Provided by:
shorewall-common_4.0.6-1_all 
NAME
masq - Shorewall Masquerade/SNAT definition file
SYNOPSIS
/etc/shorewall/masq
DESCRIPTION
Use this file to define dynamic NAT (Masquerading) and to define Source
NAT (SNAT).
Warning
The entries in this file are order-sensitive. The first entry
that matches a particular connection will be the one that is
used.
Warning
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
〈shorewall-tcrules.tcml〉 (5) to do that.
The columns in the file are as follows.
INTERFACE — [+]interface[:[digit]][:[address[,address]...[exclusion]]
Outgoing interface. This is usually your internet interface. If
ADD_SNAT_ALIASES=Yes in shorewall.conf 〈shorewall.conf.html〉
(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
〈shorewall-exclusion.html〉 (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 〈shorewall-nat.html〉 (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
〈shorewall-nat.html〉 (5).
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. 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 〈shorewall-exclusion.html〉 (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
ADDRESS (Optional) — [-|[SAME:[nodst:]][address-or-address-
range[,address-or-address-range]...][:lowport-
highport][:random]|detect|random]
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 〈shorewall.conf.html〉 (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.
Warning
Support for the SAME target is scheduled for removal from the
Linux kernel in 2008.
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 listed.
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.
reqid=number
where number is specified using setkey(8) using the
’unique:number option for the SPD level.
spi=<number>
where number is the SPI of the SA used to encrypt/decrypt
packets.
proto=ah|esp|ipcomp
IPSEC Encapsulation Protocol
mss=number
sets the MSS field in TCP packets
mode=transport|tunnel
IPSEC mode
tunnel-src=address[/mask]
only available with mode=tunnel
tunnel-dst=address[/mask]
only available with mode=tunnel
strict Means that packets must match all rules.
next 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 Value of the packet or connection mark.
mask A mask to be applied to the mark before testing.
:C Designates a connection mark. If omitted, the packet
mark’s value is tested. This option is only supported by
Shorewall-perl.
EXAMPLES
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 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
Example 2:
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
Example 3:
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
Example 4:
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
Example 5:
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
Warning
The order of the above two rules is significant!
FILES
/etc/shorewall/masq
SEE ALSO
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_routes(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)
23 November 2007 shorewall-masq(5)