You don't have to have a good memory to remember a time when only large
organizations could afford to have a number of computers networked together
by a LAN. Today network technology has dropped so much in
price that two things have happened. First, LANs are now
commonplace, even in many household environments. Certainly many Linux
users will have two or more computers connected by some Ethernet.
Second, network resources, particularly IP addresses, are now
a scarce resource and while they used to be free, they are now being bought
Most people with a LAN will probably also want an Internet
connection that every computer on the LAN can use. The IP routing rules
are quite strict in how they deal with this situation. Traditional solutions
to this problem would have involved requesting an IP network address, perhaps
a class C address for small sites, assigning each host on the LAN an
address from this network and using a router to connect the LAN to the
In a commercialized Internet environment, this is quite an
expensive proposition. First, you'd be required to pay for the network
address that is assigned to you. Second, you'd probably have to pay your
Internet Service Provider for the privilege of having a suitable route
to your network put in place so that the rest of the Internet knows how to
reach you. This might still be practical for companies, but domestic
installations don't usually justify the cost.
Fortunately, Linux provides an answer to this dilemma.
This answer involves a component of a group of advanced networking features
called Network Address Translation (NAT).
NAT describes the process of modifying the network
addresses contained with datagram headers while they are in transit. This
might sound odd at first, but we'll show that it is ideal for solving
the problem we've just described and many have encountered. IP masquerade
is the name given to one type of network address translation
that allows all of the hosts on a private network to use the Internet at the
price of a single IP address.
IP masquerading allows you to use a private (reserved) IP network
address on your LAN and have your Linux-based router perform some
clever, real-time translation of IP addresses and ports. When it
receives a datagram from a computer on the LAN, it takes note of the
type of datagram it is, “TCP,” “UDP,”
“ICMP,” etc., and modifies the datagram so that it looks
like it was generated by the router machine itself (and remembers that
it has done so). It then transmits the datagram onto the Internet with
its single connected IP address. When the destination host receives
this datagram, it believes the datagram has come from the routing
host and sends any reply datagrams back to that address. When the
Linux masquerade router receives a datagram from its Internet
connection, it looks in its table of established masqueraded
connections to see if this datagram actually belongs to a computer on
the LAN, and if it does, it reverses the modification it did on the
forward path and transmits the datagram to the LAN computer.
A simple example is illustrated in Figure 11-1.
Figure 11-1. A typical IP masquerade configuration
We have a small Ethernet network using one of the reserved network addresses.
The network has a Linux-based masquerade router providing access to the
Internet. One of the workstations on the network (192.168.1.3) wishes to
establish a connection to the remote host 126.96.36.199 on port 8888. The
workstation routes its datagram to the masquerade router, which identifies
this connection request as requiring masquerade services. It accepts the
datagram and allocates a port number to use (1035), substitutes its own
IP address and port number for those of the originating host, and transmits
the datagram to the destination host. The destination host believes it has
received a connection request from the Linux masquerade host and generates
a reply datagram. The masquerade host, upon receiving this datagram,
finds the association in its masquerade table and reverses the substution
it performed on the outgoing datagram. It then transmits the reply datagram
to the originating host.
The local host believes it is speaking directly to the remote host. The
remote host knows nothing about the local host at all and believes it has
received a connection from the Linux masquerade host. The Linux masquerade
host knows these two hosts are speaking to each other, and on what ports,
and performs the address and port translations necessary to allow
This might all seem a little confusing, and it can be, but it works and
is really quite simple to configure. So don't worry if you don't understand
all the details yet.