736 lines
23 KiB
Plaintext
736 lines
23 KiB
Plaintext
Chroot-BIND HOWTO
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Scott Wunsch, scott at wunsch.org
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v1.5, 1 December 2001
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This document describes installing the BIND 9 nameserver to run in a
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chroot jail and as a non-root user, to provide added security and min
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imise the potential effects of a security compromise. Note that this
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document has been updated for BIND 9; if you still run BIND 8, you
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want the Chroot-BIND8 HOWTO instead.
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______________________________________________________________________
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Table of Contents
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1. Introduction
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1.1 What?
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1.2 Why?
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1.3 Where?
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1.4 How?
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1.5 Disclaimer
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2. Preparing the Jail
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2.1 Creating a User
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2.2 Directory Structure
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2.3 Placing the BIND Data
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2.4 System Support Files
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2.5 Logging
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2.5.1 The Ideal Solution
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2.5.2 The Other Solutions
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2.6 Tightening Permissions
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3. Compiling and Installing Your Shiny New BIND
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3.1 Doing the Compile
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4. Installing Your Shiny New BIND
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4.1 Installing the Binaries
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4.2 Setting up the Init Script
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4.3 Configuration Changes
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5. The End
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5.1 Launching BIND
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5.2 That's It!
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6. Appendix - Upgrading BIND Later
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7. Appendix - Thanks
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8. Appendix - Document Distribution Policy
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______________________________________________________________________
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1. Introduction
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This is the Chroot-BIND HOWTO; see ``Where?'' for the master site,
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which contains the latest copy. It is assumed that you already know
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how to configure and use BIND (the Berkeley Internet Name Domain). If
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not, I would recommend that you read the DNS HOWTO first. It is also
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assumed that you have a basic familiarity with compiling and
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installing software on your UNIX-like system.
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1.1. What?
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This document describes some extra security precautions that you can
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take when you install BIND. It explains how to configure BIND so that
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it resides in a ``chroot jail,'' meaning that it cannot see or access
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files outside its own little directory tree. We shall also configure
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it to run as a non-root user.
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The idea behind chroot is fairly simple. When you run BIND (or any
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other process) in a chroot jail, the process is simply unable to see
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any part of the filesystem outside the jail. For example, in this
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document, we'll set BIND up to run chrooted to the directory
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/chroot/named. Well, to BIND, the contents of this directory will
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appear to be /, the root directory. Nothing outside this directory
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will be accessible to it. You've probably encounted a chroot jail
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before, if you've ever used ftp to log into a public system.
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Because the chroot process is much simpler with BIND 9, I have started
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to expand this document slightly, to include more general tips about
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securing a BIND installation. Nevertheless, this document is not (and
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is not intended to be) a complete reference for securing BIND. If you
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do only what is outlined in this document, you're not finished
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securing your nameserver!
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1.2. Why?
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The idea behind running BIND in a chroot jail is to limit the amount
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of access any malicious individual could gain by exploiting
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vulnerabilities in BIND. It is for the same reason that we run BIND
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as a non-root user.
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This should be considered as a supplement to the normal security
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precautions (running the latest version, using access control, etc.),
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certainly not as a replacement for them.
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If you're interested in DNS security, you might also be interested in
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a few other products. Building BIND with StackGuard
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<http://www.immunix.org/products.html#stackguard> would probably be a
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good idea for even more protection. Using it is easy; it's just like
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using ordinary gcc. Also, DNScache <http://cr.yp.to/dnscache.html> is
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a secure replacement for BIND, written by Dan Bernstein. Dan is the
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author of qmail, and DNScache appears to follow a similar philosophy.
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1.3. Where?
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The latest version of this document is always available from the web
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site of the Linux/Open Source Users of Regina, Sask., at
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<http://www.losurs.org/docs/howto/Chroot-BIND.html>.
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There is now a Japanese translation of this document, maintained by
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Nakano Takeo nakano at apm.seikei.ac.jp. This is available at
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<http://www.linux.or.jp/JF/JFdocs/Chroot-BIND-HOWTO.html>.
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BIND is available from the Internet Software Consortium
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<http://www.isc.org/> at <http://www.isc.org/bind.html>. As of this
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writing, the current version of BIND 9 is 9.2.0. BIND 9 has been out
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for some time now, and many people are using it in production.
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Nevertheless, some more conservative sorts still prefer to remain with
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BIND 8. If you are such a person, please see my Chroot-BIND8 HOWTO
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(available from the same location) for details on chrooting it, but be
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warned that BIND 8 is much messier to chroot.
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Keep in mind that there are known security holes in many earlier
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versions of BIND, so make very sure that you're running the latest
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version!
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1.4. How?
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I wrote this document based on my experiences in setting BIND up in a
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chroot environment. In my case, I already had an existing BIND
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installation in the form of a package that came with my Linux
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distribution. I'll assume that most of you are probably in the same
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situation, and will simply be transferring over and modifying the
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configuration files from your existing BIND installation, and then
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removing the package before installing the new one. Don't remove the
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package yet, though; we may want some files from it first.
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If this is not the case for you, you should still be able to follow
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this document. The only difference is that, where I refer to copying
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an existing file, you first have to create it yourself. The DNS HOWTO
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may be helpful for this.
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1.5. Disclaimer
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These steps worked for me, on my system; your mileage may vary. This
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is but one way to approach this; there are other ways to set the same
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thing up (although the general approach will be the same). It just
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happens that this was the first way that I tried that worked, so I
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wrote it down.
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My BIND experience to date has been installing on Linux servers.
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However, most of the instructions in this document should be easily
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applicable to other flavours of UNIX as well, and I shall try to point
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out differences of which I am aware. I've also received suggestions
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from people using other distributions and other platforms, and I've
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tried to incorporate their comments where possible.
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If you run Linux, you need to make sure that you're running a 2.4
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kernel before attempting this. The -u switch (to run as a non-root
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user) requires this newer kernel.
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2. Preparing the Jail
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2.1. Creating a User
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As mentioned in the introduction, it's not a good idea to run BIND as
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root. So, before we begin, let's create a separate user for BIND.
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Note that you should never use an existing generic user like nobody
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for this purpose. However, some distributions, such as SuSE and Linux
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Mandrake have started providing a specific user (generally called
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named); you can simply adapt this user for our purposes, if you like.
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This requires adding a line something like the following to
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/etc/passwd:
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named:x:200:200:Nameserver:/chroot/named:/bin/false
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And one like this to /etc/group:
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named:x:200:
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This creates a user and group called named for BIND. Make sure that
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the UID and GID (both 200 in this example) are unique on your system.
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The shell is set to /bin/false because this user will never need to
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log in.
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2.2. Directory Structure
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Now, we must set up the directory structure that we will use for the
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chroot jail in which BIND will live. This can be anywhere on your
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filesystem; the truly paranoid may even want to put it on a separate
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volume. I shall assume that you will use /chroot/named. Let's start
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by creating the following directory structure:
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/chroot
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+-- named
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+-- dev
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+-- etc
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| +-- namedb
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| +-- slave
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+-- var
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+-- run
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If you use GNU mkdir (such as on a Linux system), you can create this
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directory structure like this:
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# mkdir -p /chroot/named
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# cd /chroot/named
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# mkdir -p dev etc/namedb/slave var/run
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2.3. Placing the BIND Data
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Assuming that you have already done a conventional installation of
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BIND and are using it, you will already have an existing named.conf
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and zone files. These files must now be moved (or copied, to be safe)
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into the chroot jail, so that BIND can get at them. named.conf goes
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in /chroot/named/etc, and the zone files can go in
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/chroot/named/etc/namedb. For example:
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# cp -p /etc/named.conf /chroot/named/etc/
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# cp -a /var/named/* /chroot/named/etc/namedb/
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BIND would normally need to write to the namedb directory, but in the
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interests of tightening security, we will not allow it to do this. If
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your nameserver serves as a slave for any zones, it will need to
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update these zone files, which means we'll have to store them in a
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separate directory, to which BIND does have write access.
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# chown -R named:named /chroot/named/etc/namedb/slave
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Keep in mind that'll you have to move any slave zones you have into
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this directory, and update your named.conf accordingly.
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BIND will also need to write to the /var/run directory, to put its
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pidfile and statistical information there, so let's allow it to do so:
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# chown named:named /chroot/named/var/run
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2.4. System Support Files
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Once BIND is running in the chroot jail, it will not be able to access
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files outside the jail at all. However, it needs to access a few key
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files, although not nearly as many as BIND 8 did.
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One file that BIND will need inside its jail is good ol' /dev/null.
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Note that the exact command necessary to create this device node may
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vary from system to system; check your /dev/MAKEDEV script to be sure.
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Some systems may also require /dev/zero, which can created similarly.
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It's reported that the BIND 9.2.0 release candidates now require
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/dev/random as well. For most Linux systems, we can use the following
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commands:
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# mknod /chroot/named/dev/null c 1 3
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# mknod /chroot/named/dev/random c 1 8
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# chmod 666 /chroot/named/dev/{null,random}
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For FreeBSD 4.3, this is:
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# mknod /chroot/named/dev/null c 2 2
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# mknod /chroot/named/dev/random c 2 3
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# chmod 666 /chroot/named/dev/{null,random}
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You also need another file in the /etc directory inside the jail. You
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must copy /etc/localtime (this is sometimes known as
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/usr/lib/zoneinfo/localtime on some systems) in there so that BIND
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logs things with the right time on them. The following command will
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take care of this:
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# cp /etc/localtime /chroot/named/etc/
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2.5. Logging
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Unlike a conventional jailbird, BIND can't just scribble its log
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entries on the walls :-). Normally, BIND logs through syslogd, the
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system logging daemon. However, this type of logging is performed by
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sending the log entries to the special socket /dev/log. Since this is
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outside the jail, BIND can't use it any more. Fortuantely, there are
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a couple options to work around this.
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2.5.1. The Ideal Solution
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The ideal solution to this dilemma requires a reasonably recent
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version of syslogd which supports the -a switch introduced by OpenBSD.
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Check the manpage for your syslogd(8) to see if you have such a
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version.
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If you do, all you have to do is add the switch ``-a
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/chroot/named/dev/log'' to the command line when you launch syslogd.
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On systems which use a full SysV-init (which includes most Linux
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distributions), this is typically done in the file
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/etc/rc.d/init.d/syslog. For example, on my Red Hat Linux system, I
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changed the line
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daemon syslogd -m 0
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to
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daemon syslogd -m 0 -a /chroot/named/dev/log
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Interestingly, as of Red Hat 7.2, Red Hat has apparently made this
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process even easier. There is now a file called /etc/sysconfig/syslog
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in which extra parameters for syslogd can be defined.
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On Caldera OpenLinux systems, they use a daemon launcher called ssd,
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which reads configuration from /etc/sysconfig/daemons/syslog. You
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simply need to modify the options line to look like this:
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OPTIONS_SYSLOGD="-m 0 -a /chroot/named/dev/log"
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Similarly, on SuSE systems, I'm told that the best place to add this
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switch is in the /etc/rc.config file. Changing the line
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SYSLOGD_PARAMS=""
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to read
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SYSLOGD_PARAMS="-a /chroot/named/dev/log"
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should do the trick.
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And, last but not least, for FreeBSD 4.3 you can apparently just edit
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the rc.conf file and put in the following:
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syslogd_flags="-s -l /chroot/named/dev/log"
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The -s is for security reasons, and is part of the default settings.
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The -l is a local path on which to put another logging node.
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Once you've figured out how to make this change for your system,
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simply restart syslogd, either by killing it and launching it again
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(with the extra parameters), or by using the SysV-init script to do it
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for you:
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# /etc/rc.d/init.d/syslog stop
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# /etc/rc.d/init.d/syslog start
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Once it's been restarted, you should see a ``file'' in
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/chroot/named/dev called log, that looks something like this:
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srw-rw-rw- 1 root root 0 Mar 13 20:58 log
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2.5.2. The Other Solutions
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If you have an older syslogd, then you'll have to find another way to
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do your logging. There are a couple programs out there, such as
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holelogd, which are designed to help by acting as a ``proxy'' and
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accepting log entries from the chrooted BIND and passing them out to
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the regular /dev/log socket.
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Alteratively, you can simply configure BIND to log to files instead of
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going through syslog. See the BIND documentation for more details if
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you choose to go this route.
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2.6. Tightening Permissions
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First of all, feel free to restrict access to the whole /chroot
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directory to the root user. Of course, not everybody may want to do
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this, especially if you have other software installed in that tree
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that doesn't appreciate it.
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# chown root /chroot
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# chmod 700 /chroot
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You can also safely restrict access to /chroot/named to the named
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user.
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# chown named:named /chroot/named
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# chmod 700 /chroot/named
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For even more tightening, on Linux systems we can make a few of the
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files and directories immutable, using the chattr tool on ext2
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filesystems.
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# cd /chroot/named
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# chattr +i etc etc/localtime var
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Equivalently, on FreeBSD 4.3, you want to look into chflags if you
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wish to make things immutable. As an example, the following should
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change everything in the /chroot/named/etc directory to immutable:
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# chflags schg /chroot/named/etc/*(*).
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It would be nice to do this for the dev directory too, but
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unfortunately that would prevent syslogd from creating its dev/log
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socket. You may also choose to set the immutable bit on other files
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in the jail as well, such as your primary zone files, if they aren't
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expected to change.
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3. Compiling and Installing Your Shiny New BIND
|
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|
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3.1. Doing the Compile
|
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|
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Compiling BIND 9 for use in a chroot jail should be a much more
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pleasant experience than BIND 8 was. In fact, you don't have to do
|
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anything special; the standard ./configure && make should suffice.
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Keep in mind that if you want to enable IPv6 support in BIND
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(--enable-ipv6) on Linux systems, you need matching versions of kernel
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and glibc. If you have kernel 2.2, you need glibc 2.1, and if you
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have kernel 2.4, you need glibc 2.2. BIND is quite picky about this.
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4. Installing Your Shiny New BIND
|
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I should mention that if you have an existing installation of BIND,
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such as from an RPM, you should probably remove it before installing
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the new one. On Red Hat systems, this probably means removing the
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packages bind and bind-utils, and possibly bind-devel and caching-
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nameserver, if you have them.
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You may want to save a copy of the init script (e.g.,
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/etc/rc.d/init.d/named), if any, before doing so; it'll be useful
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later on.
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If you are upgrading from an older version of BIND, such as BIND 8,
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you will want to read the migration documentation in the file
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doc/misc/migration in the BIND source package. I don't deal with any
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migration issues in this document; I simply assume that you are
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replacing an existing, working installation of BIND 9.
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4.1. Installing the Binaries
|
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This is the easy part :-). Just run make install and let it take care
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of it for you. Really, that's it!
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4.2. Setting up the Init Script
|
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|
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If you have an existing init script from your distribution, it would
|
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probably be best simply to modify it to run the new binary, with the
|
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appropriate switches. The switches are... (drumroll please...)
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|
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· -u named, which tells BIND to run as the user named, rather than
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root.
|
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· -t /chroot/named, which tells BIND to chroot itself to the jail
|
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that we've set up.
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· -c /etc/named.conf, which tells BIND where to find its
|
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configuration file within the jail.
|
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|
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The following is the init script I use with my Red Hat 6.0 system. As
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you can see, it is almost exactly the same as the way it shipped from
|
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Red Hat. I haven't tried the rndc commands yet, but I can't see any
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reason why they shouldn't work.
|
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|
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|
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|
||
______________________________________________________________________
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||
#!/bin/sh
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#
|
||
# 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.
|
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# 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 /usr/local/sbin/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 /usr/local/sbin/named -u named -t /chroot/named -c /etc/named.conf
|
||
echo
|
||
touch /var/lock/subsys/named
|
||
;;
|
||
stop)
|
||
# Stop daemons.
|
||
echo -n "Shutting down named: "
|
||
killproc named
|
||
rm -f /var/lock/subsys/named
|
||
echo
|
||
;;
|
||
status)
|
||
status named
|
||
exit $?
|
||
;;
|
||
restart)
|
||
$0 stop
|
||
$0 start
|
||
exit $?
|
||
;;
|
||
reload)
|
||
/usr/local/sbin/rndc reload
|
||
exit $?
|
||
;;
|
||
probe)
|
||
# named knows how to reload intelligently; we don't want linuxconf
|
||
# to offer to restart every time
|
||
/usr/local/sbin/rndc reload >/dev/null 2>&1 || echo start
|
||
exit 0
|
||
;;
|
||
|
||
*)
|
||
echo "Usage: named {start|stop|status|restart|reload}"
|
||
exit 1
|
||
esac
|
||
|
||
exit 0
|
||
______________________________________________________________________
|
||
|
||
|
||
|
||
As with syslogd, as of Red Hat 7.2 this process is now even easier.
|
||
There is a file called /etc/sysconfig/named in which extra parameters
|
||
for syslogd can be defined. The default /etc/rc.d/init.d/named on Red
|
||
Hat 7.2, however, will check for the existance of /etc/named.conf
|
||
before starting. You will need to correct this path.
|
||
|
||
On Caldera OpenLinux systems, you simply need to modify the variables
|
||
defined at the top, and it will apparently take care of the rest for
|
||
you:
|
||
|
||
|
||
NAME=named
|
||
DAEMON=/usr/local/sbin/$NAME
|
||
OPTIONS="-t /chroot/named -u named -c /etc/named.conf"
|
||
|
||
|
||
|
||
And for FreeBSD 4.3, you can edit the rc.conf file and put in the
|
||
following:
|
||
|
||
|
||
named_enable="YES"
|
||
named_program="chroot/named/bin/named"
|
||
named_flags="-u named -t /chroot/named -c /etc/namedb/named.conf"
|
||
|
||
|
||
|
||
4.3. 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";
|
||
statistics-file "/var/run/named.stats";
|
||
|
||
|
||
|
||
Since this file is being read by the named daemon, all the paths are
|
||
of 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 /var/run directory as well.
|
||
|
||
|
||
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.
|
||
|
||
|
||
5.2. That's It!
|
||
|
||
You can go take a nap now ;-).
|
||
|
||
|
||
6. Appendix - Upgrading BIND Later
|
||
|
||
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 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!
|
||
|
||
|
||
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.
|
||
|
||
· Robert Dalton <support at accesswest.com> for suggesting a couple
|
||
more example commands, and pointing out BIND 9.2.0's need of
|
||
/dev/random.
|
||
|
||
· Eric McCormick <hostmaster at cybertime.net> for the FreeBSD 4.3
|
||
information.
|
||
|
||
· Tan Zheng Da <tzd at pobox.com> for the details about the changes
|
||
in Red Hat 7.2 that make this a little easier.
|
||
|
||
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.
|
||
|
||
|
||
|