Chroot-BIND8 HOWTO: Preparing the Jail

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## 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.

### 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

### 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.

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