# Extension Scripts and Default Actions

Tom
Eastep
2001-2006
Thomas M. Eastep

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This article applies to Shorewall 3.0 and later. If you are running a version of Shorewall earlier than Shorewall 3.0.0 then please see the documentation for that release.

Extension scripts are user-provided scripts that are invoked at various points during firewall start, restart, stop and clear. The scripts are placed in /etc/shorewall and are processed using the Bourne shell source mechanism.

1. Be sure that you actually need to use an extension script to do what you want. Shorewall has a wide range of features that cover most requirements.

2. DO NOT SIMPLY COPY RULES THAT YOU FIND ON THE NET INTO AN EXTENSION SCRIPT AND EXPECT THEM TO WORK AND TO NOT BREAK SHOREWALL. TO USE SHOREWALL EXTENSION SCRIPTS YOU MUST KNOW WHAT YOU ARE DOING WITH RESPECT TO iptables/Netfilter AND SHOREWALL.

The following scripts can be supplied:

- init -- invoked early in shorewall start and shorewall restart

- initdone -- invoked after Shorewall has flushed all existing rules but before any rules have been added to the builtin chains.

- start -- invoked after the firewall has been started or restarted.

- started -- invoked as a first step when the firewall is being started

- stop -- invoked as a first step when the firewall is being stopped.

- stopped -- invoked after the firewall has been stopped.

- clear -- invoked after the firewall has been cleared.

- refresh -- invoked while the firewall is being refreshed but before the blacklst chains have been rebuilt.

- continue -- invoked to allow you to insert special rules to allow traffic while Shorewall is [re]starting. Any rules added in this script should be deleted in your start script. This script is invoked earlier in the [re]start process than is the initdone script described above.

- maclog -- (Added in Shorewall version 3.2.5) invoked while mac filtering rules are being created. It is invoked once for each interface having 'maclist' specified and it is invoked just before the logging rule is added to the current chain (the name of that chain will be in $CHAIN).

If your version of Shorewall doesn't have the file that you want to use from the above list, you can simply create the file yourself. You can also supply a script with the same name as any of the filter chains in the firewall and the script will be invoked after the /etc/shorewall/rules file has been processed but before the /etc/shorewall/policy file has been processed.

There are a couple of special considerations for commands in extension scripts:

- When you want to run `iptables`, use the command `run_iptables` instead. `run_iptables` will run the iptables utility passing the arguments to `run_iptables` and if the command fails, the firewall will be stopped (or restored from the last `save` command, if any).

- If you wish to generate a log message, use log_rule_limit. Parameters are:

  - Log Level

  - Chain to insert the rule into

  - Chain name to display in the message (this can be different from the preceding argument — see the Port Knocking article for an example of how to use this).

  - Disposition to report in the message (ACCEPT, DROP, etc)

  - Rate Limit (if passed as "" then $LOGLIMIT is assumed — see the LOGLIMIT option in /etc/shorewall/shorewall.conf)

  - Log Tag ("" if none)

  - Command (-A or -I for append or insert).

  - The remaining arguments are passed "as is" to iptables

- Many of the extension scripts get executed for both the shorewall start and shorewall restart commands. You can determine which command is being executed using the contents of $COMMAND.

      if [ $COMMAND = start ]; then
         ...

- Shorewall versions 3.0.x and earlier only. If you run commands other than `iptables` that must be re-run in order to restore the firewall to its current state then you must save the commands to the restore file. The restore file is a temporary file in `/var/lib/shorewall` that will be renamed `/var/lib/shorewall/restore-base` at the successful completion of the Shorewall command. The `shorewall save` command combines `/var/lib/shorewall/restore-base` with the output of `iptables-save` to produce the `/var/lib/shorewall/restore` script.

  Here are three functions that are useful when running commands other than `iptables`:

  1. save_command() -- saves the passed command to the restore file.

     Example:

         save_command echo Operation Complete

     That command would simply write "echo Operation Complete" to the restore file.

  2. run_and_save_command() -- saves the passed command to the restore file then executes it. The return value is the exit status of the command. Example:

         run_and_save_command "echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all"

     Note that as in this example, when the command involves file redirection then the entire command must be enclosed in quotes. This applies to all of the functions described here.

  3. ensure_and_save_command() -- runs the passed command. If the command fails, the firewall is restored to it's prior saved state and the operation is terminated. If the command succeeds, the command is written to the restore file

- Shorewall version 3.2.0 and later only. When compiling your firewall configuration, Shorewall copies most extension scripts directly into the "compiled" program where they are executed in-line during processing of the start, restart and restore commands. When copying a script, Shorewall indents the script to match the surrounding code; if you have 'awk' installed on the system where the configuration is being compiled, Shorewall can correctly handle line continuation in your script ("\" as the last character on a line). If you do not have awk, you may not use line continuation in your scripts. Also beware that quoted strings continued from one line to another will have extra whitespace inserted as a result of indentation.

  The `/etc/shorewall/params` script is processed during compilation and copied into the compiled script as just described. So shell variables set during compilation may be used in Shorewall configuration files while those set at run-time are available to your other extension scripts. Note that if you assign dynamic values to variables, there is no guarantee that the value calculated at compile time will be the same as what is calculated at run time. This is particularly true if you use the `shorewall compile` command to compile a program then run that program at a later time.

  Extension scripts associated with a particular chain or action are not copied into the compiled script; they are rather processed directly by the compiler using the Bourne shell "." command. For example, if A is an action then if `/etc/shorewall/A` exists then it will be processed by the compiler rather than copied into the compiled script.

You can also define a default action to be performed immediately before a policy of ACCEPT, DROP or REJECT is applied. Separate actions can be assigned to each policy type so for example you can have a different default action for DROP and REJECT policies. The most common usage of default actions is to silently drop traffic that you don't wish to have logged by the policy.

As released, Shorewall defines a number of actions which are cataloged in the `/usr/share/shorewall/actions.std` file. That file is processed before /etc/shorewall/actions. Among the entries in `/usr/share/shorewall/actions.std` are:

    Drop:DROP
    Reject:REJECT

So the action named Drop is performed immediately before DROP policies are applied and the action called Reject is performed before REJECT policies are applied. These actions are defined in the files `/usr/share/shorewall/action.Drop` and `/usr/share/shorewall/action.Reject` respectively.

You can override these defaults with entries in your /etc/shorewall/actions file. For example, if that file were to contain MyDrop:DROP then the default action for DROP policies would become MyDrop.

One final note. The chain created to perform an action has the same name as the action. You can use an extension script by that name to add rules to the action's chain in the same way as you can any other chain. So if you create the new action Dagger and define it in `/etc/shorewall/action.Dagger`, you can also have an extension script named `/etc/shorewall/Dagger` that can add rules to the Dagger chain that can't be created using `/etc/shorewall/action.Dagger`.