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(5) to do that.
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(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(5).
Shorewall allows loose matches to wildcard entries in shorewall-interfaces(5). For
ppp0 in this file
will match a shorewall-interfaces(5)
entry that defines
Where more that one internet providers shares a single interface, 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(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)). 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).
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.
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
Note that a colon (":") must appear between an
interface name and the
In that example traffic from eth1 would be masqueraded unless it came from 192.168.1.4 or 184.108.40.206/27
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. 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.
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.
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.
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
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
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.
Only locally-generated connections will match if this column is non-empty.
When this column is non-empty, the rule applies 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.
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
#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 ipsec0:10.1.1.0/24 220.127.116.11/24
You want all outgoing traffic from 192.168.1.0/24 through eth0 to use source address 18.104.22.168 which is NOT the primary address of eth0. You want 22.214.171.124 to be added to eth0 with name eth0:0.
#INTERFACE SOURCE ADDRESS eth0:0 192.168.1.0/24 126.96.36.199
You want all outgoing SMTP traffic entering the firewall on eth1 to be sent from eth0 with source IP address 188.8.131.52. You want all other outgoing traffic from eth1 to be sent from eth0 with source IP address 184.108.40.206.
#INTERFACE SOURCE ADDRESS PROTO PORT(S) eth0 eth1 220.127.116.11 tcp smtp eth0 eth1 18.104.22.168
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)