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412 lines
18 KiB
XML
412 lines
18 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article>
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<!--$Id$-->
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<articleinfo>
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<title>Extension Scripts and Default Actions</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
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<copyright>
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<year>2001-2007</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<caution>
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<para><emphasis role="bold">This article applies to Shorewall 3.0 and
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later. If you are running a version of Shorewall earlier than Shorewall
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3.0.0 then please see the documentation for that
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release.</emphasis></para>
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</caution>
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<para>Extension scripts are user-provided scripts that are invoked at
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various points during firewall start, restart, stop and clear. The scripts
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are placed in /etc/shorewall and are processed using the Bourne shell
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<quote>source</quote> mechanism.</para>
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<caution>
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<orderedlist>
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<listitem>
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<para>Be sure that you actually need to use an extension script to do
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what you want. Shorewall has a wide range of features that cover most
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requirements.</para>
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</listitem>
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<listitem>
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<para>DO NOT SIMPLY COPY RULES THAT YOU FIND ON THE NET INTO AN
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EXTENSION SCRIPT AND EXPECT THEM TO WORK AND TO NOT BREAK SHOREWALL.
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TO USE SHOREWALL EXTENSION SCRIPTS YOU MUST KNOW WHAT YOU ARE DOING
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WITH RESPECT TO iptables/Netfilter AND SHOREWALL.</para>
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</listitem>
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</orderedlist>
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</caution>
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<para>The following scripts can be supplied:</para>
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<itemizedlist>
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<listitem>
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<para>init -- invoked early in <quote>shorewall start</quote> and
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<quote>shorewall restart</quote></para>
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</listitem>
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<listitem>
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<para>initdone -- invoked after Shorewall has flushed all existing rules
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but before any rules have been added to the builtin chains.</para>
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</listitem>
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<listitem>
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<para>start -- invoked after the firewall has been started or
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restarted.</para>
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</listitem>
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<listitem>
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<para>started -- invoked after the firewall has been marked as
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'running'.</para>
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</listitem>
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<listitem>
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<para>stop -- invoked as a first step when the firewall is being
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stopped.</para>
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</listitem>
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<listitem>
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<para>stopped -- invoked after the firewall has been stopped.</para>
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</listitem>
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<listitem>
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<para>clear -- invoked after the firewall has been cleared.</para>
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</listitem>
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<listitem>
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<para>refresh -- invoked while the firewall is being refreshed but
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before the blacklst chains have been rebuilt (Not used by Shorewall
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Perl).</para>
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</listitem>
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<listitem>
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<para>continue -- invoked to allow you to insert special rules to allow
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traffic while Shorewall is [re]starting. Any rules added in this script
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should be deleted in your <emphasis>start</emphasis> script. This script
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is invoked earlier in the [re]start process than is the
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<emphasis>initdone</emphasis> script described above (Not used by
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Shorewall Perl).</para>
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</listitem>
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<listitem>
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<para>maclog -- (Added in Shorewall version 3.2.5) invoked while mac
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filtering rules are being created. It is invoked once for each interface
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having 'maclist' specified and it is invoked just before the logging
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rule is added to the current chain (the name of that chain will be in
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$CHAIN).</para>
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</listitem>
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</itemizedlist>
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<para><emphasis role="bold">If your version of Shorewall doesn't have the
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file that you want to use from the above list, you can simply create the
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file yourself.</emphasis> You can also supply a script with the same name as
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any of the filter chains in the firewall and the script will be invoked
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after the /etc/shorewall/rules file has been processed but before the
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/etc/shorewall/policy file has been processed.</para>
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<para>There are a couple of special considerations for commands in extension
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scripts:</para>
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<itemizedlist>
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<listitem>
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<para>When you want to run <command>iptables</command>, use the command
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<command>run_iptables</command> instead. <command>run_iptables</command>
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will run the iptables utility passing the arguments to
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<command>run_iptables</command> and if the command fails, the firewall
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will be stopped (or restored from the last <command>save</command>
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command, if any).</para>
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</listitem>
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<listitem>
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<para>If you wish to generate a log message, use <emphasis
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role="bold">log_rule_limit</emphasis>. Parameters are:</para>
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<itemizedlist>
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<listitem>
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<para>Log Level</para>
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</listitem>
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<listitem>
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<para>Chain to insert the rule into</para>
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</listitem>
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<listitem>
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<para>Chain name to display in the message (this can be different
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from the preceding argument — see the <ulink
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url="PortKnocking.html">Port Knocking article</ulink> for an example
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of how to use this).</para>
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</listitem>
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<listitem>
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<para>Disposition to report in the message (ACCEPT, DROP,
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etc)</para>
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</listitem>
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<listitem>
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<para>Rate Limit (if passed as "" then $LOGLIMIT is assumed — see
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the LOGLIMIT option in <ulink
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url="Documentation.htm#Conf">/etc/shorewall/shorewall.conf</ulink>)</para>
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</listitem>
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<listitem>
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<para>Log Tag ("" if none)</para>
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</listitem>
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<listitem>
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<para>Command (-A or -I for append or insert).</para>
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</listitem>
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<listitem>
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<para>The remaining arguments are passed "as is" to iptables</para>
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</listitem>
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</itemizedlist>
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</listitem>
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<listitem>
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<para>Many of the extension scripts get executed for both the shorewall
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start and shorewall restart commands. You can determine which command is
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being executed using the contents of $COMMAND.</para>
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<programlisting>if [ $COMMAND = start ]; then
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...</programlisting>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Shorewall versions 3.0.x and earlier
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only.</emphasis> If you run commands other than
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<command>iptables</command> that must be re-run in order to restore the
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firewall to its current state then you must save the commands to the
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<firstterm>restore file</firstterm>. The restore file is a temporary
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file in <filename class="directory">/var/lib/shorewall</filename> that
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will be renamed <filename>/var/lib/shorewall/restore-base</filename> at
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the successful completion of the Shorewall command. The
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<command>shorewall save</command> command combines
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<filename>/var/lib/shorewall/restore-base</filename> with the output of
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<command>iptables-save</command> to produce the
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<filename>/var/lib/shorewall/restore</filename> script.</para>
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<para>Here are three functions that are useful when running commands
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other than <command>iptables</command>:</para>
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<orderedlist>
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<listitem>
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<para><emphasis role="bold">save_command() </emphasis>-- saves the
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passed command to the restore file.</para>
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<para>Example: <programlisting>save_command echo Operation Complete</programlisting></para>
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<para>That command would simply write "echo Operation Complete" to
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the restore file.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">run_and_save_command()</emphasis> --
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saves the passed command to the restore file then executes it. The
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return value is the exit status of the command. Example:
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<programlisting>run_and_save_command "echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all"</programlisting></para>
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<para>Note that as in this example, when the command involves file
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redirection then the entire command must be enclosed in quotes. This
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applies to all of the functions described here.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">ensure_and_save_command()</emphasis> --
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runs the passed command. If the command fails, the firewall is
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restored to its prior saved state and the operation is terminated.
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If the command succeeds, the command is written to the restore
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file</para>
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</listitem>
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</orderedlist>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Shorewall version 3.2.0 - 3.2.8
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only.</emphasis> When compiling your firewall configuration, Shorewall
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copies most extension scripts directly into the "compiled" program where
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they are executed in-line during processing of the start, restart and
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restore commands. When copying a script, Shorewall indents the script to
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match the surrounding code; if you have 'awk' installed on the system
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where the configuration is being compiled, Shorewall can correctly
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handle line continuation in your script ("\" as the last character on a
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line). If you do not have awk, you may not use line continuation in your
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scripts. Also beware that quoted strings continued from one line to
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another will have extra whitespace inserted as a result of
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indentation.</para>
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<note>
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<para>The <filename>/etc/shorewall/params</filename> script is
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processed during compilation <emphasis role="bold">and</emphasis>
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copied into the compiled script as just described. So shell variables
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set during compilation may be used in Shorewall configuration files
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while those set at run-time are available to your other extension
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scripts. Note that if you assign dynamic values to variables, there is
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no guarantee that the value calculated at compile time will be the
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same as what is calculated at run time. This is particularly true if
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you use the <command>shorewall compile</command> command to compile a
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program then run that program at a later time.</para>
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</note>
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<note>
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<para>Extension scripts associated with a particular chain or action
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are not copied into the compiled script; they are rather processed
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directly by the compiler using the Bourne shell "." command. For
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example, if A is an action then if <filename
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class="directory">/etc/shorewall/A</filename> exists then it will be
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processed by the compiler rather than copied into the compiled
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script.</para>
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</note>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Shorewall version 3.2.9 (3.4.0 RC2) and
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later (Shorewall-shell).</emphasis> When compiling your firewall
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configuration, Shorewall copies most extension scripts directly into the
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"compiled" program where they are executed in-line during processing of
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the start, restart and restore commands. When copying a script,
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Shorewall indents the script to match the surrounding code; if you have
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'awk' installed on the system where the configuration is being compiled,
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Shorewall can correctly handle line continuation in your script ("\" as
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the last character on a line). If you do not have awk, you may not use
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line continuation in your scripts. Also beware that quoted strings
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continued from one line to another will have extra whitespace inserted
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as a result of indentation.</para>
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<note>
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<para>The <filename>/etc/shorewall/params</filename> script is
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processed only during compilation if EXPORTPARAMS=No in
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<filename>shorewall.conf</filename>. So shell variables set in that
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file may be used in Shorewall configuration files only. Any variables
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that your extension scripts require at run-time on the firewall system
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should be set in the <filename>init</filename> extension script (if
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you need variable values in the <filename>stop</filename> or
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<filename>stopped</filename> scripts, you will need to set their value
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in <filename>stop</filename> since <filename>init</filename> is not
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invoked when processing the <command>stop</command> and
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<command>clear</command> commands).</para>
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<para>When EXPORTPARAMS=Yes (the default), the
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<filename>/etc/shorewall/params</filename> script is processed during
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compilation <emphasis role="bold">and</emphasis> copied into the
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compiled script as described above. So shell variables set during
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compilation may be used in Shorewall configuration files while those
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set at run-time are available to your other extension scripts.Note
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that if you assign dynamic values to variables, there is no guarantee
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that the value calculated at compile time will be the same as what is
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calculated at run time. This is particularly true if you use the
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<command>shorewall compile</command> command to compile a program then
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run that program at a later time or if you use Shorewall Lite.</para>
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</note>
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<note>
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<para>Extension scripts associated with a particular chain or action
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are not copied into the compiled script; they are rather processed
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directly by the compiler using the Bourne shell "." command. For
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example, if A is an action then if <filename
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class="directory">/etc/shorewall/A</filename> exists then it will be
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processed by the compiler rather than copied into the compiled
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script.</para>
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</note>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Shorewall-perl</emphasis>. Because the
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compiler is now written in Perl, your compile-time extension scripts
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from earlier versions will no longer work. Compile-time extension
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scripts are executed using the Perl 'eval `cat <file>`' mechanism.
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Be sure that each script returns a 'true' value; otherwise, the compiler
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will assume that the script failed and will abort the
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compilation.</para>
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<para>All scripts will need to begin with the following
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line:<programlisting>use Shorewall::Chains;</programlisting> For more
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complex scripts, you may need to 'use' other Shorewall Perl modules --
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browse <filename
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class="directory">/usr/share/shorewall-perl/Shorewall/</filename> to see
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what's available.</para>
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<para>When a script is invoked, the $chainref scalar variable will hold
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a reference to a chain table entry.<simplelist>
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<member>$chainref->{name} contains the name of the chain</member>
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<member>$chainref->{table} holds the table name</member>
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</simplelist>To add a rule to the chain:<programlisting>add_rule( $chainref, <the rule> );</programlisting>Where<simplelist>
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<member><the rule> is a scalar argument holding the rule text.
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Do not include "-A <chain name>"</member>
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</simplelist>Example:<programlisting>add_rule( $chainref, '-j ACCEPT' );</programlisting>To
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insert a rule into the chain:<programlisting> insert_rule( $chainref, <rulenum>, <the rule> );</programlisting>The
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log_rule_limit function works like it does in the shell compiler with
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two exceptions:<itemizedlist>
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<listitem>
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<para>You pass the chain reference rather than the name of the
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chain.</para>
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</listitem>
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<listitem>
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<para>The commands are 'add' and 'insert' rather than '-A' and
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'-I'.</para>
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</listitem>
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<listitem>
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<para>There is only a single "pass as-is to iptables" argument (so
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you must quote that part).</para>
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</listitem>
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</itemizedlist>Example:<programlisting>log_rule_limit(
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'info' ,
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$chainref ,
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$chainref->{name},
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'DROP' ,
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'', #Limit
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'' , #Log tag
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'add', #Command
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'-p tcp' #Pass as-is
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);</programlisting>Some run-time scripts have been converted to
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compile time scripts:<simplelist>
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<member>initdone</member>
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<member>maclog</member>
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</simplelist>Note that in the 'initdone' script, there is no default
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chain ($chainref). You can objtain a reference to a standard chain
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by:<programlisting>my $chainref = $chain_table{<table>}{<chain name>};</programlisting>Example:<programlisting>my $chainref = $chain_table{filter}{INPUT};</programlisting>Some
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run-time scripts are simply eliminated because they no longer make any
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sense under Shorewall-perl:<itemizedlist>
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<listitem>
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<para>continue - This script was designed to allow you to add
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special temporary rules during [re]start. Shorewall-perl doesn't
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need such rules.</para>
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</listitem>
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<listitem>
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<para>refresh - The <command>refresh</command> command is the same
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as <command>restart</command></para>
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</listitem>
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</itemizedlist></para>
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</listitem>
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</itemizedlist>
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</article> |