35c66a5bd4
There was TAB damage in some of the examples, causing text that should line up in real life to not line up. Fixed, by replacing the tabs with an appropriate number of spaces.
1620 lines
66 KiB
Text
1620 lines
66 KiB
Text
<!--
|
|
The FreeBSD Documentation Project
|
|
|
|
$Id: chapter.sgml,v 1.13 1999-04-08 21:33:23 nik Exp $
|
|
-->
|
|
|
|
<chapter id="security">
|
|
<title>Security</title>
|
|
|
|
<sect1 id="crypt">
|
|
<title>DES, MD5, and Crypt</title>
|
|
|
|
<para><emphasis>Contributed by &a.wollman; 24 September
|
|
1995.</emphasis></para>
|
|
|
|
<para>In order to protect the security of passwords on UN*X systems from
|
|
being easily exposed, passwords have traditionally been scrambled in
|
|
some way. Starting with Bell Labs' Seventh Edition Unix, passwords were
|
|
encrypted using what the security people call a “one-way hash
|
|
function”. That is to say, the password is transformed in such a
|
|
way that the original password cannot be regained except by brute-force
|
|
searching the space of possible passwords. Unfortunately, the only
|
|
secure method that was available to the AT&T researchers at the time
|
|
was based on DES, the Data Encryption Standard. This causes only
|
|
minimal difficulty for commercial vendors, but is a serious problem for
|
|
an operating system like FreeBSD where all the source code is freely
|
|
available, because national governments in many places like to place
|
|
restrictions on cross-border transport of DES and other encryption
|
|
software.</para>
|
|
|
|
<para>So, the FreeBSD team was faced with a dilemma: how could we provide
|
|
compatibility with all those UNIX systems out there while still not
|
|
running afoul of the law? We decided to take a dual-track approach: we
|
|
would make distributions which contained only a non-regulated password
|
|
scrambler, and then provide as a separate add-on library the DES-based
|
|
password hash. The password-scrambling function was moved out of the C
|
|
library to a separate library, called <filename>libcrypt</filename>
|
|
because the name of the C function to implement it is
|
|
<function>crypt</function>. In FreeBSD 1.x and some pre-release 2.0
|
|
snapshots, the non-regulated scrambler uses an insecure function written
|
|
by Nate Williams; in subsequent releases this was replaced by a
|
|
mechanism using the RSA Data Security, Inc., MD5 one-way hash function.
|
|
Because neither of these functions involve encryption, they are believed
|
|
to be exportable from the US and importable into many other
|
|
countries.</para>
|
|
|
|
<para>Meanwhile, work was also underway on the DES-based password hash
|
|
function. First, a version of the <function>crypt</function> function
|
|
which was written outside the US was imported, thus synchronizing the US
|
|
and non-US code. Then, the library was modified and split into two; the
|
|
DES <filename>libcrypt</filename> contains only the code involved in
|
|
performing the one-way password hash, and a separate
|
|
<filename>libcipher</filename> was created with the entry points to
|
|
actually perform encryption. The code was partitioned in this way to
|
|
make it easier to get an export license for the compiled library.</para>
|
|
|
|
<sect2>
|
|
<title>Recognizing your <command>crypt</command> mechanism</title>
|
|
|
|
<para>It is fairly easy to recognize whether a particular password
|
|
string was created using the DES- or MD5-based hash function. MD5
|
|
password strings always begin with the characters
|
|
<literal>$1$</literal>. 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>$</literal> character, so a relatively short
|
|
string which doesn't begin with a dollar sign is very likely a DES
|
|
password.</para>
|
|
|
|
<para>Determining which library is being used on your system is fairly
|
|
easy for most programs, except for those like <command>init</command>
|
|
which are statically linked. (For those programs, the only way is to
|
|
try them on a known password and see if it works.) Programs which use
|
|
<function>crypt</function> are linked against
|
|
<filename>libcrypt</filename>, which for each type of library is a
|
|
symbolic link to the appropriate implementation. For example, on a
|
|
system using the DES versions:</para>
|
|
|
|
<screen>&prompt.user; <userinput> cd /usr/lib</userinput>
|
|
&prompt.user; <userinput>ls -l /usr/lib/libcrypt*</userinput>
|
|
lrwxr-xr-x 1 bin bin 13 Sep 5 12:50 libcrypt.a -> libdescrypt.a
|
|
lrwxr-xr-x 1 bin bin 18 Sep 5 12:50 libcrypt.so.2.0 -> libdescrypt.so.2.0
|
|
lrwxr-xr-x 1 bin bin 15 Sep 5 12:50 libcrypt_p.a -> libdescrypt_p.a</screen>
|
|
|
|
<para>On a system using the MD5-based libraries, the same links will be
|
|
present, but the target will be <filename>libscrypt</filename> rather
|
|
than <filename>libdescrypt</filename>.</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="skey">
|
|
<!--
|
|
Copyright 1995 Massachusetts Institute of Technology
|
|
|
|
Permission to use, copy, modify, and distribute this software and
|
|
its documentation for any purpose and without fee is hereby
|
|
granted, provided that both the above copyright notice and this
|
|
permission notice appear in all copies, that both the above
|
|
copyright notice and this permission notice appear in all
|
|
supporting documentation, and that the name of M.I.T. not be used
|
|
in advertising or publicity pertaining to distribution of the
|
|
software without specific, written prior permission. M.I.T. makes
|
|
no representations about the suitability of this software for any
|
|
purpose. It is provided "as is" without express or implied
|
|
warranty.
|
|
|
|
THIS SOFTWARE IS PROVIDED BY M.I.T. ``AS IS''. M.I.T. DISCLAIMS
|
|
ALL EXPRESS OR IMPLIED WARRANTIES WITH REGARD TO THIS SOFTWARE,
|
|
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
|
|
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
|
|
SHALL M.I.T. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
|
|
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
|
|
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
|
|
USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
|
|
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
|
|
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
|
|
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
SUCH DAMAGE.
|
|
-->
|
|
|
|
<title>S/Key</title>
|
|
|
|
<para><emphasis>Contributed by &a.wollman; 25 September
|
|
1995.</emphasis></para>
|
|
|
|
<para>S/Key is a one-time password scheme based on a one-way hash function
|
|
(in our version, this is MD4 for compatibility; other versions have used
|
|
MD5 and DES-MAC). S/Key has been a standard part of all FreeBSD
|
|
distributions since version 1.1.5, and is also implemented on a large
|
|
and growing number of other systems. S/Key is a registered trademark of
|
|
Bell Communications Research, Inc.</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 “UNIX password”. The
|
|
second sort is the one-time password which is generated by the S/Key
|
|
<command>key</command> program and accepted by the
|
|
<command>keyinit</command> program and the login prompt; we will call
|
|
this a “one-time password”. The final sort of password is
|
|
the secret password which you give to the <command>key</command> program
|
|
(and sometimes the <command>keyinit</command> program) which it uses to
|
|
generate one-time passwords; we will call it a “secret
|
|
password” or just unqualified “password”.</para>
|
|
|
|
<para>The secret password does not necessarily have anything to do with
|
|
your UNIX password (while they can be the same, this is not
|
|
recommended). While UNIX passwords are limited to eight characters in
|
|
length, your S/Key secret password can be as long as you like; I use
|
|
seven-word phrases. In general, the S/Key system operates completely
|
|
independently of the UNIX password system.</para>
|
|
|
|
<para>There are in addition two other sorts of data involved in the S/Key
|
|
system; one is called the “seed” or (confusingly)
|
|
“key”, and consists of two letters and five digits, and the
|
|
other is the “iteration count” and is a number between 100
|
|
and 1. S/Key constructs a one-time password from these components by
|
|
concatenating the seed and the secret password, then applying a one-way
|
|
hash (the RSA Data Security, Inc., MD4 secure hash function)
|
|
iteration-count times, and turning the result into six short English
|
|
words. The <command>login</command> and <command>su</command> programs
|
|
keep 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 function is used, it is not
|
|
possible to generate future one-time passwords having overheard one
|
|
which was successfully used; the iteration count is decremented after
|
|
each successful login to keep the user and login program in sync. (When
|
|
you get the iteration count down to 1, it is time to reinitialize
|
|
S/Key.)</para>
|
|
|
|
<para>There are four programs involved in the S/Key system which we will
|
|
discuss below. The <command>key</command> program accepts an iteration
|
|
count, a seed, and a secret password, and generates a one-time password.
|
|
The <command>keyinit</command> program is used to initialized S/Key, and
|
|
to change passwords, iteration counts, or seeds; it takes either a
|
|
secret password, or an iteration count, seed, and one-time password.
|
|
The <command>keyinfo</command> program examines the
|
|
<filename>/etc/skeykeys</filename> file and prints out the invoking
|
|
user's current iteration count and seed. Finally, the
|
|
<command>login</command> and <command>su</command> programs contain the
|
|
necessary logic to accept S/Key one-time passwords for authentication.
|
|
The <command>login</command> program is also capable of disallowing the
|
|
use of UNIX passwords on connections coming from specified
|
|
addresses.</para>
|
|
|
|
<para>There are four different sorts of operations we will cover. The
|
|
first is using the <command>keyinit</command> program over a secure
|
|
connection to set up S/Key for the first time, or to change your
|
|
password or seed. The second operation is using the
|
|
<command>keyinit</command> program over an insecure connection, in
|
|
conjunction with the <command>key</command> program over a secure
|
|
connection, to do the same. The third is using the
|
|
<command>key</command> program to log in over an insecure connection.
|
|
The fourth is using the <command>key</command> program 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
|
|
(like at a conference).</para>
|
|
|
|
<sect2>
|
|
<title>Secure connection initialization</title>
|
|
|
|
<para>To initialize S/Key, change your password, or change your seed
|
|
while logged in over a secure connection (e.g., on the console of a
|
|
machine), use the <command>keyinit</command> command without any
|
|
parameters while logged in as yourself:</para>
|
|
|
|
<screen>&prompt.user; keyinit
|
|
Updating wollman: ) these will not appear if you
|
|
Old key: ha73895 ) have not used S/Key before
|
|
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.
|
|
<prompt>Enter secret password:</prompt> ) I typed my pass phrase here
|
|
<prompt>Again secret password:</prompt> ) I typed it again ID
|
|
|
|
wollman s/key is 99 ha73896 ) discussed below SAG
|
|
HAS FONT GOUT FATE BOOM )</screen>
|
|
|
|
<para>There is a lot of information here. At the<prompt>Enter secret
|
|
password:</prompt> prompt, you should enter some password or phrase
|
|
(I use phrases of minimum seven words) which will be needed to
|
|
generate login keys. The line starting `ID' gives the parameters of
|
|
your particular S/Key instance: your login name, the iteration count,
|
|
and seed. When logging in with S/Key, 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 S/Key or change your password or seed over an
|
|
insecure connection, you will need to already have a secure connection
|
|
to some place where you can run the <command>key</command> program;
|
|
this might be in the form of a desk accessory on a Macintosh, or a
|
|
shell prompt on a machine you trust (we will show the latter). 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 wollman: Old key: kh94741
|
|
Reminder you need the 6 English words from the skey command.
|
|
<prompt>Enter sequence count from 1 to 9999:</prompt> <userinput>100</userinput> ) I typed this
|
|
<prompt>Enter new key [default kh94742]:</prompt>
|
|
s/key 100 kh94742</screen>
|
|
|
|
<para>To accept the default seed (which the <command>keyinit</command>
|
|
program confusingly calls a <literal>key</literal>), press return.
|
|
Then move over to your secure connection or S/Key desk accessory, and
|
|
give it the same parameters:</para>
|
|
|
|
<screen>&prompt.user; <userinput>key 100 kh94742</userinput>
|
|
Reminder - Do not use this program while logged in via telnet or rlogin.
|
|
<prompt>Enter secret password:</prompt> ) I typed my secret password
|
|
HULL NAY YANG TREE TOUT VETO</screen>
|
|
|
|
<para>Now switch back over to the insecure connection, and copy the
|
|
one-time password generated by <command>key</command> over to the
|
|
<command>keyinit</command> program:</para>
|
|
|
|
<screen><prompt>s/key access password:</prompt> <userinput>HULL NAY YANG TREE TOUT VETO</userinput>
|
|
ID wollman s/key is 100 kh94742
|
|
HULL NAY YANG TREE TOUT VETO</screen>
|
|
|
|
<para>The rest of the description from the previous section applies here
|
|
as well.</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Diversion: a login prompt</title>
|
|
|
|
<para>Before explaining how to generate one-time passwords, we should go
|
|
over an S/Key login prompt:</para>
|
|
|
|
<screen>&prompt.user; <userinput>telnet himalia</userinput>
|
|
Trying 18.26.0.186...
|
|
Connected to himalia.lcs.mit.edu.
|
|
Escape character is '^]'.
|
|
s/key 92 hi52030
|
|
<prompt>Password:</prompt></screen>
|
|
|
|
<para>Note that, before prompting for a password, the login program
|
|
prints out the iteration number and seed which you will need in order
|
|
to generate the appropriate key. You will also find a useful feature
|
|
(not shown here): if you press return at the password prompt, the
|
|
login program will turn echo on, so you can see what you are typing.
|
|
This can be extremely useful if you are attempting to type in an S/Key
|
|
by hand, such as from a printout.</para>
|
|
|
|
<para>If this machine were configured to disallow UNIX passwords over a
|
|
connection from my machine, the prompt would have also included the
|
|
annotation <literal>(s/key required)</literal>, indicating that only
|
|
S/Key one-time passwords will be accepted.</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Generating a single one-time password</title>
|
|
|
|
<para>Now, to generate the one-time password needed to answer this login
|
|
prompt, we use a trusted machine and the <command>key</command>
|
|
program. (There are versions of the <command>key</command> program
|
|
from DOS and Windows machines, and there is an S/Key desk accessory
|
|
for Macintosh computers as well.) The command-line
|
|
<command>key</command> program takes as its parameters the iteration
|
|
count and seed; you can cut-and-paste right from the login prompt
|
|
starting at <literal>key</literal> to the end of the line.
|
|
Thus:</para>
|
|
|
|
<screen>&prompt.user; <userinput>key 92 hi52030</userinput> ) pasted from previous section
|
|
Reminder - Do not use this program while logged in via telnet or rlogin.
|
|
<prompt>Enter secret password:</prompt> ) I typed my secret password
|
|
ADEN BED WOLF HAW HOT STUN</screen>
|
|
|
|
<para>And in the other window:</para>
|
|
|
|
<screen>s/key 92 hi52030 ) from previous section
|
|
<prompt>Password:</prompt>
|
|
(turning echo on)
|
|
<prompt>Password:</prompt>ADEN BED WOLF HAW HOT STUN
|
|
Last login: Wed Jun 28 15:31:00 from halloran-eldar.l
|
|
[etc.]</screen>
|
|
|
|
<para>This is the easiest mechanism <emphasis>if</emphasis> you have a
|
|
trusted machine. There is a Java S/Key <command>key</command> applet,
|
|
<ulink URL="http://www.cs.umd.edu/~harry/jotp/src.html">The Java OTP
|
|
Calculator</ulink>, that you can download and run locally on any
|
|
Java supporting brower.</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Generating multiple one-time passwords</title>
|
|
|
|
<para>Sometimes we have to go places where no trusted machines or
|
|
connections are available. In this case, it is possible to use the
|
|
<command>key</command> command to generate a number of one-time
|
|
passwords in the same command; these can then be printed out. For
|
|
example:</para>
|
|
|
|
<screen>&prompt.user; <userinput>key -n 25 57 zz99999</userinput>
|
|
Reminder - Do not use this program while logged in via telnet or rlogin.
|
|
<prompt>Enter secret password:</prompt>
|
|
33: WALT THY MALI DARN NIT HEAD
|
|
34: ASK RICE BEAU GINA DOUR STAG
|
|
…
|
|
56: AMOS BOWL LUG FAT CAIN INCH
|
|
57: GROW HAYS TUN DISH CAR BALM</screen>
|
|
|
|
<para>The <option>-n 25</option> requests twenty-five keys in sequence;
|
|
the <option>57</option> indicates the <emphasis>ending</emphasis>
|
|
iteration number; and the rest is as before. 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>The configuration file <filename>/etc/skey.access</filename> can
|
|
be used to configure restrictions on the use of UNIX passwords based
|
|
on the host name, user name, terminal port, or IP address of a login
|
|
session. The complete format of the file is documented in the
|
|
&man.skey.access.5; manual page; there are also some security
|
|
cautions there which should be read before depending on this file for
|
|
security.</para>
|
|
|
|
<para>If there is no <filename>/etc/skey.access</filename> file (which
|
|
is the default state as FreeBSD is shipped), 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 18.26.0.0 255.255.0.0
|
|
permit user jrl
|
|
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 user 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">
|
|
<title>Kerberos</title>
|
|
|
|
<para><emphasis>Contributed by &a.markm; (based on contribution by
|
|
&a.md;).</emphasis></para>
|
|
|
|
<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>
|
|
|
|
<para>In FreeBSD, the Kerberos is not that from the original 4.4BSD-Lite,
|
|
distribution, but eBones, which had been previously ported to FreeBSD
|
|
1.1.5.1, and was sourced from outside the USA/Canada, and is thus
|
|
available to system owners outside those countries.</para>
|
|
|
|
<para>For those needing to get a legal foreign distribution of this
|
|
software, please <emphasis>do not</emphasis> get it from a USA or Canada
|
|
site. You will get that site in <emphasis>big</emphasis> trouble! A
|
|
legal copy of this is available from <hostid
|
|
role="fqdn">ftp.internat.freebsd.org</hostid>, which is in South
|
|
Africa and an official FreeBSD mirror site.</para>
|
|
|
|
<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, of 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>GRONDAR.ZA</filename> and the
|
|
server is <filename>grunt.grondar.za</filename>. We edit or create
|
|
the <filename>krb.conf</filename> file:</para>
|
|
|
|
<screen>&prompt.root; <userinput>cat krb.conf</userinput>
|
|
GRONDAR.ZA
|
|
GRONDAR.ZA grunt.grondar.za 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
|
|
“key distribution centre”. The words <literal>admin
|
|
server</literal> following a hosts name means that host also
|
|
provides an administrative database server. For further explanation
|
|
of these terms, please consult the Kerberos man pages.</para>
|
|
|
|
<para>Now we have to add <hostid role="fqdn">grunt.grondar.za</hostid>
|
|
to the <filename>GRONDAR.ZA</filename> realm and also add an entry to
|
|
put all hosts in the <hostid role="domainname">.grondar.za</hostid>
|
|
domain in the <filename>GRONDAR.ZA</filename> realm. The
|
|
<filename>krb.realms</filename> file would be updated as
|
|
follows:</para>
|
|
|
|
<screen>&prompt.root; <userinput>cat krb.realms</userinput>
|
|
grunt.grondar.za GRONDAR.ZA
|
|
.grondar.za GRONDAR.ZA
|
|
.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 Centre). 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>GRONDAR.ZA</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's 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>
|
|
|
|
<Not found>, <prompt>Create [y] ?</prompt> <userinput>y</userinput>
|
|
|
|
Principal: passwd, Instance: grunt, kdc_key_ver: 1
|
|
<prompt>New Password:</prompt> <---- enter RANDOM here
|
|
Verifying password
|
|
|
|
<prompt>New Password:</prompt> <---- 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>
|
|
|
|
<Not found>, <prompt>Create [y] ?</prompt>
|
|
|
|
Principal: rcmd, Instance: grunt, kdc_key_ver: 1
|
|
<prompt>New Password:</prompt> <---- enter RANDOM here
|
|
Verifying password
|
|
|
|
<prompt>New Password:</prompt> <---- 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> <---- 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
|
|
/etc/kerberosIV 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 server 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's 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>
|
|
|
|
<Not found>, <prompt>Create [y] ?</prompt> <userinput>y</userinput>
|
|
|
|
Principal: jane, Instance: , kdc_key_ver: 1
|
|
<prompt>New Password:</prompt> <---- enter a secure password here
|
|
Verifying password
|
|
|
|
<prompt>New Password:</prompt> <---- 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> <---- 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 &</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: GRONDAR.ZA
|
|
&prompt.root; <userinput>kadmind -n &</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.grondar.za)
|
|
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@GRONDAR.ZA
|
|
|
|
Issued Expires Principal
|
|
Apr 30 11:23:22 Apr 30 19:23:22 krbtgt.GRONDAR.ZA@GRONDAR.ZA</screen>
|
|
|
|
<para>Now try changing the password using <command>passwd</command> to
|
|
check if the kpasswd daemon can get authorization to the Kerberos
|
|
database:</para>
|
|
|
|
<screen>&prompt.user; <userinput>passwd</userinput>
|
|
realm GRONDAR.ZA
|
|
<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 root 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>
|
|
|
|
<Not found>, Create [y] ? y
|
|
|
|
Principal: jane, Instance: root, kdc_key_ver: 1
|
|
<prompt>New Password:</prompt> <---- enter a SECURE password here
|
|
Verifying password
|
|
|
|
<prompt>New Password:</prompt> <---- 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> <--- Keep this short!
|
|
<prompt>Attributes [ 0 ] ?</prompt>
|
|
Edit O.K.
|
|
<prompt>Principal name:</prompt> <---- 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.grondar.za)
|
|
Kerberos Initialization for "jane.root"
|
|
<prompt>Password:</prompt></screen>
|
|
|
|
<para>Now we need to add the user to root's <filename>.klogin</filename>
|
|
file:</para>
|
|
|
|
<screen>&prompt.root; <userinput>cat /root/.klogin</userinput>
|
|
jane.root@GRONDAR.ZA</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@GRONDAR.ZA
|
|
|
|
Issued Expires Principal
|
|
May 2 20:43:12 May 3 04:43:12 krbtgt.GRONDAR.ZA@GRONDAR.ZA</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><principal>.<instance></literal> of the form
|
|
<literal><username>.</literal><literal>root</literal> will allow
|
|
that <literal><username></literal> to <command>su</command> to
|
|
root 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@GRONDAR.ZA</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@GRONDAR.ZA
|
|
jack@GRONDAR.ZA</screen>
|
|
|
|
<para>This allows anyone in the <filename>GRONDAR.ZA</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, Jane now logs into another system, using
|
|
Kerberos:</para>
|
|
|
|
<screen>&prompt.user; <userinput>kinit</userinput>
|
|
MIT Project Athena (grunt.grondar.za)
|
|
<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 (Jane 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.grondar.za)
|
|
<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">
|
|
<title>Firewalls</title>
|
|
|
|
<para><emphasis>Contributed by &a.gpalmer; and &a.alex;.</emphasis></para>
|
|
|
|
<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 companies
|
|
internal network and the “Big Bad Internet” will solve all
|
|
your security problems.</para>
|
|
|
|
<para>It may help, but a poorly setup firewall system is more of a
|
|
security risk than not having one at all. A firewall can only add
|
|
another layer of security to your systems, but they will not be able
|
|
to 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
|
|
bit 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 <emphasis>proxy
|
|
servers</emphasis>, rely 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
|
|
<application>IPFW</application>), 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
|
|
document.</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 in it, which defaults to
|
|
forwarding all packets. To enable the filters, you need to define a
|
|
set of rules for the filtering code, so that it can decide if the
|
|
packet should be allowed to pass or not.</para>
|
|
|
|
<para>To decide if a packet should be passed on or not, 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 “rule
|
|
chain”.</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 (telnetd, ftpd, 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 “one-shot” 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>
|
|
|
|
<para><application>IPFW</application>, 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 <application>IPFW</application>.
|
|
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 <application>IPFW</application> is
|
|
designed, you can use <application>IPFW</application> on non-router
|
|
machines to perform packet filtering on incoming and outgoing
|
|
connections. This is a special case of the more general use of
|
|
<application>IPFW</application>, and the same commands and techniques
|
|
should be used in this situation.</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Enabling IPFW on FreeBSD</title>
|
|
|
|
<para>As the main part of the <application>IPFW</application> 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 <link linkend="kernelconfig">reconfiguring
|
|
the kernel</link> 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>
|
|
|
|
<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>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Configuring IPFW</title>
|
|
|
|
<para>The configuration of the <application>IPFW</application> 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 <application>IPFW</application> 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 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/mark</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 “any IP
|
|
address”.</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>…</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> (timestamp). 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 set 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 ipfw</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> by being forwarded
|
|
by the router:</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 I 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, I 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 of `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. syslogd with 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>As currently supplied, FreeBSD does not have the ability to load
|
|
firewall rules at boot time. My suggestion is to put a call to a
|
|
shell script in the <filename>/etc/netstart</filename> script. Put
|
|
the call early enough in the netstart file so that the firewall is
|
|
configured before any of the IP interfaces are configured. This means
|
|
that there is no window during which time your network is open.</para>
|
|
|
|
<para>The actual script used to load the rules is entirely up to you.
|
|
There is currently no support in the <command>ipfw</command> utility
|
|
for loading multiple rules in the one command. The system I use is to
|
|
use the command:</para>
|
|
|
|
<screen>&prompt.root; <userinput>ipfw list</userinput></screen>
|
|
|
|
<para>to write a list of the current rules out to a file, and then use a
|
|
text editor to prepend <literal>ipfw </literal> before all the lines.
|
|
This will allow the script to be fed into /bin/sh and reload the rules
|
|
into the kernel. Perhaps not the most efficient way, but it
|
|
works.</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'll have to allow
|
|
packets coming from ports 191 and 1525 to any internal UDP port
|
|
through the firewall. ntp 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="ftp://ftp.cert.org/pub/tech_tips/packet_filtering">ftp://ftp.cert.org/pub/tech_tips/packet_filtering</ulink></para>
|
|
|
|
<para>As I said above, these are only <emphasis>guidelines</emphasis>.
|
|
You will have to decide what filter rules you want to use on your
|
|
firewall yourself. I cannot accept ANY responsibility if someone
|
|
breaks into your network, even if you follow the advice given
|
|
above.</para>
|
|
</sect2>
|
|
</sect1>
|
|
</chapter>
|
|
|
|
<!--
|
|
Local Variables:
|
|
mode: sgml
|
|
sgml-declaration: "../chapter.decl"
|
|
sgml-indent-data: t
|
|
sgml-omittag: nil
|
|
sgml-always-quote-attributes: t
|
|
sgml-parent-document: ("../handbook.sgml" "part" "chapter")
|
|
End:
|
|
-->
|
|
|