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<!--
The FreeBSD Documentation Project
$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/security/chapter.sgml,v 1.26 2000/03/10 22:42:58 phantom Exp $
$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/security/chapter.sgml,v 1.27 2000/03/23 09:22:27 unfurl Exp $
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<chapter id="security">
<title>Security</title>
<para><emphasis>Parts rewritten and updated by &a.unfurl;, 21 March
2000.</emphasis></para>
<para><emphasis>Much of this chapter has been taken from the
&man.security.7; man page, originally written by
&a.dillon;.</emphasis></para>
<sect1>
<title>Synopsis</title>
<para>The following chapter will provide a basic introduction to
system security concepts, some general good rules of thumb, and some
advanced topics such as S/Key, OpenSSL, Kerberos, and others.</para>
</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 &ldquo;honest&rdquo; 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 ever bigger issue.</para>
<para>Security is best implemented through a layered
&ldquo;onion&rdquo; 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 interefere 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 schg flags (see &man.chflags.1;) on every system binary because
while this may temporarily protect the binaries, it prevents a
hacker who has broken in from making an easily detectable change
that may result in your security mechanisms not detecting the hacker
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 break root. 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>
<para>A denial of service attack is an action that deprives the
machine of needed resources. Typically, D.O.S. attacks are
brute-force mechanisms that attempt to crash or otherwise make a
machine unusable by overwhelming its servers or network stack. Some
D.O.S. attacks try to take advantages 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 fill up internet
pipe.</para>
<para>A user account compromise is even more common then a D.O.S.
attack. Many sysadmins still run standard telnetd, rlogind, rshd,
and ftpd 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 root. 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 root. The
distinction is important because without access to root the attacker
cannot generally hide his tracks and may, at best, be able to do
nothing more then 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>
<para>System administrators must keep in mind that there are
potentially many ways to break root on a machine. The attacker may
know the root password, the attacker may find a bug in a root-run
server and be able to break root over a network connection to that
server, or the attacker may know of a bug in an suid-root program
that allows the attacker to break root once he has broken into a
user's account. If an attacker has found a way to break root on a
machine, the attacker may not have a need to install Many of the
root holes found and closed to date involve a considerable amount of
work by the hacker to cleanup after himself, so most hackers do
install backdoors. This gives you a convienient way to detect the
hacker. Making it impossible for a hacker to install a backdoor may
actually be detrimental to your security because it will not close
off the hole the hacker found to break in in the first place.</para>
<para>Security remedies should always be implemented with a
multi-layered &ldquo;onion peel&rdquo; approach and can be
categorized as follows:</para>
<orderedlist>
<listitem>
<para>Securing root and staff accounts.</para>
</listitem>
<listitem>
<para>Securing root &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>
<sect1 id="securing-freebsd">
<title>Securing FreeBSD</title>
<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 root account and staff accounts</title>
<para>First off, do not bother securing staff accounts if you have
not secured the root account. Most systems have a password
assigned to the root 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 unsecure in the
<filename>/etc/ttys</filename> file so that direct root 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 root logins
are disabled there as well. Consider every access method &ndash;
services such as ftp often fall through the cracks. Direct root
logins should only be allowed via the system console.</para>
<para>Of course, as a sysadmin you have to be able to get to root,
so we open up a few holes. But we make sure these holes require
additional password verification to operate. One way to make root
accessible is to add appropriate staff accounts to the
<literal>wheel</literal> group (in
<filename>/etc/group</filename>). The staff members placed in the
<literal>wheel</literal> group are allowed to
<literal>su</literal> to root. You should never give staff
members native wheel access by putting them in the
<literal>wheel</literal> group in their password entry. Staff
accounts should be placed in a <literal>staff</literal> group, and
then added to the <literal>wheel</literal> group via the
<filename>/etc/group</filename> file. Only those staff members
who actually need to have root access should be placed in the
<literal>wheel</literal> group. It is also possible, when using
an authentication method such as kerberos, to use kerberos's
<filename>.k5login</filename> file in the root account to allow a
&man.ksu.1; to root without having to place anyone at all in the
<literal>wheel</literal> group. This may be the better solution
since the <literal>wheel</literal> mechanism still allows an
intruder to break root if the intruder has gotten hold of your
password file and can break into a staff account. While having
the <literal>wheel</literal> mechanism is better then having
nothing at all, it is not necessarily the safest option.</para>
<para>An indirect way to secure the root account is to secure your
staff accounts by using an alternative login access method and
<literal>*</literal>'ing out the crypted password for the staff
accounts. This way an intruder may be able to steal the password
file but will not be able to break into any staff accounts (or,
indirectly, root, even if root has a crypted password associated
with it). Staff members get into their staff accounts through a
secure login mechanism such as &man.kerberos.1; or &man.ssh.1;
using a private/public key pair. When you use something like
kerberos, you generally must secure the machines which run the
kerberos servers and your desktop workstation. When you use a
public/private key pair with <application>ssh</application>, you
must generally secure the machine you are logging in
<emphasis>from</emphasis> (typically your workstation), but you
can also add an additional layer of protection to the key pair by
password protecting the keypair when you create it with
&man.ssh-keygen.1;. Being able to <literal>*</literal> out the
passwords for staff accounts also guarantees that staff members can
only login through secure access methods that you have setup. You
can thus force all staff members to use secure, encrypted
connections for all of their sessions which closes an important
hole used by many intruders: That of 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>
<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 machine the staff member
may have an account on. 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>
<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 imapd or
popper is like giving a universal root 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 root. For
example, the <application>ntalk</application>,
<application>comsat</application>, and
<application>finger</application> daemons can be run in special
user <literal>sandboxes</literal>. A sandbox isn't perfect unless
you go to 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 root, 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;.
The default <filename>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>
<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 then you are willing to
perform (the convenience factor strikes again). You may have to
run these servers as root and rely on other mechanisms to detect
break-ins that might occur through them.</para>
<para>The other big potential root hole 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, root holes are
occasionally found in these binaries. A root 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 then 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 crypted 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 tty 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
<literal>*</literal> 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
<application>ssh</application> 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 <application>ssh</application> or
kerberos for access to those accounts. Even though the crypted
password file (<filename>/etc/spwd.db</filename>) can only be read
by root, 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 <link
linkend="security-integrity">Checking file integrity</link>
below).</para>
</sect2>
<sect2>
<title>Securing the Kernel Core, Raw Devices, and
Filesystems</title>
<para>If an attacker breaks root 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 should not have the bpf device compiled in.</para>
<para>But even if you turn off the bpf device, you still have
<filename>/dev/mem</filename> and <filename>/dev/kmem</filename>
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 bpf 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 <literal>kern.securelevel</literal> variable. Once you have
set the securelevel to 1, write access to raw devices will be
denied and special chflags 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: Binaires, 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 then 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 hackers, 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 <application>ssh</application> keypairs to
allow the limit-access box to <application>ssh</application> 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
<application>ssh</application> may be the better choice even with
the audit-trail tracks that <application>ssh</application>
lays.</para>
<para>Once you give a limit-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
boxes 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 <application>ssh</application> rather then NFS,
writing the security script is much more difficult. You
essentially have to scp 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> daemon on the
client box may already be compromised. All in all, using
<application>ssh</application> may be necessary when running over
unsecure links, but it's 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. I would 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 I recommend
using 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 do 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 hacker who also
has access to this document.</para>
</sect2>
<sect2>
<title>Denial of Service Attacks</title>
<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. Inetd (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 inetd 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 inetd, 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 no 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 realtime
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 that 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
root compromise. Always configure an exclusive firewall, i.e.,
&ldquo;firewall everything <emphasis>except</emphasis> ports A, B,
C, D, and M-Z&rdquo;. 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 &ldquo;close&rdquo; 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 <literal>net.inet.ip.portrange</literal>
<command>sysctl</command>'s (<command>sysctl -a | fgrep
portrange</command>), which can also ease the complexity of your
firewall's configuration. I usually use a normal first/last range
of 4000 to 5000, and a hiport range of 49152 to 65535, then block
everything under 4000 off in my firewall (except for certain
specific internet-accessible ports, of course).</para>
<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 then overload 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 ICMP_BANDLIM which limits the effectiveness
of these sorts of attacks. The last major class of springboard
attacks is related to certain internal inetd 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 chargen 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 <literal>net.inet.ip.rtexpire</literal>,
<literal>rtminexpire</literal>, and <literal>rtmaxcache</literal>
<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 rtexpire but will never decrease it to less then
rtminexpire. 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 <literal>rtminexpire</literal> 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
<literal>rtexpire</literal> and <literal>rtminexpire</literal>
via &man.sysctl.8;. Never set either parameter to zero (unless
you want to crash the machine <!-- smiley -->:-). 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>
<para>There are a few issues with both kerberos and
<application>ssh</application> that need to be addressed if you
intend to use them. Kerberos V is an excellent authentication
protocol but the kerberized <application>telnet</application> and
<application>rlogin</application> suck rocks. There are bugs 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><application>ssh</application> 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
<application>ssh</application> to an unsecure machine, your keys
becomes exposed. The actual keys themselves are not exposed, but
<application>ssh</application> installs a forwarding port for the
duration of your login and if a hacker has broken root on the
unsecure 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 <application>ssh</application> 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
<application>ssh</application> keys while at the same time
protecting passwords via kerberos. <application>ssh</application>
keys should only be used for automated tasks from secure machines
(something that kerberos is unsuited to). We also recommend that
you either turn off key-forwarding in the
<application>ssh</application> configuration, or that you make use
of the <literal>from=IP/DOMAIN</literal> option that
<application>ssh</application> allows in its
<filename>authorized_keys</filename> file to make the key only
useable to entities logging in from specific machines.</para>
</sect2>
</sect1>
<sect1 id="crypt">
<title>DES, MD5, and Crypt</title>
<para><emphasis>Parts rewritten and updated by &a.unfurl;, 21 March
2000.</emphasis></para>
<para>Every user on a UNIX system has a password associated with their
account, obviously these passwords need to be known only to
the user and the actual operating system. In order to keep

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<chapter id="security">
<title>Security</title>
<para><emphasis>Parts rewritten and updated by &a.unfurl;, 21 March
2000.</emphasis></para>
<para><emphasis>Much of this chapter has been taken from the
&man.security.7; man page, originally written by
&a.dillon;.</emphasis></para>
<sect1>
<title>Synopsis</title>
<para>The following chapter will provide a basic introduction to
system security concepts, some general good rules of thumb, and some
advanced topics such as S/Key, OpenSSL, Kerberos, and others.</para>
</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 &ldquo;honest&rdquo; 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 ever bigger issue.</para>
<para>Security is best implemented through a layered
&ldquo;onion&rdquo; 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 interefere 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 schg flags (see &man.chflags.1;) on every system binary because
while this may temporarily protect the binaries, it prevents a
hacker who has broken in from making an easily detectable change
that may result in your security mechanisms not detecting the hacker
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 break root. 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>
<para>A denial of service attack is an action that deprives the
machine of needed resources. Typically, D.O.S. attacks are
brute-force mechanisms that attempt to crash or otherwise make a
machine unusable by overwhelming its servers or network stack. Some
D.O.S. attacks try to take advantages 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 fill up internet
pipe.</para>
<para>A user account compromise is even more common then a D.O.S.
attack. Many sysadmins still run standard telnetd, rlogind, rshd,
and ftpd 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 root. 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 root. The
distinction is important because without access to root the attacker
cannot generally hide his tracks and may, at best, be able to do
nothing more then 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>
<para>System administrators must keep in mind that there are
potentially many ways to break root on a machine. The attacker may
know the root password, the attacker may find a bug in a root-run
server and be able to break root over a network connection to that
server, or the attacker may know of a bug in an suid-root program
that allows the attacker to break root once he has broken into a
user's account. If an attacker has found a way to break root on a
machine, the attacker may not have a need to install Many of the
root holes found and closed to date involve a considerable amount of
work by the hacker to cleanup after himself, so most hackers do
install backdoors. This gives you a convienient way to detect the
hacker. Making it impossible for a hacker to install a backdoor may
actually be detrimental to your security because it will not close
off the hole the hacker found to break in in the first place.</para>
<para>Security remedies should always be implemented with a
multi-layered &ldquo;onion peel&rdquo; approach and can be
categorized as follows:</para>
<orderedlist>
<listitem>
<para>Securing root and staff accounts.</para>
</listitem>
<listitem>
<para>Securing root &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>
<sect1 id="securing-freebsd">
<title>Securing FreeBSD</title>
<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 root account and staff accounts</title>
<para>First off, do not bother securing staff accounts if you have
not secured the root account. Most systems have a password
assigned to the root 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 unsecure in the
<filename>/etc/ttys</filename> file so that direct root 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 root logins
are disabled there as well. Consider every access method &ndash;
services such as ftp often fall through the cracks. Direct root
logins should only be allowed via the system console.</para>
<para>Of course, as a sysadmin you have to be able to get to root,
so we open up a few holes. But we make sure these holes require
additional password verification to operate. One way to make root
accessible is to add appropriate staff accounts to the
<literal>wheel</literal> group (in
<filename>/etc/group</filename>). The staff members placed in the
<literal>wheel</literal> group are allowed to
<literal>su</literal> to root. You should never give staff
members native wheel access by putting them in the
<literal>wheel</literal> group in their password entry. Staff
accounts should be placed in a <literal>staff</literal> group, and
then added to the <literal>wheel</literal> group via the
<filename>/etc/group</filename> file. Only those staff members
who actually need to have root access should be placed in the
<literal>wheel</literal> group. It is also possible, when using
an authentication method such as kerberos, to use kerberos's
<filename>.k5login</filename> file in the root account to allow a
&man.ksu.1; to root without having to place anyone at all in the
<literal>wheel</literal> group. This may be the better solution
since the <literal>wheel</literal> mechanism still allows an
intruder to break root if the intruder has gotten hold of your
password file and can break into a staff account. While having
the <literal>wheel</literal> mechanism is better then having
nothing at all, it is not necessarily the safest option.</para>
<para>An indirect way to secure the root account is to secure your
staff accounts by using an alternative login access method and
<literal>*</literal>'ing out the crypted password for the staff
accounts. This way an intruder may be able to steal the password
file but will not be able to break into any staff accounts (or,
indirectly, root, even if root has a crypted password associated
with it). Staff members get into their staff accounts through a
secure login mechanism such as &man.kerberos.1; or &man.ssh.1;
using a private/public key pair. When you use something like
kerberos, you generally must secure the machines which run the
kerberos servers and your desktop workstation. When you use a
public/private key pair with <application>ssh</application>, you
must generally secure the machine you are logging in
<emphasis>from</emphasis> (typically your workstation), but you
can also add an additional layer of protection to the key pair by
password protecting the keypair when you create it with
&man.ssh-keygen.1;. Being able to <literal>*</literal> out the
passwords for staff accounts also guarantees that staff members can
only login through secure access methods that you have setup. You
can thus force all staff members to use secure, encrypted
connections for all of their sessions which closes an important
hole used by many intruders: That of 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>
<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 machine the staff member
may have an account on. 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>
<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 imapd or
popper is like giving a universal root 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 root. For
example, the <application>ntalk</application>,
<application>comsat</application>, and
<application>finger</application> daemons can be run in special
user <literal>sandboxes</literal>. A sandbox isn't perfect unless
you go to 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 root, 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;.
The default <filename>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>
<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 then you are willing to
perform (the convenience factor strikes again). You may have to
run these servers as root and rely on other mechanisms to detect
break-ins that might occur through them.</para>
<para>The other big potential root hole 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, root holes are
occasionally found in these binaries. A root 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 then 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 crypted 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 tty 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
<literal>*</literal> 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
<application>ssh</application> 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 <application>ssh</application> or
kerberos for access to those accounts. Even though the crypted
password file (<filename>/etc/spwd.db</filename>) can only be read
by root, 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 <link
linkend="security-integrity">Checking file integrity</link>
below).</para>
</sect2>
<sect2>
<title>Securing the Kernel Core, Raw Devices, and
Filesystems</title>
<para>If an attacker breaks root 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 should not have the bpf device compiled in.</para>
<para>But even if you turn off the bpf device, you still have
<filename>/dev/mem</filename> and <filename>/dev/kmem</filename>
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 bpf 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 <literal>kern.securelevel</literal> variable. Once you have
set the securelevel to 1, write access to raw devices will be
denied and special chflags 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: Binaires, 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 then 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 hackers, 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 <application>ssh</application> keypairs to
allow the limit-access box to <application>ssh</application> 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
<application>ssh</application> may be the better choice even with
the audit-trail tracks that <application>ssh</application>
lays.</para>
<para>Once you give a limit-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
boxes 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 <application>ssh</application> rather then NFS,
writing the security script is much more difficult. You
essentially have to scp 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> daemon on the
client box may already be compromised. All in all, using
<application>ssh</application> may be necessary when running over
unsecure links, but it's 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. I would 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 I recommend
using 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 do 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 hacker who also
has access to this document.</para>
</sect2>
<sect2>
<title>Denial of Service Attacks</title>
<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. Inetd (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 inetd 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 inetd, 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 no 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 realtime
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 that 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
root compromise. Always configure an exclusive firewall, i.e.,
&ldquo;firewall everything <emphasis>except</emphasis> ports A, B,
C, D, and M-Z&rdquo;. 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 &ldquo;close&rdquo; 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 <literal>net.inet.ip.portrange</literal>
<command>sysctl</command>'s (<command>sysctl -a | fgrep
portrange</command>), which can also ease the complexity of your
firewall's configuration. I usually use a normal first/last range
of 4000 to 5000, and a hiport range of 49152 to 65535, then block
everything under 4000 off in my firewall (except for certain
specific internet-accessible ports, of course).</para>
<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 then overload 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 ICMP_BANDLIM which limits the effectiveness
of these sorts of attacks. The last major class of springboard
attacks is related to certain internal inetd 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 chargen 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 <literal>net.inet.ip.rtexpire</literal>,
<literal>rtminexpire</literal>, and <literal>rtmaxcache</literal>
<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 rtexpire but will never decrease it to less then
rtminexpire. 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 <literal>rtminexpire</literal> 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
<literal>rtexpire</literal> and <literal>rtminexpire</literal>
via &man.sysctl.8;. Never set either parameter to zero (unless
you want to crash the machine <!-- smiley -->:-). 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>
<para>There are a few issues with both kerberos and
<application>ssh</application> that need to be addressed if you
intend to use them. Kerberos V is an excellent authentication
protocol but the kerberized <application>telnet</application> and
<application>rlogin</application> suck rocks. There are bugs 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><application>ssh</application> 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
<application>ssh</application> to an unsecure machine, your keys
becomes exposed. The actual keys themselves are not exposed, but
<application>ssh</application> installs a forwarding port for the
duration of your login and if a hacker has broken root on the
unsecure 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 <application>ssh</application> 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
<application>ssh</application> keys while at the same time
protecting passwords via kerberos. <application>ssh</application>
keys should only be used for automated tasks from secure machines
(something that kerberos is unsuited to). We also recommend that
you either turn off key-forwarding in the
<application>ssh</application> configuration, or that you make use
of the <literal>from=IP/DOMAIN</literal> option that
<application>ssh</application> allows in its
<filename>authorized_keys</filename> file to make the key only
useable to entities logging in from specific machines.</para>
</sect2>
</sect1>
<sect1 id="crypt">
<title>DES, MD5, and Crypt</title>
<para><emphasis>Parts rewritten and updated by &a.unfurl;, 21 March
2000.</emphasis></para>
<para>Every user on a UNIX system has a password associated with their
account, obviously these passwords need to be known only to
the user and the actual operating system. In order to keep