# User-defined Actions

### Tom Eastep

Copyright © 2003, 2004, 2005 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”.

2005-09-12

## Creating a New Action

Prior to Shorewall version 1.4.9, rules in `/etc/shorewall/rules` were limited to those defined by Netfilter (ACCEPT, DROP, REJECT, etc.). Beginning with Shorewall version 1.4.9, users may use sequences of these elementary operations to define more complex actions.

To define a new action:

1. Add a line to ``/etc/shorewall/actions`` that names your new action. Action names must be valid shell variable names ((must begin with a letter and be composed of letters, digits and underscore characters) as well as valid Netfilter chain names. If you intend to log from the action, the name must have a maximum of 11 characters. It is recommended that the name you select for a new action begins with a capital letter; that way, the name won't conflict with a Shorewall-defined chain name.

   Beginning with Shorewall-2.0.0-Beta1, the name of the action may be optionally followed by a colon (“:”) and ACCEPT, DROP or REJECT. When this is done, the named action will become the common action for policies of type ACCEPT, DROP or REJECT respectively. The common action is applied immediately before the policy is enforced (before any logging is done under that policy) and is used mainly to suppress logging of uninteresting traffic which would otherwise clog your logs. The same policy name can appear in multiple actions; the last such action for each policy name is the one which Shorewall will use.

   Shorewall includes pre-defined actions for DROP and REJECT -- see below.

2. Once you have defined your new action name (ActionName), then copy /usr/share/shorewall/action.template to `/etc/shorewall/action.ActionName` (for example, if your new action name is “Foo” then copy `/usr/share/shorewall/action.template` to `/etc/shorewall/action.Foo`).

3. Now modify the new file to define the new action.

Columns in the action.template file are as follows:

- TARGET - Must be ACCEPT, DROP, REJECT, LOG, CONTINUE, QUEUE or <action> where <action> is a previously-defined action (that is, it must precede the action being defined in this file in your `/etc/shorewall/actions` file). These actions have the same meaning as they do in the `/etc/shorewall/rules` file (CONTINUE terminates processing of the current action and returns to the point where that action was invoked). The TARGET may optionally be followed by a colon (“:”) and a syslog log level (e.g, REJECT:info or ACCEPT:debugging). This causes the packet to be logged at the specified level. You may also specify ULOG (must be in upper case) as a log level.This will log to the ULOG target for routing to a separate log through use of ulogd (http://www.gnumonks.org/projects/ulogd).

- SOURCE - Source hosts to which the rule applies. A comma-separated list of subnets and/or hosts. Hosts may be specified by IP or MAC address; mac addresses must begin with “~” and must use “-” as a separator.

  Alternatively, clients may be specified by interface name. For example, eth1 specifies a client that communicates with the firewall system through eth1. This may be optionally followed by another colon (“:”) and an IP/MAC/subnet address as described above (e.g., eth1:192.168.1.5).

- DEST - Location of Server. Same as above with the exception that MAC addresses are not allowed.

  Unlike in the SOURCE column, you may specify a range of up to 256 IP addresses using the syntax <first ip>-<last ip>.

- PROTO - Protocol - Must be “tcp”, “udp”, “icmp”, a number, or “all”.

- DEST PORT(S) - Destination Ports. A comma-separated list of Port names (from `/etc/services`), port numbers or port ranges; if the protocol is “icmp”, this column is interpreted as the destination icmp-type(s).

  A port range is expressed as <low port>:<high port>.

  This column is ignored if PROTOCOL = all but must be entered if any of the following fields are supplied. In that case, it is suggested that this field contain “-”.

  If your kernel contains multi-port match support, then only a single Netfilter rule will be generated if in this list and in the CLIENT PORT(S) list below:

  1. There are 15 or less ports listed.

  2. No port ranges are included.

  Otherwise, a separate rule will be generated for each port.

- SOURCE PORT(S) - Port(s) used by the client. If omitted, any source port is acceptable. Specified as a comma-separated list of port names, port numbers or port ranges.

  If you don't want to restrict client ports but need to specify an ADDRESS in the next column, then place "-" in this column.

  If your kernel contains multi-port match support, then only a single Netfilter rule will be generated if in this list and in the DEST PORT(S) list above:

  1. There are 15 or less ports listed.

  2. No port ranges are included.

  Otherwise, a separate rule will be generated for each port.

- RATE LIMIT - You may rate-limit the rule by placing a value in this column:

           <rate>/<interval>[:<burst>]

  where <rate> is the number of connections per <interval> (“sec” or “min”) and <burst> is the largest burst permitted. If no <burst> is given, a value of 5 is assumed. There may be no whitespace embedded in the specification.

           Example: 10/sec:20

- USER/GROUP - For output rules (those with the firewall as their source), you may control connections based on the effective UID and/or GID of the process requesting the connection. This column can contain any of the following:

  [!]<user number>[:]
  [!]<user name>[:]
  [!]:<group number>
  [!]:<group name>
  [!]<user number>:<group number>
  [!]<user name>:<group number>
  [!]<user inumber>:<group name>
  [!]<user name>:<group name>

Omitted column entries should be entered using a dash ("-:).

Example:

`/etc/shorewall/actions`:

         LogAndAccept

`/etc/shorewall/action.LogAndAccept`

         LOG:info
         ACCEPT

To use your action, in `/etc/shorewall/rules` you might do something like:

    #ACTION      SOURCE      DEST        PROTO    DEST PORT(S)
    LogAndAccept loc         $FW         tcp      22

Prior to Shorewall 2.1.2, specifying a log level (and optionally a log tag) on a rule that specified a user-defined (or Shorewall-defined) action would log all traffic passed to the action. Beginning with Shorewall 2.1.2, specifying a log level in a rule that specifies a user- or Shorewall-defined action will cause each rule in the action to be logged with the specified level (and tag).

The extent to which logging of action rules occur is governed by the following:

1. When you invoke an action and specify a log level, only those rules in the action that have no log level will be changed to log at the level specified at the action invocation.

   Example:

   /etc/shorewall/action.foo

       #TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
       ACCEPT       -          -        tcp      22
       bar:info

   /etc/shorewall/rules:

       #ACTION      SOURCE     DEST     PROTO    DEST PORT(S)
       foo:debug    $FW        net

   Logging in the invoke 'foo' action will be as if foo had been defined as:

       #TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
       ACCEPT:debug -          -        tcp      22
       bar:info

2. If you follow the log level with "!" then logging will be at that level for all rules recursively invoked by the action.

   Example:

   /etc/shorewall/action.foo

       #TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
       ACCEPT       -          -        tcp      22
       bar:info

   /etc/shorewall/rules:

       #ACTION      SOURCE     DEST     PROTO    DEST PORT(S)
       foo:debug!    $FW        net

   Logging in the invoke 'foo' action will be as if foo had been defined as:

       #TARGET      SOURCE     DEST     PROTO    DEST PORT(S)
       ACCEPT:debug -          -        tcp      22
       bar:debug

The change in Shorewall 2.1.2 has an effect on extension scripts used with user-defined actions. If you define an action 'acton' and you have an `/etc/shorewall/acton` script then when that script is invoked, the following three variables will be set for use by the script:

- $CHAIN = the name of the chain where your rules are to be placed. When logging is used on an action invocation, Shorewall creates a chain with a slightly different name from the action itself.

- $LEVEL = Log level. If empty, no logging was specified.

- $TAG = Log Tag.

Example:

`/etc/shorewall/rules`:

    #ACTION          SOURCE           DEST
    acton:info:test  $FW              net

Your /etc/shorewall/acton file will be run with:

- $CHAIN="%acton1"

- $LEVEL="info"

- $TAG="test"

## Standard Actions In Shorewall 2.0

Beginning with Shorewall 2.0.0-Beta1, Shorewall includes a number of pre-defined actions. These defined actions are listed in `/usr/share/shorewall/actions.std`.

Example 1. Example of Using a Standard Action

Suppose that you wish to enable ftp from your local network to your firewall. In `/etc/shorewall/rules`:

    #ACTION   SOURCE      DEST        PROTO    ...
    AllowFTP  loc         $FW

`/usr/share/shorewall/actions.std` is processed before `/etc/shorewall/actions` and if you have any actions defined with the same name as one in `/usr/share/shorewall/actions.std`, your version in `/etc/shorewall` will be the one used. So if you wish to modify a standard action, simply copy the associated action file from `/usr/share/shorewall` to `/etc/shorewall and modify` it to suit your needs. The next shorewall restart will cause your action to be installed in place of the standard one. In particular, if you want to modify the common actions “Drop” or “Reject”, simply copy `action.Drop` or `Action.Reject` to `/etc/shorewall` and modify that copy as desired.

### Note

Some of the standard actions are built-ins. This means that there is no corresponding action.* file and that Shorewall constructs the rules for the actions using direct iptables commands. If you need to modify one of these built-in actions, you will need to use the Extension Script mechanism described below and you will need to give the action a different name.

## Common Actions

Also beginning with Shorewall version 2.2.0-Beta1, when an ACCEPT, DROP or REJECT policy is about to be enforced, a common action can first be invoked. In /etc/shorewall/actions.std are found these two entries:

    Drop:DROP       #Common Action for DROP policy
    Reject:REJECT   #Common Action for REJECT policy

These entries designate the action named Drop as the common action for DROP policies and the common action Reject as the common action for REJECT policies.

The purpose of common actions is:

- To avoid filling your log with useless clutter. For example, one of the things that the Drop action does is to silently drop SMB traffic by invoking the DropSMB action.

- To ensure proper behavior. For example, both the Drop and Reject actions invoke the RejectAuth action to REJECT connection requests on TCP port 113. If these requests are simply dropped, connection timeouts can occur when you connect to a server that uses AUTH identification.

It should be stressed that the common actions do not cause any traffic to be dropped or rejected that isn't about to be dropped or rejected anyway (remember that these actions are invoked just before the connection request is going to be dropped or rejected by policy anyway). Their main function is to avoid log clutter.

## Creating an Action using an Extension Script

There may be cases where you wish to create a chain with rules that can't be constructed using the tools defined in the action.template. In that case, you can use an extension script.

### Note

If you actually need an action to drop broadcast packets, use the dropBcast standard action rather than create one like this.

Example 2. An action to drop all broadcast packets

/etc/shorewall/actions

    DropBcasts

/etc/shorewall/action.DropBcasts

    # This file is empty

/etc/shorewall/DropBcasts

    run_iptables -A DropBcasts -m pkttype --pkttype broadcast -j DROP