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IP Masquerading lets you run multiple machines on a network behind a
Linux box so that all the machines (including the Linux masquerade box)
appear as a single IP address to the outside world.

The main use of masquerading is when your ISP only gives you one IP
address and wants to charge like a wounded bull for multiple IP
addresses.  Instead of paying your ISP large amounts of money for a
separate address for each of your machines, funnel them all through a
Linux box running IP masquerading.  Even when you have multiple IP
addresses, you can still use masquerading if you want to hide your
internal networks from the rest of the world.

To activate IP masquerading, compile the kernel with IP Forwarding, IP
Firewalling and IP Masquerading, the first two options must be on
before you can see the masquerade option.  Also consider using the
ipautofw and ICMP masquerading suboptions.

Some of the masq code is in the kernel, some is in modules so you have
to make zImage and make modules.  There are masq helper modules to
handle special protocols, you only need to load a helper module if you
want to use the corresponding protocol.  Helper modules have to be
explicitly loaded (usually from somewhere in /etc/rc.d), they cannot be
loaded using kerneld.  The current helper modules are ip_masq_ftp,
ip_masq_irc, ip_masq_raudio, ip_masq_cuseeme, ip_masq_vdolive,
ip_masq_quake.

All of the modules can take a parameter specifying the port they work
on - ie ftp handles connections to port 21 by default.  This parameter,
which can be ommitted to take the default port(s) makes the command
line look like this
        insmod ip_masq_raudio.o ports=7070,7071,7072
Up to 12 ports can be specified (this value can be changed if you 
recompile).

Masquerading is more of a server function than a single user function.
Using it correctly requires some knowledge of TCP, UDP, IP and a high
level understanding of some protocols.  For more details on IP
masquerading, visit 
        http://www.indyramp.com/masq/
and read the HOWTO.

There is a mailing list covering use of masqueraing, information can
be found on the indyramp web site given above - please read the basic
information before posting to the mailing list.

Other information on masquerading can be found at
        http://www.wwonline.com/~achau/ipmasq/

                                                        March 5, 1997
                                                        Keith Owens
                                                        Nigel Metheringham