Provided by: shorewall_5.1.12.2-1_all 
NAME
snat - Shorewall SNAT/Masquerade definition file
SYNOPSIS
/etc/shorewall[6]/snat
DESCRIPTION
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 It superseded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.
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 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.
ACTION
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:
action[+][(parameter,...)][:level]
where action is an action declared in shorewall-actions(5)[5] with the nat option. See
www.shorewall.net/Actions.html[6] for further information.
CONTINUE[+]:level
Causes matching packets to be exempted from any following rules in the file.
LOG:level
Added in Shorewall 5.1.9. Simply log the packet and continue with the next rule.
MASQUERADE[+][([lowport[-highport]][random])][:level]
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.
NFLOG[(nflog-parameters)]
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.
SNAT[+]([address-or-address-range][:lowport[-highport]][:random][:persistent]|detect)[: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
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.
Example: 206.124.146.177-206.124.146.180
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.
Examples:
192.0.2.4:5000-6000
:4000-5000
You may also specify a single port number, which will be assigned to the outgoing connection, if
possible.
ULOG[(ulog-parameters)]
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 shorewall-logging(5)[4].
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 - {interface[:digit][,interface[:digit]]...|$FW}[:[dest-address[,dest-address]...[exclusion]]
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:
eth0(Avvanta)
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:
eth0(Avvanta):
eth2::192.0.2.32/27
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 protocols.
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.
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
yes
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
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.
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).
Examples:
joe
program must be run by joe
:kids
program must be run by a member of the 'kids' group
!:kids
program must not be run by a member of the 'kids' group
+upnpd
#program named upnpd
Important
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 >
/proc/net/nf_condition/switch-name
To turn it off again:
echo 0 >
/proc/net/nf_condition/switch-name
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.
EXAMPLES
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 192.168.0.0/24.
Your entry in the file will be:
#ACTION SOURCE DEST
MASQUERADE 192.168.0.0/24 eth0
IPv4 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:
#ACTION SOURCE DEST
MASQUERADE 192.168.0.0/24 eth0
MASQUERADE 192.168.1.0/24 eth0
IPv4 Example 3:
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.
#ACTION SOURCE DEST
SNAT(206.124.146.176) 192.168.1.0/24 eth0:0
IPv4 Example 4:
You want all outgoing SMTP traffic entering the firewall from 172.20.1.0/29 to be sent from eth0 with
source IP address 206.124.146.177. You want all other outgoing traffic from 172.20.1.0/29 to be sent
from eth0 with source IP address 206.124.146.176.
#INTERFACE SOURCE ADDRESS PROTO DPORT
eth0 172.20.1.0/29 206.124.146.177 tcp smtp
eth0 172.20.1.0/29 206.124.146.176
#ACTION SOURCE DEST PROTO PORT
SNAT(206.124.146.177) 172.20.1.0/29 eth0 tcp smtp
SNAT(206.124.146.176) 172.20.1.0/29 eth0
Warning
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 206.124.146.177.
#ACTION SOURCE DEST
SNAT(206.124.146.177) - eth0:+myset[dst]
IPv4 Example 6:
SNAT outgoing connections on eth0 from 192.168.1.0/24 in round-robin fashion between addresses
1.1.1.1, 1.1.1.3, and 1.1.1.9 (Shorewall 4.5.9 and later).
/etc/shorewall/tcrules:
#ACTION SOURCE DEST PROTO DPORT SPORT USER TEST
1-3:CF 192.168.1.0/24 eth0 ; state=NEW
/etc/shorewall/snat:
#ACTION SOURCE DEST
SNAT(1.1.1.1) 192.168.1.0/24 eth0 { mark=1:C }
SNAT(1.1.1.3) 192.168.1.0/24 eth0 { mark=2:C }
SNAT(1.1.1.9) 192.168.1.0/24 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.
/etc/shorewall/snat:
#ACTION SOURCE DEST
SNAT(2001:470:a:227::1) ::/0 sit1 { probability=0.50 }
SNAT(2001:470:a:227::2) ::/0 sit
FILES
/etc/shorewall/snat
/etc/shorewall6/snat
SEE ALSO
http://www.shorewall.net/configuration_file_basics.htm#Pairs[13]
shorewall(8)
NOTES
1. shorewall-masq
http://www.shorewall.net/manpages/shorewall-masq.html
2. shorewall-rtrules
http://www.shorewall.net/manpages/shorewall-rtrules.html
3. shorewall-mangle
http://www.shorewall.net/manpages/shorewall-mangle.html
4. shorewall-logging(5)
http://www.shorewall.netshorewall-logging.html
5. shorewall-actions(5)
http://www.shorewall.net/manpages/shorewall-actions.html
6. www.shorewall.net/Actions.html
http://www.shorewall.net/Actions.html
7. shorewall.conf
http://www.shorewall.net/manpages/shorewall.conf.html
8. shorewall-nat
http://www.shorewall.net/manpages/shorewall-nat.html
9. shorewall-interfaces
http://www.shorewall.net/manpages/shorewall-interfaces.html
10. more that one internet provider share a single interface
http://www.shorewall.net/4.4/MultiISP.html#Shared
11. shorewall-exclusion
http://www.shorewall.net/manpages/shorewall-exclusion.html
12. shorewall-rules(5)
http://www.shorewall.net/manpages/shorewall-rules.html
13. http://www.shorewall.net/configuration_file_basics.htm#Pairs
http://www.shorewall.net/configuration_file_basics.htm#Pairs