Outgoing interfacelist. This may be a comma-separated list of interface names. 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:

        eth0(Avvanta)

In that case, you will want to specify the interface's address for that provider in the ADDRESS column.

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

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, prefix the interface name with "+":

        +eth0
        +eth0:192.0.2.32/27
        +eth0:2

This feature should only be required if you need to insert rules in this file that preempt entries in shorewall-nat(5).

Comments may be attached to Netfilter rules generated from entries in this file through the use of COMMENT lines. These lines begin with the word 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 with only the word COMMENT.

Beginning with Shorewall 4.6.0, a new syntax is also accepted. With the exception of the leading '+', the interfacelist and qualifiers may appear within the parentheses of INLINE(...).

Example:

        +INLINE(eth0)

When this is done, you may augment the rule generated by Shorewall with iptables matches of your own. These matches appear after a semicolon (';') at the end of the line.

See example 8 below.

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.

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 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 first.ip.in.range-last.ip.in.range. 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.

Example: 206.124.146.177-206.124.146.180

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. Unlike random, persistent may not be used by itself.

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.

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

This column may not contain DNS Names.

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" or "udp" in the PROTO column.

Examples:

        192.0.2.4:5000-6000
        :4000-5000

If you simply place NONAT in this column, no rewriting of the source IP address or port number will be performed. This is useful if you want particular traffic to be exempt from the entries that follow in the file.

If you want to leave this column empty but you need to specify the next column then place a hyphen ("-") here.

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.

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 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.

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.

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:

        #INTERFACE   SOURCE
        eth0    192.168.0.0/24

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:

        #INTERFACE   SOURCE
        eth0         192.168.1.0/24

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:

        #INTERFACE              SOURCE
        ipsec0:10.1.1.0/24      196.168.1.0/24

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.

        #INTERFACE              SOURCE          ADDRESS
        eth0:0                  192.168.1.0/24  206.124.146.176

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   PORT(S)
        eth0         172.20.1.0/29    206.124.146.177 tcp     smtp
        eth0         172.20.1.0/29    206.124.146.176

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.

        #INTERFACE              SOURCE          ADDRESS
        eth0:+myset[dst]        -               206.124.146.177

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).

/etc/shorewall/tcrules:

       #ACTION   SOURCE         DEST         PROTO   PORT(S)       SOURCE  USER    TEST
       #                                                           PORT(S)
       1-3:CF    192.168.1.0/24 eth0 ; state=NEW

/etc/shorewall/masq:

       #INTERFACE SOURCE         ADDRESS     ...
       eth0       192.168.1.0/24 1.1.1.1 ; mark=1:C
       eth0       192.168.1.0/24 1.1.1.3 ; mark=2:C
       eth0       192.168.1.0/24 1.1.1.4 ; mark=3:C

Your eth1 has two public IP addresses: 70.90.191.121 and 70.90.191.123. You want to use the iptables statistics match to masquerade outgoing connections evenly between these two addresses.

/etc/shorewall/masq:

       #INTERFACE    SOURCE         ADDRESS 
       INLINE(eth1)  0.0.0.0/0      70.90.191.121 ;  -m statistic --mode random --probability 0.50
       eth1          0.0.0.0/0      70.90.191.123 

If INLINE_MATCHES=Yes in shorewall.conf(5), then these rules may be specified as follows:

/etc/shorewall/masq:

       #INTERFACE    SOURCE         ADDRESS 
       eth1          0.0.0.0/0      70.90.191.121 ;  -m statistic --mode random --probability 0.50
       eth1          0.0.0.0/0      70.90.191.123