Re: tlug: It works!

[Jim Tittsler <jwt-tlug@example.com>]

On Fri, Oct 09, 1998 at 12:35:10PM +0000, Darren Cook wrote:
> Am I right in thinking those extra machines (eg. 192.168.1.2) can't do web
> browsing, ftp, connect to a POP server, etc? So the only external machine
> that can see them is the one with the two network cards?

With Linux's IP Masquerading, it (almost*) all works transparently.  The
server keeps track of which of the private network machines made which
request/connection, and automagically routes reply packets back to the
appropriate private network host.  Things like web browsing, POP, IMAP,
telnet, etc. all work transparently.  The private network host thinks it is
talking directly to its destination host, but the masquerading host acting
as the gateway, is rewriting the addresses so it looks like the requests from
the outside world are coming from it.

(*) It gets trickier with protocols like FTP that set up a connection on one
port and then perform communications on another.  There are modules that
understand many of the "interesting" protocols and will keep track of the
necessary private connections.  There is a catch-all program that attempts
to handle still other odd protocols that may not have a specific module to
support them.

> What I'm thinking is that when a web server on the net gets a request from
> 192.168.1.2 how will it know where to send the reply to? Or am I
> misunderstanding something (again :-)?

The server will have rewritten the address to its real address, when the
reply comes back, it puts the private address of the requesting machine
back.

IP Masquerade (and Network Address Translation, the larger category) info:
  http://w3.clat.hi-tech.ac.jp/LDP/HOWTO/mini/IP-Masquerade.html
  http://ipmasq.home.ml.org/
  http://linas.org/linux/load.html

(This feature is also present in many ISDN routers, such as the MN128-SOHO
family.)
-- 
Jim Tittsler, Tokyo   ICQ: 5981586


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