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IPNAT(5)                      File Formats Manual                     IPNAT(5)

       ipnat, ipnat.conf - IP NAT file format

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

       ipmap :: = mapblock | redir | map .

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

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

       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 .
       rdrproto ::= tcpudp | protocol .

       rr ::= "round-robin" .
       age ::= "age" decnumber [ "/" decnumber ] .
       clamp ::= "mssclamp" decnumber .
       tcpudp ::= "tcp/udp" | protocol .
       mapproxy ::= "proxy" "port" port proxy-name '/' protocol
       rdrproxy ::= "proxy" proxy-name .

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

       numbers ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' .

       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

       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-num-

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

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

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

       bimap  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

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

       map de0 ->

       or as

       map de0 from to any ->

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

       To  the  right  of the "->" is the address and port specification which
       will be written into the packet providing it has  already  successfully
       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 speci-
       fied 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 allotted range.

       ICMP  messages  can be divided into two groups: "errors" and "queries".
       ICMP errors are generated as a response of another IP packet. IP Filter
       will  take  care  that ICMP errors that are the response of a NAT-ed IP
       packet are handled properly.

       For 4 types of ICMP queries (echo request, timestamp request,  informa-
       tion request and address mask request) IP Filter supports an additional
       mapping called "ICMP id mapping". All these 4 types of ICMP queries use
       a  unique  identifier called the ICMP id. This id is set by the process
       sending the ICMP query and it is usually equal to the process  id.  The
       receiver  of  the ICMP query will use the same id in its response, thus
       enabling the sender to  recognize  that  the  incoming  ICMP  reply  is
       intended for him and is an answer to a query that he made. The "ICMP id
       mapping" feature modifies these ICMP id in a way identical  to  portmap
       for TCP or UDP.

       The  reason  that  you  might want this, is that using this feature you
       don't need an IP address per host behind the NAT box, that wants to  do
       ICMP  queries.   The  two  numbers behind the icmpidmap keyword are the
       first and the last icmp id number that can be used. There is one impor-
       tant  caveat:  if  you map to an IP address that belongs to the NAT box
       itself (notably if you have only a single public IP address), then  you
       must  ensure that the NAT box does not use the icmpidmap range that you
       specified in the map rule.  Since the ICMP id is  usually  the  process
       id,  it is wise to restrict the largest permittable process id (PID) on
       your operating system to e.g. 63999 and use the range  64000:65535  for
       ICMP id mapping. Changing the maximal PID is system dependent. For most
       BSD  derived   systems   can   be   done   by   changing   PID_MAX   in
       /usr/include/sys/proc.h and then rebuild the system.

       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.  The current state of the
       proxies is listed below, as one of three states:

       Aging - protocol is roughly understood from the time at which the proxy
              was written but it is not well tested or maintained;

       Developmental  - basic functionality exists, works most of the time but
              may be problematic in extended real use;

       Experimental - rough support for the protocol at best, may or  may  not
              work  as testing has been at best sporadic, possible large scale
              changes to the code in order to properly support the protocol.

       Mature - well tested, protocol is properly understood by the proxy;

       The currently compiled in proxy list is as follows:

       FTP - Mature

       IRC - Experimental

       rpcbind - Experimental

       H.323 - Experimental

       Real Audio (PNA) - Aging

       IPsec - Developmental

       netbios - Experimental

       R-command - Mature

       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 spec-
       ify 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 accom-
       plishing  this,  the rule is removed from the top of the list and added
       to the end, automatically, as required.  This will not effect the  dis-
       play 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 outgo-
       ing 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 hav-
       ing 252 ports of its own.  As opposed to the above use of map,  if  for
       some  reason the user of (say) wanted 260 simultaneous con-
       nections 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.
       /usr/share/examples/ipf  Directory with examples.

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