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

named:x:200:

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
       +-- dev
       +-- etc
       |    +-- namedb
       |         +-- slave
       +-- 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 would normally need to write to the 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.

# chown -R named:named /chroot/named/etc/namedb/slave

Keep in mind that'll you have to move any slave zones you have into this directory, and update your named.conf accordingly.

BIND will also need to write to the /var/run directory, to put its pidfile and statistical information 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, although not nearly as many as BIND 8 did.

One file that BIND will need inside its jail is good ol' /dev/null. Note that 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, which can created similarly. For most Linux systems, we can use the following command:

# mknod /chroot/named/dev/null c 1 3

You also need another file in the /etc directory inside the jail. You must copy /etc/localtime (this is sometimes known as /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:

# cp /etc/localtime /chroot/named/etc/

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.

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

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=""

SYSLOGD_PARAMS="-a /chroot/named/dev/log"

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

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.

2.6 Tightening Permissions

First of all, feel free to restrict access to the whole /chroot directory to the 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.

# chown root /chroot
# chmod 700 /chroot

You can also safely restrict access to /chroot/named to the named user.

# chown named:named /chroot/named
# chmod 700 /chroot/named

For even more tightening, on Linux systems we can make a few of the files and directories immutable, using the chattr tool on ext2 filesystems.

# cd /chroot/named
# chattr +i etc etc/localtime var

It would be nice to do this for the dev directory too, but unfortunately that would prevent syslogd from creating its 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.