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

If you have more than one ISP link, adding entries to this file will not force connections to go out through a particular link. You must use entries in shorewall-rtrules[2](5) or PREROUTING entries in shorewall-mangle[3](5) to do that.

Choices for ACTION are:

action[+][(parameter,...)][:level]

CONTINUE[+]:level

LOG:level

MASQUERADE[+][([lowport[-highport]][random])][:level]

NFLOG[(nflog-parameters)]

SNAT[+]([address-or-address-range][:lowport[-highport]][:random][:persistent]|detect)[:level]

        192.0.2.4:5000-6000


        :4000-5000

ULOG[(ulog-parameters)]

Normally Masq/SNAT rules are evaluated after those for one-to-one NAT (defined in shorewall-nat[8](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[8](5).

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.

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[9](5). Shorewall allows loose matches to wildcard entries in shorewall-interfaces[9](5). For example, ppp0 in this file will match a shorewall-interfaces[9](5) entry that defines ppp+.

Where more that one internet provider share a single interface[10], 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[11](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.

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.

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.

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.

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.

reqid=number

spi=<number>

proto=ah|esp|ipcomp

mss=number

mode=transport|tunnel

tunnel-src=address[/mask]

tunnel-dst=address[/mask]

strict

next

yes

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

!

value

mask

:C

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:

joe

:kids

!:kids

+upnpd

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:

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.

This column was formerly labelled ORIGINAL DEST.

Your entry in the file will be:

        #ACTION    SOURCE              DEST


        MASQUERADE 192.168.0.0/24      eth0

Your entry in the file will be:

        #ACTION      SOURCE                  DEST


        MASQUERADE   2001:470:b:787::0/64    eth0