Re: Routing decision?

[Wim Ceulemans <wim.ceulemans@able.be>]

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Henrik Nordstrom wrote:

>On Mon, 15 Sep 2003, Wim Ceulemans wrote:
>
>  
>
>>If I understand correct, if the local endpoint of the socket is not 
>>bound, then:
>>1. The routing decision is taken, and the source IP address is assigned 
>>to the IP address of the interface
>>    through which the packet would leave the firewall.
>>    
>>
>
>Yes. Or to be more precise to the source IP address of the route by which 
>the packet would leave the host, or interface address if no route source 
>address is defined in the routing table.
>
>  
>
>>2. The packet travels through the OUTPUT chain and does not pass the 
>>routing decision anymore, because
>>     the routing decision was already taken before going to the OUTPUT 
>>chain.
>>    
>>
>
>Yes.
>
>However on the next packet of the connection (assuming it is a TCP
>connection) the situation is the reverse. The local endpoint of the socket
>is now bound and there is no need for a routing decision during packet
>construction as the source IP address is already know. So now routing
>occurs after OUTPUT like it logically should be.
>
>  
>
>>Is there any specific reason why the packet doesn't pass the routing 
>>decision the second time?
>>    
>>
>
>Why should it if the routing has already been called once?
>
Here's why: if you want to mark packets in the OUTPUT chain that are 
http traffic, based
on the destination port. And then based on that mark send the packets to 
another routing table
added with the ip command.

>
>iptables does route the packet a second time if you mangle or NAT the
>packet.
>
>Regards
>Henrik
>
>
>  
>


-- 
Wim Ceulemans
R&D Engineer

Secure Internet Communication with aXs Guard

Able NV
Leuvensesteenweg 282 - B-3190 Boortmeerbeek - Belgium
Phone: + 32 15 50.44.00 - Fax: + 32 15 50.44.09
E-mail: wim.ceulemans@able.be



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