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Re: iptables port forwarding



* Vesa Salento (vsalento@cc.hut.fi) [021111 06:14]:
> Hi,
> 
> I'd like to know whether it is possible to do this kind of thing with
> iptables. I'd like to configure my firewall/router so that all the packets
> to the port 80 from the Internet (from specific addresses) are forwarded
> automatically to the ssh-port on another host on the Internet (not on my
> home lan).

No, not that easily.  NAT cannot solve this problem; you'll need a proxy
of some sort.  The reason is that the return packets from the actual
destination host would never be un-NAT-ed; you'd get something like
this:

B  => SYN => A
A* => SYN => C  * A forwards the packet with the source address as B
C  => SYN/ACK => B
B  => RST => C

Where A is your firewall, C is the destination (the one running sshd)
and B is the source.  When C tries to reply to B, B ignores it and
continues to wait for replies from A, which never come.  The way it
works when using DNAT to a host on your LAN (not elsewhere on the
internet) is that C's return packet (the SYN/ACK above) passes back
through A, who rewrites the source address to A, and C happily thinks
it's communicating with A directly, with no knowledge that A is actually
just forwarding and rewriting packets.

I have a feeling I didn't explain that too well; ask if it doesn't make
any sense and I (or someone else) can try again.

good times,
Vineet
-- 
http://www.doorstop.net/
-- 
"Extremism in the defense of liberty is no vice.
Moderation in the pursuit of justice is no virtue."  -- Barry Goldwater 

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