Provided by: shorewall_5.2.8-6_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superseded shorewall-masq[1](5) in Shorewall 5.0.14.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.
Beginning with Shorewall 5.2.6, the snat file supports two different formats:
1. The SPORT (source port) column is omitted. This is the default unless a "?FORMAT 2"
compiler directive is included.
2. The SPORT column immediately follows the DPORT column.
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 https://shorewall.org/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.
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|DPORT} (Optional) - {-|[!]port-name-or-number[,port-name-or-number]...|+ipset}
The column was renamed to DPORT in Shorewall 5.2.6. Beginning with that release, both
PORT and DPORT are accepted in the alternative input format,
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.
SPORT {-|[!]port-name-or-number[,port-name-or-number]...|+ipset}
FORMAT 2 only.
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.
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 randomly to addresses 1.1.1.1,
1.1.1.3, and 1.1.1.9 (Shorewall 5.0.0 and later).
/etc/shorewall/snat:
#ACTION SOURCE DEST
SNAT(1.1.1.1) 192.168.1.0/24 eth0 { probability=0.33 }
SNAT(1.1.1.3) 192.168.1.0/24 eth0 { probability=0.50 }
SNAT(1.1.1.9) 192.168.1.0/24 eth0
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
https://shorewall.org/configuration_file_basics.htm#Pairs[13]
shorewall(8)
NOTES
1. shorewall-masq
https://shorewall.org/manpages/shorewall-masq.html
2. shorewall-rtrules
https://shorewall.org/manpages/shorewall-rtrules.html
3. shorewall-mangle
https://shorewall.org/manpages/shorewall-mangle.html
4. shorewall-logging(5)
https://shorewall.org/manpages/shorewall-logging.html
5. shorewall-actions(5)
https://shorewall.org/manpages/shorewall-actions.html
6. https://shorewall.org/Actions.html
https://shorewall.org/Actions.html
7. shorewall.conf
https://shorewall.org/manpages/shorewall.conf.html
8. shorewall-nat
https://shorewall.org/manpages/shorewall-nat.html
9. shorewall-interfaces
https://shorewall.org/manpages/shorewall-interfaces.html
10. more that one internet provider share a single interface
https://shorewall.org/4.4/MultiISP.html#Shared
11. shorewall-exclusion
https://shorewall.org/manpages/shorewall-exclusion.html
12. shorewall-rules(5)
https://shorewall.org/manpages/shorewall-rules.html
13. https://shorewall.org/configuration_file_basics.htm#Pairs
https://shorewall.org/configuration_file_basics.htm#Pairs