ipnat, ipnat.conf − IP NAT file format


The format for files accepted by ipnat is described by the following grammar:

ipmap :: = mapblock | redir | map .

map ::= mapit ifname ipmask "->" dstipmask [ mapport ] mapoptions.
map ::= mapit ifname fromto "->" dstipmask [ mapport ] mapoptions.
mapblock ::= "map-block" ifname ipmask "->" ipmask [ ports ] mapoptions.
redir ::= "rdr" ifname ipmask dport "->" ip [ "," ip ] rdrport rdroptions .

dport ::= "port" number [ "-" number ] .
ports ::= "ports" number | "auto" .
rdrport ::= "port" number .
mapit ::= "map" | "bimap" .
fromto ::= "from" object "to" object .
ipmask ::= ip "/" bits | ip "/" mask | ip "netmask" mask .
dstipmask ::= ipmask | "range" ip "-" ip .
mapport ::= "portmap" tcpudp portspec .
mapoptions ::= [ tcpudp ] [ "frag" ] [ age ] [ clamp ] .
rdroptions ::= [ tcpudp | protocol ] [ rr ] [ "frag" ] [ age ] [ clamp ] .

object :: = addr [ port-comp | port-range ] .
addr :: = "any" | nummask | host-name [ "mask" ipaddr | "mask" hexnumber ] .
port-comp :: = "port" compare port-num .
port-range :: = "port" port-num range port-num .

rr ::= "round-robin" .
age ::= "age" decnumber [ "/" decnumber ] .
clamp ::= "mssclamp" decnumber .
tcpudp ::= "tcp/udp" | "tcp" | "udp" .

protocol ::= protocol-name | decnumber .
nummask ::= host-name [ "/" number ] .
portspec ::= "auto" | number ":" number .
ifname ::= ’A’ - ’Z’ { ’A’ - ’Z’ } numbers .

number ::= numbers [ number ] .
numbers ::= ’0’ | ’1’ | ’2’ | ’3’ | ’4’ | ’5’ | ’6’ | ’7’ | ’8’ | ’9’ .

In addition to this, # is used to mark the start of a comment and may appear at the end of a line with a NAT rule (as described above) or on its own lines. Blank lines are ignored.

For standard NAT functionality, a rule should start with map and then proceeds to specify the interface for which outgoing packets will have their source address rewritten.

Packets which will be rewritten can only be selected by matching the original source address. A netmask must be specified with the IP address.

The address selected for replacing the original is chosen from an IP#/netmask pair. A netmask of all 1’s indicating a hostname is valid. A netmask of 31 1’s ( is considered invalid as there is no space for allocating host IP#’s after consideration for broadcast and network addresses.

When remapping TCP and UDP packets, it is also possible to change the source port number. Either TCP or UDP or both can be selected by each rule, with a range of port numbers to remap into given as port-number:port-number.


There are four commands recognised by IP Filter’s NAT code:


that is used for mapping one address or network to another in an unregulated round robin fashion;


that is used for redirecting packets to one IP address and port pair to another;


for setting up bidirectional NAT between an external IP address and an internal IP address and


which sets up static IP address based translation, based on a algorithm to squeeze the addresses to be translated into the destination range.


For basic NAT and redirection of packets, the address subject to change is used along with its protocol to check if a packet should be altered. The packet matching part of the rule is to the left of the "->" in each rule.

Matching of packets has now been extended to allow more complex compares. In place of the address which is to be translated, an IP address and port number comparison can be made using the same expressions available with ipf. A simple NAT rule could be written as:

map de0 ->

or as

map de0 from to any ->

For even greater control, one may negate either of the "from" or "to" clauses with a preceding exclamation mark ("!"). Please note that one may not use a negated "from" within a map rule or a negated "to" within a rdr rule. Such a rule might look like the following:

+map de0 from ! to ->

Only IP address and port numbers can be compared against. This is available with all NAT rules.


At the end of each rule, a number of qualifiers can be used to change how the rule works. They are as follows:

A specific protocol may be given either by its name (as found in /etc/protocols) or its number. A special case for supporting both TCP and UDP is allowed with the name tcp/udp.


Once a rule with this term has been successfully used, it is put at the bottom of the list of those available so that each one will get used, in turn, in a list of matching left hand sides.


This qualifier is currently has no impact on NAT operation.


If more refined timeouts are required than those available globally for NAT settings, this allows you to set them for non-TCP use.


To the right of the "->" is the address and port specification which will be written into the packet providing it has already successful matched the prior constraints. The case of redirections (rdr) is the simplest: the new destination address is that specified in the rule. For map rules, the destination address will be one for which the tuple combining the new source and destination is known to be unique. If the packet is either a TCP or UDP packet, the destination and source ports come into the equation too. If the tuple already exists, IP Filter will increment the port number first, within the available range specified with portmap and if there exists no unique tuple, the source address will be incremented within the specified netmask. If a unique tuple cannot be determined, then the packet will not be translated. The map-block is more limited in how it searches for a new, free and unique tuple, in that it will used an algorithm to determine what the new source address should be, along with the range of available ports - the IP address is never changed and nor does the port number ever exceed its alloted range.


IP Filter comes with a few, simple, proxies built into the code that is loaded into the kernel to allow secondary channels to be opened without forcing the packets through a user program.


True transparent proxying should be performed using the redirect (rdr) rules directing ports to localhost ( with the proxy program doing a lookup through /dev/ipnat to determine the real source and address of the connection.


Two options for use with rdr are available to support primitive, round-robin based load balancing. The first option allows for a rdr to specify a second destination, as follows:

rdr le0 port 80 ->, port 80 tcp

This would send alternate connections to either or In scenarios where the load is being spread amongst a larger set of servers, you can use:

rdr le0 port 80 ->, port 80 tcp round-robin
rdr le0 port 80 -> port 80 tcp round-robin

In this case, a connection will be redirected to, then and then before going back to In accomplishing this, the rule is removed from the top of the list and added to the end, automatically, as required. This will not effect the display of rules using "ipnat -l", only the internal application order.


This section deals with the map command and its variations.

To change IP#’s used internally from network 10 into an ISP provided 8 bit subnet at through the ppp0 interface, the following would be used:

map ppp0 ->

The obvious problem here is we’re trying to squeeze over 16,000,000 IP addresses into a 254 address space. To increase the scope, remapping for TCP and/or UDP, port remapping can be used;

map ppp0 -> portmap tcp/udp 1025:65000

which falls only 527,566 ’addresses’ short of the space available in network 10. If we were to combine these rules, they would need to be specified as follows:

map ppp0 -> portmap tcp/udp 1025:65000
map ppp0 ->

so that all TCP/UDP packets were port mapped and only other protocols, such as ICMP, only have their IP# changed. In some instances, it is more appropriate to use the keyword auto in place of an actual range of port numbers if you want to guarantee simultaneous access to all within the given range. However, in the above case, it would default to 1 port per IP address, since we need to squeeze 24 bits of address space into 8. A good example of how this is used might be:

map ppp0 -> portmap tcp/udp auto

which would result in each IP address being given a small range of ports to use (252). The problem here is that the map directive tells the NAT code to use the next address/port pair available for an outgoing connection, resulting in no easily discernible relation between external addresses/ports and internal ones. This is overcome by using map-block as follows:

map-block ppp0 -> ports auto

For example, this would result in being mapped to with each address, from to having 252 ports of its own. As opposed to the above use of map, if for some reason the user of (say) wanted 260 simultaneous connections going out, they would be limited to 252 with map-block but would just move on to the next IP address with the map command.




ipnat(4), hosts(5), ipf(5), services(5), ipf(8), ipnat(8)