		&man.syslogd.8;.  Without this option, even if you specify
	      that packets should be logged in the filter rules, nothing will
	      happen.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>options IPFIREWALL_VERBOSE_LIMIT=10</literal></term>
		
	  <listitem>
	    <para>Limits the number of packets logged through
		&man.syslogd.8; on a per entry basis.  You may wish to use
	      this option in hostile environments in which you want to log
	      firewall activity, but do not want to be open to a denial of
	      service attack via syslog flooding.</para>

	    <para>When a chain entry reaches the packet limit specified,
	      logging is turned off for that particular entry.  To resume
	      logging, you will need to reset the associated counter using the
		&man.ipfw.8; utility:</para>
	    
	    <screen>&prompt.root; <userinput>ipfw zero 4500</userinput></screen>
	    <para>Where 4500 is the chain entry you wish to continue
	      logging.</para>
	  </listitem>
	</varlistentry>
      </variablelist>
      
      <note><para>Previous versions of FreeBSD contained an
	<literal>IPFIREWALL_ACCT</literal>  option.  This is now obsolete as
	the firewall code automatically  includes accounting
	facilities.</para>
      </note>
    </sect2>

    <sect2>
      <title>Configuring IPFW</title>
      <indexterm>
        <primary><command>ipfw</command></primary>
        <secondary>configuring</secondary>
      </indexterm>
      
      <para>The configuration of the IPFW software
	is done through the &man.ipfw.8; utility.  The syntax for this
	command looks quite complicated, but it is relatively simple once you
	understand its structure.</para>

      <para>There are currently four different command categories used by the
	utility: addition/deletion, listing, flushing, and clearing.
	Addition/deletion is used to build the rules that control how packets
	are accepted, rejected, and logged.  Listing is used to examine the
	contents of your rule set (otherwise known as the chain) and packet
	counters (accounting).  Flushing is used to remove all entries from
	the chain.  Clearing is used to zero out one or more accounting
	entries.</para>
	  
      <sect3>
	<title>Altering the IPFW Rules</title>

	<para>The syntax for this form of the command is:
	  <cmdsynopsis>
	    <command>ipfw</command>
	    <arg>-N</arg>
	    <arg choice="plain">command</arg>
	    <arg>index</arg>
	    <arg choice="plain">action</arg>
	    <arg>log</arg>
	    <arg choice="plain">protocol</arg>
	    <arg choice="plain">addresses</arg>
	    <arg>options</arg>
	  </cmdsynopsis></para>

	<para>There is one valid flag when using this form of the
	  command:</para>

	<variablelist>
	  <varlistentry>
	    <term>-N</term>

	    <listitem>
	      <para>Resolve addresses and service names in output.</para>
	    </listitem>
	  </varlistentry>
	</variablelist>

	<para>The <emphasis>command</emphasis> given can be shortened to the
	  shortest unique form.  The valid <emphasis>commands</emphasis>
	  are:</para>
	    
	<variablelist>
	  <varlistentry>
	    <term>add</term>

	    <listitem>
	      <para>Add an entry to the firewall/accounting rule list</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>delete</term>
	    
	    <listitem>
	      <para>Delete an entry from the firewall/accounting rule
		list</para>
	    </listitem>
	  </varlistentry>
	</variablelist>

	<para>Previous versions of IPFW used
	  separate firewall and accounting entries.  The present version
	  provides packet accounting with each firewall entry.</para>
	    
	<para>If an <emphasis>index</emphasis> value is supplied, it is used to
	  place the entry at a specific point in the chain. Otherwise, the
	  entry is placed at the end of the chain at an index 100 greater than
	  the last chain entry (this does not include the default policy, rule
	  65535, deny).</para>
	    
	<para>The <literal>log</literal> option causes matching rules to be
	  output to the system console if the kernel was compiled with
	  <literal>IPFIREWALL_VERBOSE</literal>.</para>
	    
	<para>Valid <emphasis>actions</emphasis> are:</para>
	    
	<variablelist>
	  <varlistentry>
	    <term>reject</term>

	    <listitem>
	      <para>Drop the packet, and send an ICMP host or port unreachable
		(as appropriate) packet to the source.</para>
	    </listitem>
	  </varlistentry>
		
	  <varlistentry>
	    <term>allow</term>
	    
	    <listitem>
	      <para>Pass the packet on as normal.  (aliases:
		<literal>pass</literal> and
		<literal>accept</literal>)</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>deny</term>
	    
	    <listitem>
	      <para>Drop the packet.  The source is not notified via an
		ICMP message (thus it appears that the packet never
		arrived at the destination).</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>count</term>
	    
	    <listitem>
	      <para>Update packet counters but do not allow/deny the packet
		based on this rule.  The search continues with the next chain
		entry.</para>
	    </listitem>
	  </varlistentry>
	</variablelist>

	<para>Each <emphasis>action</emphasis> will be recognized by the
	  shortest unambiguous prefix.</para>
	    
	<para>The <emphasis>protocols</emphasis> which can be specified
	  are:</para>
	    
	<variablelist>
	  <varlistentry>
	    <term>all</term>

	    <listitem>
	      <para>Matches any IP packet</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>icmp</term>
	    
	    <listitem>
	      <para>Matches ICMP packets</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>tcp</term>
	    
	    <listitem>
	      <para>Matches TCP packets</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>udp</term>
	    
	    <listitem>
	      <para>Matches UDP packets</para>
	    </listitem>
	  </varlistentry>
	</variablelist>

	<para>The <emphasis>address</emphasis> specification is:</para>

	<cmdsynopsis>
	  <arg choice="plain">from</arg>
	  <arg choice="plain"><replaceable>address/mask</replaceable></arg><arg><replaceable>port</replaceable></arg>
	  <arg choice="plain">to</arg>
	  <arg choice="plain"><replaceable>address/mask</replaceable></arg><arg><replaceable>port</replaceable></arg>
	  <arg>via <replaceable>interface</replaceable></arg>
	</cmdsynopsis>
	    
	<para>You can only specify <replaceable>port</replaceable> in
	  conjunction with <emphasis>protocols</emphasis> which support ports
	  (UDP and TCP).</para>
	    
	<para>The <option>via</option> is optional and may specify the IP
	  address or domain name of a local IP interface, or an interface name
	  (e.g.  <devicename>ed0</devicename>) to match only packets coming
	  through this interface.  Interface unit numbers can be specified
	  with an optional wildcard.  For example, <literal>ppp*</literal>
	  would match all kernel PPP interfaces.</para>
	    
	<para>The syntax used to specify an
	  <replaceable>address/mask</replaceable> is:
	  
	  <screen><replaceable>address</replaceable></screen>
	      
	  or
	      
	  <screen><replaceable>address</replaceable>/<replaceable>mask-bits</replaceable></screen>
	      
	  or
	      
	  <screen><replaceable>address</replaceable>:<replaceable>mask-pattern</replaceable></screen>
	</para>

	<para>A valid hostname may be specified in place of the IP address.
	  <option><replaceable>mask-bits</replaceable></option> is a decimal
	  number representing how many bits in the address mask should be set.
	  e.g.  specifying <literal>192.216.222.1/24</literal> will create a
	  mask which will allow any address in a class C subnet (in this case,
	  192.216.222) to be matched.
	  <option><replaceable>mask-pattern</replaceable></option> is an IP
	  address which will be logically AND'ed with the address given. The
	  keyword <literal>any</literal> may be used to specify <quote>any IP
	  address</quote>.</para>
	    
	<para>The port numbers to be blocked are specified as:
	  
	  <cmdsynopsis>
	    <arg choice="plain"><replaceable>port</replaceable><arg>,<replaceable>port</replaceable><arg>,<replaceable>port</replaceable><arg>&hellip;</arg></arg></arg></arg>
	  </cmdsynopsis>

	  to specify either a single port or a list of ports, or
	  
	  <cmdsynopsis>
	    <arg choice="plain"><replaceable>port</replaceable>-<replaceable>port</replaceable></arg>
	  </cmdsynopsis>

	  to specify a range of ports.  You may also combine a single range
	  with a list, but the range must always be specified first.</para>
	    
	<para>The <emphasis>options</emphasis> available are:</para>

	<variablelist>
	  <varlistentry>
	    <term>frag</term>

	    <listitem>
	      <para>Matches if the packet is not the first fragment of the
		datagram.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>in</term>
	    
	    <listitem>
	      <para>Matches if the packet is on the way in.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>out</term>
	    
	    <listitem>
	      <para>Matches if the packet is on the way out.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>ipoptions <replaceable>spec</replaceable></term>
	    
	    <listitem>
	      <para>Matches if the IP header contains the comma separated list
		of options specified in <replaceable>spec</replaceable>.  The
		supported list of IP options are: <literal>ssrr</literal>
		(strict source route), <literal>lsrr</literal> (loose source
		route), <literal>rr</literal> (record packet route), and
		<literal>ts</literal> (time stamp).  The absence of a
		particular option may be denoted with a leading
		<literal>!</literal>.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>established</term>
	    
	    <listitem>
	      <para>Matches if the packet is part of an already established
		TCP connection (i.e. it has the RST or ACK bits set).  You can
		optimize the performance of the firewall by placing
		<emphasis>established</emphasis> rules early in the
		chain.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>setup</term>
	    
	    <listitem>
	      <para>Matches if the packet is an attempt to establish a TCP
		connection (the SYN bit is set but the ACK bit is
		not).</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>tcpflags <replaceable>flags</replaceable></term>
	    
	    <listitem>
	      <para>Matches if the TCP header contains the comma separated
		list of <replaceable>flags</replaceable>.  The supported flags
		are <literal>fin</literal>, <literal>syn</literal>,
		<literal>rst</literal>, <literal>psh</literal>,
		<literal>ack</literal>, and <literal>urg</literal>.  The
		absence of a particular flag may be indicated by a leading
		<literal>!</literal>.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>icmptypes <replaceable>types</replaceable></term>
	    
	    <listitem>
	      <para>Matches if the ICMP type is present in the list
		<replaceable>types</replaceable>.  The list may be specified
		as any combination of ranges and/or individual types separated
		by commas.  Commonly used ICMP types are: <literal>0</literal>
		echo reply (ping reply), <literal>3</literal> destination
		unreachable, <literal>5</literal> redirect,
		<literal>8</literal> echo request (ping request), and
		<literal>11</literal> time exceeded (used to indicate TTL
		expiration as with &man.traceroute.8;).</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </sect3>
      
      <sect3>
	<title>Listing the IPFW Rules</title>

	<para>The syntax for this form of the command is:
	  <cmdsynopsis>
	    <command>ipfw</command>
	    <arg>-a</arg>
	    <arg>-t</arg>
	    <arg>-N</arg>
	    <arg choice="plain">l</arg>
	  </cmdsynopsis></para>

	<para>There are three valid flags when using this form of the
	  command:</para>
	    
	<variablelist>
	  <varlistentry>
	    <term>-a</term>

	    <listitem>
	      <para>While listing, show counter values.  This option is the
		only way to see accounting counters.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>-t</term>
	    
	    <listitem>
	      <para>Display the last match times for each chain entry. The
		time listing is incompatible with the input syntax used by the
		  &man.ipfw.8; utility.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>-N</term>
	    
	    <listitem>
	      <para>Attempt to resolve given addresses and service
		names.</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </sect3>
      
      <sect3>
	<title>Flushing the IPFW Rules</title>

	<para>The syntax for flushing the chain is:
	  <cmdsynopsis>
	    <command>ipfw</command>
	    <arg choice="plain">flush</arg>
	  </cmdsynopsis></para>

	<para>This causes all entries in the firewall chain to be removed
	  except the fixed default policy enforced by the kernel (index
	  65535).  Use caution when flushing rules, the default deny policy
	  will leave your system cut off from the network until allow entries
	  are added to the chain.</para>
      </sect3>
      
      <sect3>
	<title>Clearing the IPFW Packet Counters</title>

	<para>The syntax for clearing one or more packet counters is:
	  <cmdsynopsis>
	    <command>ipfw</command>
	    <arg choice="plain">zero</arg>
	    <arg choice="opt"><replaceable>index</replaceable></arg>
	  </cmdsynopsis></para>

	<para>When used without an <replaceable>index</replaceable> argument,
	  all packet counters are cleared.  If an
	  <replaceable>index</replaceable> is supplied, the clearing operation
	  only affects a specific chain entry.</para>
      </sect3>
    </sect2>

    <sect2>
      <title>Example Commands for <application>ipfw</application></title>
      
      <para>This command will deny all packets from the host <hostid
	  role="fqdn">evil.crackers.org</hostid> to the telnet port of the
	host <hostid role="fqdn">nice.people.org</hostid>:</para>
	  	  
      <screen>&prompt.root; <userinput>ipfw add deny tcp from evil.crackers.org to nice.people.org 23</userinput></screen>
	  
      <para>The next example denies and logs any TCP traffic from the entire
	<hostid role="domainname">crackers.org</hostid> network (a class C) to
	the <hostid role="fqdn">nice.people.org</hostid> machine (any
	port).</para>
	  
      <screen>&prompt.root; <userinput>ipfw add deny log tcp from evil.crackers.org/24 to nice.people.org</userinput></screen>
	  
      <para>If you do not want people sending X sessions to your internal
	network (a subnet of a class C), the following command will do the
	necessary filtering:</para>
	  
      <screen>&prompt.root; <userinput>ipfw add deny tcp from any to my.org/28 6000 setup</userinput></screen>
	  
      <para>To see the accounting records:
	    
	<screen>&prompt.root; <userinput>ipfw -a list</userinput></screen>

	or in the short form
	    
	<screen>&prompt.root; <userinput>ipfw -a l</userinput></screen>
      </para>

      <para>You can also see the last time a chain entry was matched
	with:</para>
      
      <screen>&prompt.root; <userinput>ipfw -at l</userinput></screen>
    </sect2>
    
    <sect2>
      <title>Building a Packet Filtering Firewall</title>
      
      <note>
	<para>The following suggestions are just that: suggestions.  The
	  requirements of each firewall are different and we cannot tell you
	  how to build a firewall to meet your particular requirements.</para>
      </note>
	  
      <para>When initially setting up your firewall, unless you have a test
	bench setup where you can configure your firewall host in a controlled
	environment, it is strongly recommend you use the logging version of the
	commands and enable logging in the kernel.  This will allow you to
	quickly identify problem areas and cure them without too much
	disruption.  Even after the initial setup phase is complete, I
	recommend using the logging for `deny' as it allows tracing of
	possible attacks and also modification of the firewall rules if your
	requirements alter.</para>
	  
      <note>
	<para>If you use the logging versions of the <command>accept</command>
	  command, it can generate <emphasis>large</emphasis> amounts of log
	  data as one log line will be generated for every packet that passes
	  through the firewall, so large FTP/http transfers, etc, will really
	  slow the system down.  It also increases the latencies on those
	  packets as it requires more work to be done by the kernel before the
	  packet can be passed on.  <application>syslogd</application> will 
	  also start using up a lot
	  more processor time as it logs all the extra data to disk, and it
	  could quite easily fill the partition <filename>/var/log</filename>
	  is located on.</para>
      </note>
	  
      <para>You should enable your firewall from
	<filename>/etc/rc.conf.local</filename> or
	<filename>/etc/rc.conf</filename>.  The associated manual page explains
	which knobs to fiddle and lists some preset firewall configurations.
	If you do not use a preset configuration, <command>ipfw list</command>
	will output the current ruleset into a file that you can
	pass to <filename>rc.conf</filename>.  If you do not use
	<filename>/etc/rc.conf.local</filename> or
	<filename>/etc/rc.conf</filename> to enable your firewall,
	it is important to make sure your firewall is enabled before
	any IP interfaces are configured.</para>
	  
      <para>The next problem is what your firewall should actually
	<emphasis>do</emphasis>! This is largely dependent on what access to
	your network you want to allow from the outside, and how much access
	to the outside world you want to allow from the inside. Some general
	rules are:</para>
      
      <itemizedlist>
	<listitem>
	  <para>Block all incoming access to ports below 1024 for TCP. This is
	    where most of the security sensitive services are, like finger,
	    SMTP (mail) and telnet.</para>
	</listitem>

	<listitem>
	  <para>Block <emphasis>all</emphasis> incoming UDP traffic.  There
	    are very few useful services that travel over UDP, and what useful
	    traffic there is, is normally a security threat (e.g. Suns RPC and
	    NFS protocols).  This has its disadvantages also, since UDP is a
	    connectionless protocol, denying incoming UDP traffic also blocks
	    the replies to outgoing UDP traffic.  This can cause a problem for
	    people (on the inside) using external archie (prospero) servers.
	    If you want to allow access to archie, you will have to allow
	    packets coming from ports 191 and 1525 to any internal UDP port
	    through the firewall.  <application>ntp</application> is another
	    service you may consider allowing through, which comes from port
	    123.</para>
	</listitem>
	      
	<listitem>
	  <para>Block traffic to port 6000 from the outside.  Port 6000 is the
	    port used for access to X11 servers, and can be a security threat
	    (especially if people are in the habit of doing <command>xhost
	      +</command> on their workstations).  X11 can actually use a
	    range of ports starting at 6000, the upper limit being how many X
	    displays you can run on the machine.  The upper limit as defined
	    by RFC 1700 (Assigned Numbers) is 6063.</para>
	</listitem>
	      
	<listitem>
	  <para>Check what ports any internal servers use (e.g. SQL servers,
	    etc).  It is probably a good idea to block those as well, as they
	    normally fall outside the 1-1024 range specified above.</para>
	</listitem>
      </itemizedlist>
	  
      <para>Another checklist for firewall configuration is available from
	CERT at <ulink
	  url="http://www.cert.org/tech_tips/packet_filtering.html">http://www.cert.org/tech_tips/packet_filtering.html</ulink></para>
	  
      <para>As stated above, these are only <emphasis>guidelines</emphasis>.
	You will have to decide what filter rules you want to use on your
	firewall yourself.  We cannot accept ANY responsibility if someone
	breaks into your network, even if you follow the advice given
	above.</para>
    </sect2>

    <sect2 id="ipfw-overhead">
      <title>IPFW Overhead and Optimization</title>

      <para>Many people want to know how much overhead IPFW adds to a
	system. The answer to this depends mostly on your rule set and
	processor speed. For most applications dealing with Ethernet
	and small rule sets, the answer is
	<quote>negligible</quote>. For those of you that need actual
	measurements to satisfy your curiosity, read on.</para>

      <para>The following measurements were made using 2.2.5-STABLE on
	a 486-66.  (While IPFW has changed slightly in later releases
	of FreeBSD, it still performs with similar speed.) IPFW was
	modified to measure the time spent within the
	<literal>ip_fw_chk</literal> routine, displaying the results
	to the console every 1000 packets.</para>

      <para>Two rule sets, each with 1000 rules were tested.  The
	first set was designed to demonstrate a worst case scenario by
	repeating the rule:</para>

      <screen>&prompt.root; <userinput>ipfw add deny tcp from any to any 55555</userinput></screen>

      <para>This demonstrates worst case by causing most of IPFW's
	packet check routine to be executed before finally deciding
	that the packet does not match the rule (by virtue of the port
	number). Following the 999th iteration of this rule was an
	<literal>allow ip from any to any</literal>.</para>

      <para>The second set of rules were designed to abort the rule
	check quickly:</para>

      <screen>&prompt.root; <userinput>ipfw add deny ip from 1.2.3.4 to 1.2.3.4</userinput></screen>

      <para>The non-matching source IP address for the above rule
	causes these rules to be skipped very quickly.  As before, the
	1000th rule was an <literal>allow ip from any to
	any</literal>.</para>

      <para>The per-packet processing overhead in the former case was
	approximately 2.703ms/packet, or roughly 2.7 microseconds per
	rule. Thus the theoretical packet processing limit with these
	rules is around 370 packets per second. Assuming 10Mbps
	Ethernet and a ~1500 byte packet size, we would only be able
	to achieve a 55.5% bandwidth utilization.</para>

      <para>For the latter case each packet was processed in
	approximately 1.172ms, or roughly 1.2 microseconds per rule.
	The theoretical packet processing limit here would be about
	853 packets per second, which could consume 10Mbps Ethernet
	bandwidth.</para>

      <para>The excessive number of rules tested and the nature of
	those rules do not provide a real-world scenario -- they were
	used only to generate the timing information presented here.
	Here are a few things to keep in mind when building an
	efficient rule set:</para>

      <itemizedlist>
	<listitem>
	  <para>Place an <literal>established</literal> rule early on
	    to handle the majority of TCP traffic.  Do not put any
	    <literal>allow tcp</literal> statements before this
	    rule.</para>
	</listitem>

	<listitem>
	  <para>Place heavily triggered rules earlier in the rule set
	    than those rarely used (<emphasis>without changing the
	    permissiveness of the firewall</emphasis>, of course).
	    You can see which rules are used most often by examining
	    the packet counting statistics with <command>ipfw -a
	    l</command>.</para>
	</listitem>
      </itemizedlist>
    </sect2>
  </sect1>

  <sect1 id="openssl">
    <title>OpenSSL</title>
    <indexterm>
      <primary>security</primary>
      <secondary>OpenSSL</secondary>
    </indexterm>
    <indexterm><primary>OpenSSL</primary></indexterm>

    <para>As of FreeBSD 4.0, the OpenSSL toolkit is a part of the base
      system.  <ulink url="http://www.openssl.org/">OpenSSL</ulink>
      provides a general-purpose cryptography library, as well as the
      Secure Sockets Layer v2/v3 (SSLv2/SSLv3) and Transport Layer
      Security v1 (TLSv1) network security protocols.</para>

    <para>However, one of the algorithms (specifically IDEA)
      included in OpenSSL is protected by patents in the USA and
      elsewhere, and is not available for unrestricted use.
      IDEA is included in the OpenSSL sources in FreeBSD, but it is not
      built by default.  If you wish to use it, and you comply with the
      license terms, enable the <literal>MAKE_IDEA</literal> switch in
      <filename>/etc/make.conf</filename> and
      rebuild your sources using <command>make world</command>.</para>

    <para>Today, the RSA algorithm is free for use in USA and other
      countries.  In the past it was protected by a patent.</para>

    <indexterm>
      <primary>OpenSSL</primary>
      <secondary>install</secondary>
    </indexterm>

    <sect2>
      <title>Source Code Installations</title>
      
      <para>OpenSSL is part of the <literal>src-crypto</literal> and
	<literal>src-secure</literal> cvsup collections.  See the <link
	linkend="mirrors">Obtaining FreeBSD</link> section for more
	information about obtaining and updating FreeBSD source
	code.</para>
    </sect2>
  </sect1>

  <sect1 id="ipsec">
    <sect1info>
      <authorgroup>
	<author>
	  <firstname>Yoshinobu</firstname>
	  <surname>Inoue</surname>
	  <contrib>Contributed by </contrib>
	</author>
	<!-- 5 Mar 2000 -->
      </authorgroup>
    </sect1info>

    <title>IPsec</title>
    <indexterm><primary>IPsec</primary></indexterm>
    <indexterm>
      <primary>security</primary>
      <secondary>IPsec</secondary>
    </indexterm>

    <note>
      <title>Terminating Characters</title>
      <para>Throughout examples in this section, and other sections,
	you will notice that there is a <quote>^D</quote> at the end
	of some examples. This means to hold down the <keycap>Control</keycap>
	key and hit the <keycap>D</keycap> key. Another commonly used
	character is <quote>^C</quote>, which respectively means to hold
	down <keycap>Control</keycap> and press <keycap>C</keycap>.</para>
    </note>

    <tip>
      <para>For other HOWTOs detailing IPSec implementation in
        FreeBSD, take a look at <ulink
        url="http://www.daemonnews.org/200101/ipsec-howto.html"></ulink>
        and <ulink
        url="http://www.freebsddiary.org/ipsec.php"></ulink>.</para>
    </tip>

    <para>The IPsec mechanism provides secure communication for IP
      layer and socket layer communication.  This section should
      explain how to use them.  For implementation details, please
      refer to <ulink
      url="../developers-handbook/ipv6.html">The
      Developers' Handbook</ulink>.</para>

    <para>The current IPsec implementation supports both transport mode
      and tunnel mode.  However, tunnel mode comes with some restrictions.
      <ulink url="http://www.kame.net/newsletter/">http://www.kame.net/newsletter/
      </ulink> has more comprehensive examples.</para>

    <para>Please be aware that in order to use this functionality, you
      must have the following options compiled into your kernel:</para>

    <programlisting>options          IPSEC              #IP security
options          IPSEC_ESP          #IP security (crypto; define w/IPSEC)</programlisting>

    <sect2>
      <title>Transport Mode Example with IPv4</title>

      <para>Let us setup security association to deploy a secure channel
        between HOST A (10.2.3.4) and HOST B (10.6.7.8).  Here we show a little
        complicated example.  From HOST A to HOST B, only old AH is used.
        From HOST B to HOST A, new AH and new ESP are combined.</para>

      <para>Now we should choose an algorithm to be used corresponding to
        <quote>AH</quote>/<quote>new AH</quote>/<quote>ESP</quote>/
	<quote>new ESP</quote>.  Please refer to the &man.setkey.8; man
        page to know algorithm names.  Our choice is MD5 for AH, new-HMAC-SHA1
        for new AH, and new-DES-expIV with 8 byte IV for new ESP.</para>

      <para>Key length highly depends on each algorithm.  For example, key
        length must be equal to 16 bytes for MD5, 20 for new-HMAC-SHA1,
        and 8 for new-DES-expIV.  Now we choose <quote>MYSECRETMYSECRET</quote>,
        <quote>KAMEKAMEKAMEKAMEKAME</quote>, <quote>PASSWORD</quote>,
	respectively.</para>

      <para>OK, let us assign SPI (Security Parameter Index) for each protocol.
        Please note that we need 3 SPIs for this secure channel since three
        security headers are produced (one for from HOST A to HOST B, two for
        from HOST B to HOST A).  Please also note that SPI MUST be greater
        than or equal to 256.  We choose, 1000, 2000, and 3000,
	respectively.</para>

      <screen>
	         (1)
	HOST A ------> HOST B

	(1)PROTO=AH
		ALG=MD5(RFC1826)
		KEY=MYSECRETMYSECRET
		SPI=1000

	         (2.1)
	HOST A <------ HOST B
	       <------
	         (2.2)

	(2.1)
	PROTO=AH
		ALG=new-HMAC-SHA1(new AH)
		KEY=KAMEKAMEKAMEKAMEKAME
		SPI=2000

	(2.2)
	PROTO=ESP
		ALG=new-DES-expIV(new ESP)
			IV length = 8
		KEY=PASSWORD
		SPI=3000
</screen>

      <para>Now, let us setup security association.  Execute &man.setkey.8;
        on both HOST A and B:</para>

      <screen>
&prompt.root; <command>setkey -c</command>
add 10.2.3.4 10.6.7.8 ah-old  1000 -m transport -A keyed-md5 "MYSECRETMYSECRET" ;
add 10.6.7.8 10.2.3.4 ah  2000 -m transport -A hmac-sha1 "KAMEKAMEKAMEKAMEKAME" ;
add 10.6.7.8 10.2.3.4 esp 3000 -m transport -E des-cbc "PASSWORD" ;
^D
</screen>

     <para>Actually, IPsec communication does not process until security policy
       entries are defined.  In this case, you must setup each host.</para>

     <screen>
At A:

&prompt.root; <command>setkey -c</command>
spdadd 10.2.3.4 10.6.7.8 any -P out ipsec
	ah/transport/10.2.3.4-10.6.7.8/require ;
^D

At B:

&prompt.root; <command>setkey -c</command>
spdadd 10.6.7.8 10.2.3.4 any -P out ipsec
	esp/transport/10.6.7.8-10.2.3.4/require ;
spdadd 10.6.7.8 10.2.3.4 any -P out ipsec
	ah/transport/10.6.7.8-10.2.3.4/require ;
^D


   HOST A --------------------------------------> HOST E
  10.2.3.4                                       10.6.7.8
          |                                     |
          ========== old AH keyed-md5 ==========>

          <========= new AH hmac-sha1 ===========
          <========= new ESP des-cbc ============
</screen>
    </sect2>

    <sect2>
      <title>Transport Mode Example with IPv6</title>

      <para>Another example using IPv6.</para>

      <para>ESP transport mode is recommended for TCP port number 110 between
        Host-A and Host-B.</para>

      <screen>
              ============ ESP ============
              |                           |
           Host-A                        Host-B
          fec0::10 -------------------- fec0::11
</screen>

      <para>Encryption algorithm is blowfish-cbc whose key is
	<quote>kamekame</quote>, and authentication algorithm is hmac-sha1
	whose key is <quote>this is the test key</quote>.
	Configuration at Host-A:</para>

      <screen>
        &prompt.root; <command>setkey -c</command> &lt;&lt;<filename>EOF</filename>
        spdadd fec0::10[any] fec0::11[110] tcp -P out ipsec
                esp/transport/fec0::10-fec0::11/use ;
        spdadd fec0::11[110] fec0::10[any] tcp -P in ipsec
                esp/transport/fec0::11-fec0::10/use ;
        add fec0::10 fec0::11 esp 0x10001
                -m transport
                -E blowfish-cbc "kamekame"
                -A hmac-sha1 "this is the test key" ;
        add fec0::11 fec0::10 esp 0x10002
                -m transport
                -E blowfish-cbc "kamekame"
                -A hmac-sha1 "this is the test key" ;
        EOF
</screen>

      <para>and at Host-B:</para>

      <screen>&prompt.root; <command>setkey -c</command> &lt;&lt;<filename>EOF</filename>
        spdadd fec0::11[110] fec0::10[any] tcp -P out ipsec
                esp/transport/fec0::11-fec0::10/use ;
        spdadd fec0::10[any] fec0::11[110] tcp -P in ipsec
                esp/transport/fec0::10-fec0::11/use ;
        add fec0::10 fec0::11 esp 0x10001 -m transport
                -E blowfish-cbc "kamekame"
                -A hmac-sha1 "this is the test key" ;
        add fec0::11 fec0::10 esp 0x10002 -m transport
                -E blowfish-cbc "kamekame"
                -A hmac-sha1 "this is the test key" ;
        EOF
</screen>

      <para>Note the direction of SP.</para>
    </sect2>

    <sect2>
      <title>Tunnel Mode Example with IPv4</title>

      <para>Tunnel mode between two security gateways</para>

      <para>Security protocol is old AH tunnel mode, i.e. specified by
        RFC1826, with keyed-md5 whose key is <quote>this is the test</quote> as
        authentication algorithm.</para>

      <screen>
                             ======= AH =======
                             |                |
         Network-A       Gateway-A        Gateway-B        Network-B
        10.0.1.0/24 ---- 172.16.0.1 ----- 172.16.0.2 ---- 10.0.2.0/24
</screen>

      <para>Configuration at Gateway-A:</para>

      <screen>
        &prompt.root; <command>setkey -c</command> &lt;&lt;<filename>EOF</filename>
        spdadd 10.0.1.0/24 10.0.2.0/24 any -P out ipsec
                ah/tunnel/172.16.0.1-172.16.0.2/require ;
        spdadd 10.0.2.0/24 10.0.1.0/24 any -P in ipsec
                ah/tunnel/172.16.0.2-172.16.0.1/require ;
        add 172.16.0.1 172.16.0.2 ah-old 0x10003 -m any
                -A keyed-md5 "this is the test" ;
        add 172.16.0.2 172.16.0.1 ah-old 0x10004 -m any
                -A keyed-md5 "this is the test" ;

        EOF
</screen>

      <para>If the port number field is omitted such as above then
	<literal>[any]</literal> is employed. <literal>-m</literal>
	specifies the mode of SA to be used. <literal>-m any</literal> means
	wild-card of mode of security protocol. You can use this SA for both
	tunnel and transport mode.</para>

      <para>and at Gateway-B:</para>

      <screen>
        &prompt.root; <command>setkey -c</command> &lt;&lt;<filename>EOF</filename>
        spdadd 10.0.2.0/24 10.0.1.0/24 any -P out ipsec
                ah/tunnel/172.16.0.2-172.16.0.1/require ;
        spdadd 10.0.1.0/24 10.0.2.0/24 any -P in ipsec
                ah/tunnel/172.16.0.1-172.16.0.2/require ;
        add 172.16.0.1 172.16.0.2 ah-old 0x10003 -m any
                -A keyed-md5 "this is the test" ;
        add 172.16.0.2 172.16.0.1 ah-old 0x10004 -m any
                -A keyed-md5 "this is the test" ;

        EOF
</screen>

      <para>Making SA bundle between two security gateways</para>

      <para>AH transport mode and ESP tunnel mode is required between
        Gateway-A and Gateway-B. In this case, ESP tunnel mode is applied first,
        and AH transport mode is next.</para>

      <screen>
                            ========== AH =========
                            |  ======= ESP =====  |
                            |  |               |  |
       Network-A          Gateway-A        Gateway-B           Network-B
    fec0:0:0:1::/64 --- fec0:0:0:1::1 ---- fec0:0:0:2::1 --- fec0:0:0:2::/64
</screen>
    </sect2>

    <sect2>
      <title>Tunnel Mode Example with IPv6</title>

      <para>Encryption algorithm is 3des-cbc, and authentication algorithm
        for ESP is hmac-sha1.  Authentication algorithm for AH is hmac-md5.
        Configuration at Gateway-A:</para>

      <screen>
        &prompt.root; <command>setkey -c</command> &lt;&lt;<filename>EOF</filename>
        spdadd fec0:0:0:1::/64 fec0:0:0:2::/64 any -P out ipsec
                esp/tunnel/fec0:0:0:1::1-fec0:0:0:2::1/require
                ah/transport/fec0:0:0:1::1-fec0:0:0:2::1/require ;
        spdadd fec0:0:0:2::/64 fec0:0:0:1::/64 any -P in ipsec
                esp/tunnel/fec0:0:0:2::1-fec0:0:0:1::1/require
                ah/transport/fec0:0:0:2::1-fec0:0:0:1::1/require ;
        add fec0:0:0:1::1 fec0:0:0:2::1 esp 0x10001 -m tunnel
                -E 3des-cbc "kamekame12341234kame1234"
                -A hmac-sha1 "this is the test key" ;
        add fec0:0:0:1::1 fec0:0:0:2::1 ah 0x10001 -m transport
                -A hmac-md5 "this is the test" ;
        add fec0:0:0:2::1 fec0:0:0:1::1 esp 0x10001 -m tunnel
                -E 3des-cbc "kamekame12341234kame1234"
                -A hmac-sha1 "this is the test key" ;
        add fec0:0:0:2::1 fec0:0:0:1::1 ah 0x10001 -m transport
                -A hmac-md5 "this is the test" ;

        EOF
</screen>

      <para>Making SAs with the different end</para>

      <para>ESP tunnel mode is required between Host-A and Gateway-A. Encryption
        algorithm is cast128-cbc, and authentication algorithm for ESP is
        hmac-sha1.  ESP transport mode is recommended between Host-A and Host-B.
        Encryption algorithm is rc5-cbc, and authentication algorithm for ESP is
        hmac-md5.</para>

      <screen>
              ================== ESP =================
              |  ======= ESP =======                 |
              |  |                 |                 |
             Host-A            Gateway-A           Host-B
          fec0:0:0:1::1 ---- fec0:0:0:2::1 ---- fec0:0:0:2::2
</screen>

      <para>Configuration at Host-A:</para>

      <screen>
        &prompt.root; <command>setkey -c</command> &lt;&lt;<filename>EOF</filename>
        spdadd fec0:0:0:1::1[any] fec0:0:0:2::2[80] tcp -P out ipsec
                esp/transport/fec0:0:0:1::1-fec0:0:0:2::2/use
                esp/tunnel/fec0:0:0:1::1-fec0:0:0:2::1/require ;
        spdadd fec0:0:0:2::1[80] fec0:0:0:1::1[any] tcp -P in ipsec
                esp/transport/fec0:0:0:2::2-fec0:0:0:l::1/use
                esp/tunnel/fec0:0:0:2::1-fec0:0:0:1::1/require ;
        add fec0:0:0:1::1 fec0:0:0:2::2 esp 0x10001
                -m transport
                -E cast128-cbc "12341234"
                -A hmac-sha1 "this is the test key" ;
        add fec0:0:0:1::1 fec0:0:0:2::1 esp 0x10002
                -E rc5-cbc "kamekame"
                -A hmac-md5 "this is the test" ;
        add fec0:0:0:2::2 fec0:0:0:1::1 esp 0x10003
                -m transport
                -E cast128-cbc "12341234"
                -A hmac-sha1 "this is the test key" ;
        add fec0:0:0:2::1 fec0:0:0:1::1 esp 0x10004
                -E rc5-cbc "kamekame"
                -A hmac-md5 "this is the test" ;

        EOF
</screen>
    </sect2>
  </sect1>

  <sect1 id="openssh">
    <sect1info>
      <authorgroup>
	<author>
	  <firstname>Chern</firstname>
	  <surname>Lee</surname>
	  <contrib>Contributed by </contrib>
	</author>
	<!-- 21 April 2001 -->
      </authorgroup>
    </sect1info>

    <title>OpenSSH</title>
    <indexterm><primary>OpenSSH</primary></indexterm>
    <indexterm>
      <primary>security</primary>
      <secondary>OpenSSH</secondary>
    </indexterm>

    <para>Secure shell is a set of network connectivity tools used to
      access remote machines securely.  It can be used as a direct
      replacement for <command>rlogin</command>,
      <command>rsh</command>, <command>rcp</command>, and
      <command>telnet</command>.  Additionally, any other TCP/IP
      connections can be tunneled/forwarded securely through ssh.
      ssh encrypts all traffic to effectively eliminate eavesdropping,
      connection hijacking, and other network-level attacks.</para>

    <para>OpenSSH is maintained by the OpenBSD project, and is based
      upon SSH v1.2.12 with all the recent bug fixes and updates.  It
      is compatible with both SSH protocols 1 and 2.  OpenSSH has been
      in the base system since FreeBSD 4.0.</para>

    <sect2>
      <title>Advantages of Using OpenSSH</title>
  
      <para>Normally, when using &man.telnet.1; or &man.rlogin.1;,
        data is sent over the network in an clear, un-encrypted form.
        Network sniffers anywhere in between the client and server can
        steal your user/password information or data transferred in
        your session.  OpenSSH offers a variety of authentication and
        encryption methods to prevent this from happening.</para>
    </sect2>

    <sect2>
      <title>Enabling sshd</title>
      <indexterm>
        <primary>OpenSSH</primary>
        <secondary>enabling</secondary>
      </indexterm>

      <para>Be sure to make the following additions to your 
        <filename>rc.conf</filename> file:</para>
      <screen>sshd_enable="YES"</screen>
      <para>This will load the <application>ssh</application> daemon
	the next time your system initializes.  Alternatively, you can
	simply run the <application>sshd</application> daemon.</para>
    </sect2>

    <sect2>
      <title>SSH Client</title>
      <indexterm>
        <primary>OpenSSH</primary>
        <secondary>client</secondary>
      </indexterm>

      <para>The &man.ssh.1; utility works similarly to 
        &man.rlogin.1;.</para>

      <screen>&prompt.root; <userinput>ssh <replaceable>user@example.com</replaceable></userinput>
Host key not found from the list of known hosts.
Are you sure you want to continue connecting (yes/no)? <userinput>yes</userinput>
Host 'example.com' added to the list of known hosts.
user@example.com's password: <userinput>*******</userinput></screen>

      <para>The login will continue just as it would have if a session was
        created using <command>rlogin</command> or
        <command>telnet</command>.  SSH utilizes a key fingerprint
        system for verifying the authenticity of the server when the 
        client connects.  The user is prompted to enter
	<literal>yes</literal> only when
        connecting for the first time.  Future attempts to login are all
        verified against the saved fingerprint key.  The SSH client
        will alert you if the saved fingerprint differs from the
        received fingerprint on future login attempts.  The fingerprints
        are saved in <filename>~/.ssh/known_hosts</filename>, or
	<filename>~/.ssh/known_hosts2</filename> for SSH v2
	fingerprints.</para>

      <para>By default, OpenSSH servers are configured to accept both
	SSH v1 and SSH v2 connections.  The client, however, can choose
	between the two.  Version 2 is known to be more robust and
	secure than its predecessor.</para>

      <para><command>ssh</command> can be forced to use either protocol
	by passing it the <option>-1</option> or <option>-2</option> argument
	for v1 and v2, respectively.</para>
    </sect2>
    
    <sect2>
      <title>Secure Copy</title>
      <indexterm>
        <primary>OpenSSH</primary>
        <secondary>secure copy</secondary>
      </indexterm>
      <indexterm><primary><command>scp</command></primary></indexterm>

      <para>The <command>scp</command> command works similarly to
	<command>rcp</command>; it copies a file to or from a remote machine,
	except in a secure fashion.</para>

      <screen>&prompt.root; <userinput> scp <replaceable>user@example.com:/COPYRIGHT COPYRIGHT</replaceable></userinput>
user@example.com's password: 
COPYRIGHT            100% |*****************************|  4735       
00:00    
&prompt.root;</screen>
      <para>Since the fingerprint was already saved for this host in the
        previous example, it is verified when using <command>scp</command>
        here.</para>

      <para>The arguments passed to <command>scp</command> are similar
	to <command>cp</command>, with the file or files in the first
	argument, and the destination in the second.  Since the file is
	fetched over the network, through SSH, one or more of the file
	arguments takes on the form
	<option>user@host:&lt;path_to_remote_file></option>.</para>

    </sect2>

    <sect2>
      <title>Configuration</title>
      <indexterm>
        <primary>OpenSSH</primary>
        <secondary>configuration</secondary>
      </indexterm>

      <para>The system-wide configuration files for both the OpenSSH 
        daemon and client reside within the <filename>/etc/ssh</filename> 
        directory.</para>

      <para><filename>ssh_config</filename> configures the client 
        settings, while <filename>sshd_config</filename> configures the 
        daemon.</para>

      <para>Additionally, the <option>sshd_program</option>
	(<filename>/usr/sbin/sshd</filename> by default), and
	<option>sshd_flags</option> <filename>rc.conf</filename>
	options can provide more levels of configuration.</para>
    </sect2>

    <sect2>
      <title>ssh-keygen</title>

      <para>Instead of using passwords, &man.ssh-keygen.1; can
        be used to generate RSA keys to authenticate a user.</para>

      <screen>&prompt.user; <userinput>ssh-keygen</userinput>
Initializing random number generator...
Generating p:  .++ (distance 66)
Generating q:  ..............................++ (distance 498)
Computing the keys...
Key generation complete.
Enter file in which to save the key (/home/user/.ssh/identity):
Enter passphrase:
Enter the same passphrase again:
Your identification has been saved in /home/user/.ssh/identity.
...</screen>

      <para>&man.ssh-keygen.1; will create a public and private
        key pair for use in authentication.  The private key is stored in
        <filename>~/.ssh/identity</filename>, whereas the public key is
        stored in <filename>~/.ssh/identity.pub</filename>.  The public
        key must be placed in <filename>~/.ssh/authorized_keys</filename>
        of the remote machine in order for the setup to work.</para>

      <para>This will allow connection to the remote machine based upon
        RSA authentication instead of passwords.</para>

      <para>If a passphrase is used in &man.ssh-keygen.1;, the user
        will be prompted for a password each time in order to use the private
        key.</para>

      <para>A SSH v2 DSA key can be created for the same purpose by using
	the <command>ssh-keygen -d</command> command (or
	<command>ssh-keygen -t dsa</command> for FreeBSD &os.current;).
	This will
	create a public/private DSA key for use in SSH v2 sessions only.
	The public key is stored in <filename>~/.ssh/id_dsa.pub</filename>,
	while the private key is in <filename>~/.ssh/id_dsa</filename>.</para>

      <para>DSA public keys are placed in
	<filename>~/.ssh/authorized_keys2</filename> on the remote
	machine.</para>

      <para>&man.ssh-agent.1; and &man.ssh-add.1; are 
        utilities used in managing multiple passworded private keys.</para>
    </sect2>

    <sect2>
      <title>SSH Tunneling</title>
      <indexterm>
        <primary>OpenSSH</primary>
        <secondary>tunneling</secondary>
      </indexterm>

      <para>OpenSSH has the ability to create a tunnel to encapsulate
        another protocol in an encrypted session.</para>

      <para>The following command tells &man.ssh.1; to create a tunnel 
         for telnet.</para>

       <screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>5023:localhost:23 user@foo.example.com</replaceable></userinput>
&prompt.user;</screen>

      <para>The <command>ssh</command> command is used with the
	following options:</para>

      <variablelist>
	<varlistentry>
	  <term><option>-2</option></term>
	  
	  <listitem>
	    <para>Forces <command>ssh</command> to use version 2 of
	      the protocol. (Do not use if you are working with older
	      ssh servers)</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-N</option></term>

	  <listitem>
	    <para>Indicates no command, or tunnel only.  If omitted,
	      <command>ssh</command> would initiate a normal
	      session.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-f</option></term>

	  <listitem>
	    <para>Forces <command>ssh</command> to run in the
	      background.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-L</option></term>

	  <listitem>
	    <para>Indicates a local tunnel in
	      <replaceable>localport:remotehost:remoteport</replaceable>
	      fashion.</para>
	  </listitem>
	  </varlistentry>

	<varlistentry>
	  <term><option>user@foo.example.com</option></term>

	  <listitem>
	    <para>The remote SSH server.</para>
	  </listitem>
	</varlistentry>
      </variablelist>


      <para>An SSH tunnel works by creating a listen socket on
	<hostid>localhost</hostid> on the specified port.
	It then forwards any connection received
	on the local host/port via the SSH connection to the specified
	remote host and port.</para>

      <para>In the example, port <replaceable>5023</replaceable> on
	<hostid>localhost</hostid> is being forwarded to port
	<replaceable>23</replaceable> on <hostid>localhost</hostid>
	of the remote machine.  Since <replaceable>23</replaceable> is telnet,
	this would create a secure telnet session through an SSH tunnel.</para>

      <para>This can be used to wrap any number of insecure TCP protocols 
        such as SMTP, POP3, FTP, etc.</para>

      <example>
	<title>Using SSH to create a secure tunnel for SMTP</title>

        <screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>5025:localhost:25 user@mailserver.example.com</replaceable></userinput>
user@mailserver.example.com's password: <userinput>*****</userinput>
&prompt.user; <userinput>telnet localhost 5025</userinput>
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mailserver.example.com ESMTP</screen>     

        <para>This can be used in conjunction with an
          &man.ssh-keygen.1; and additional user accounts to create a
          more seamless/hassle-free SSH tunneling environment.  Keys
          can be used in place of typing a password, and the tunnels
          can be run as a separate user.</para>
      </example>

      <sect3>
	<title>Practical SSH Tunneling Examples</title>

	<sect4>
	  <title>Secure Access of a POP3 server</title>

	  <para>At work, there is an SSH server that accepts
	    connections from the outside.  On the same office network
	    resides a mail server running a POP3 server.  The network,
	    or network path between your home and office may or may not
	    be completely trustable.  Because of this, you need to check
	    your e-mail in a secure manner.  The solution is to create
	    an SSH connection to your office's SSH server, and tunnel
	    through to the mail server.</para>

	  <screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>2110:mail.example.com:110 user@ssh-server.example.com</replaceable></userinput>
user@ssh-server.example.com's password: ******</screen>

	  <para>When the tunnel is up and running, you can point your
	    mail client to send POP3 requests to <hostid>localhost</hostid>
	    port 2110.  A connection here will be forwarded securely across
	    the tunnel to <hostid>mail.example.com</hostid>.</para>
	</sect4>

	<sect4>
	  <title>Bypassing a Draconian Firewall</title>

	  <para>Some network administrators impose extremely Draconian
	    firewall rules, filtering not only incoming connections,
	    but outgoing connections.  You may be only given access
	    to contact remote machines on ports 22 and 80 for SSH
	    and web surfing.</para>

	  <para>You may wish to access another (perhaps non-work
	    related) service, such as an Ogg Vorbis server to stream
	    music. If this Ogg Vorbis server is streaming on some other
	    port than 22 or 80, you will not be able to access it.</para>

	  <para>The solution is to create an SSH connection to a machine
	    outside of your network's firewall, and use it to tunnel to
	    the Ogg Vorbis server.</para>

	  <screen>&prompt.user; <userinput>ssh -2 -N -f -L <replaceable>8888:music.example.com:8000 user@unfirewalled.myserver.com</replaceable></userinput>
user@unfirewalled.myserver.com's password: *******</screen>

	  <para>Your streaming client can now be pointed to
	    <hostid>localhost</hostid> port 8888, which will be
	    forwarded over to <hostid>music.example.com</hostid> port
	    8000, successfully evading the firewall.</para>
        </sect4>
      </sect3>
    </sect2>

    <sect2>
      <title>Further Reading</title>
      <para><ulink url="http://www.openssh.com/">OpenSSH</ulink></para>
      <para>&man.ssh.1; &man.scp.1; &man.ssh-keygen.1; 
        &man.ssh-agent.1; &man.ssh-add.1;</para>
      <para>&man.sshd.8; &man.sftp-server.8;</para>
    </sect2>
  </sect1>

</chapter>

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