Chroot-BIND8 HOWTO
Scott Wunsch, scott at wunsch.org
v1.4, 1 July 2001
This document describes installing the BIND 8 nameserver to run in a
chroot jail and as a non-root user, to provide added security and min
imise the potential effects of a security compromise. This version of
the document covers the old but still popular BIND 8; there is another
document which provides similar information for BIND 9.
______________________________________________________________________
Table of Contents
1. Introduction
1.1 What?
1.2 Why?
1.3 Where?
1.4 How?
1.5 Disclaimer
2. Preparing the Jail
2.1 Creating a User
2.2 Directory Structure
2.3 Placing the BIND Data
2.4 System Support Files
2.5 Logging
2.5.1 The Ideal Solution
2.5.2 The Other Solutions
3. Compiling BIND
3.1 Modifying Paths
3.2 Doing the Build
4. Installing Your Shiny New BIND
4.1 Installing the Tools Outside the Jail
4.2 Installing the Binaries in the Jail
4.3 Setting up the Init Script
4.4 Configuration Changes
5. The End
5.1 Launching BIND
5.2 That's It!
6. Appendix - Upgrading BIND Later
7. Appendix - Thanks
8. Appendix - Document Distribution Policy
______________________________________________________________________
1. Introduction
This is the Chroot-BIND8 HOWTO; see ``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.
1.1. What?
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 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 process) in a chroot jail, the process is simply unable to see
any part of the filesystem outside the jail. For example, in this
document, we'll set BIND up to run chrooted to the directory
/chroot/named. Well, to BIND, the contents of this directory will
appear to be /, 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.
1.2. Why?
The idea behind running BIND in a chroot jail is to limit the amount
of access 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 them.
If you're interested in DNS security, you might also be interested in
a few other products. Building BIND with StackGuard
<http://www.immunix.org/products.html#stackguard> would probably be a
good idea for even more protection. Using it is easy; it's just like
using ordinary gcc. Also, DNScache <http://cr.yp.to/dnscache.html> is
a secure replacement for BIND, written by Dan Bernstein. Dan is the
author of qmail, and DNScache appears to follow a similar philosophy.
1.3. Where?
The latest version of this document is always available from the web
site of the Linux/Open Source Users of Regina, Sask., at
<http://www.losurs.org/docs/howto/Chroot-BIND8.html>.
There is now a Japanese translation of this document, maintained by
nakano at apm.seikei.ac.jp. This is available at
<http://www.linux.or.jp/JF/JFdocs/Chroot-BIND8-HOWTO.html>.
BIND is available from the Internet Software Consortium
<http://www.isc.org/> at <http://www.isc.org/bind.html>. As of this
writing, the current version of BIND 8 is 8.2.4. BIND 9.x has now
been released, and has been around for a little while. You may
consider upgrading to it; the chroot process is certainly much simpler
and cleaner. If you are running BIND 9, then you want the Chroot-BIND
HOWTO, which should be available from the same location as this
document.
Keep in mind that there are known security holes in all versions of
BIND 8 less than 8.2.3, so make very sure that you're running the
latest version!
1.4. How?
I wrote this document based on my experiences in setting BIND up in a
chroot environment. In my case, I already had an existing BIND
installation in the form of a package that came with my Linux
distribution. I'll assume that most of you are probably in the same
situation, and will simply be transferring over and modifying the
configuration files from your existing BIND installation, and then
removing the package before installing the new one. Don't remove the
package yet, though; we may want some files from it first.
If this is not the case for you, you should still be able to follow
this document. The only difference is that, where I refer to copying
an existing file, you first have to create it yourself. The DNS HOWTO
may be helpful for this.
1.5. Disclaimer
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.
My BIND experience to date has been installing on Linux servers.
However, most 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.
2. Preparing the Jail
2.1. Creating a User
As mentioned in the introduction, it's not a good idea to run BIND as
root. So, before we begin, let's create a separate user for BIND.
Note that you should never use an existing generic user like nobody
for this purpose. However, some distributions, such as SuSE and Linux
Mandrake have started providing a specific user (generally called
named); you can simply adapt this user for our purposes, if you like.
This requires adding a line something like the following to
/etc/passwd:
named:x:200:200:Nameserver:/chroot/named:/bin/false
And one like this to /etc/group:
named:x:200:
This creates a user and group called named for BIND. Make sure that
the UID and GID (both 200 in this example) are unique on your system.
The shell is set to /bin/false because this user will never need to
log in.
2.2. Directory Structure
Now, we must set up the directory structure that we will use for the
chroot jail in which BIND will live. This can be anywhere on your
filesystem; the truly paranoid may even want to put it on a separate
volume. I shall assume that you will use /chroot/named. Let's start
by creating the following directory structure:
/chroot
+-- named
+-- bin
+-- dev
+-- etc
| +-- namedb
+-- lib
+-- var
+-- run
2.3. Placing the BIND Data
Assuming that you have already done a conventional installation of
BIND and are using it, you will already have an existing named.conf
and zone files. These files must now be moved (or copied, to be safe)
into the chroot jail, so that BIND can get at them. named.conf goes
in /chroot/named/etc, and the zone files can go in
/chroot/named/etc/namedb. For example:
# cp -p /etc/named.conf /chroot/named/etc/
# cp -a /var/named/* /chroot/named/etc/namedb/
BIND will likely need to write to the 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 named user the owner of this
directory and its contents:
# chown -R named:named /chroot/named/etc/namedb
BIND will also need to write to the /var/run directory, to put its
pidfile and ndc socket there, so let's allow it to do so:
# chown named:named /chroot/named/var/run
2.4. System Support Files
Once BIND is running in the chroot jail, it will not be able to access
files outside the jail 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:
# 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
As an alternative, you could simply build statically-linked versions
of the BIND binaries to put in your chroot jail. You should also copy
ldconfig into the jail, and run it to create an etc/ld.so.cache for
the jail environment. The following commands could take care of this:
# cp /sbin/ldconfig /chroot/named/bin/
# chroot /chroot/named /bin/ldconfig -v
BIND needs one more system file in its jail: good ol' /dev/null.
Again, the exact command necessary to create this device node may vary
from system to system; check your /dev/MAKEDEV script to be sure.
Some systems may also require /dev/zero. For most Linux systems, we
can use the following command:
# mknod /chroot/named/dev/null c 1 3
Finally, you need a couple extra files in the /etc directory inside
the jail. In particular, you must copy /etc/localtime (this sometimes
known as /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 group file with the named group in it. The following two
commands will take care of this:
# cp /etc/localtime /chroot/named/etc/
# echo 'named:x:200:' > /chroot/named/etc/group
Keep in mind that the GID, 200 in this example, must match the one you
defined in the real /etc/group above.
2.5. Logging
Unlike a conventional jailbird, BIND can't just scribble its log
entries on the walls :-). Normally, BIND logs through syslogd, the
system logging daemon. However, this type of logging is performed by
sending the log entries to the special socket /dev/log. Since this is
outside the jail, BIND can't use it any more. Fortuantely, there are
a couple options to work around this.
2.5.1. The Ideal Solution
The ideal solution to this dilemma requires a reasonably recent
version of syslogd which supports the -a switch introduced by OpenBSD.
Check the manpage for your syslogd(8) to see if you have such a
version.
If you do, all you have to do is add the switch ``-a
/chroot/named/dev/log'' to the command line when you launch syslogd.
On systems which use a full SysV-init (which includes most Linux
distributions), this is typically done in the file
/etc/rc.d/init.d/syslog. For example, on my Red Hat Linux system, I
changed the line
daemon syslogd -m 0
to
daemon syslogd -m 0 -a /chroot/named/dev/log
On Caldera OpenLinux systems, they use a daemon launcher called ssd,
which reads configuration from /etc/sysconfig/daemons/syslog. You
simply need to modify the options line to look like this:
OPTIONS_SYSLOGD="-m 0 -a /chroot/named/dev/log"
Similarly, on SuSE systems, I'm told that the best place to add this
switch is in the /etc/rc.config file. Changing the line
SYSLOGD_PARAMS=""
to read
SYSLOGD_PARAMS="-a /chroot/named/dev/log"
should do the trick.
Once you've figured out how to make this change for your system,
simply restart syslogd, either by killing it and launching it again
(with the extra parameters), or by using the SysV-init script to do it
for you:
# /etc/rc.d/init.d/syslog stop
# /etc/rc.d/init.d/syslog start
Once it's been restarted, you should see a ``file'' in
/chroot/named/dev called log, that looks something like this:
srw-rw-rw- 1 root root 0 Mar 13 20:58 log
2.5.2. The Other Solutions
If you have an older syslogd, then you'll have to find another way to
do your logging. There are a couple programs out there, such as
holelogd, which are designed to help by acting as a ``proxy'' and
accepting log entries from the chrooted BIND and passing them out to
the regular /dev/log socket.
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.
3. Compiling BIND
You should be able to find the BIND source by visiting
<http://www.isc.org/bind.html>. You need the bind-src.tar.gz package.
Be sure to get the latest version!
3.1. Modifying Paths
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 too much :-).
The main directory that we have to worry about here is /var/run,
because its contents are required for both the main named daemon
(inside the jail), and the 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 src/port/linux/Makefile.set
(substitute your port's directory if you're not running Linux), and
change the line
DESTRUN=/var/run
to
DESTRUN=/chroot/named/var/run
While you're in there, you may want to change the other destination
paths from /usr to /usr/local.
Now everything should be able to find that directory... except the
named daemon itself, to which it's still just /var/run inside the
jail. We can get around this by making a small change in the named
source. In the file src/bin/named/named.h, find the line
#include "pathnames.h"
and add the following line immediately after it
#define _PATH_NDCSOCK "/var/run/ndc"
This way, named will ignore our definition of DESTRUN over in Make
file.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.
3.2. Doing the Build
You should now be able to compile BIND as normal, following the
instructions in the INSTALL file. At this stage, we only want to
compile BIND, not install it. Don't go too far when following the
INSTALL file. Essentially, it's just make clean, make depend, and
make.
4. Installing Your Shiny New BIND
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 bind and bind-utils, and possibly bind-devel and caching-
nameserver, if you have them.
You may want to save a copy of the init script (e.g.,
/etc/rc.d/init.d/named), if any, before doing so; it'll be useful
later on.
4.1. Installing the Tools Outside the Jail
This is the easy part :-). Just run make install and let it take care
of it for you. You may want to chmod 000 /usr/local/sbin/named
afterwards, to make sure you don't accidentally run the non-chrooted
copy of BIND. (This is /usr/sbin/named if you didn't tell it to go in
/usr/local/sbin like I suggested.)
4.2. Installing the Binaries in the Jail
Only two parts of the package have to live inside the chroot jail:
the main named daemon itself, and named-xfer, which it uses for zone
transfers. You can simply copy them in from the source tree:
# cp src/bin/named/named /chroot/named/bin
# cp src/bin/named-xfer/named-xfer /chroot/named/bin
4.3. Setting up the Init Script
If you have an existing init script from your distribution, it would
probably be best simply to modify it to run /chroot/named/bin/named,
with the appropriate switches. The switches are... (drumroll
please...)
· -u named, which tells BIND to run as the user named, rather than
root.
· -g named, to run BIND under the group named too, rather than root
or wheel.
· -t /chroot/named, which tells BIND to chroot itself to the jail
that we've set up.
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 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.
______________________________________________________________________
#!/bin/sh
#
# named This shell script takes care of starting and stopping
# named (BIND DNS server).
#
# chkconfig: 345 55 45
# description: named (BIND) is a Domain Name Server (DNS) \
# that is used to resolve host names to IP addresses.
# probe: true
# Source function library.
. /etc/rc.d/init.d/functions
# Source networking configuration.
. /etc/sysconfig/network
# Check that networking is up.
[ ${NETWORKING} = "no" ] && exit 0
[ -f /chroot/named/bin/named ] || exit 0
[ -f /chroot/named/etc/named.conf ] || exit 0
# See how we were called.
case "$1" in
start)
# Start daemons.
echo -n "Starting named: "
daemon /chroot/named/bin/named -u named -g named -t /chroot/named
echo
touch /var/lock/subsys/named
;;
stop)
# Stop daemons.
echo -n "Shutting down named: "
killproc named
rm -f /var/lock/subsys/named
echo
;;
status)
/usr/local/sbin/ndc status
exit $?
;;
restart)
/usr/local/sbin/ndc -n /chroot/named/bin/named "restart -u named -g named -t /chroot/named"
exit $?
;;
reload)
/usr/local/sbin/ndc reload
exit $?
;;
probe)
# named knows how to reload intelligently; we don't want linuxconf
# to offer to restart every time
/usr/local/sbin/ndc reload >/dev/null 2>&1 || echo start
exit 0
;;
*)
echo "Usage: named {start|stop|status|restart}"
exit 1
esac
exit 0
______________________________________________________________________
On Caldera OpenLinux systems, you simply need to modify the variables
defined at the top, and it will apparently take care of the rest for
you:
NAME=named
DAEMON=/chroot/named/bin/$NAME
OPTIONS="-t /chroot/named -u named -g named"
4.4. Configuration Changes
You will also have to add or change a few options in your named.conf
to keep the various directories straight. In particular, you should
add (or change, if you already have them) the following directives in
the options section:
directory "/etc/namedb";
pid-file "/var/run/named.pid";
named-xfer "/bin/named-xfer";
Since this file is being read by the 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
named.conf to get ndc working properly:
controls {
unix "/var/run/ndc" perm 0600 owner 0 group 0;
};
5. The End
5.1. Launching BIND
Everything should be set up, and you should be ready to put your new,
more secure BIND into action. Assuming you set up a SysV-style init
script, you can simply launch it as:
# /etc/rc.d/init.d/named start
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 ``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 named. If
not, you have a problem.
5.2. That's It!
You can go take a nap now ;-).
6. Appendix - Upgrading BIND Later
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?
Nope. In fact, you really just need the section on ``Compiling BIND''
and the first two parts of the section on ``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!
7. Appendix - Thanks
I'd like to thank the following people for their assistance in the
creation of this HOWTO:
· Lonny Selinger <lonny at abyss.za.org> for "testing" the first
version of this HOWTO and making sure that I didn't miss any steps.
· Chirik <chirik at CastleFur.COM>, Dwayne Litzenberger <dlitz at
dlitz.net>, Phil Bambridge <phil.b at cableinet.co.uk>, Robert Cole
<rcole at metrum-datatape.com>, Colin MacDonald <colinm at
telus.net>, and others for pointing out errors, omissions, and
providing other useful advice to make this HOWTO even better.
· Erik Wallin <erikw at sec.se> and Brian Cervenka <brian at
zerobelow.org> for providing good suggestions for further
tightening the jail.
And last but certainly not least, I'd like to thank Nakano Takeo
<nakano at apm.seikei.ac.jp> for translating the Chroot-BIND HOWTO
into Japanese. You can find his translation at
<http://www.linux.or.jp/JF/JFdocs/Chroot-BIND-HOWTO.html>.
8. Appendix - Document Distribution Policy
Copyright © Scott Wunsch, 2000-2001. This document may be distributed
only subject to the terms set forth in the LDP licence at
<http://metalab.unc.edu/LDP/COPYRIGHT.html>.
This HOWTO is free documentation; you can redistribute it and/or
modify it under the terms of the LDP licence. It is distributed in
the hope that it will be useful, but without any warranty; without
even the impled warranty of merchantability or fitness for a
particular purpose. See the LDP licence for more details.
