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" 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 .
mapport ::= "portmap" tcpudp portspec .
mapoptions ::= [ tcpudp ] [ "frag" ] [ age ] [ clamp ] .
rdroptions ::= [ tcpudp ] [ 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" | protocol .
protocol ::= protocol-name | decnumber .
nummask ::= host-name [ "/" decnumber ] .
portspec ::= "auto" | portnumber ":" portnumber .
portnumber ::= number { numbers } .
ifname ::= 'A' - 'Z' { 'A' - 'Z' } numbers .
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 (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-num-
ber:port-number.
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
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.
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
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 10.1.0.0/16 ! to 10.1.0.0/16 -> 201.2.3.4/32
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:
protocol
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.
round-robin
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.
frag This qualifier is currently has no impact on NAT operation.
age 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 specificaton which
will be written into the packet providing it has already successful
matched the prior constraints. The case of redirections (rdr) is the
simpliest: 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 (127.0.0.1) 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 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.
This section deals with the map command and it's 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 instaces, 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). 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 discernable 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
ipnat(4), hosts(5), ipf(5), services(5), ipf(8), ipnat(8)
IPNAT(5)
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