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.

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)).

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

        eth0:
        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).

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

In order to exclude a address of the specified SOURCE, you may append an exclusion ("!" and a comma-separated list of IP addresses (host or net) that you wish to exclude (see shorewall-exclusion(5))). Note that with Shorewall-perl, a colon (":") must appear between an interface name and the exclusion;

Example (shorewall-shell): eth1!192.168.1.4,192.168.32.0/27

Example (shorewall-perl): eth1:!192.168.1.4,192.168.32.0/27

In that example traffic from eth1 would be masqueraded unless it came from 192.168.1.4 or 196.168.32.0/27

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. Beginning with Shorewall 4.0.6, 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 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

You can invoke the SAME target rather than the SNAT target by prefixing the column contents with SAME:.

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.

If the nodst: option is included, then the same source address is used for a given internal system regardless of which remote system is involved.

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.

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 (lowport:highport).

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.

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.

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 can be either:

        #INTERFACE   SOURCE
        eth0         eth1

or

        #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 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.

        #INTERFACE   SOURCE  ADDRESS         PROTO   PORT(S)
        eth0         eth1    206.124.146.177 tcp     smtp
        eth0         eth1    206.124.146.176