shorewall-snat(5)
NAME
snat - Shorewall SNAT/Masquerade definition file
SYNOPSIS
/etc/shorewall/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 It superseded shorewall-masq[1](5) in Shorewall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. Choices are:
MASQUERADE[+][([lowport-highport][random])]
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 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.
SNAT[+]([address-or-address-range][:lowport-highport][:random][:persistent]|detect|
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[4](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.
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. This feature replaces the SAME modifier which was removed
from Shorewall in version 4.4.0.
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
CONTINUE[+]
Causes matching packets to be exempted from any following rules
in the file.
action[+][(parameter,...)]
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.
Normally Masq/SNAT rules are evaluated after those for one-to-one
NAT (defined in shorewall-nat[7](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[7](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 (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).
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]][:[dest-address[,dest-address]...[exclusion]]}
Outgoing interface. This is usually your internet interface. If
ADD_SNAT_ALIASES=Yes in shorewall.conf[4](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[8](5).
Shorewall allows loose matches to wildcard entries in
shorewall-interfaces[8](5). For example, ppp0 in this file will
match a shorewall-interfaces[8](5) entry that defines ppp+.
Where more that one internet provider share a single interface[9],
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[10](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.
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
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
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
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
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!
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]
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 }
Example 7:
Your eth1 has two public IP addresses: 70.90.191.121 and
70.90.191.123. You want to use the iptables statistics match to
masquerade outgoing connections evenly between these two addresses.
/etc/shorewall/snat:
#ACTION SOURCE DEST
SNAT(70.90.191.121) - eth1 { probability=.50 }
SNAT(70.90.191.123) - eth1
FILES
/etc/shorewall/snat
SEE ALSO
http://www.shorewall.net/configuration_file_basics.htm#Pairs[11] shorewall(8), shorewall-accounting(5), shorewall-actions(5), shorewall-blacklist(5), shorewall-exclusion(5), shorewall-hosts(5), shorewall_interfaces(5), shorewall-ipsets(5), shorewall-maclist(5), shorewall-nat(5), shorewall-netmap(5), shorewall-params(5), shorewall-policy(5), shorewall-providers(5), shorewall-proxyarp(5), shorewall-rtrules(5), shorewall-routestopped(5), shorewall-rules(5), shorewall.conf(5), shorewall-secmarks(5), shorewall-tcclasses(5), shorewall-tcdevices(5), shorewall-mangle(5), shorewall-tos(5), shorewall-tunnels(5), shorewall-zones(5)
NOTES
1. shorewall-masq
http://www.shorewall.netshorewall-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.conf
http://www.shorewall.net/manpages/shorewall.conf.html
5. shorewall-actions(5)
http://www.shorewall.netshorewall-actions.html
6. www.shorewall.net/Actions.html
http://www.shorewall.net/Actions.html
7. shorewall-nat
http://www.shorewall.net/manpages/shorewall-nat.html
8. shorewall-interfaces
http://www.shorewall.net/manpages/shorewall-interfaces.html
9. more that one internet provider share a single interface
http://www.shorewall.net/4.4/MultiISP.html#Shared
10. shorewall-exclusion
http://www.shorewall.net/manpages/shorewall-exclusion.html
11. http://www.shorewall.net/configuration_file_basics.htm#Pairs
http://www.shorewall.net/configuration_file_basics.htm#Pairs
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