old-www/HOWTO/text/Chroot-BIND8-HOWTO

  Chroot-BIND8 HOWTO
  Scott Wunsch, scott at wunsch.org
  v1.4, 1 July 2001

  This document describes installing the BIND 8 nameserver to run in a
  chroot jail and as a non-root user, to provide added security and min­
  imise the potential effects of a security compromise.  This version of
  the document covers the old but still popular BIND 8; there is another
  document which provides similar information for BIND 9.
  ______________________________________________________________________

  Table of Contents


  1. Introduction

     1.1 What?
     1.2 Why?
     1.3 Where?
     1.4 How?
     1.5 Disclaimer

  2. Preparing the Jail

     2.1 Creating a User
     2.2 Directory Structure
     2.3 Placing the BIND Data
     2.4 System Support Files
     2.5 Logging
        2.5.1 The Ideal Solution
        2.5.2 The Other Solutions

  3. Compiling BIND

     3.1 Modifying Paths
     3.2 Doing the Build

  4. Installing Your Shiny New BIND

     4.1 Installing the Tools Outside the Jail
     4.2 Installing the Binaries in the Jail
     4.3 Setting up the Init Script
     4.4 Configuration Changes

  5. The End

     5.1 Launching BIND
     5.2 That's It!

  6. Appendix - Upgrading BIND Later

  7. Appendix - Thanks

  8. Appendix - Document Distribution Policy



  ______________________________________________________________________

  1.  Introduction

  This is the Chroot-BIND8 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.


  1.1.  What?

  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 ftped into a public system.


  1.2.  Why?

  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.),
  not 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
  <http://www.immunix.org/products.html#stackguard> would probably be a
  good idea for even more protection.  Using it is easy; it's just like
  using ordinary gcc.  Also, DNScache <http://cr.yp.to/dnscache.html> is
  a secure replacement for BIND, written by Dan Bernstein.  Dan is the
  author of qmail, and DNScache appears to follow a similar philosophy.


  1.3.  Where?

  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-BIND8.html>.

  There is now a Japanese translation of this document, maintained by
  nakano at apm.seikei.ac.jp.  This is available at
  <http://www.linux.or.jp/JF/JFdocs/Chroot-BIND8-HOWTO.html>.

  BIND is available from the Internet Software Consortium
  <http://www.isc.org/> at  <http://www.isc.org/bind.html>.  As of this
  writing, the current version of BIND 8 is 8.2.4.  BIND 9.x has now
  been released, and has been around for a little while.  You may
  consider upgrading to it; the chroot process is certainly much simpler
  and cleaner.  If you are running BIND 9, then you want the Chroot-BIND
  HOWTO, which should be available from the same location as this
  document.

  Keep in mind that there are known security holes in all versions of
  BIND 8 less than 8.2.3, so make very sure that you're running the
  latest version!



  1.4.  How?

  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.


  1.5.  Disclaimer

  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.


  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.


  2.5.1.  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



  2.5.2.  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.


  3.  Compiling BIND

  You should be able to find the BIND source by visiting
  <http://www.isc.org/bind.html>.  You need the bind-src.tar.gz package.
  Be sure to get the latest version!


  3.1.  Modifying Paths

  Things can get a bit confusing at this point, because different parts
  of the BIND package will be referring to the same directories by
  different names (depending on whether or not they're running inside
  the jail).  I'll try not to confuse you too much :-).

  The main directory that we have to worry about here is /var/run,
  because its contents are required for both the main named daemon
  (inside the jail), and the ndc utility (on the outside).  We'll start
  by setting everything up to find this directory from the outside
  world.  To do this, we need to modify src/port/linux/Makefile.set
  (substitute your port's directory if you're not running Linux), and
  change the line


       DESTRUN=/var/run



  to


       DESTRUN=/chroot/named/var/run



  While you're in there, you may want to change the other destination
  paths from /usr to /usr/local.

  Now everything should be able to find that directory... except the
  named daemon itself, to which it's still just /var/run inside the
  jail.  We can get around this by making a small change in the named
  source.  In the file src/bin/named/named.h, find the line


       #include "pathnames.h"



  and add the following line immediately after it


       #define _PATH_NDCSOCK    "/var/run/ndc"



  This way, named will ignore our definition of DESTRUN over in Make­
  file.set and use the correct location (from its perspective in the
  chroot jail).  You will notice some warnings about redefinitions of
  _PATH_NDCSOCK when you do the build; just ignore them.


  3.2.  Doing the Build

  You should now be able to compile BIND as normal, following the
  instructions in the INSTALL file.  At this stage, we only want to
  compile BIND, not install it.  Don't go too far when following the
  INSTALL file.  Essentially, it's just make clean, make depend, and
  make.


  4.  Installing Your Shiny New BIND

  I should mention that if you have an existing installation of BIND,
  such as from an RPM, you should probably remove it before installing
  the new one.  On Red Hat systems, this probably means removing the
  packages bind and bind-utils, and possibly bind-devel and caching-
  nameserver, if you have them.

  You may want to save a copy of the init script (e.g.,
  /etc/rc.d/init.d/named), if any, before doing so; it'll be useful
  later on.


  4.1.  Installing the Tools Outside the Jail

  This is the easy part :-).  Just run make install and let it take care
  of it for you.  You may want to chmod 000 /usr/local/sbin/named
  afterwards, to make sure you don't accidentally run the non-chrooted
  copy of BIND.  (This is /usr/sbin/named if you didn't tell it to go in
  /usr/local/sbin like I suggested.)


  4.2.  Installing the Binaries in the Jail

  Only two parts of the package have to live inside the chroot jail:
  the main named daemon itself, and named-xfer, which it uses for zone
  transfers.  You can simply copy them in from the source tree:


  # cp src/bin/named/named /chroot/named/bin

  # cp src/bin/named-xfer/named-xfer /chroot/named/bin



  4.3.  Setting up the Init Script

  If you have an existing init script from your distribution, it would
  probably be best simply to modify it to run /chroot/named/bin/named,
  with the appropriate switches.  The switches are... (drumroll
  please...)

  ·  -u named, which tells BIND to run as the user named, rather than
     root.

  ·  -g named, to run BIND under the group named too, rather than root
     or wheel.

  ·  -t /chroot/named, which tells BIND to chroot itself to the jail
     that we've set up.

  The following is the init script I use with my Red Hat 6.0 system.  As
  you can see, it is almost exactly the same as the way it shipped from
  Red Hat.  I have also modified the ndc restart command so that it
  restarts the server properly, and keeps it chrooted.  You should
  probably do the same in your init script, even if you don't copy this
  one.



  ______________________________________________________________________
  #!/bin/sh
  #
  # 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.
  # 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 /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
  ______________________________________________________________________
  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=/chroot/named/bin/$NAME
       OPTIONS="-t /chroot/named -u named -g named"



  4.4.  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";
       named-xfer "/bin/named-xfer";



  Since this file is being read by the 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
  named.conf to get ndc working properly:


       controls {
           unix "/var/run/ndc" perm 0600 owner 0 group 0;
       };



  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.

  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 ``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 named.  If
  not, you have a problem.
  5.2.  That's It!

  You can go take a nap now ;-).


  6.  Appendix - Upgrading BIND Later

  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 ``Compiling BIND''
  and the first two parts of the section on ``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!


  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.

  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.