shorewall_code/Shorewall/manpages/shorewall-snat.xml

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<refentry>
  <refmeta>
    <refentrytitle>shorewall-snat</refentrytitle>

    <manvolnum>5</manvolnum>

    <refmiscinfo>Configuration Files</refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname>snat</refname>

    <refpurpose>Shorewall SNAT/Masquerade definition file</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>/etc/shorewall[6]/snat</command>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>This file is used to define dynamic NAT (Masquerading) and to define
    Source NAT (SNAT). It superseded <ulink
    url="shorewall-masq.html">shorewall-masq</ulink>(5) in Shorewall
    5.0.14.</para>

    <warning>
      <para>The entries in this file are order-sensitive. The first entry that
      matches a particular connection will be the one that is used.</para>
    </warning>

    <warning>
      <para>If you have more than one ISP link, adding entries to this file
      will <emphasis role="bold">not</emphasis> force connections to go out
      through a particular link. You must use entries in <ulink
      url="shorewall-rtrules.html">shorewall-rtrules</ulink>(5) or PREROUTING
      entries in <ulink
      url="shorewall-mangle.html">shorewall-mangle</ulink>(5) to do
      that.</para>
    </warning>

    <para>Beginning with Shorewall 5.2.6, the snat file supports two different
    formats:</para>

    <orderedlist>
      <listitem>
        <para>The SPORT (source port) column is omitted. This is the default
        unless a "?FORMAT 2" compiler directive is included.</para>
      </listitem>

      <listitem>
        <para>The SPORT column immediately follows the DPORT column.</para>
      </listitem>
    </orderedlist>

    <para>The columns in the file are as follows.</para>

    <variablelist>
      <varlistentry>
        <term><emphasis role="bold">ACTION</emphasis></term>

        <listitem>
          <para>Defines the type of rule to generate. Beginning with Shorewall
          5.1.9, with the exception of NFLOG and ULOG, the action may be
          followed by a colon (":") and a <replaceable>log level</replaceable>
          (see <ulink
          url="shorewall-logging.html">shorewall-logging(5)</ulink>).</para>

          <para>Choices for ACTION are:</para>

          <variablelist>
            <varlistentry>
              <term><emphasis
              role="bold"><replaceable>action</replaceable></emphasis>[+][(<replaceable>parameter</replaceable>,...)][:<replaceable>level</replaceable>]</term>

              <listitem>
                <para>where <replaceable>action</replaceable> is an action
                declared in <ulink
                url="shorewall-actions.html">shorewall-actions(5)</ulink> with
                the <option>nat</option> option. See <ulink
                url="../Actions.html">https://shorewall.org/Actions.html</ulink>
                for further information.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">CONTINUE</emphasis>[+]:<replaceable>level</replaceable></term>

              <listitem>
                <para>Causes matching packets to be exempted from any
                following rules in the file.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">LOG:<replaceable>level</replaceable></emphasis></term>

              <listitem>
                <para>Added in Shorewall 5.1.9. Simply log the packet and
                continue with the next rule.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">MASQUERADE[+]</emphasis>[([<replaceable>lowport</replaceable>[-<replaceable>highport</replaceable>]][<option>random</option>])][:<replaceable>level</replaceable>]</term>

              <listitem>
                <para>Causes matching outgoing packages to have their source
                IP address set to the primary IP address of the interface
                specified in the DEST column. if
                <replaceable>lowport</replaceable>-<replaceable>highport</replaceable>
                is given, that port range will be used to assign a source
                port. If only <replaceable>lowport</replaceable> is given,
                that port will be assigned, if possible. If option
                <option>random</option> is used then port mapping will be
                randomized. MASQUERADE should only be used when the DEST
                interface has a dynamic IP address. Otherwise, SNAT should be
                used and should specify the interface's static address.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">NFLOG</emphasis>[(<replaceable>nflog-parameters</replaceable>)]</term>

              <listitem>
                <para>Added in Shorewall 5.1.9. Queues matching packets to a
                back end logging daemon via a netlink socket then continues to
                the next rule. See <ulink
                url="shorewall-logging.html">shorewall-logging(5)</ulink>.</para>

                <para>The <replaceable>nflog-parameters</replaceable> are a
                comma-separated list of up to 3 numbers:</para>

                <itemizedlist>
                  <listitem>
                    <para>The first number specifies the netlink group
                    (0-65535). If omitted (e.g., NFLOG(,0,10)) then a value of
                    0 is assumed.</para>
                  </listitem>

                  <listitem>
                    <para>The second number specifies the maximum number of
                    bytes to copy. If omitted, 0 (no limit) is assumed.</para>
                  </listitem>

                  <listitem>
                    <para>The third number specifies the number of log
                    messages that should be buffered in the kernel before they
                    are sent to user space. The default is 1.</para>
                  </listitem>
                </itemizedlist>

                <para>NFLOG is similar to<emphasis role="bold">
                LOG:NFLOG</emphasis>[(<replaceable>nflog-parameters</replaceable>)],
                except that the log level is not changed when this ACTION is
                used in an action or macro body and the invocation of that
                action or macro specifies a log level.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">SNAT[+]</emphasis>([<emphasis>address-or-address-range</emphasis>][:<emphasis>lowport</emphasis><emphasis
              role="bold">[-</emphasis><emphasis>highport</emphasis>]][<emphasis
              role="bold">:random</emphasis>][:<option>persistent</option>]|<emphasis
              role="bold">detect</emphasis>)[:<replaceable>level</replaceable>]</term>

              <listitem>
                <para>If you specify an address here, matching packets will
                have their source address set to that address. If
                ADD_SNAT_ALIASES is set to Yes or yes in <ulink
                url="shorewall.conf.html">shorewall.conf</ulink>(5) then
                Shorewall will automatically add this address to the INTERFACE
                named in the first column (IPv4 only).</para>

                <para>You may also specify a range of up to 256 IP addresses
                if you want the SNAT address to be assigned from that range in
                a round-robin fashion by connection. The range is specified by
                <emphasis>first.ip.in.range</emphasis>-<emphasis>last.ip.in.range</emphasis>.
                You may follow the port range with<emphasis role="bold">
                :random</emphasis> in which case assignment of ports from the
                list will be random. <emphasis role="bold">random</emphasis>
                may also be specified by itself in this column in which case
                random local port assignments are made for the outgoing
                connections.</para>

                <para>Example: 206.124.146.177-206.124.146.180</para>

                <para>You may follow the port range (or <emphasis
                role="bold">:random</emphasis>) with <emphasis
                role="bold">:persistent</emphasis>. This is only useful when
                an address range is specified and causes a client to be given
                the same source/destination IP pair.</para>

                <para>You may also use the special value
                <option>detect</option> which causes Shorewall to determine
                the IP addresses configured on the interface named in the DEST
                column and substitute them in this column.</para>

                <para>Finally, you may also specify a comma-separated list of
                ranges and/or addresses in this column.</para>

                <para>DNS Names names are not allowed.</para>

                <para>Normally, Netfilter will attempt to retain the source
                port number. You may cause netfilter to remap the source port
                by following an address or range (if any) by ":" and a port
                range with the format
                <emphasis>lowport</emphasis>-<emphasis>highport</emphasis>. If
                this is done, you must specify "tcp", "udp", "dccp" or "stcp"
                in the PROTO column.</para>

                <para>Examples:</para>

                <programlisting>        192.0.2.4:5000-6000
        :4000-5000</programlisting>

                <para>You may also specify a single port number, which will be
                assigned to the outgoing connection, if possible.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">ULOG</emphasis>[(<replaceable>ulog-parameters</replaceable>)]</term>

              <listitem>
                <para>IPv4 only. Added in Shorewall 5.1.9. Queues matching
                packets to a back end logging daemon via a netlink socket then
                continues to the next rule. See <ulink
                url="shorewall-logging.html">shorewall-logging(5)</ulink>.</para>

                <para>Similar to<emphasis role="bold">
                LOG:ULOG</emphasis>[(<replaceable>ulog-parameters</replaceable>)],
                except that the log level is not changed when this ACTION is
                used in an action or macro body and the invocation of that
                action or macro specifies a log level.</para>
              </listitem>
            </varlistentry>
          </variablelist>

          <para>Normally Masq/SNAT rules are evaluated after those for
          one-to-one NAT (defined in <ulink
          url="shorewall-nat.html">shorewall-nat</ulink>(5)). If you want the
          rule to be applied before one-to-one NAT rules, follow the action
          name with "+": This feature should only be required if you need to
          insert rules in this file that preempt entries in <ulink
          url="shorewall-nat.html">shorewall-nat</ulink>(5).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">SOURCE</emphasis> (Optional) -
        [<emphasis>interface</emphasis>|<emphasis>address</emphasis>[<emphasis
        role="bold">,</emphasis><emphasis>address</emphasis>...][<emphasis>exclusion</emphasis>]]</term>

        <listitem>
          <para>Set of hosts that you wish to masquerade. You can specify this
          as an <emphasis>address</emphasis> (net or host) or as an
          <emphasis>interface</emphasis>. Unless you want to perform SNAT in
          the INPUT chain (see DEST below), if you give the name of an
          interface (deprecated), the interface must be up before you start
          the firewall and the Shorewall rules compiler will warn you of that
          fact. (Shorewall will use your main routing table to determine the
          appropriate addresses to masquerade).</para>

          <para>The preferred way to specify the SOURCE is to supply one or
          more host or network addresses separated by comma. You may use ipset
          names preceded by a plus sign (+) to specify a set of hosts.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">DEST</emphasis> -
        {<emphasis>interface</emphasis>[<emphasis
        role="bold">:</emphasis><emphasis>digit][,</emphasis><emphasis>interface</emphasis>[<emphasis
        role="bold">:</emphasis><emphasis>digit</emphasis>]]...|$FW}[<emphasis
        role="bold">:</emphasis>[<emphasis>dest-address</emphasis>[<emphasis
        role="bold">,</emphasis><emphasis>dest-address</emphasis>]...[<emphasis>exclusion</emphasis>]]</term>

        <listitem>
          <para>Outgoing <emphasis>interface</emphasis>s and destination
          networks. Multiple interfaces may be listed when the ACTION is
          MASQUERADE, but this is usually just your internet interface. If
          ADD_SNAT_ALIASES=Yes in <ulink
          url="shorewall.conf.html">shorewall.conf</ulink>(5), you may add ":"
          and a <emphasis>digit</emphasis> to indicate that you want the alias
          added with that name (e.g., eth0:0). This will allow the alias to be
          displayed with ifconfig. <emphasis role="bold">That is the only use
          for the alias name; it may not appear in any other place in your
          Shorewall configuration.</emphasis></para>

          <para>Beginning with Shorewall 5.1.12, SNAT may be performed in the
          nat table's INPUT chain by specifying $FW rather than one or more
          interfaces.</para>

          <para>Each interface must match an entry in <ulink
          url="shorewall-interfaces.html">shorewall-interfaces</ulink>(5).
          Shorewall allows loose matches to wildcard entries in <ulink
          url="shorewall-interfaces.html">shorewall-interfaces</ulink>(5). For
          example, <filename class="devicefile">ppp0</filename> in this file
          will match a <ulink
          url="shorewall-interfaces.html">shorewall-interfaces</ulink>(5)
          entry that defines <filename
          class="devicefile">ppp+</filename>.</para>

          <para>Where <ulink url="../4.4/MultiISP.html#Shared">more that one
          internet provider share a single interface</ulink>, the provider is
          specified by including the provider name or number in
          parentheses:</para>

          <programlisting>        eth0(Avvanta)</programlisting>

          <para>In that case, you will want to specify the interface's address
          for that provider as the SNAT parameter.</para>

          <para>The interface may be qualified by adding the character ":"
          followed by a comma-separated list of destination host or subnet
          addresses to indicate that you only want to change the source IP
          address for packets being sent to those particular destinations.
          Exclusion is allowed (see <ulink
          url="shorewall-exclusion.html">shorewall-exclusion</ulink>(5)) as
          are ipset names preceded by a plus sign '+';</para>

          <para>If you wish to inhibit the action of ADD_SNAT_ALIASES for this
          entry then include the ":" but omit the digit:</para>

          <programlisting>        eth0(Avvanta):
        eth2::192.0.2.32/27</programlisting>

          <para>Comments may be attached to Netfilter rules generated from
          entries in this file through the use of ?COMMENT lines. These lines
          begin with ?COMMENT; the remainder of the line is treated as a
          comment which is attached to subsequent rules until another ?COMMENT
          line is found or until the end of the file is reached. To stop
          adding comments to rules, use a line containing only
          ?COMMENT.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">PROTO</emphasis> (Optional) - {<emphasis
        role="bold">-</emphasis>|[!]{<emphasis>protocol-name</emphasis>|<emphasis>protocol-number</emphasis>}[,...]|+<replaceable>ipset</replaceable>}</term>

        <listitem>
          <para>If you wish to restrict this entry to a particular protocol
          then enter the protocol name (from protocols(5)) or number here. See
          <ulink url="shorewall-rules.html">shorewall-rules(5)</ulink> for
          details.</para>

          <para>Beginning with Shorewall 4.5.12, this column can accept a
          comma-separated list of protocols.</para>

          <para>Beginning with Shorewall 4.6.0, an
          <replaceable>ipset</replaceable> name can be specified in this
          column. This is intended to be used with
          <firstterm>bitmap:port</firstterm> ipsets.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">{PORT|DPORT}</emphasis> (Optional) -
        {-|[!]<emphasis>port-name-or-number</emphasis>[,<emphasis>port-name-or-number</emphasis>]...|+<replaceable>ipset</replaceable>}</term>

        <listitem>
          <para>The column was renamed to DPORT in Shorewall 5.2.6. Beginning
          with that release, both PORT and DPORT are accepted in the
          alternative input format,</para>

          <para>If the PROTO column specifies TCP (6), UDP (17), DCCP (33),
          SCTP (132) or UDPLITE (136) then you may list one or more port
          numbers (or names from services(5)) or port ranges separated by
          commas.</para>

          <para>Port ranges are of the form
          <emphasis>lowport</emphasis>:<emphasis>highport</emphasis>.</para>

          <para>Beginning with Shorewall 4.6.0, an
          <replaceable>ipset</replaceable> name can be specified in this
          column. This is intended to be used with
          <firstterm>bitmap:port</firstterm> ipsets.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">SPORT
        {-|[!]<replaceable>port-name-or-number</replaceable>[,<replaceable>port-name-or-number</replaceable>]...|+<replaceable>ipset</replaceable>}</emphasis></term>

        <listitem>
          <para>FORMAT 2 only.</para>

          <para>If the PROTO column specifies TCP (6), UDP (17), DCCP (33),
          SCTP (132) or UDPLITE (136) then you may list one or more port
          numbers (or names from services(5)) or port ranges separated by
          commas.</para>

          <para>Port ranges are of the form
          <emphasis>lowport</emphasis>:<emphasis>highport</emphasis>.</para>

          <para>An <replaceable>ipset</replaceable> name can be specified in
          this column. This is intended to be used with
          <firstterm>bitmap:port</firstterm> ipsets.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">IPSEC</emphasis> (Optional) -
        [<emphasis>option</emphasis>[<emphasis
        role="bold">,</emphasis><emphasis>option</emphasis>]...]</term>

        <listitem>
          <para>If you specify a value other than "-" in this column, you must
          be running kernel 2.6 and your kernel and iptables must include
          policy match support.</para>

          <para>Comma-separated list of options from the following. Only
          packets that will be encrypted via an SA that matches these options
          will have their source address changed.</para>

          <variablelist>
            <varlistentry>
              <term><emphasis
              role="bold">reqid=</emphasis><emphasis>number</emphasis></term>

              <listitem>
                <para>where <emphasis>number</emphasis> is specified using
                setkey(8) using the 'unique:<emphasis>number</emphasis> option
                for the SPD level.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis role="bold">spi=</emphasis>&lt;number&gt;</term>

              <listitem>
                <para>where <emphasis>number</emphasis> is the SPI of the SA
                used to encrypt/decrypt packets.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis role="bold">proto=</emphasis><emphasis
              role="bold">ah</emphasis>|<emphasis
              role="bold">esp</emphasis>|<emphasis
              role="bold">ipcomp</emphasis></term>

              <listitem>
                <para>IPSEC Encapsulation Protocol</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">mss=</emphasis><emphasis>number</emphasis></term>

              <listitem>
                <para>sets the MSS field in TCP packets</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis role="bold">mode=</emphasis><emphasis
              role="bold">transport</emphasis>|<emphasis
              role="bold">tunnel</emphasis></term>

              <listitem>
                <para>IPSEC mode</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">tunnel-src=</emphasis><emphasis>address</emphasis>[/<emphasis>mask</emphasis>]</term>

              <listitem>
                <para>only available with mode=tunnel</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis
              role="bold">tunnel-dst=</emphasis><emphasis>address</emphasis>[/<emphasis>mask</emphasis>]</term>

              <listitem>
                <para>only available with mode=tunnel</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis role="bold">strict</emphasis></term>

              <listitem>
                <para>Means that packets must match all rules.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis role="bold">next</emphasis></term>

              <listitem>
                <para>Separates rules; can only be used with strict</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis role="bold">yes</emphasis></term>

              <listitem>
                <para>When used by itself, causes all traffic that will be
                encrypted/encapsulated to match the rule.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">MARK</emphasis> - [<emphasis
        role="bold">!</emphasis>]<emphasis>value</emphasis>[/<emphasis>mask</emphasis>][<emphasis
        role="bold">:C</emphasis>]</term>

        <listitem>
          <para>Defines a test on the existing packet or connection mark. The
          rule will match only if the test returns true.</para>

          <para>If you don't want to define a test but need to specify
          anything in the following columns, place a "-" in this field.</para>

          <variablelist>
            <varlistentry>
              <term>!</term>

              <listitem>
                <para>Inverts the test (not equal)</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis>value</emphasis></term>

              <listitem>
                <para>Value of the packet or connection mark.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis>mask</emphasis></term>

              <listitem>
                <para>A mask to be applied to the mark before testing.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><emphasis role="bold">:C</emphasis></term>

              <listitem>
                <para>Designates a connection mark. If omitted, the packet
                mark's value is tested.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">USER</emphasis> (Optional) - [<emphasis
        role="bold">!</emphasis>][<emphasis>user-name-or-number</emphasis>][<emphasis
        role="bold">:</emphasis><emphasis>group-name-or-number</emphasis>][<emphasis
        role="bold">+</emphasis><emphasis>program-name</emphasis>]</term>

        <listitem>
          <para>This column was formerly labelled USER/GROUP.</para>

          <para>Only locally-generated connections will match if this column
          is non-empty.</para>

          <para>When this column is non-empty, the rule matches only if the
          program generating the output is running under the effective
          <emphasis>user</emphasis> and/or <emphasis>group</emphasis>
          specified (or is NOT running under that id if "!" is given).</para>

          <para>Examples:</para>

          <variablelist>
            <varlistentry>
              <term>joe</term>

              <listitem>
                <para>program must be run by joe</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>:kids</term>

              <listitem>
                <para>program must be run by a member of the 'kids'
                group</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>!:kids</term>

              <listitem>
                <para>program must not be run by a member of the 'kids'
                group</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>+upnpd</term>

              <listitem>
                <para>#program named upnpd</para>

                <important>
                  <para>The ability to specify a program name was removed from
                  Netfilter in kernel version 2.6.14.</para>
                </important>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">SWITCH -
        [!]<replaceable>switch-name</replaceable>[={0|1}]</emphasis></term>

        <listitem>
          <para>Added in Shorewall 4.5.1 and allows enabling and disabling the
          rule without requiring <command>shorewall restart</command>.</para>

          <para>The rule is enabled if the value stored in
          <filename>/proc/net/nf_condition/<replaceable>switch-name</replaceable></filename>
          is 1. The rule is disabled if that file contains 0 (the default). If
          '!' is supplied, the test is inverted such that the rule is enabled
          if the file contains 0.</para>

          <para>Within the <replaceable>switch-name</replaceable>, '@0' and
          '@{0}' are replaced by the name of the chain to which the rule is a
          added. The <replaceable>switch-name</replaceable> (after '@...'
          expansion) must begin with a letter and be composed of letters,
          decimal digits, underscores or hyphens. Switch names must be 30
          characters or less in length.</para>

          <para>Switches are normally <emphasis role="bold">off</emphasis>. To
          turn a switch <emphasis role="bold">on</emphasis>:</para>

          <simplelist>
            <member><command>echo 1 &gt;
            /proc/net/nf_condition/<replaceable>switch-name</replaceable></command></member>
          </simplelist>

          <para>To turn it <emphasis role="bold">off</emphasis> again:</para>

          <simplelist>
            <member><command>echo 0 &gt;
            /proc/net/nf_condition/<replaceable>switch-name</replaceable></command></member>
          </simplelist>

          <para>Switch settings are retained over <command>shorewall
          restart</command>.</para>

          <para>Beginning with Shorewall 4.5.10, when the
          <replaceable>switch-name</replaceable> is followed by
          <option>=0</option> or <option>=1</option>, then the switch is
          initialized to off or on respectively by the
          <command>start</command> command. Other commands do not affect the
          switch setting.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">ORIGDEST</emphasis> - [<emphasis
        role="bold">-</emphasis>|<emphasis>address</emphasis>[,<emphasis>address</emphasis>]...[<emphasis>exclusion</emphasis>]|<emphasis>exclusion</emphasis>]</term>

        <listitem>
          <para>(Optional) Added in Shorewall 4.5.6. This column may be
          included and may contain one or more addresses (host or network)
          separated by commas. Address ranges are not allowed. When this
          column is supplied, rules are generated that require that the
          original destination address matches one of the listed addresses. It
          is useful for specifying that SNAT should occur only for connections
          that were acted on by a DNAT when they entered the firewall.</para>

          <para>This column was formerly labelled ORIGINAL DEST.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><emphasis role="bold">PROBABILITY</emphasis> -
        [<replaceable>probability</replaceable>]</term>

        <listitem>
          <para>Added in Shorewall 5.0.0. When non-empty, requires the
          <firstterm>Statistics Match</firstterm> capability in your kernel
          and ip6tables and causes the rule to match randomly but with the
          given <replaceable>probability</replaceable>. The
          <replaceable>probability</replaceable> is a number 0 &lt;
          <replaceable>probability</replaceable> &lt;= 1 and may be expressed
          at up to 8 decimal points of precision.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <variablelist>
      <varlistentry>
        <term>IPv4 Example 1:</term>

        <listitem>
          <para>You have a simple masquerading setup where eth0 connects to a
          DSL or cable modem and eth1 connects to your local network with
          subnet 192.168.0.0/24.</para>

          <para>Your entry in the file will be:</para>

          <programlisting>        #ACTION    SOURCE              DEST
        MASQUERADE 192.168.0.0/24      eth0</programlisting>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>IPv4 Example 2:</term>

        <listitem>
          <para>You add a router to your local network to connect subnet
          192.168.1.0/24 which you also want to masquerade. You then add a
          second entry for eth0 to this file:</para>

          <programlisting>        #ACTION    SOURCE              DEST
        MASQUERADE 192.168.0.0/24      eth0
        MASQUERADE 192.168.1.0/24      eth0</programlisting>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>IPv4 Example 3:</term>

        <listitem>
          <para>You want all outgoing traffic from 192.168.1.0/24 through eth0
          to use source address 206.124.146.176 which is NOT the primary
          address of eth0. You want 206.124.146.176 to be added to eth0 with
          name eth0:0.</para>

          <programlisting>        #ACTION                 SOURCE          DEST
        SNAT(206.124.146.176)   192.168.1.0/24  eth0:0</programlisting>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>IPv4 Example 4:</term>

        <listitem>
          <para>You want all outgoing SMTP traffic entering the firewall from
          172.20.1.0/29 to be sent from eth0 with source IP address
          206.124.146.177. You want all other outgoing traffic from
          172.20.1.0/29 to be sent from eth0 with source IP address
          206.124.146.176.</para>

          <programlisting>        #INTERFACE   SOURCE           ADDRESS         PROTO   DPORT
        eth0         172.20.1.0/29    206.124.146.177 tcp     smtp
        eth0         172.20.1.0/29    206.124.146.176</programlisting>

          <programlisting>        #ACTION                 SOURCE          DEST        PROTO     PORT
        SNAT(206.124.146.177)   172.20.1.0/29   eth0        tcp       smtp
        SNAT(206.124.146.176)   172.20.1.0/29   eth0</programlisting>

          <warning>
            <para>The order of the above two rules is significant!</para>
          </warning>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>IPv4 Example 5:</term>

        <listitem>
          <para>Connections leaving on eth0 and destined to any host defined
          in the ipset <emphasis>myset</emphasis> should have the source IP
          address changed to 206.124.146.177.</para>

          <programlisting>        #ACTION                 SOURCE          DEST
        SNAT(206.124.146.177)   -               eth0:+myset[dst]</programlisting>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>IPv4 Example 6:</term>

        <listitem>
          <para>SNAT outgoing connections on eth0 from 192.168.1.0/24 in
          round-robin fashion between addresses 1.1.1.1, 1.1.1.3, and 1.1.1.9
          (Shorewall 4.5.9 and later).</para>

          <programlisting>/etc/shorewall/tcrules:

       #ACTION   SOURCE         DEST         PROTO   DPORT         SPORT    USER    TEST
       1-3:CF    192.168.1.0/24 eth0 ; state=NEW

/etc/shorewall/snat:

       #ACTION                 SOURCE          DEST
       SNAT(1.1.1.1)           192.168.1.0/24  eth0  { mark=1:C }
       SNAT(1.1.1.3)           192.168.1.0/24  eth0  { mark=2:C }
       SNAT(1.1.1.9)           192.168.1.0/24  eth0  { mark=3:C }</programlisting>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>IPv6 Example 1:</term>

        <listitem>
          <para>You have a simple 'masquerading' setup where eth0 connects to
          a DSL or cable modem and eth1 connects to your local network with
          subnet 2001:470:b:787::0/64</para>

          <para>Your entry in the file will be:</para>

          <programlisting>        #ACTION      SOURCE                  DEST
        MASQUERADE   2001:470:b:787::0/64    eth0</programlisting>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>IPv6 Example 2:</term>

        <listitem>
          <para>Your sit1 interface has two public IP addresses:
          2001:470:a:227::1 and 2001:470:b:227::1. You want to use the
          iptables statistics match to masquerade outgoing connections evenly
          between these two addresses.</para>

          <programlisting>/etc/shorewall/snat:

       #ACTION                      SOURCE     DEST
       SNAT(2001:470:a:227::1)      ::/0       sit1              { probability=0.50 }
       SNAT(2001:470:a:227::2)      ::/0       sit</programlisting>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>FILES</title>

    <para>/etc/shorewall/snat</para>

    <para>/etc/shorewall6/snat</para>
  </refsect1>

  <refsect1>
    <title>See ALSO</title>

    <para><ulink
    url="../configuration_file_basics.htm#Pairs">https://shorewall.org/configuration_file_basics.htm#Pairs</ulink></para>

    <para>shorewall(8)</para>
  </refsect1>
</refentry>