LARTC - Feb 2006 - Multiple providers routing

HI folks,


   I have a linux router connected to two separate internet 
connection from an ISP. There is a third interface ( ip -> 
192.168.1.1 ) in the router connected to the local network. 
Configured the routing tables and added the rules and everything 
seems to be working fine from the routing box. Traceroute to 
external internet sites reveal that traffic is being routed 
correctly and that the failover mechanism is working.

     Now in my internal machines the gateway address is the set to 
the third interface of the router and the internal machines can 
ping the router ( 192.168.1.1 ). The problem is that the internal 
machines cant connect to the net. A quick check with pings  and 
tcpdump revealed that the packets from the internal machines are 
arriving at the router and are being routed correctly... but are 
not coming BACK from the router to the internal machines.

Any pointers as to why this is happening would be useful....


Thanks,




Sameer Gurung

samgurung@rediffmail.com
g_sameer75@yahoo.com

Greetings Sameer,

 : I have a linux router connected to two separate internet 
 : connection from an ISP. There is a third interface ( ip -> 
 : 192.168.1.1 ) in the router connected to the local network. 
 : Configured the routing tables and added the rules and everything 
 : seems to be working fine from the routing box. Traceroute to 
 : external internet sites reveal that traffic is being routed 
 : correctly and that the failover mechanism is working.
 : 
 : Now in my internal machines the gateway address is the set to the 
 : third interface of the router and the internal machines can ping 
 : the router ( 192.168.1.1 ). The problem is that the internal 
 : machines cant connect to the net. A quick check with pings and 
 : tcpdump revealed that the packets from the internal machines are 
 : arriving at the router and are being routed correctly... but are 
 : not coming BACK from the router to the internal machines.
 : 
 : Any pointers as to why this is happening would be useful....

Quick, experienced guess:

  # sysctl net.ipv4.conf.default.rp_filter

If the answer provided is:
 
  net.ipv4.conf.default.rp_filter = 1

Then, you''ll need to flip the reverse path filtering toggle [0].  
When this sysctl is set to 1, the kernel automatically drops packets 
incoming from the "wrong" interface according to the primary 
(''main'') routing table.

Good luck,

-Martin

 [0]
http://ipsysctl-tutorial.frozentux.net/chunkyhtml/theconfvariables.html#AEN634

-- 
Martin A. Brown --- Wonderfrog Enterprises --- martin@wonderfrog.net