This is the Chroot-BIND 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.
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 used 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!
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.), certainly not as 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 would probably be a good idea for even more protection. Using it is easy; it's just like using ordinary gcc. Also, DNScache is a secure replacement for BIND, written by Dan Bernstein. Dan is the author of qmail, and DNScache appears to follow a similar philosophy.
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-BIND.html.
There is now a Japanese translation of this document, maintained by Nakano Takeo nakano at apm.seikei.ac.jp. This is available at http://www.linux.or.jp/JF/JFdocs/Chroot-BIND-HOWTO.html.
BIND is available from the Internet Software Consortium at http://www.isc.org/bind.html. As of this writing, the current version of BIND 9 is 9.2.0. BIND 9 has been out for some time now, and many people are using it in production. Nevertheless, some more conservative sorts still prefer to remain with BIND 8. If you are such a person, please see my Chroot-BIND8 HOWTO (available from the same location) for details on chrooting it, but be warned that BIND 8 is much messier to chroot.
Keep in mind that there are known security holes in many earlier versions of BIND, so make very sure that you're running the latest version!
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.
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. I've also received suggestions from people using other distributions and other platforms, and I've tried to incorporate their comments where possible.
If you run Linux, you need to make sure that you're running a 2.4 kernel before attempting this. The -u switch (to run as a non-root user) requires this newer kernel.