ipnat(5)

NAME

ipnat, ipnat.conf - IP NAT file format

DESCRIPTION

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

ipmap :: = mapblock | redir | map .

map ::= mapit ifname lhs "->" dstipmask [ mapicmp | map

port | 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 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
(255.255.255.254) 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.

COMMANDS

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 exter

nal IP address and an internal IP address and

map-block

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

MATCHING

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 10.1.0.0/16 -> 201.2.3.4/32

or as

map de0 from 10.1.0.0/16 to any -> 201.2.3.4/32

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

TRANSLATION

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 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 mapblock 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.

ICMPIDMAP

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, information 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 important
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.

KERNEL PROXIES

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

TRANSPARENT PROXIES

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

LOAD-BALANCING

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 203.1.2.3/32 port 80 -> 203.1.2.3,203.1.2.4 port

80 tcp

This would send alternate connections to either 203.1.2.3
or 203.1.2.4. In scenarios where the load is being spread
amongst a larger set of servers, you can use:

rdr le0 203.1.2.3/32 port 80 -> 203.1.2.3,203.1.2.4 port

80 tcp round-robin
rdr le0 203.1.2.3/32 port 80 -> 203.1.2.5 port 80 tcp

round-robin

In this case, a connection will be redirected to
203.1.2.3, then 203.1.2.4 and then 203.1.2.5 before going
back to 203.1.2.3. 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.

EXAMPLES

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 209.1.2.0 through the ppp0
interface, the following would be used:

map ppp0 10.0.0.0/8 -> 209.1.2.0/24

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 10.0.0.0/8 -> 209.1.2.0/24 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 10.0.0.0/8 -> 209.1.2.0/24 portmap tcp/udp

1025:65000
map ppp0 10.0.0.0/8 -> 209.1.2.0/24

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 172.192.0.0/16 -> 209.1.2.0/24 portmap tcp/udp

auto

which would result in each IP address being given a small
range of ports to use (252). In all cases, the new port
number that is used is deterministic. That is, port X
will always map to port Y. WARNING: It is not advisable
to use the auto feature if you are map'ing to a /32 (i.e.
0/32) because the NAT code will try to map multiple hosts
to the same port number, outgoing and ultimately this will
only succeed for one of them. 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 172.192.0.0/16 -> 209.1.2.0/24 ports auto

For example, this would result in 172.192.0.0/24 being
mapped to 209.1.2.0/32 with each address, from 172.192.0.0
to 172.192.0.255 having 252 ports of its own. As opposed
to the above use of map, if for some reason the user of
(say) 172.192.0.2 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. /dev/ipnat
/etc/services
/etc/hosts

SEE ALSO

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

IPNAT(5)