Routing Incoming and Outgoing Packets Differently

Routing Incoming and Outgoing Packets Differently

[Jim Doble]

Is there a way to use one set of routing rules for incoming packets and 
a different set of routing rules for outgoing packets? Or is there a way 
to set a socket option so that packets sent on a given socket are routed 
using a different routing table?

Jim Doble
Tavve SoftwareCompany

Re: Routing Incoming and Outgoing Packets Differently

[Pascal Hambourg]

Hello,

Jim Doble a écrit :
> Is there a way to use one set of routing rules for incoming packets and 
> a different set of routing rules for outgoing packets?

Yes, with advanced routing.

Quote from ip manpage (http://lartc.org/manpages/ip.txt) :

    ip rule add - insert a new rule
[...]
        iif NAME
               select the incoming device to match.  If the inter-
               face  is  loopback,  the  rule only matches packets
               originating from this host.  This  means  that  you
               may  create  separate  routing tables for forwarded
               and local packets and, hence, completely  segregate
               them.
[end of quote]

So you can create an alternate routing table for locally-generated 
packets. Incoming packets will use the default "main" routing table.

ip rule add iif lo lookup <output_table_number> prio 32765
ip route add <destination1> gw <gateway> table <output_table_number>
ip route add <destination2> dev <interface> table <output_table_number>
...

You need to have the following options enabled in your kernel to use 
this feature.
CONFIG_IP_ADVANCED_ROUTER=y
CONFIG_IP_MULTIPLE_TABLES=y