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shorewall_code/Shorewall/manpages/shorewall-snat.xml
Tom Eastep a9a379c5a5
Implement INPUT SNAT 
Signed-off-by: Tom Eastep <teastep@shorewall.net>
2018-01-22 16:37:38 -08:00

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<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 It superseded 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="/manpages/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="/manpages/shorewall-rtrules.html">shorewall-rtrules</ulink>(5) or
PREROUTING entries in <ulink
url="/manpages/shorewall-mangle.html">shorewall-mangle</ulink>(5) to do
that.</para>
</warning>
<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="/manpages/shorewall-actions.html">shorewall-actions(5)</ulink>
with the <option>nat</option> option. See <ulink
url="/Actions.html">www.shorewall.net/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="/manpages/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="/manpages/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="/manpages/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="/manpages/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="/manpages/shorewall-interfaces.html">shorewall-interfaces</ulink>(5).
Shorewall allows loose matches to wildcard entries in <ulink
url="/manpages/shorewall-interfaces.html">shorewall-interfaces</ulink>(5).
For example, <filename class="devicefile">ppp0</filename> in this
file will match a <ulink
url="/manpages/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="/manpages/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="/manpages/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</emphasis> (Optional) -
{-|[!]<emphasis>port-name-or-number</emphasis>[,<emphasis>port-name-or-number</emphasis>]...|+<replaceable>ipset</replaceable>}</term>
<listitem>
<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">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">http://www.shorewall.net/configuration_file_basics.htm#Pairs</ulink></para>
<para>shorewall(8)</para>
</refsect1>
</refentry>
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