Provided by: shorewall-common_4.2.10-1_all bug


       masq - Shorewall Masquerade/SNAT definition file




       Use this file to define dynamic NAT (Masquerading) and to define Source
       NAT (SNAT).

       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, adding entries to this file will *not*
       force connections to go out through a particular ISP. You must use
       PREROUTING entries in shorewall-tcrules[1](5) to do that.

       The columns in the file are as follows.

       INTERFACE -
           Outgoing interfacelist. Prior to Shorewall 4.1.4, this must be a
           single interface name; in 4.1.4 and later, this may be a
           comma-separated list of interface names. This is usually your
           internet interface. If ADD_SNAT_ALIASES=Yes in
           shorewall.conf[2](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[3](5).
           Prior to Shorewall 4.1.4, this must be an exact match.
           Shorewall-perl 4.1.4 and later allow loose matches to wildcard
           entries in shorewall-interfaces[3](5). For example, ppp0 in this
           file will match a shorewall-interfaces[3](5) entry that defines

           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[4](5)).

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

           Normally Masq/SNAT rules are evaluated after those for one-to-one
           NAT (defined in shorewall-nat[5](5)). If you want the rule to be
           applied before one-to-one NAT rules, prefix the interface name with

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

       SOURCE (Formerly called SUBNET) -
           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[4](5))). Note that with Shorewall-perl, a colon
           (":") must appear between an interface name and the exclusion;

           Example (shorewall-shell): eth1!,

           Example (shorewall-perl): eth1:!,

           In that example traffic from eth1 would be masqueraded unless it
           came from or

       ADDRESS (Optional) -
           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[2](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
  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.


           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.


           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..sp
           Support for the SAME target is scheduled for removal from the Linux
           kernel in 2008.

           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.

       PROTO (Optional) - {-|[!]protocol-name|[!]protocol-number}
           If you wish to restrict this entry to a particular protocol then
           enter the protocol name (from protocols(5)) or number here.

       PORT(S) (Optional) - [[!]port-name-or-number[,port-name-or-number]...]
           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

       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

               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

       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. This option is only supported by


       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 can be either:

                       #INTERFACE   SOURCE
                       eth0         eth1

                       #INTERFACE   SOURCE

       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:

                       #INTERFACE   SOURCE

       Example 3:
           You have an IPSEC tunnel through ipsec0 and you want to masquerade
           packets coming from but only if these packets are
           destined for hosts in

                       #INTERFACE              SOURCE

       Example 4:
           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

                       #INTERFACE              SOURCE          ADDRESS

       Example 5:
           You want all outgoing SMTP traffic entering the firewall on eth1 to
           be sent from eth0 with source IP address You want
           all other outgoing traffic from eth1 to be sent from eth0 with
           source IP address

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

           The order of the above two rules is significant!




       shorewall(8), shorewall-accounting(5), shorewall-actions(5),
       shorewall-blacklist(5), shorewall-exclusion(5), shorewall-hosts(5),
       shorewall-interfaces(5), shorewall-ipsec(5), shorewall-maclist(5),
       shorewall-nat(5), shorewall-netmap(5), shorewall-params(5),
       shorewall-policy(5), shorewall-providers(5), shorewall-proxyarp(5),
       shorewall-route_rules(5), shorewall-routestopped(5),
       shorewall-rules(5), shorewall.conf(5), shorewall-tcclasses(5),
       shorewall-tcdevices(5), shorewall-tcrules(5), shorewall-tos(5),
       shorewall-tunnels(5), shorewall-zones(5)


        1. shorewall-tcrules

        2. shorewall.conf

        3. shorewall-interfaces

        4. shorewall-exclusion

        5. shorewall-nat

                                  06/18/2009                 SHOREWALL-MASQ(5)