mirror of https://github.com/tLDP/LDP
608 lines
23 KiB
Plaintext
608 lines
23 KiB
Plaintext
<!doctype linuxdoc system>
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<article>
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<!-- Title information -->
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<title>Chroot-BIND8 HOWTO
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<author>Scott Wunsch, <tt>scott at wunsch.org</>
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<date>v1.4, 1 July 2001
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<abstract>
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This document describes installing the BIND 8 nameserver to run in a chroot
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jail and as a non-root user, to provide added security and minimise the
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potential effects of a security compromise. This version of the document
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covers the old but still popular BIND 8; there is another document which
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provides similar information for BIND 9.
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</abstract>
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<!-- Table of contents -->
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<toc>
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<!-- Begin the document -->
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<sect>Introduction
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<p>
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This is the Chroot-BIND8 HOWTO; see <ref id="where" Name="Where?"> for the
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master site, which contains the latest copy. It is assumed that you already
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know 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 assumed
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that you have a basic familiarity with compiling and installing software on
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your UNIX-like system.
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<sect1>What?
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<p>
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This document describes some extra security precautions that you can take when
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you install BIND. It explains how to configure BIND so that it resides in a
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``chroot jail'', meaning that it cannot see or access files outside its own
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little directory tree. We shall also configure 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 other
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process) in a chroot jail, the process is simply unable to see any part of the
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filesystem outside the jail. For example, in this document, we'll set BIND up
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to run chrooted to the directory <tt>/chroot/named</>. Well, to BIND, the
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contents of this directory will appear to be <tt>/</>, the root directory.
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Nothing outside this directory will be accessible to it. You've probably
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encounted a chroot jail before, if you've ever ftped into a public system.
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<sect1>Why?
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<p>
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The idea behind running BIND in a chroot jail is to limit the amount of access
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any malicious individual could gain by exploiting vulnerabilities in BIND. It
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is for the same reason that we run BIND as a non-root user.
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This should be considered as a supplement to the normal security precautions
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(running the latest version, using access control, etc.), not a replacement for
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them.
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If you're interested in DNS security, you might also be interested in a few
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other products. Building BIND with <url
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url="http://www.immunix.org/products.html#stackguard" name="StackGuard"> would
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probably be a good idea for even more protection. Using it is easy; it's
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just like using ordinary gcc. Also, <url
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url="http://cr.yp.to/dnscache.html" name="DNScache"> is a secure replacement
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for BIND, written by Dan Bernstein. Dan is the author of qmail, and DNScache
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appears to follow a similar philosophy.
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<sect1>Where?<label id="where">
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<p>
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The latest version of this document is always available from the web site of the
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Linux/Open Source Users of Regina, Sask., at <url
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url="http://www.losurs.org/docs/howto/Chroot-BIND8.html">.
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There is now a Japanese translation of this document, maintained by <tt>nakano
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at apm.seikei.ac.jp</>. This is available at <url
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url="http://www.linux.or.jp/JF/JFdocs/Chroot-BIND8-HOWTO.html">.
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BIND is available from <url url="http://www.isc.org/" name="the Internet
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Software Consortium"> at <url url="http://www.isc.org/bind.html">. As of this
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writing, the current version of BIND 8 is 8.2.4. BIND 9.x has now been
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released, and has been around for a little while. You may consider upgrading
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to it; the chroot process is certainly much simpler and cleaner. If you are
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running BIND 9, then you want the Chroot-BIND HOWTO, which should be available
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from the same location as this document.
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Keep in mind that there are <bf>known</> security holes in all
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versions of BIND 8 less than <bf>8.2.3</>, so make very sure that you're
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running the latest version!
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<sect1>How?
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<p>
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I wrote this document based on my experiences in setting BIND up in a chroot
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environment. In my case, I already had an existing BIND installation in the
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form of a package that came with my Linux distribution. I'll assume that most
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of you are probably in the same situation, and will simply be transferring over
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and modifying the configuration files from your existing BIND installation, and
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then 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 this
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document. The only difference is that, where I refer to copying an existing
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file, you first have to create it yourself. The DNS HOWTO may be helpful for
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this.
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<sect1>Disclaimer
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<p>
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These steps worked for me, on my system. Your mileage may vary. This is but
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one way to approach this; there are other ways to set the same thing up
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(although the general approach will be the same). It just happens that this
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was the first way that I tried that worked, so I wrote it down.
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My BIND experience to date has been installing on Linux servers. However, most
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of the instructions in this document should be easily applicable to other
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flavours of UNIX as well, and I shall try to point out differences of which I am
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aware.
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<sect>Preparing the Jail
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<sect1>Creating a User
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<p>
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As mentioned in the introduction, it's not a good idea to run BIND as root. So,
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before we begin, let's create a separate user for BIND. Note that you should
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never use an existing generic user like <tt>nobody</> for this purpose.
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However, some distributions, such as SuSE and Linux Mandrake have started
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providing a specific user (generally called <tt>named</>); you can simply adapt
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this user for our purposes, if you like.
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This requires adding a line something like the following to <tt>/etc/passwd</>:
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<tscreen><verb>
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named:x:200:200:Nameserver:/chroot/named:/bin/false
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</verb></tscreen>
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And one like this to <tt>/etc/group</>:
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<tscreen><verb>
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named:x:200:
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</verb></tscreen>
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This creates a user and group called <tt>named</> for BIND. Make sure that the
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UID and GID (both 200 in this example) are unique on your system. The shell is
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set to <tt>/bin/false</> because this user will never need to log in.
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<sect1>Directory Structure
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<p>
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Now, we must set up the directory structure that we will use for the chroot jail
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in which BIND will live. This can be anywhere on your filesystem; the truly
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paranoid may even want to put it on a separate volume. I shall assume that you
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will use <tt>/chroot/named</>. Let's start by creating the following directory
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structure:
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<tscreen><verb>
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/chroot
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+-- named
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+-- bin
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+-- dev
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+-- etc
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+-- lib
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+-- var
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+-- run
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</verb></tscreen>
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<sect1>Placing the BIND Data
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<p>
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Assuming that you have already done a conventional installation of BIND and are
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using it, you will already have an existing <tt>named.conf</> and zone files.
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These files must now be moved (or copied, to be safe) into the chroot jail, so
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that BIND can get at them. <tt>named.conf</> goes in <tt>/chroot/named/etc</>,
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and the zone files can go in <tt>/chroot/named/etc/namedb</>. For example:
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<tscreen><verb>
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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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</verb></tscreen>
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BIND will likely need to write to the <tt>namedb</> directory, and probably some
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of the files in it. For example, if your DNS serves as a slave for a zone, it
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will have to update that zone file. Also, BIND can dump statistical
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information, and does so in this directory. For that reason, you should
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probably make the <tt>named</> user the owner of this directory and its contents:
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<tscreen><verb>
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# chown -R named:named /chroot/named/etc/namedb
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</verb></tscreen>
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BIND will also need to write to the <tt>/var/run</> directory, to put its
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pidfile and ndc socket there, so let's allow it to do so:
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<tscreen><verb>
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# chown named:named /chroot/named/var/run
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</verb></tscreen>
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<sect1>System Support Files
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<p>
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Once BIND is running in the chroot jail, it will not be able to access files
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outside the jail <bf>at all</>. However, it needs to access a few key files, such
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as the system's C library. Exactly what libraries are required will depend on
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your flavour of UNIX. For most modern Linux systems, the following commands
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will be sufficient to put the necessary libraries in place:
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<tscreen><verb>
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# cd /chroot/named/lib
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# cp -p /lib/libc-2.*.so .
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# ln -s libc-2.*.so libc.so.6
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# cp -p /lib/ld-2.*.so .
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# ln -s ld-2.*.so ld-linux.so.2
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</verb></tscreen>
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As an alternative, you could simply build statically-linked versions of the BIND
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binaries to put in your chroot jail. You should also copy <tt>ldconfig</> into
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the jail, and run it to create an <tt>etc/ld.so.cache</> for the jail environment.
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The following commands could take care of this:
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<tscreen><verb>
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# cp /sbin/ldconfig /chroot/named/bin/
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# chroot /chroot/named /bin/ldconfig -v
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</verb></tscreen>
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BIND needs one more system file in its jail: good ol' <tt>/dev/null</>. Again,
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the exact command necessary to create this device node may vary from system to
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system; check your <tt>/dev/MAKEDEV</> script to be sure. Some systems may also
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require <tt>/dev/zero</>. For most Linux systems, we can use the following
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command:
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<tscreen><verb>
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# mknod /chroot/named/dev/null c 1 3
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</verb></tscreen>
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Finally, you need a couple extra files in the <tt>/etc</> directory inside the
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jail. In particular, you must copy <tt>/etc/localtime</> (this sometimes known
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as <tt>/usr/lib/zoneinfo/localtime</> on some systems) in there so that BIND
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logs things with the right time on them, and you must make a simple <tt>group</>
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file with the <tt>named</> group in it. The following two commands will take
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care of this:
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<tscreen><verb>
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# cp /etc/localtime /chroot/named/etc/
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# echo 'named:x:200:' > /chroot/named/etc/group
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</verb></tscreen>
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Keep in mind that the GID, 200 in this example, must match the one you defined
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in the real <tt>/etc/group</> above.
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<sect1>Logging<label id="logging">
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<p>
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Unlike a conventional jailbird, BIND can't just scribble its log entries on the
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walls :-). Normally, BIND logs through <tt>syslogd</>, the system logging daemon.
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However, this type of logging is performed by sending the log entries to the
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special socket <tt>/dev/log</>. Since this is outside the jail, BIND can't use
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it any more. Fortuantely, there are a couple options to work around this.
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<sect2>The Ideal Solution
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<p>
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The ideal solution to this dilemma requires a reasonably recent version of
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<tt>syslogd</> which supports the <tt>-a</> switch introduced by OpenBSD.
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Check the manpage for your <tt>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 ``<tt>-a
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/chroot/named/dev/log</>'' to the command line when you launch <tt>syslogd</>.
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On systems which use a full SysV-init (which includes most Linux distributions),
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this is typically done in the file <tt>/etc/rc.d/init.d/syslog</>. For example,
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on my Red Hat Linux system, I changed the line
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<tscreen><verb>
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daemon syslogd -m 0
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</verb></tscreen>
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to
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<tscreen><verb>
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daemon syslogd -m 0 -a /chroot/named/dev/log
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</verb></tscreen>
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On Caldera OpenLinux systems, they use a daemon launcher called <tt>ssd</>,
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which reads configuration from <tt>/etc/sysconfig/daemons/syslog</>. You
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simply need to modify the options line to look like this:
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<tscreen><verb>
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OPTIONS_SYSLOGD="-m 0 -a /chroot/named/dev/log"
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</verb></tscreen>
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Similarly, on SuSE systems, I'm told that the best place to add this switch
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is in the <tt>/etc/rc.config</> file. Changing the line
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<tscreen><verb>
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SYSLOGD_PARAMS=""
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</verb></tscreen>
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to read
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<tscreen><verb>
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SYSLOGD_PARAMS="-a /chroot/named/dev/log"
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</verb></tscreen>
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should do the trick.
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Once you've figured out how to make this change for your system, simply
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restart <tt>syslogd</>, either by killing it and launching it again (with
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the extra parameters), or by using the SysV-init script to do it for you:
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<tscreen><verb>
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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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</verb></tscreen>
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Once it's been restarted, you should see a ``file'' in <tt>/chroot/named/dev</>
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called <tt>log</>, that looks something like this:
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<verb>srw-rw-rw- 1 root root 0 Mar 13 20:58 log</verb>
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<sect2>The Other Solutions
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<p>
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If you have an older <tt>syslogd</>, then you'll have to find another way to do
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your logging. There are a couple programs out there, such as <tt>holelogd</>,
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which are designed to help by acting as a ``proxy'' and accepting log entries
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from the chrooted BIND and passing them out to the regular <tt>/dev/log</>
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socket.
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Alteratively, you can simply configure BIND to log to files instead of going
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through syslog. See the BIND documentation for more details if you choose to go
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this route.
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<sect>Compiling BIND<label id="compiling">
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<p>
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You should be able to find the BIND source by visiting <url
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url="http://www.isc.org/bind.html">. You need the <tt>bind-src.tar.gz</>
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package. Be sure to get the latest version!
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<sect1>Modifying Paths
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<p>
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Things can get a bit confusing at this point, because different parts of the
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BIND package will be referring to the same directories by different names
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(depending on whether or not they're running inside the jail). I'll try not to
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confuse you <bf>too</> much :-).
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The main directory that we have to worry about here is <tt>/var/run</>, because
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its contents are required for both the main <tt>named</> daemon (inside the
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jail), and the <tt>ndc</> utility (on the outside). We'll start by setting
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everything up to find this directory from the outside world. To do this, we
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need to modify <tt>src/port/linux/Makefile.set</> (substitute your port's
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directory if you're not running Linux), and change the line
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<tscreen><verb>
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DESTRUN=/var/run
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</verb></tscreen>
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to
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<tscreen><verb>
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DESTRUN=/chroot/named/var/run
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</verb></tscreen>
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While you're in there, you may want to change the other destination paths from
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<tt>/usr</> to <tt>/usr/local</>.
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Now everything should be able to find that directory... except the <tt/named/
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daemon itself, to which it's still just <tt>/var/run</> inside the jail. We can
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get around this by making a small change in the <tt/named/ source. In the file
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<tt>src/bin/named/named.h</>, find the line
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<tscreen><verb>
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#include "pathnames.h"
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</verb></tscreen>
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and add the following line immediately after it
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<tscreen><verb>
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#define _PATH_NDCSOCK "/var/run/ndc"
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</verb></tscreen>
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This way, <tt/named/ will ignore our definition of <tt/DESTRUN/ over in
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<tt/Makefile.set/ and use the correct location (from its perspective in the
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chroot jail). You will notice some warnings about redefinitions of
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_PATH_NDCSOCK when you do the build; just ignore them.
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<sect1>Doing the Build
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<p>
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You should now be able to compile BIND as normal, following the instructions in
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the <tt/INSTALL/ file. At this stage, we only want to compile BIND, not install
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it. Don't go too far when following the <tt/INSTALL/ file. Essentially, it's
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just <tt/make clean/, <tt/make depend/, and <tt/make/.
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<sect>Installing Your Shiny New BIND<label id="installing">
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<p>
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I should mention that if you have an existing installation of BIND, such as from
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an RPM, you should probably remove it before installing the new one. On Red Hat
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systems, this probably means removing the packages <tt/bind/ and
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<tt/bind-utils/, and possibly <tt/bind-devel/ and <tt/caching-nameserver/, if
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you have them.
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You may want to save a copy of the init script (e.g.,
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<tt>/etc/rc.d/init.d/named</>), if any, before doing so; it'll be useful later
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on.
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<sect1>Installing the Tools Outside the Jail
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<p>
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This is the easy part :-). Just run <tt/make install/ and let it take care of
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it for you. You may want to <tt>chmod 000 /usr/local/sbin/named</> afterwards,
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to make sure you don't accidentally run the non-chrooted copy of BIND. (This
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is <tt>/usr/sbin/named</> if you didn't tell it to go in <tt>/usr/local/sbin</>
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like I suggested.)
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<sect1>Installing the Binaries in the Jail
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<p>
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Only two parts of the package have to live inside the chroot jail: the main
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<tt/named/ daemon itself, and <tt/named-xfer/, which it uses for zone transfers.
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You can simply copy them in from the source tree:
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<tscreen><verb>
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# cp src/bin/named/named /chroot/named/bin
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# cp src/bin/named-xfer/named-xfer /chroot/named/bin
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</verb></tscreen>
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<sect1>Setting up the Init Script
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<p>
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If you have an existing init script from your distribution, it would probably be
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best simply to modify it to run <tt>/chroot/named/bin/named</>, with the
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appropriate switches. The switches are... <it/(drumroll please...)/
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<itemize>
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<item><tt/-u named/, which tells BIND to run as the user <tt/named/, rather than
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<tt/root/.
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<item><tt/-g named/, to run BIND under the group <tt/named/ too, rather than
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<tt/root/ or <tt/wheel/.
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<item><tt>-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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</itemize>
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The following is the init script I use with my Red Hat 6.0 system. As you can
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see, it is almost exactly the same as the way it shipped from Red Hat. I have
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also modified the <tt>ndc restart</> command so that it restarts the server
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properly, and keeps it chrooted. You should probably do the same in your init
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script, even if you don't copy this one.
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<tscreen><code>
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#!/bin/sh
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#
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# named This shell script takes care of starting and stopping
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# named (BIND DNS server).
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#
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# chkconfig: 345 55 45
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# description: named (BIND) is a Domain Name Server (DNS) \
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# that is used to resolve host names to IP addresses.
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# probe: true
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# Source function library.
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. /etc/rc.d/init.d/functions
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# Source networking configuration.
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. /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 $?
|
|
;;
|
|
probe)
|
|
# named knows how to reload intelligently; we don't want linuxconf
|
|
# to offer to restart every time
|
|
/usr/local/sbin/ndc reload >/dev/null 2>&1 || echo start
|
|
exit 0
|
|
;;
|
|
|
|
*)
|
|
echo "Usage: named {start|stop|status|restart}"
|
|
exit 1
|
|
esac
|
|
|
|
exit 0
|
|
</code></tscreen>
|
|
|
|
On Caldera OpenLinux systems, you simply need to modify the variables defined
|
|
at the top, and it will apparently take care of the rest for you:
|
|
<tscreen><verb>
|
|
NAME=named
|
|
DAEMON=/chroot/named/bin/$NAME
|
|
OPTIONS="-t /chroot/named -u named -g named"
|
|
</verb></tscreen>
|
|
|
|
<sect1>Configuration Changes
|
|
|
|
<p>
|
|
You will also have to add or change a few options in your <tt>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 <tt>options</>
|
|
section:
|
|
<tscreen><verb>
|
|
directory "/etc/namedb";
|
|
pid-file "/var/run/named.pid";
|
|
named-xfer "/bin/named-xfer";
|
|
</verb></tscreen>
|
|
Since this file is being read by the <tt>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
|
|
<tt>named.conf</> to get <tt>ndc</> working properly:
|
|
<tscreen><verb>
|
|
controls {
|
|
unix "/var/run/ndc" perm 0600 owner 0 group 0;
|
|
};
|
|
</verb></tscreen>
|
|
|
|
<sect>The End
|
|
|
|
<sect1>Launching BIND
|
|
|
|
<p>
|
|
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:
|
|
<tscreen><verb>
|
|
# /etc/rc.d/init.d/named start
|
|
</verb></tscreen>
|
|
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 <ref
|
|
id="logging" name="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 <tt>named</>. If not, you have a problem.
|
|
|
|
<sect1>That's It!
|
|
|
|
<p>
|
|
You can go take a nap now ;-).
|
|
|
|
<sect>Appendix - Upgrading BIND Later<label id="upgrading">
|
|
|
|
<p>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 <ref id="compiling"
|
|
name="Compiling BIND"> and the first two parts of the section on <ref
|
|
id="installing" name="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!
|
|
|
|
<sect>Appendix - Thanks<label id="thanks">
|
|
|
|
<p>
|
|
I'd like to thank the following people for their assistance in the creation
|
|
of this HOWTO:
|
|
|
|
<itemize>
|
|
|
|
<item>Lonny Selinger <tt><lonny at abyss.za.org></> for "testing" the
|
|
first version of this HOWTO and making sure that I didn't miss any steps.
|
|
|
|
<item>Chirik <tt><chirik at CastleFur.COM></>, Dwayne Litzenberger
|
|
<tt><dlitz at dlitz.net></>, Phil Bambridge <tt><phil.b at
|
|
cableinet.co.uk></>, Robert Cole <tt><rcole at metrum-datatape.com></>,
|
|
Colin MacDonald <tt><colinm at telus.net></>, and others for pointing out
|
|
errors, omissions, and providing other useful advice to make this HOWTO even
|
|
better.
|
|
|
|
<item>Erik Wallin <tt><erikw at sec.se></> and Brian Cervenka
|
|
<tt><brian at zerobelow.org></> for providing good suggestions for
|
|
further tightening the jail.
|
|
|
|
</itemize>
|
|
|
|
And last but certainly not least, I'd like to thank Nakano Takeo <tt><nakano
|
|
at apm.seikei.ac.jp></> for translating the Chroot-BIND HOWTO into
|
|
Japanese. You can find his translation at <url
|
|
url="http://www.linux.or.jp/JF/JFdocs/Chroot-BIND-HOWTO.html">.
|
|
|
|
<sect>Appendix - Document Distribution Policy<label id="legal">
|
|
|
|
<p>
|
|
Copyright © Scott Wunsch, 2000-2001. This document may be distributed only
|
|
subject to the terms set forth in the LDP licence at <url
|
|
url="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 <bf>without any warranty</>; without even the impled warranty of
|
|
merchantability or fitness for a particular purpose. See the LDP licence for
|
|
more details.
|
|
|
|
</article>
|