Compiled Firewall Programs and Shorewall Lite

Compiled Firewall Programs and Shorewall Lite Tom Eastep 2006 Thomas M. 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.

Overview Beginning with Shorewall version 3.1, Shorewall has the capability to compile a Shorewall configuration and produce a runnable firewall program script. The script is a complete program which can be placed on a system with Shorewall Lite installed and can serve as the firewall creation script for that system.

Restrictions While compiled Shorewall programs are useful in many cases, there are some important restrictions that you should be aware of before attempting to use them. The detectnets interface option is not supported. DYNAMIC_ZONES=Yes in shorewall.conf is not supported. All extension scripts used are copied into the program. The ramifications of this are: If you update an extension script, the compiled program will not use the updated script. The /etc/shorewall/params extension script is executed at compile time as well as at run time. Running the script at compile time allows variable expansion (expanding $variable to it's defined value) of variables used in Shorewall configuration files to occur at compile time. Running it at run-time allows your extension scripts to use the variables that it creates. BUT -- for any given variable, the value at compile time may be different from the value at run-time unless you only assign constant values. For example, if you have: EXT_IP=$(find_first_interface_address eth0) in /etc/shorewall/params then all occurrences of $EXT_IP in Shorewall configuration files will be replaced with eth0's IP address when the program is being compiled. On the other hand, if you use $EXT_IP in your /etc/shorewall/start script, the value will be the IP address of eth0 when the program is run. Bottom line: You probably want to use only constant values for variables set in /etc/shorewall/params. You must install Shorewall Lite on the system where you want to run the script. You then install the compiled program in /usr/share/shorewall/firewall and use the /sbin/shorewall program included with Shorewall Lite to control the firewall just as if the full Shorewall distribution was installed.

The "shorewall compile" command A compiled script is produced using the compile command:

shorewall compile [ -e ] [ <directory name> ] <path name>

where

-e Indicates that the program is to be "exported" to another system. When this flag is set, neither the "detectnets" interface option nor DYNAMIC_ZONES=Yes in shorewall.conf are allowed. The created program may be run on a system that has only Shorewall Lite installed When this flag is given, Shorewall does not probe the current system to determine the kernel/iptables features that it supports. It rather reads those capabilities from /etc/shorewall/capabilities. See below for details. <directory name> specifies a directory to be searched for configuration files before those directories listed in the CONFIG_PATH variable in shorewall.conf. When -e <directory-name> is included, only the SHOREWALL_SHELL and VERBOSITY settings from /etc/shorewall/shorewall.conf are used and these apply only to the compiler itself. The settings used by the compiled firewall script are determined by the contents of <directory name>/shorewall.conf. <path name> specifies the name of the script to be created.

Shorewall Lite (Added in version 3.2.0 RC 1) The following information applies to Shorewall 3.2.2 and later. Users running versions of Shorewall and Shorewall Lite earlier than 3.2.2 are urged to upgrade. Shorewall Lite is a companion product to Shorewall and is designed to allow you to maintain all Shorewall configuration information on a single system within your network. You install the full Shorewall release on one system within your network. You need not configure Shorewall there and you may totally disable startup of Shorewall in your init scripts. For ease of reference, we call this system the 'administrative system'. On each system where you wish to run a Shorewall-generated firewall, you install Shorewall Lite. For ease of reference, we will call these systems the 'firewall systems'. The firewall systems do NOT need to have the full Shorewall product installed but rather only the Shorewall Lite product. Shorewall and Shorewall LIte may be installed on the same system but that isn't encouraged. On the administrative system you create a separate 'export directory' for each firewall system. You copy the contents of /usr/share/shorewall/configfiles into each export directory. On each firewall system, you run: /sbin/shorewall-lite show -f capabilities > capabilities scp capabilities <admin system>:<this system's config dir> If you are running Debian or one of its derivatives like Ubuntu then edit /etc/default/shorewall-lite and set startup=1. On the administrative system, for each firewall system you do the following (this may be done by a non-root user who has root ssh access to the firewall system): modify the files in the corresponding export directory appropriately. It's a good idea to include the IP address of the administrative system in the routestopped file. It is important to understand that with Shorewall Lite, the firewall's export directory on the administrative system acts as /etc/shorewall for that firewall. So when the Shorewall documentation gives instructions for placing entries in files in the firewall's /etc/shorewall, when using Shorewall Lite you make those changes in the firewall's export directory on the administrative system. The CONFIG_PATH variable is treated as follows: The value of CONFIG_PATH in /etc/shorewall/shorewall.conf is ignored when compiling for export (the -e option in given) and when the load or reload command is being executed (see below). The value of CONFIG_PATH in the shorewall.conf file in the export directory is used to search for configuration files during compilation of that configuration. The value of CONFIG_PATH used when the script is run on the firewall system is "/etc/shorewall-lite:/usr/share/shorewall-lite". cd <export directory> /sbin/shorewall load firewall The load command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and starts Shorewall Lite on the remote system via ssh. Example (firewall's DNS name is 'gateway'): /sbin/shorewall load gateway If you later need to change the firewall's configuration, change the appropriate files in the firewall's export directory then: cd <export directory> /sbin/shorewall reload firewall The reload command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and restarts Shorewall Lite on the remote system via ssh. There is a shorewall.conf file installed as part of Shorewall Lite (/etc/shorewall-lite/shorewall.conf). You can use that file on the firewall system to override some of the settings from the shorewall.conf file in the export directory. Settings that you can override are:

VERBOSITY LOGFILE LOGFORMAT IPTABLES PATH SHOREWALL_SHELL SUBSYSLOCK RESTOREFILE

You will normally not need to touch /etc/shorewall-lite/shorewall.conf. The /sbin/shorewall-lite program included with Shorewall Lite supports the same set of commands as the /sbin/shorewall program in a full Shorewall installation with the following exceptions:

add compile delete refresh reload try safe-start safe-restart show actions show macros

On systems with only Shorewall Lite installed, I recommend that you create a symbolic link /sbin/shorewall and point it at /sbin/shorewall-lite. That way, you can use shorewall as the command regardless of which product is installed.

ln -sf shorewall-lite /sbin/shorewall

Converting a system from Shorewall to Shorewall Lite Converting a firewall system that is currently running Shorewall to run Shorewall Lite instead is straight-forward. On the administrative system, create an export directory for the firewall system. Copy the contents of /etc/shorewall/ from the firewall system to the export directory on the administrative system. On the firewall system: Be sure that the IP address of the administrative system is included in /etc/shorewall/routestopped. shorewall stop We recommend that you uninstall Shorewall at this point. Install Shorewall Lite on the firewall system. If you are running Debian or one of its derivatives like Ubuntu then edit /etc/default/shorewall-lite and set startup=1. On the firewall system: /sbin/shorewall-lite show -f capabilities > capabilities scp capabilities <admin system>:<this system's config dir> On the administrative system: It's a good idea to include the IP address of the administrative system in the firewall system's routestopped file. Also, edit the shorewall.conf file in the firewall's export directory and change the CONFIG_PATH setting to remove /etc/shorewall. You can replace it with /usr/share/shorewall/configfiles if you like. Example:

Before editing: CONFIG_PATH=/etc/shorewall:/usr/share/shorewall After editing: CONFIG_PATH=/usr/share/shorewall/configfiles:/usr/share/shorewall

Changing CONFIG_PATH will ensure that subsequent compilations using the export directory will not include any files from /etc/shorewall. After having made the above changes to the firewall's export directory, execute the following commands: cd <export directory> /sbin/shorewall load <firewall system> Example (firewall's DNS name is 'gateway'): /sbin/shorewall load gateway The load command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and starts Shorewall Lite on the remote system via ssh. If you later need to change the firewall's configuration, change the appropriate files in the firewall's export directory then: cd <export directory> /sbin/shorewall reload firewall The reload command compiles a firewall script from the configuration files in the current working directory (using shorewall compile -e), copies that file to the remote system via scp and restarts Shorewall Lite on the remote system via ssh.

The /etc/shorewall/capabilities file and the shorecap program As mentioned above, the /etc/shorewall/capabilities file specifies that kernel/iptables capabilities of the target system. Here is a sample file:

NAT_ENABLED=Yes # NAT MANGLE_ENABLED=Yes # Packet Mangling MULTIPORT=Yes # Multi-port Match XMULTIPORT=Yes # Extended Multi-port Match CONNTRACK_MATCH=Yes # Connection Tracking Match USEPKTTYPE= # Packet Type Match POLICY_MATCH=Yes # Policy Match PHYSDEV_MATCH=Yes # Physdev Match LENGTH_MATCH=Yes # Packet Length Match IPRANGE_MATCH=Yes # IP range Match RECENT_MATCH=Yes # Recent Match OWNER_MATCH=Yes # Owner match IPSET_MATCH= # Ipset Match CONNMARK=Yes # CONNMARK Target XCONNMARK=Yes # Extended CONNMARK Target CONNMARK_MATCH=Yes # Connmark Match XCONNMARK_MATCH=Yes # Extended Connmark Match RAW_TABLE=Yes # Raw Table IPP2P_MATCH= # IPP2P Match CLASSIFY_TARGET=Yes # CLASSIFY Target ENHANCED_REJECT=Yes # Extended REJECT KLUDGEFREE= # iptables accepts multiple "-m iprange" or "-m physdev" in a single command MARK=Yes # MARK Target Support XMARK=YES # Extended MARK Target Support MANGLE_FORWARD # Mangle table has FORWARD chain

As you can see, the file contains a simple list of shell variable assignments -- the variables correspond to the capabilities listed by the shorewall show capabilities command and they appear in the same order as the output of that command. To aid in creating this file, Shorewall Lite includes a shorecap program. The program is installed in the /usr/share/shorewall-lite/ directory and may be run as follows:

[ IPTABLES=<iptables binary> ] [ MODULESDIR=<kernel modules directory> ] /usr/share/shorewall-lite/shorecap > capabilities

The IPTABLES and MODULESDIR options have their usual Shorewall default values. The capabilities file may then be copied to a system with Shorewall installed and used when compiling firewall programs to run on the remote system. Beginning with Shorewall Lite version 3.2.2, the capabilities file may also be creating using /sbin/shorewall-lite:

shorewall-lite show -f capabilities > capabilities

Running compiled programs directly Compiled firewall programs are complete programs that support the following run-line commands:

<program> [ -q ] [ -v ] [ -n ] start <program> [ -q ] [ -v ] [ -n ] stop <program> [ -q ] [ -v ] [ -n ] clear <program> [ -q ] [ -v ] [ -n ] restart <program> [ -q ] [ -v ] [ -n ] status <program> [ -q ] [ -v ] [ -n ] version

The options have their same meaning is when they are passed to /sbin/shorewall itself. The default VERBOSITY level is the level specified in the shorewall.conf file used when then program was compiled.