snat — Shorewall SNAT/Masquerade definition file




This file is used to define dynamic NAT (Masquerading) and to define Source NAT (SNAT). It superseded shorewall-masq(5) in Shorewall 5.0.14.


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(5) or PREROUTING entries in shorewall-mangle(5) to do that.

The columns in the file are as follows.


Defines the type of rule to generate. Choices are:


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 lowport-highport is given, that port range will be used to assign a source port. If option random 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.


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 shorewall.conf(5) then Shorewall will automatically add this address to the INTERFACE named in the first column.

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 You may follow the port range with :random in which case assignment of ports from the list will be random. random may also be specified by itself in this column in which case random local port assignments are made for the outgoing connections.


You may follow the port range (or :random) with :persistent. This is only useful when an address range is specified and causes a client to be given the same source/destination IP pair. This feature replaces the SAME modifier which was removed from Shorewall in version 4.4.0.

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

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

DNS Names names are not allowed.

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 lowport-highport. If this is done, you must specify "tcp", "udp", "dccp" or "stcp" in the PROTO column.


Causes matching packets to be exempted from any following rules in the file.


where action is an action declared in shorewall-actions(5) with the nat option. See for further information.

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).

SOURCE (Optional) - [interface|address[,address...][exclusion]]

Set of hosts that you wish to masquerade. You can specify this as an address (net or host) or as an interface (use of an interface is deprecated). If you give the name of an interface, 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.

DEST - {[+]interface[:[digit]][:[dest-address[,dest-address]...[exclusion]]}

Outgoing interface. This is usually 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.

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:


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:


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.

PROTO (Optional) - {-|[!]{protocol-name|protocol-number}[,...]|+ipset}

If you wish to restrict this entry to a particular protocol then enter the protocol name (from protocols(5)) or number here.

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.

PORT (Optional) - {-|[!]port-name-or-number[,port-name-or-number]...|+ipset}

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.

IPSEC (Optional) - [option[,option]...]

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.


where number is specified using setkey(8) using the 'unique:number option for the SPD level.


where number is the SPI of the SA used to encrypt/decrypt packets.


IPSEC Encapsulation Protocol


sets the MSS field in TCP packets


IPSEC mode


only available with mode=tunnel


only available with mode=tunnel


Means that packets must match all rules.


Separates rules; can only be used with strict


When used by itself, causes all traffic that will be encrypted/encapsulated to match the rule.

MARK - [!]value[/mask][:C]

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.


Inverts the test (not equal)


Value of the packet or connection mark.


A mask to be applied to the mark before testing.


Designates a connection mark. If omitted, the packet mark's value is tested.

USER (Optional) - [!][user-name-or-number][:group-name-or-number][+program-name]

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).



program must be run by joe


program must be run by a member of the 'kids' group


program must not be run by a member of the 'kids' group


#program named upnpd


The ability to specify a program name was removed from Netfilter in kernel version 2.6.14.

SWITCH - [!]switch-name[={0|1}]

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:

echo 1 > /proc/net/nf_condition/switch-name

To turn it off again:

echo 0 > /proc/net/nf_condition/switch-name

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.

ORIGDEST - [-|address[,address]...[exclusion]|exclusion]

(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.

PROBABILITY - [probability]

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.


Example 1:

You have a simple masquerading setup where eth0 connects to a DSL or cable modem and eth1 connects to your local network with subnet

Your entry in the file will be:

        #ACTION    SOURCE              DEST
        MASQUERADE      eth0
Example 2:

You add a router to your local network to connect subnet which you also want to masquerade. You then add a second entry for eth0 to this file:

        #ACTION    SOURCE              DEST
        MASQUERADE      eth0
        MASQUERADE      eth0
Example 3:

You want all outgoing traffic from through eth0 to use source address which is NOT the primary address of eth0. You want to be added to eth0 with name eth0:0.

        #ACTION                 SOURCE          DEST
        SNAT(  eth0:0
Example 4:

You want all outgoing SMTP traffic entering the firewall from to be sent from eth0 with source IP address You want all other outgoing traffic from to be sent from eth0 with source IP address

        #INTERFACE   SOURCE           ADDRESS         PROTO   DPORT
        eth0 tcp     smtp
        #ACTION                 SOURCE          DEST        PROTO     PORT
        SNAT(   eth0        tcp       smtp
        SNAT(   eth0


The order of the above two rules is significant!

Example 5:

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

        #ACTION                 SOURCE          DEST
        SNAT(   -               eth0+myset[dst]
Example 6:

SNAT outgoing connections on eth0 from in round-robin fashion between addresses,, and (Shorewall 4.5.9 and later).


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


       #ACTION                 SOURCE          DEST
       SNAT(   eth0  { mark=1:C }
       SNAT(   eth0  { mark=2:C }
       SNAT(   eth0  { mark=3:C }
Example 7:

Your eth1 has two public IP addresses: and You want to use the iptables statistics match to masquerade outgoing connections evenly between these two addresses.


       #ACTION                 SOURCE           DEST
       SNAT(     -                eth1 { probability=.50 }
       SNAT(     -                eth1




shorewall(8), shorewall-accounting(5), shorewall-actions(5), shorewall-blacklist(5), shorewall-exclusion(5), shorewall-hosts(5), shorewall_interfaces(5), shorewall-ipsets(5), shorewall-maclist(5), shorewall-nat(5), shorewall-netmap(5), shorewall-params(5), shorewall-policy(5), shorewall-providers(5), shorewall-proxyarp(5), shorewall-rtrules(5), shorewall-routestopped(5), shorewall-rules(5), shorewall.conf(5), shorewall-secmarks(5), shorewall-tcclasses(5), shorewall-tcdevices(5), shorewall-mangle(5), shorewall-tos(5), shorewall-tunnels(5), shorewall-zones(5)