Provided by: shorewall_4.4.26.1-1_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 link, adding entries to this file will not force
           connections to go out through a particular link. You must use entries in
           shorewall-route_rules[1](5) or PREROUTING entries in shorewall-tcrules[2](5) to do

       The columns in the file are as follows.

           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[3](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

           Each interface must match an entry in shorewall-interfaces[4](5). Shorewall allows
           loose matches to wildcard entries in shorewall-interfaces[4](5). For example, ppp0 in
           this file will match a shorewall-interfaces[4](5) entry that defines ppp+.

           Where more that one internet provider share a single interface[5], the provider is
           specified by including the provider name or number in parentheses:


           In that case, you will want to specify the interfaces'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[6](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[7](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[7](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.

       SOURCE (Formerly called SUBNET) - {interface[:exclusion]|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).

           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[6](5))). Note that a colon (":") must appear between an
           interface name and the exclusion;

           Example: eth1:!,

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

       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[3](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. 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.



           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 (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.

       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/GROUP (Optional) - [!][user-name-or-number][:group-name-or-number][+program-name]
           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.


       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:

                       #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 eth0:0.

                       #INTERFACE              SOURCE          ADDRESS

       Example 5:
           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   PORT(S)
                       eth0 tcp     smtp

               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-ipsets(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-secmarks(5), shorewall-tcclasses(5), shorewall-tcdevices(5),
       shorewall-tcrules(5), shorewall-tos(5), shorewall-tunnels(5), shorewall-zones(5)


        1. shorewall-route_rules

        2. shorewall-tcrules

        3. shorewall.conf

        4. shorewall-interfaces

        5. more that one internet provider share a single interface

        6. shorewall-exclusion

        7. shorewall-nat

[FIXME: source]                             12/13/2011                          SHOREWALL-MASQ(5)