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LDP/howto/linuxdoc/Chroot-BIND-HOWTO.sgml
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@ -6,11 +6,13 @@
<title>Chroot-BIND HOWTO
<author>Scott Wunsch, <tt>scott at wunsch.org</>
<date>v1.3, 24 February 2001
<date>v1.4, 1 July 2001
<abstract>
This document describes installing the BIND 8 nameserver to run in a chroot
This document describes installing the BIND 9 nameserver to run in a chroot
jail and as a non-root user, to provide added security and minimise the
potential effects of a security compromise.
potential effects of a security compromise. Note that this document has
been updated for BIND 9; if you still run BIND 8, you want the Chroot-BIND8
HOWTO instead.
</abstract>
<!-- Table of contents -->
@ -21,19 +23,19 @@ potential effects of a security compromise.
<sect>Introduction
<p>
This is the Chroot-BIND HOWTO; see <ref id="where" Name="Where?"> for the master
site, which contains the latest copy. It is assumed that you already know how
to configure and use BIND (the Berkeley Internet Name Domain). If not, I would
recommend that you read the DNS HOWTO first. It is also assumed that you have
a basic familiarity with compiling and installing software on your UNIX-like
system.
This is the Chroot-BIND HOWTO; see <ref id="where" Name="Where?"> for the
master site, which contains the latest copy. It is assumed that you already
know how to configure and use BIND (the Berkeley Internet Name Domain). If
not, I would recommend that you read the DNS HOWTO first. It is also assumed
that you have a basic familiarity with compiling and installing software on
your UNIX-like system.
<sect1>What?
<p>
This document describes some extra security precautions that you can take when
you install BIND. It explains how to configure BIND so that it resides in a
``chroot jail'', meaning that it cannot see or access files outside its own
``chroot jail,'' meaning that it cannot see or access files outside its own
little directory tree. We shall also configure it to run as a non-root user.
The idea behind chroot is fairly simple. When you run BIND (or any other
@ -42,7 +44,14 @@ filesystem outside the jail. For example, in this document, we'll set BIND up
to run chrooted to the directory <tt>/chroot/named</>. Well, to BIND, the
contents of this directory will appear to be <tt>/</>, the root directory.
Nothing outside this directory will be accessible to it. You've probably
encounted a chroot jail before, if you've ever ftped into a public system.
encounted a chroot jail before, if you've ever used <tt>ftp</> to log into a
public system.
Because the chroot process is much simpler with BIND 9, I have started to expand
this document slightly, to include more general tips about securing a BIND
installation. Nevertheless, this document is not (and is not intended to be) a
complete reference for securing BIND. If you do only what is outlined in this
document, you're not finished securing your nameserver!
<sect1>Why?
@ -52,7 +61,8 @@ any malicious individual could gain by exploiting vulnerabilities in BIND. It
is for the same reason that we run BIND as a non-root user.
This should be considered as a supplement to the normal security precautions
(running the latest version, using access control, etc.), not a replacement for
(running the latest version, using access control, etc.), certainly not as a
replacement for
them.
If you're interested in DNS security, you might also be interested in a few
@ -71,18 +81,21 @@ The latest version of this document is always available from the web site of the
Linux/Open Source Users of Regina, Sask., at <url
url="http://www.losurs.org/docs/howto/Chroot-BIND.html">.
There is now a Japanese translation of this document, maintained by Nakano
Takeo <tt>nakano at apm.seikei.ac.jp</>. This is available at <url
url="http://www.linux.or.jp/JF/JFdocs/Chroot-BIND-HOWTO.html">.
BIND is available from <url url="http://www.isc.org/" name="the Internet
Software Consortium"> at <url url="http://www.isc.org/bind.html">. As of this
writing, the current version of BIND8 is 8.2.3. BIND 9.x has now been
released, but I haven't had time to look at it in depth yet, so I don't know
how much of this document will be applicable. Some administrators are now
putting BIND9 into production use, but it still requires a taste for living on
the bleeding edge (and upgrading every couple weeks), so you may be better off
staying with BIND8 for now.
writing, the current version of BIND 9 is 9.1.2. BIND 9 has been out for some
time now, and many people are using it in production. Nevertheless, if you
are the conservative sort, you may prefer to remain with BIND 8. If this is
the case, please see my Chroot-BIND8 HOWTO (available from the same loation)
for details on chrooting it. Be warned that BIND 8 is much messier to chroot
though.
Keep in mind that there are <bf>known</> security holes in all
versions of BIND8 less than <bf>8.2.3</>, so make very sure that you're
running the latest version!
Keep in mind that there are <bf>known</> security holes in many earlier
versions of BIND, so make very sure that you're running the latest version!
<sect1>How?
@ -103,7 +116,7 @@ this.
<sect1>Disclaimer
<p>
These steps worked for me, on my system. Your mileage may vary. This is but
These steps worked for me, on my system; your mileage may vary. This is but
one way to approach this; there are other ways to set the same thing up
(although the general approach will be the same). It just happens that this
was the first way that I tried that worked, so I wrote it down.
@ -113,6 +126,10 @@ of the instructions in this document should be easily applicable to other
flavours of UNIX as well, and I shall try to point out differences of which I am
aware.
If you run Linux, you need to make sure that you're running a 2.4 kernel before
attempting this. The <tt>-u</> switch (to run as a non-root user) requires
this newer kernel.
<sect>Preparing the Jail
<sect1>Creating a User
@ -149,11 +166,10 @@ structure:
<tscreen><verb>
/chroot
+-- named
+-- bin
+-- dev
+-- etc
| +-- namedb
+-- lib
| +-- slave
+-- var
+-- run
</verb></tscreen>
@ -172,16 +188,20 @@ and the zone files can go in <tt>/chroot/named/etc/namedb</>. For example:
# cp -a /var/named/* /chroot/named/etc/namedb/
</verb></tscreen>
BIND will likely need to write to the <tt>namedb</> directory, and probably some
of the files in it. For example, if your DNS serves as a slave for a zone, it
will have to update that zone file. Also, BIND can dump statistical
information, and does so in this directory. For that reason, you should
probably make the <tt>named</> user the owner of this directory and its contents:
BIND would normally need to write to the <tt>namedb</> directory, but in the
interests of tightening security, we will not allow it to do this. If your
nameserver serves as a slave for any zones, it will need to update these zone
files, which means we'll have to store them in a separate directory, to which
BIND does have write access.
<tscreen><verb>
# chown -R named:named /chroot/named/etc/namedb
# chown -R named:named /chroot/named/etc/namedb/slave
</verb></tscreen>
Keep in mind that'll you have to move any slave zones you have into this
directory, and update your <tt>named.conf</> accordingly.
BIND will also need to write to the <tt>/var/run</> directory, to put its
pidfile and ndc socket there, so let's allow it to do so:
pidfile and statistical information there, so let's allow it to do so:
<tscreen><verb>
# chown named:named /chroot/named/var/run
</verb></tscreen>
@ -190,50 +210,27 @@ pidfile and ndc socket there, so let's allow it to do so:
<p>
Once BIND is running in the chroot jail, it will not be able to access files
outside the jail <bf>at all</>. However, it needs to access a few key files, such
as the system's C library. Exactly what libraries are required will depend on
your flavour of UNIX. For most modern Linux systems, the following commands
will be sufficient to put the necessary libraries in place:
<tscreen><verb>
# cd /chroot/named/lib
# cp -p /lib/libc-2.*.so .
# ln -s libc-2.*.so libc.so.6
# cp -p /lib/ld-2.*.so .
# ln -s ld-2.*.so ld-linux.so.2
</verb></tscreen>
As an alternative, you could simply build statically-linked versions of the BIND
binaries to put in your chroot jail. You should also copy <tt>ldconfig</> into
the jail, and run it to create an <tt>etc/ld.so.cache</> for the jail environment.
The following commands could take care of this:
<tscreen><verb>
# cp /sbin/ldconfig /chroot/named/bin/
# chroot /chroot/named /bin/ldconfig -v
</verb></tscreen>
outside the jail <bf>at all</>. However, it needs to access a few key files,
although not nearly as many as BIND 8 did.
BIND needs one more system file in its jail: good ol' <tt>/dev/null</>. Again,
the exact command necessary to create this device node may vary from system to
system; check your <tt>/dev/MAKEDEV</> script to be sure. Some systems may also
require <tt>/dev/zero</>. For most Linux systems, we can use the following
command:
One file that BIND will need inside its jail is good ol' <tt>/dev/null</>.
Note that the exact command necessary to create this device node may vary from
system to system; check your <tt>/dev/MAKEDEV</> script to be sure. Some
systems may also require <tt>/dev/zero</>, which can created similarly. For
most Linux systems, we can use the following command:
<tscreen><verb>
# mknod /chroot/named/dev/null c 1 3
</verb></tscreen>
Finally, you need a couple extra files in the <tt>/etc</> directory inside the
jail. In particular, you must copy <tt>/etc/localtime</> (this sometimes known
as <tt>/usr/lib/zoneinfo/localtime</> on some systems) in there so that BIND
logs things with the right time on them, and you must make a simple <tt>group</>
file with the <tt>named</> group in it. The following two commands will take
care of this:
You also need another file in the <tt>/etc</> directory inside the jail. You
must copy <tt>/etc/localtime</> (this is sometimes known as
<tt>/usr/lib/zoneinfo/localtime</> on some systems) in there so that BIND logs
things with the right time on them. The following command will take care
of this:
<tscreen><verb>
# cp /etc/localtime /chroot/named/etc/
# echo 'named:x:200:' > /chroot/named/etc/group
</verb></tscreen>
Keep in mind that the GID, 200 in this example, must match the one you defined
in the real <tt>/etc/group</> above.
<sect1>Logging<label id="logging">
<p>
@ -308,115 +305,99 @@ Alteratively, you can simply configure BIND to log to files instead of going
through syslog. See the BIND documentation for more details if you choose to go
this route.
<sect>Compiling BIND<label id="compiling">
<sect1>Tightening Permissions<label id="perm">
<p>
You should be able to find the BIND source by visiting <url
url="http://www.isc.org/bind.html">. You need the <tt>bind-src.tar.gz</>
package. Be sure to get the latest version!
First of all, feel free to restrict access to the whole <tt>/chroot</>
directory to the <tt>root</> user. Of course, not everybody may want to do
this, especially if you have other software installed in that tree that
doesn't appreciate it.
<sect1>Modifying Paths
<tscreen><verb>
# chown root /chroot
# chmod 700 /chroot
</verb></tscreen>
You can also safely restrict access to <tt>/chroot/named</> to the <tt>named</>
user.
<tscreen><verb>
# chown named:named /chroot/named
# chmod 700 /chroot/named
</verb></tscreen>
For even more tightening, on Linux systems we can make a few of the files and
directories immutable, using the <tt>chattr</> tool on ext2 filesystems.
<tscreen><verb>
# cd /chroot/named
# chattr +i etc etc/localtime var
</verb></tscreen>
It would be nice to do this for the <tt>dev</> directory too, but unfortunately
that would prevent <tt>syslogd</> from creating its <tt>dev/log</> socket.
You may also choose to set the immutable bit on other files in the jail as
well, such as your primary zone files, if they aren't expected to change.
<sect>Compiling and Installing Your Shiny New BIND<label id="compiling">
<p>
Things can get a bit confusing at this point, because different parts of the
BIND package will be referring to the same directories by different names
(depending on whether or not they're running inside the jail). I'll try not to
confuse you <bf>too</> much :-).
The main directory that we have to worry about here is <tt>/var/run</>, because
its contents are required for both the main <tt>named</> daemon (inside the
jail), and the <tt>ndc</> utility (on the outside). We'll start by setting
everything up to find this directory from the outside world. To do this, we
need to modify <tt>src/port/linux/Makefile.set</> (substitute your port's
directory if you're not running Linux), and change the line
<tscreen><verb>
DESTRUN=/var/run
</verb></tscreen>
to
<tscreen><verb>
DESTRUN=/chroot/named/var/run
</verb></tscreen>
While you're in there, you may want to change the other destination paths from
<tt>/usr</> to <tt>/usr/local</>.
Now everything should be able to find that directory... except the <tt/named/
daemon itself, to which it's still just <tt>/var/run</> inside the jail. We can
get around this by making a small change in the <tt/named/ source. In the file
<tt>src/bin/named/named.h</>, find the line
<tscreen><verb>
#include "pathnames.h"
</verb></tscreen>
and add the following line immediately after it
<tscreen><verb>
#define _PATH_NDCSOCK "/var/run/ndc"
</verb></tscreen>
This way, <tt/named/ will ignore our definition of <tt/DESTRUN/ over in
<tt/Makefile.set/ and use the correct location (from its perspective in the
chroot jail). You will notice some warnings about redefinitions of
_PATH_NDCSOCK when you do the build; just ignore them.
<sect1>Doing the Build
<sect1>Doing the Compile
<p>
You should now be able to compile BIND as normal, following the instructions in
the <tt/INSTALL/ file. At this stage, we only want to compile BIND, not install
it. Don't go too far when following the <tt/INSTALL/ file. Essentially, it's
just <tt/make clean/, <tt/make depend/, and <tt/make/.
Compiling BIND 9 for use in a chroot jail should be a much more pleasant
experience than BIND 8 was. In fact, you don't have to do anything special;
the standard <tt>./configure && make</> should suffice.
Keep in mind that if you want to enable IPv6 support in BIND
(<tt>--enable-ipv6</>) on Linux systems, you need matching versions of kernel
and glibc. If you have kernel 2.2, you need glibc 2.1, and if you have kernel
2.4, you need glibc 2.2. BIND is quite picky about this.
<sect>Installing Your Shiny New BIND<label id="installing">
<p>
I should mention that if you have an existing installation of BIND, such as from
an RPM, you should probably remove it before installing the new one. On Red Hat
systems, this probably means removing the packages <tt/bind/ and
<tt/bind-utils/, and possibly <tt/bind-devel/ and <tt/caching-nameserver/, if
you have them.
systems, this probably means removing the packages <tt>bind</> and
<tt>bind-utils</>, and possibly <tt>bind-devel</> and <tt>caching-nameserver</>,
if you have them.
You may want to save a copy of the init script (e.g.,
<tt>/etc/rc.d/init.d/named</>), if any, before doing so; it'll be useful later
on.
<sect1>Installing the Tools Outside the Jail
If you are upgrading from an older version of BIND, such as BIND 8, you will
want to read the migration documentation in the file <tt>doc/misc/migration</>
in the BIND source package. I don't deal with any migration issues in this
document; I simply assume that you are replacing an existing, working
installation of BIND 9.
<sect1>Installing the Binaries
<p>
This is the easy part :-). Just run <tt/make install/ and let it take care of
it for you. You may want to <tt>chmod 000 /usr/local/sbin/named</> afterwards,
to make sure you don't accidentally run the non-chrooted copy of BIND. (This
is <tt>/usr/sbin/named</> if you didn't tell it to go in <tt>/usr/local/sbin</>
like I suggested.)
<sect1>Installing the Binaries in the Jail
<p>
Only two parts of the package have to live inside the chroot jail: the main
<tt/named/ daemon itself, and <tt/named-xfer/, which it uses for zone transfers.
You can simply copy them in from the source tree:
<tscreen><verb>
# cp src/bin/named/named /chroot/named/bin
# cp src/bin/named-xfer/named-xfer /chroot/named/bin
</verb></tscreen>
This is the easy part :-). Just run <tt>make install</> and let it take care of
it for you. Really, that's it!
<sect1>Setting up the Init Script
<p>
If you have an existing init script from your distribution, it would probably be
best simply to modify it to run <tt>/chroot/named/bin/named</>, with the
appropriate switches. The switches are... <it/(drumroll please...)/
If you have an existing init script from your distribution, it would probably
be best simply to modify it to run the new binary, with the appropriate
switches. The switches are... <it>(drumroll please...)</>
<itemize>
<item><tt/-u named/, which tells BIND to run as the user <tt/named/, rather than
<tt/root/.
<item><tt/-g named/, to run BIND under the group <tt/named/ too, rather than
<tt/root/ or <tt/wheel/.
<item><tt>-u named</>, which tells BIND to run as the user <tt>named</>, rather
than <tt>root</>.
<item><tt>-t /chroot/named</>, which tells BIND to chroot itself to the jail
that we've set up.
<item><tt>-c /etc/named.conf</>, which tells BIND where to find its
configuration file within the jail.
</itemize>
The following is the init script I use with my Red Hat 6.0 system. As you can
see, it is almost exactly the same as the way it shipped from Red Hat. I have
also modified the <tt>ndc restart</> command so that it restarts the server
properly, and keeps it chrooted. You should probably do the same in your init
script, even if you don't copy this one.
see, it is almost exactly the same as the way it shipped from Red Hat. I
haven't tried the <tt>rndc</> commands yet, but I can't see any reason why
they shouldn't work.
<tscreen><code>
#!/bin/sh
#
@ -437,7 +418,7 @@ script, even if you don't copy this one.
# Check that networking is up.
[ ${NETWORKING} = "no" ] && exit 0
[ -f /chroot/named/bin/named ] || exit 0
[ -f /usr/local/sbin/named ] || exit 0
[ -f /chroot/named/etc/named.conf ] || exit 0
@ -446,7 +427,7 @@ case "$1" in
start)
# Start daemons.
echo -n "Starting named: "
daemon /chroot/named/bin/named -u named -g named -t /chroot/named
daemon /usr/local/sbin/named -u named -t /chroot/named -c /etc/named.conf
echo
touch /var/lock/subsys/named
;;
@ -458,26 +439,27 @@ case "$1" in
echo
;;
status)
/usr/local/sbin/ndc status
status named
exit $?
;;
restart)
/usr/local/sbin/ndc -n /chroot/named/bin/named "restart -u named -g named -t /chroot/named"
$0 stop
$0 start
exit $?
;;
reload)
/usr/local/sbin/ndc reload
/usr/local/sbin/rndc reload
exit $?
;;
probe)
# named knows how to reload intelligently; we don't want linuxconf
# to offer to restart every time
/usr/local/sbin/ndc reload >/dev/null 2>&1 || echo start
/usr/local/sbin/rndc reload >/dev/null 2>&1 || echo start
exit 0
;;
*)
echo "Usage: named {start|stop|status|restart}"
echo "Usage: named {start|stop|status|restart|reload}"
exit 1
esac
@ -488,8 +470,8 @@ On Caldera OpenLinux systems, you simply need to modify the variables defined
at the top, and it will apparently take care of the rest for you:
<tscreen><verb>
NAME=named
DAEMON=/chroot/named/bin/$NAME
OPTIONS="-t /chroot/named -u named -g named"
DAEMON=/usr/local/sbin/$NAME
OPTIONS="-t /chroot/named -u named -c /etc/named.conf"
</verb></tscreen>
<sect1>Configuration Changes
@ -502,18 +484,14 @@ section:
<tscreen><verb>
directory "/etc/namedb";
pid-file "/var/run/named.pid";
named-xfer "/bin/named-xfer";
statistics-file "/var/run/named.stats";
</verb></tscreen>
Since this file is being read by the <tt>named</> daemon, all the paths are of
course relative to the chroot jail.
Some people have also reported having to add an extra block to their
<tt>named.conf</> to get <tt>ndc</> working properly:
<tscreen><verb>
controls {
unix "/var/run/ndc" perm 0600 owner 0 group 0;
};
</verb></tscreen>
course relative to the chroot jail. As of this writing, BIND 9 does not support
many of the statistics and dump files that previous versions did. Presumably
later versions will; if you are running such a version, you may have to add
additional entries to cause BIND to write them to the <tt>/var/run</> directory
as well.
<sect>The End
@ -528,12 +506,6 @@ simply launch it as:
</verb></tscreen>
Make sure you kill any old versions of BIND still running before doing this.
If you take a look at your logs, you should find the initialisation messages
that BIND spits out when it loads. (If not, there's a problem with your <ref
id="logging" name="logging configuration"> that you need to fix.) Amongst those
messages, BIND should tell you that it chrooted successfully, and that it is
running as the user and group <tt>named</>. If not, you have a problem.
<sect1>That's It!
<p>
@ -541,34 +513,44 @@ You can go take a nap now ;-).
<sect>Appendix - Upgrading BIND Later<label id="upgrading">
<p>So, you had BIND 8.2.2_P7 all nicely chrooted and tweaked to your taste...
and then you hear this nasty rumour that there's a remotely-exploitable root
hole in that version too, and you need to upgrade to 8.2.3 right away. Do
you have to go through this whole long process to install this new version?
<p>So, you had BIND 9.1.2 all nicely chrooted and tweaked to your taste... and
then you hear this nasty rumour that BIND 9.1.3 is finally out, and you just
have to give it a try right away. Do you have to go through this whole
long process to install this new version?
Nope. In fact, you really just need the section on <ref id="compiling"
name="Compiling BIND"> and the first two parts of the section on <ref
id="installing" name="Installing BIND"> (installing the binaries outside and
inside the jail, respectively).
The rest of the HOWTO deals with setting up the jail and other things like
that, which shouldn't need to be altered between versions of BIND. You can
just dump the new binaries in over top of the old ones, and you're good to go.
But don't forget to kill and restart BIND afterwards, or the old, vulnerable
version will still be running!
Nope. In fact, you really just need to compile the new BIND and install it
over top of the old one. Just don't forget to kill the old version and restart
BIND, or it'll still be the old version running!
<sect>Appendix - Thanks<label id="thanks">
<p>
I'd like to thank Lonny Selinger <tt>&lt;lonny at abyss.za.org&gt;</> for
"testing" this HOWTO and making sure that I didn't miss any steps. I'd also
like to thank Chirik <tt>&lt;chirik at CastleFur.COM&gt;</>, Dwayne Litzenberger
<tt>&lt;dlitz at cheerful.com&gt;</>, Phil Bambridge <tt>&lt;phil.b at
I'd like to thank the following people for their assistance in the creation
of this HOWTO:
<itemize>
<item>Lonny Selinger <tt>&lt;lonny at abyss.za.org&gt;</> for "testing" the
first version of this HOWTO and making sure that I didn't miss any steps.
<item>Chirik <tt>&lt;chirik at CastleFur.COM&gt;</>, Dwayne Litzenberger
<tt>&lt;dlitz at dlitz.net&gt;</>, Phil Bambridge <tt>&lt;phil.b at
cableinet.co.uk&gt;</>, Robert Cole <tt>&lt;rcole at metrum-datatape.com&gt;</>,
Colin MacDonald <tt>&lt;colinm at telus.net&gt;</>, and others for pointing out
errors, omissions, and providing other useful advice to make this HOWTO even
better.
<item>Erik Wallin <tt>&lt;erikw at sec.se&gt;</> and Brian Cervenka
<tt>&lt;brian at zerobelow.org&gt;</> for providing good suggestions for
further tightening the jail.
</itemize>
And last but certainly not least, I'd like to thank Nakano Takeo <tt>&lt;nakano
at apm.seikei.ac.jp&gt;</> for translating the Chroot-BIND HOWTO into
Japanese. You can find his translation at <url
url="http://www.linux.or.jp/JF/JFdocs/Chroot-BIND-HOWTO.html">.
<sect>Appendix - Document Distribution Policy<label id="legal">
<p>