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 log level (see shorewall-logging(5)).

Choices for ACTION are:

Normally Masq/SNAT rules are evaluated after those for one-to-one NAT (defined in shorewall-nat(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 shorewall-nat(5).

Set of hosts that you wish to masquerade. You can specify this as an address (net or host) or as an interface. 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).

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.

Outgoing interfaces 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 shorewall.conf(5), you may add ":" and a digit 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. That is the only use for the alias name; it may not appear in any other place in your Shorewall configuration.

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.

Each interface must match an entry in shorewall-interfaces(5). Shorewall allows loose matches to wildcard entries in shorewall-interfaces(5). For example, ppp0 in this file will match a shorewall-interfaces(5) entry that defines ppp+.

Where more that one internet provider share a single interface, the provider is specified by including the provider name or number in parentheses:

        eth0(Avvanta)

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

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 shorewall-exclusion(5)) as are ipset names preceded by a plus sign '+';

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

        eth0(Avvanta):
        eth2::192.0.2.32/27

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.

If you wish to restrict this entry to a particular protocol then enter the protocol name (from protocols(5)) or number here. See shorewall-rules(5) for details.

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

Beginning with Shorewall 4.6.0, an ipset name can be specified in this column. This is intended to be used with bitmap:port ipsets.

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,

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.

Port ranges are of the form lowport:highport.

Beginning with Shorewall 4.6.0, an ipset name can be specified in this column. This is intended to be used with bitmap:port ipsets.

FORMAT 2 only.

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.

Port ranges are of the form lowport:highport.

An ipset name can be specified in this column. This is intended to be used with bitmap:port ipsets.

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.

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.

Defines a test on the existing packet or connection mark. The rule will match only if the test returns true.

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

This column was formerly labelled USER/GROUP.

Only locally-generated connections will match if this column is non-empty.

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

Examples:

Added in Shorewall 4.5.1 and allows enabling and disabling the rule without requiring shorewall restart.

The rule is enabled if the value stored in /proc/net/nf_condition/switch-name 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.

Within the switch-name, '@0' and '@{0}' are replaced by the name of the chain to which the rule is a added. The switch-name (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.

Switches are normally off. To turn a switch on:

To turn it off again:

Switch settings are retained over shorewall restart.

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

(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.

This column was formerly labelled ORIGINAL DEST.

Added in Shorewall 5.0.0. When non-empty, requires the Statistics Match capability in your kernel and ip6tables and causes the rule to match randomly but with the given probability. The probability is a number 0 < probability <= 1 and may be expressed at up to 8 decimal points of precision.

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.

Your entry in the file will be:

        #ACTION    SOURCE              DEST
        MASQUERADE 192.168.0.0/24      eth0

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:

        #ACTION    SOURCE              DEST
        MASQUERADE 192.168.0.0/24      eth0
        MASQUERADE 192.168.1.0/24      eth0

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.

        #ACTION                 SOURCE          DEST
        SNAT(206.124.146.176)   192.168.1.0/24  eth0:0

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.

        #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

        #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

Connections leaving on eth0 and destined to any host defined in the ipset myset should have the source IP address changed to 206.124.146.177.

        #ACTION                 SOURCE          DEST
        SNAT(206.124.146.177)   -               eth0:+myset[dst]

SNAT outgoing connections on eth0 from 192.168.1.0/24 randomly to addresses 1.1.1.1, 1.1.1.3, and 1.1.1.9 (Shorewall 5.0.0 and later).

/etc/shorewall/snat:

       #ACTION                 SOURCE          DEST
       SNAT(1.1.1.1)           192.168.1.0/24  eth0  { probability=0.33 }
       SNAT(1.1.1.3)           192.168.1.0/24  eth0  { probability=0.50 }
       SNAT(1.1.1.9)           192.168.1.0/24  eth0

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

Your entry in the file will be:

        #ACTION      SOURCE                  DEST
        MASQUERADE   2001:470:b:787::0/64    eth0

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.

/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