Re: Forward internal packets as though they're external

[Jon Heese <netfilter@jonheese.com>]

Hmm. Nope.

I added this rule:

iptables -vt nat -A PREROUTING -p tcp --dport 6969 -s 192.168.0.0/24 \
      -d 192.168.0.100 -j SNAT --to 65.9.134.4

And I get this error:

SNAT  tcp opt -- in * out *  192.168.0.0/24  -> 192.168.0.100  tcp 
dpt:6969 to:65.9.252.78
iptables: Invalid argument

Seems like it doesn't like the "SNAT" target in the PREROUTING chain (if 
I replace it with "DROP", the rule inserts fine).  Your idea makes 
sense, Rob, but seems like something's still one step away...

Regards,
Jon Heese

/dev/rob0 wrote:
> On Thursday 2005-October-27 08:07, Jon Heese wrote:
> 
>>>Change your DNAT rule to match all the packets you want to match:
>>>
>>>iptables -vt nat -A PREROUTING -d 65.9.134.4 -p tcp --dport 6969 \
>>>    -j DNAT --to 192.168.0.100
>>
>>Except for the --verbose, that's exactly what I'm already doing to
>>DNAT everything from the outside through to castor's 6969.  This rule
>>does not seem to be catching traffic from the inside.  Do I have to
>>do something special to get internal traffic into the PREROUTING
> 
> 
> After I sent this (and went to bed :) ) I realised what must be 
> happening: it's getting the original packet there, but replies are 
> going direct to the IP of the originator.
> 
> Suppose 192.168.0.129 connects to 65.9.134.4:6969 ... the router passes 
> it faithfully on to 192.168.0.100. Now 192.168.0.100 has a packet from 
> 192.168.0.129, and it won't send the reply to 65.9.134.4. The client at 
> 192.168.0.129 is going to be confused. This cannot be a reply to my 
> 65.9.134.4:6969 connection attempt, go away, Castor.
> 
> Perhaps you need a SNAT rule in POSTROUTING:
> iptables -vt nat -A PREROUTING -p tcp --dport 6969 -s 192.168.0.0/24 \
>     -d 192.168.0.100 -j SNAT --to 65.9.134.4
> 
> A cleaner solution (not sure if applicable to BitTorrent) would be an 
> alternate DNS view, so that external clients resolve the name to 
> 65.9.134.4 whilst internal ones resolve to 192.168.0.100.