<?xml version="1.0" encoding="UTF-8"?>
<refentry>
<refmeta>
<refentrytitle>shorewall-masq</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>masq</refname>
<refpurpose>Shorewall Masquerade/SNAT definition file</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>/etc/shorewall/masq</command>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>Use this file to define dynamic NAT (Masquerading) and to define
Source NAT (SNAT).</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, adding entries to this file will
*not* force connections to go out through a particular ISP. You must use
PREROUTING entries in <ulink
url="shorewall-tcrules.tcml">shorewall-tcrules</ulink>(5) to do
that.</para>
</warning>
<para>The columns in the file are as follows.</para>
<variablelist>
<varlistentry>
<term><emphasis role="bold">INTERFACE</emphasis> — [<emphasis
role="bold">+</emphasis>]<emphasis>interface</emphasis>[<emphasis
role="bold">:</emphasis>[<emphasis>digit</emphasis>]][<emphasis
role="bold">:</emphasis>[<emphasis>address</emphasis>[<emphasis
role="bold">,</emphasis><emphasis>address</emphasis>]...][<emphasis>exclusion</emphasis>]</term>
<listitem>
<para>Outgoing <emphasis>interface</emphasis>. This is usually 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 configuratio</emphasis>n.</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)).</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:
eth2::192.0.2.32/27</programlisting>
<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, prefix the interface
name with "+":</para>
<programlisting> +eth0
+eth0:192.0.2.32/27
+eth0:2</programlisting>
<para>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> (Formerly called SUBNET)
—
{<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>. If you give the name of an
interface, the interface must be up before you start the firewall
(Shorewall will use your main routing table to determine the
appropriate addresses to masquerade).</para>
<para>In order to exclude a address of the specified SOURCE, you may
append an <emphasis>exclusion</emphasis> ("!" and a comma-separated
list of IP addresses (host or net) that you wish to exclude (see
<ulink
url="shorewall-exclusion.html">shorewall-exclusion</ulink>(5))).</para>
<para>Example: eth1!192.168.1.4,192.168.32.0/27</para>
<para>In that example traffic from eth1 would be masqueraded unless
it came from 192.168.1.4 or 196.168.32.0/27</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">ADDRESS</emphasis> (Optional) — [<emphasis
role="bold">-</emphasis>|[<emphasis
role="bold">SAME:</emphasis>[<emphasis
role="bold">nodst:</emphasis>]][<emphasis>address-or-address-range</emphasis>[,<emphasis>address-or-address-range</emphasis>]...][:<emphasis>lowport</emphasis><emphasis
role="bold">-</emphasis><emphasis>highport</emphasis>]|<emphasis
role="bold">detect</emphasis>]</term>
<listitem>
<para>If you specify an address here, SNAT will be used and this
will be the source 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.</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>.</para>
<para>Example: 206.124.146.177-206.124.146.180</para>
<para>You may also use the special value "detect" which causes
Shorewall to determine the IP addresses configured on the interface
named in the INTERFACES 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>This column may not contain DNS Names.</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" or "udp" in the PROTO column.</para>
<para>Examples:</para>
<programlisting> 192.0.2.4:5000-6000
:4000-5000</programlisting>
<para>You can invoke the SAME target rather than the SNAT target by
prefixing the column contents with <option>SAME:</option>.</para>
<para>SAME works like SNAT with the exception that the same local IP
address is assigned to each connection from a local address to a
given remote address.</para>
<para>If the <option>nodst:</option> option is included, then the
same source address is used for a given internal system regardless
of which remote system is involved.</para>
<para>If you want to leave this column empty but you need to specify
the next column then place a hyphen ("-") here.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">PROTO</emphasis> (Optional) — {<emphasis
role="bold">-</emphasis>|<emphasis>protocol-name</emphasis>|<emphasis>protocol-number</emphasis>}</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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">PORT(S)</emphasis> (Optional) —
[<emphasis>port-name-or-number</emphasis>[,<emphasis>port-name-or-number</emphasis>]...]</term>
<listitem>
<para>If the PROTO column specifies TCP (protocol 6) or UDP
(protocol 17) then you may list one or more port numbers (or names
from services(5)) separated by commas or you may list a single port
range
(<emphasis>lowport</emphasis>:<emphasis>highport</emphasis>).</para>
<para>Where a comma-separated list is given, your kernel and
iptables must have multiport match support and a maximum of 15 ports
may be listed.</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><number></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>
</variablelist>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Examples</title>
<variablelist>
<varlistentry>
<term>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 can be either:</para>
<programlisting> #INTERFACE SOURCE
eth0 eth1</programlisting>
<para>or</para>
<programlisting> #INTERFACE SOURCE
eth0 192.168.0.0/24</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>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> #INTERFACE SOURCE
eth0 192.168.1.0/24</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>Example 3:</term>
<listitem>
<para>You have an IPSEC tunnel through ipsec0 and you want to
masquerade packets coming from 192.168.1.0/24 but only if these
packets are destined for hosts in 10.1.1.0/24:</para>
<programlisting> #INTERFACE SOURCE
ipsec0:10.1.1.0/24 196.168.1.0/24</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>Example 4:</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> #INTERFACE SOURCE ADDRESS
eth0:0 192.168.1.0/24 206.124.146.176</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>Example 5:</term>
<listitem>
<para>You want all outgoing SMTP traffic entering the firewall on
eth1 to be sent from eth0 with source IP address 206.124.146.177.
You want all other outgoing traffic from eth1 to be sent from eth0
with source IP address 206.124.146.176.</para>
<programlisting> #INTERFACE SOURCE ADDRESS PROTO PORT(S)
eth0 eth1 206.124.146.177 tcp smtp
eth0 eth1 206.124.146.176</programlisting>
<warning>
<para>The order of the above two rules is significant!</para>
</warning>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>FILES</title>
<para>/etc/shorewall/masq</para>
</refsect1>
<refsect1>
<title>See ALSO</title>
<para><ulink
url="http://www.shorewall.net/Documentation.htm#Masq">http://www.shorewall.net/Documentation.htm#Masq</ulink></para>
<para>shorewall(8), shorewall-accounting(5), shorewall-actions(5),
shorewall-blacklist(5), shorewall-exclusion(5), shorewall-hosts(5),
shorewall-interfaces(5), shorewall-ipsec(5), shorewall-maclist(5),
shorewall-nat(5), shorewall-netmap(5), shorewall-params(5),
shorewall-policy(5), shorewall-providers(5), shorewall-proxyarp(5),
shorewall-route_routes(5), shorewall-routestopped(5), shorewall-rules(5),
shorewall.conf(5), shorewall-tcclasses(5), shorewall-tcdevices(5),
shorewall-tcrules(5), shorewall-tos(5), shorewall-tunnels(5),
shorewall-zones(5)</para>
</refsect1>
</refentry>