Provided by: shorewall_5.2.3.4-1_all bug


       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       superseded shorewall-masq[1](5) in Shorewall 5.0.14.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[2](5) or PREROUTING entries in shorewall-mangle[3](5) to do that.

       The columns in the file are as follows.

           Defines the type of rule to generate. Beginning with Shorewall 5.1.9, with the
           exception of NFLOG and ULOG, the action may be followed by a colon (":") and a log
           level (see shorewall-logging(5)[4]).

           Choices for ACTION are:

               where action is an action declared in shorewall-actions(5)[5] with the nat option.
               See[6] for further information.

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

               Added in Shorewall 5.1.9. Simply log the packet and continue with the next rule.

               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 only lowport is given, that port will be assigned, if possible. 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.

               Added in Shorewall 5.1.9. Queues matching packets to a back end logging daemon via
               a netlink socket then continues to the next rule. See shorewall-logging(5)[4].

               The nflog-parameters are a comma-separated list of up to 3 numbers:

               •   The first number specifies the netlink group (0-65535). If omitted (e.g.,
                   NFLOG(,0,10)) then a value of 0 is assumed.

               •   The second number specifies the maximum number of bytes to copy. If omitted, 0
                   (no limit) is assumed.

               •   The third number specifies the number of log messages that should be buffered
                   in the kernel before they are sent to user space. The default is 1.

               NFLOG is similar to LOG:NFLOG[(nflog-parameters)], except that the log level is
               not changed when this ACTION is used in an action or macro body and the invocation
               of that action or macro specifies a log level.

               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[7](5) then Shorewall will automatically add this address to the
               INTERFACE named in the first column (IPv4 only).

               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.

               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.



               You may also specify a single port number, which will be assigned to the outgoing
               connection, if possible.

               IPv4 only. Added in Shorewall 5.1.9. Queues matching packets to a back end logging
               daemon via a netlink socket then continues to the next rule. See

               Similar to LOG:ULOG[(ulog-parameters)], except that the log level is not changed
               when this ACTION is used in an action or macro body and the invocation of that
               action or macro specifies a log level.

           Normally Masq/SNAT rules are evaluated after those for one-to-one NAT (defined in
           shorewall-nat[8](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[8](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. Unless you want to perform SNAT in the INPUT chain (see DEST
           below), if you give the name of an interface (deprecated), 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 -
           Outgoing interfaces and destination networks. Multiple interfaces may be listed when
           the ACTION is MASQUERADE, but this is usually just your internet interface. If
           ADD_SNAT_ALIASES=Yes in shorewall.conf[7](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.

           Beginning with Shorewall 5.1.12, SNAT may be performed in the nat table's INPUT chain
           by specifying $FW rather than one or more interfaces.

           Each interface must match an entry in shorewall-interfaces[9](5). Shorewall allows
           loose matches to wildcard entries in shorewall-interfaces[9](5). For example, ppp0 in
           this file will match a shorewall-interfaces[9](5) entry that defines ppp+.

           Where more that one internet provider share a single interface[10], 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[11](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. See shorewall-rules(5)[12] for details.

           Beginning with Shorewall 4.5.12, this column can accept a comma-separated list of

           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 >
           To turn it off again:
               echo 0 >
           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.


       IPv4 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

       IPv4 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

       IPv4 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

       IPv4 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!

       IPv4 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]

       IPv4 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 }

       IPv6 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 2001:470:b:787::0/64

           Your entry in the file will be:

                       #ACTION      SOURCE                  DEST
                       MASQUERADE   2001:470:b:787::0/64    eth0

       IPv6 Example 2:
           Your sit1 interface has two public IP addresses: 2001:470:a:227::1 and
           2001:470:b:227::1. You want to use the iptables statistics match to masquerade
           outgoing connections evenly between these two addresses.


                      #ACTION                      SOURCE     DEST
                      SNAT(2001:470:a:227::1)      ::/0       sit1              { probability=0.50 }
                      SNAT(2001:470:a:227::2)      ::/0       sit







        1. shorewall-masq

        2. shorewall-rtrules

        3. shorewall-mangle

        4. shorewall-logging(5)

        5. shorewall-actions(5)


        7. shorewall.conf

        8. shorewall-nat

        9. shorewall-interfaces

       10. more that one internet provider share a single interface

       11. shorewall-exclusion

       12. shorewall-rules(5)