Using NAT to relay traffic

Using NAT to relay traffic

[Jared Cook]

I have two servers on two different networks.  I am running a service on 
box A that I am transitioning to box B.  While I wait on DNS to 
propagate, I would like to do some iptables magic to send traffic from 
box A to box B using NAT.  For instance, when pop3 email users connect 
to box A, I would like box A to send the request to box B 
transparantly.  Is this possible?  I have had success doing port 
forwarding to the local machine, but when I specify box B as the "--to", 
it doesn't work.  Any help would be greatly appreciated.

Thanks,
Jared

Re: Using NAT to relay traffic

[Grant Taylor]

The problem that you are having when you port forward traffic from Box A to Box B is that the returning traffic comes directly from Box B to the client that sent the traffic in the first place thus you have an incorrect communications path.  Ironically I just had to work on a situation sort of similar to this one.  What I did in my situation to accomplish this was to DNAT the traffic destined to Box A over to Box B, like you have done.  You also need to SNAT the traffic leaving Box A on it's way Box B to be from Box A's IP so that when Box B replies it will reply back to Box A which will in turn reply back to the client system.  Thus you no longer have a triangle of client to Box a to Box B to client but rather client to Box A to Box B to Box A to client.  Let me know what your network config looks like if you would like me to come up with some iptables rules for you.

Reference my replies to "HELP! Transparent Proxy using bridging 2.6.9 and REDIRECT on	different subnet" thread for an example or email me and I'll try to provide more help.



Grant. . . .

Jared Cook wrote:
> I have two servers on two different networks.  I am running a service on 
> box A that I am transitioning to box B.  While I wait on DNS to 
> propagate, I would like to do some iptables magic to send traffic from 
> box A to box B using NAT.  For instance, when pop3 email users connect 
> to box A, I would like box A to send the request to box B 
> transparantly.  Is this possible?  I have had success doing port 
> forwarding to the local machine, but when I specify box B as the "--to", 
> it doesn't work.  Any help would be greatly appreciated.
> 
> Thanks,
> Jared
> 

RE: Using NAT to relay traffic

[Sietse van Zanen]

 Indeed.

It would make much more sense, that if you want a fast turnover, to lower
the TTL of your DNS records to a few seconds.

-----Original Message-----
From: netfilter-bounces@lists.netfilter.org
[mailto:netfilter-bounces@lists.netfilter.org] On Behalf Of Grant Taylor
Sent: 24 March 2005 01:37
To: Jared Cook
Cc: netfilter@lists.netfilter.org
Subject: Re: Using NAT to relay traffic

The problem that you are having when you port forward traffic from Box A to
Box B is that the returning traffic comes directly from Box B to the client
that sent the traffic in the first place thus you have an incorrect
communications path.  Ironically I just had to work on a situation sort of
similar to this one.  What I did in my situation to accomplish this was to
DNAT the traffic destined to Box A over to Box B, like you have done.  You
also need to SNAT the traffic leaving Box A on it's way Box B to be from Box
A's IP so that when Box B replies it will reply back to Box A which will in
turn reply back to the client system.  Thus you no longer have a triangle of
client to Box a to Box B to client but rather client to Box A to Box B to
Box A to client.  Let me know what your network config looks like if you
would like me to come up with some iptables rules for you.

Reference my replies to "HELP! Transparent Proxy using bridging 2.6.9 and
REDIRECT on	different subnet" thread for an example or email me and I'll
try to provide more help.



Grant. . . .

Jared Cook wrote:
> I have two servers on two different networks.  I am running a service 
> on box A that I am transitioning to box B.  While I wait on DNS to 
> propagate, I would like to do some iptables magic to send traffic from 
> box A to box B using NAT.  For instance, when pop3 email users connect 
> to box A, I would like box A to send the request to box B 
> transparantly.  Is this possible?  I have had success doing port 
> forwarding to the local machine, but when I specify box B as the 
> "--to", it doesn't work.  Any help would be greatly appreciated.
> 
> Thanks,
> Jared
> 

Re: Using NAT to relay traffic

[Jared Cook]

That is a good option for services such as http or ftp, but I run into 
an issue with mail clients that stay up 24 hours a day.  Outlook will 
cache DNS information, and wont look up again until the client has been 
restarted in my experience.  I think this can give me a decent 
transition period that will give me less support headaches.

Jared

Sietse van Zanen wrote:

> Indeed.
>
>It would make much more sense, that if you want a fast turnover, to lower
>the TTL of your DNS records to a few seconds.
>
>-----Original Message-----
>From: netfilter-bounces@lists.netfilter.org
>[mailto:netfilter-bounces@lists.netfilter.org] On Behalf Of Grant Taylor
>Sent: 24 March 2005 01:37
>To: Jared Cook
>Cc: netfilter@lists.netfilter.org
>Subject: Re: Using NAT to relay traffic
>
>The problem that you are having when you port forward traffic from Box A to
>Box B is that the returning traffic comes directly from Box B to the client
>that sent the traffic in the first place thus you have an incorrect
>communications path.  Ironically I just had to work on a situation sort of
>similar to this one.  What I did in my situation to accomplish this was to
>DNAT the traffic destined to Box A over to Box B, like you have done.  You
>also need to SNAT the traffic leaving Box A on it's way Box B to be from Box
>A's IP so that when Box B replies it will reply back to Box A which will in
>turn reply back to the client system.  Thus you no longer have a triangle of
>client to Box a to Box B to client but rather client to Box A to Box B to
>Box A to client.  Let me know what your network config looks like if you
>would like me to come up with some iptables rules for you.
>
>Reference my replies to "HELP! Transparent Proxy using bridging 2.6.9 and
>REDIRECT on	different subnet" thread for an example or email me and I'll
>try to provide more help.
>
>
>
>Grant. . . .
>
>Jared Cook wrote:
>  
>
>>I have two servers on two different networks.  I am running a service 
>>on box A that I am transitioning to box B.  While I wait on DNS to 
>>propagate, I would like to do some iptables magic to send traffic from 
>>box A to box B using NAT.  For instance, when pop3 email users connect 
>>to box A, I would like box A to send the request to box B 
>>transparantly.  Is this possible?  I have had success doing port 
>>forwarding to the local machine, but when I specify box B as the 
>>"--to", it doesn't work.  Any help would be greatly appreciated.
>>
>>Thanks,
>>Jared
>>
>>    
>>
>
>
>  
>

Re: Using NAT to relay traffic

[Jared Cook]

Thanks.  The SNAT piece indeed was the part I was missing.  It all works 
great now.


Grant Taylor wrote:

> The problem that you are having when you port forward traffic from Box 
> A to Box B is that the returning traffic comes directly from Box B to 
> the client that sent the traffic in the first place thus you have an 
> incorrect communications path.  Ironically I just had to work on a 
> situation sort of similar to this one.  What I did in my situation to 
> accomplish this was to DNAT the traffic destined to Box A over to Box 
> B, like you have done.  You also need to SNAT the traffic leaving Box 
> A on it's way Box B to be from Box A's IP so that when Box B replies 
> it will reply back to Box A which will in turn reply back to the 
> client system.  Thus you no longer have a triangle of client to Box a 
> to Box B to client but rather client to Box A to Box B to Box A to 
> client.  Let me know what your network config looks like if you would 
> like me to come up with some iptables rules for you.
>
> Reference my replies to "HELP! Transparent Proxy using bridging 2.6.9 
> and REDIRECT on    different subnet" thread for an example or email me 
> and I'll try to provide more help.
>
>
>
> Grant. . . .
>
> Jared Cook wrote:
>
>> I have two servers on two different networks.  I am running a service 
>> on box A that I am transitioning to box B.  While I wait on DNS to 
>> propagate, I would like to do some iptables magic to send traffic 
>> from box A to box B using NAT.  For instance, when pop3 email users 
>> connect to box A, I would like box A to send the request to box B 
>> transparantly.  Is this possible?  I have had success doing port 
>> forwarding to the local machine, but when I specify box B as the 
>> "--to", it doesn't work.  Any help would be greatly appreciated.
>>
>> Thanks,
>> Jared
>>

Re: Using NAT to relay traffic

[Grant Taylor]

You are welcome.  I'm glad that I could help.  :)

I do ask a favor, have a good day for me as I was up WAY too late working on things.  Now where did I put my sleeping bag at work???



Grant. . . .

Jared Cook wrote:

> Thanks.  The SNAT piece indeed was the part I was missing.  It all works 
> great now.