ipmap :: = mapblock | redir | map .
map ::= mapit ifname ipmask "->" ipmask [ mapport ] .
mapblock ::= "map-block" ifname ipmask "->" ipmask [ ports ] .
redir ::= "rdr" ifname [ fromspec ] ipmask "->" ip [ ports ] [ tcpudp ] .
ports ::= "ports" numports | "auto" .
mapit ::= "map" | "bimap" .
ipmask ::= ip "/" bits | ip "/" mask | ip "netmask" mask .
mapport ::= "portmap" tcpudp portnumber ":" portnumber .
fromspec ::= "from" ip "/" ipmask .
tcpudp ::= "tcp" | "udp" | "tcp/udp" .
portnumber ::= number { numbers } | "auto" .
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.
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. In the case of redirects, it is also possible to select packets on a source address basis using the from keyword, as well as the manditory destination port. The packet matching part of the rule is to the left of the "->" in each rule.
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.
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