.Dd October 10, 1998
.Dt IPNAT 1
.Os
.Sh NAME
.Nm ipnat
.Nd User interface to the NAT
.Sh SYNOPSIS
.Nm ipnat
.Op Fl CFlnrsv
.Op Fl f Ar filename
.Sh DESCRIPTION
.Nm ipnat
provides control over the kernel's network address translation (NAT). The NAT remaps IP addresses from one range to another. In other words, when properly configured on a gateway, the NAT provides internet access to connected computers lacking officially assigned IP addresses. It is discussed in RFC 1631.
.Pp
Options are as follows:
.Bl -tag -width Ds
.It Fl C
Delete all entries in the NAT list.
.It Fl F
Flush all active mappings from the NAT table. 
.It Fl l
Display the current entries and mappings.
.It Fl n
Do not alter the NAT table.
.It Fl r
Remove, rather than add, entries specified in the rule list. 
.It Fl s
Display statistics.
.It Fl v
Verbosity, displays detailed information pertaining to rule processing. 
.El
.Pp
Certain configuration requirements must be met before
.Nm ipnat
will work. These are listed in
.Pa /usr/share/ipf/nat.2 .
.Pp
.Nm ipnat
operates on a list of rules, specified by 
.Ar filename .
This file is typically 
.Pa /etc/ipnat.rules ;
stdin is represented by "\-". Each rule is parsed, then sequentially added to
the kernel's internal NAT list. Like 
.Xr ipf 1 ,
if an entry contradicts another previously added, the newer will take precedence. 
.Pp
Comments (beginning with a ``#'') and blank lines are ignored as
.Nm ipnat
parses the file. Entries may be separated by spaces or tabs. Each rule must begin with either
.Em map
or
.Em rdr .  
.Pp
.Em map 
tells the NAT how a range of addresses should be translated. The entries use the following format:
.Pp
.Bd -unfilled -offset indent -compact
map ifname internal/mask -> external/mask options
.Ed
.Pp
The
.Em ifname 
field is the interface to which packets are sent. A gateway with a PPP link would probably use ``ppp0'' or ``tun0'', while an ethernet connection would instead have the name of its device. 
.Pp
The address range of the LAN goes in the
.Em internal
field. This is usually one of the three blocks of address space the Internet Assigned Numbers Authority has allocated for private networks (RFC 1597):
.Pp
.Bd -unfilled -offset indent -compact
10.0.0.0    - 10.255.255.255
172.16.0.0  - 172.16.255.255
192.168.0.0 - 192.168.255.255
.Ed
.Pp
The
.Em external
address is the offically assigned IP number of the gateway or network.
.Pp
.Em mask
is the netmask of the address. This mask is 32 bits long, and is divided into four 8 bit numbers.
.Pp
.Bd -unfilled -offset indent -compact
11111111.0.0.0				Class A - 8 bits set.
11111111.11111111.0.0 		Class B - 16 bits set.
11111111.11111111.11111111.0	Class C - 24 bits set.
.Ed
.Pp
The number of bits set in the mask is placed following the IP address.
.Pp
Both
.Em internal 
and 
.Em external
may be an actual IP address, a hostname, or the name of an interface. If it is a network number, however, a problem may arise. For example:
.Pp
.Bd -unfilled -offset indent -compact
map ppp0 10.0.0.0/8 -> 209.1.2.0/24
.Ed
.Pp
16,000,000 IP addresses are being squeezed into an address space of only 254. This is solved by the
.Em portmap 
option, which remaps ports instead of IP addresses. The protocol is specified by following the option with either
.Em tcp , 
.Em udp ,
.Em tcp/udp ,
or 
.Em tcpudp
(the last two have the same effect). The syntax to assign a range of ports is ``portnumber:portnumber''. This looks like:
.Pp
.Bd -unfilled -offset indent -compact 
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
.Ed 
.Pp
That will cut the number down from ~16,000,000 addresses short to only 527,566.
.Pp
.Em rdr
tells the NAT how to redirect incoming packets. It is useful if one wishes to redirect a connection through a proxy, or to another box on the private network. The format of this directive is:
.Pp
rdr ifname external/mask port service -> internal port service protocol
.Pp
This setup is best described by an example of an actual entry:
.Pp
.Bd -unfilled -offset indent -compact
rdr xl0 0.0.0.0/0 port 25 -> 204.213.176.10 port smtp 
.Ed
.Pp
This redirects all smtp packets received on xl0 to 204.213.176.10, port 25. A netmask is not needed on the
.Em internal
address; it is always 32. The 
.Em external
and 
.Em internal
fields, similar to the
.Em map
directive, may be actual addresses, hostnames, or interfaces. Likewise, the 
.Em service
field may be the name of a service, or a port number. The 
.Em protocol
of the service may be selected by appending 
.Em tcp , 
.Em udp ,
.Em tcp/udp ,
or
.Em tcpudp
(the last two have the same effect) to the end of the line. TCP is the default.
.Sh FILES
.Bl -tag -width /usr/share/ipf/nat.1 -compact
.It Pa /dev/ipnat
.It Pa /usr/share/ipf/nat.1
Example rules.
.It Pa /usr/share/ipf/nat.2
System requirements for use of the NAT.
.It Pa /etc/ipnat.rules
Actual rule list.
.Sh SEE ALSO
.Xr ipnat 4 ,
.Xr ipnat 5 ,
.Xr ipf 8 ,
.Xr ipf 4
.br
http://coombs.anu.edu.au/ipfilter/