Dynamic Zones

Tom Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2009/08/07


Table of Contents

Overview
Installing xtables-addons
Defining a Dynamic Zone
Adding a Host to a Dynamic Zone
Deleting a Host from a Dynamic Zone
Listing the Contents of a Dynamic Zone
Dynamic Zone Contents and Shorewall stop/start/restart
Restrictions

Overview

There is sometimes a need to be able to define a zone whose members are unknown at compile-time. For example, you may wish to require authentication of internal users before allowing them access to the internet. When a user is authenticated, the user's IP address is added to the zone of users permitted web access.

Shorewall provides basic support for defining such zones. This support is based on ipset. As of this writing, ipset is not included in the standard kernel distribution; so to use dynamic zones, you must be running kernel 2.6.20 or later and have installed the xtables-addons.

Installing xtables-addons

The xtables-addons are fairly easy to install. You do not need to recompile your kernel.

Debian™ users can find xtables-addons-common and xtables-addons-source packages in testing. The kernel modules can be built and installed with the help of module-assistant. As of this writing, these packages are in the admin group rather than in the network group!!??

For other users, the basic steps are as follows:

  1. Install gcc and make

  2. Install the headers for the kernel you are running. In some distributions, such as Debian™ and Ubuntu™, the packet is called kernel-headers. For other distrubutions, such as OpenSuSE, you must install the kernel-source package.

  3. download the iptables source tarball

  4. untar the source

  5. cd to the iptables source directory

  6. run 'make'

  7. Your new iptables binary will now be installed in /usr/local/sbin. Modify shorewall.conf to specify IPTABLES=/usr/local/sbin/iptables

  8. Download the latest xtables-addons source tarball

  9. Untar the xtables-addons source

  10. cd to the xtables-addons source directory

  11. run './configure'

  12. run 'make'

  13. As root, cd to the xtables-addons directory and run 'make install'.

  14. Restart shorewall

  15. 'shorewall show capabilities' should now indicate Ipset Match: Available

You will have to repeat steps 10-13 each time that you receive a kernel upgrade from your distribution vendor. You can install xtables-addons before booting to the new kernel as follows (new-kernel-version is the version of the newly-installed kernel - example 2.6.28.11-generic. Look in the /lib/modules directory to get the full version name)

  1. cd to the xtables-addons source directory

  2. run 'make clean'

  3. run './configure --with-kbuild=/lib/modules/new-kernel-version/build --with-ksource=/lib/modules/new-kernel-version/source'

  4. run 'make'

  5. As root, cd to the xtables-addons source directory and run 'make install'.

  6. As root, run 'depmod -a new-kernel-version'

Defining a Dynamic Zone

A dynamic zone is defined by using the keyword dynamic in the zones host list.

Example:

/etc/shorewall/zones:

#NAME        TYPE             OPTIONS
loc          ipv4
webok:loc    ipv4

/etc/shorewall/interfaces:

#ZONE       INTERFACE      BROADCAST        OPTIONS
loc         eth0           -                …

/etc/shorewall/hosts:

#ZONE       HOSTS          OPTIONS
webok       eth0:dynamic

Once the above definition is added, Shorewall will automatically create an ipset named webok_eth0 the next time that Shorewall is started or restarted. Shorewall will create an ipset of type iphash. If you want to use a different type of ipset, such as macipmap, then you will want to manually create that ipset yourself before the next Shorewall start/restart.

Adding a Host to a Dynamic Zone

Adding a host to a dynamic zone is accomplished by adding the host's IP address to the appropriate ipset. Shorewall provldes a command for doing that:

shorewall add interface:address ... zone

Example:

shorewall add eth0:192.168.3.4 webok

The command can only be used when the ipset involved is of type iphash. For other ipset types, the ipset command must be used directly.

Deleting a Host from a Dynamic Zone

Deleting a host from a dynamic zone is accomplished by removing the host's IP address from the appropriate ipset. Shorewall provldes a command for doing that:

shorewall delete interface:address ... zone

Example:

shorewall delete eth0:192.168.3.4 webok

The command can only be used when the ipset involved is of type iphash. For other ipset types, the ipset command must be used directly.

Listing the Contents of a Dynamic Zone

The shorewall show command may be used to list the current contents of a dynamic zone.

shorewall show dynamic zone

Example:

shorewall show dynamic webok
eth0:
   192.168.3.4
   192.168.3.9

Dynamic Zone Contents and Shorewall stop/start/restart

The contents of a dynamic zone survive shorewall stop/shorewall start and shorewall restart. During shorewall stop, the contents of the ipsets are saved in the file ${VARDIR}/ipsets.save (usually /var/lib/shorewall/ipsets.save). During shorewall start, the contents of that file are restored to the sets. During both shorewall start and shorewall restart, any new ipsets required as a result of a configuration change are added.

Restrictions

When using dynamic zones, you may not use ipsets in your /etc/shorewall/routestopped file.