[LARTC] Wierd Policy Routing Behaviour

[Sellaro <sellaro@email.it>]

Hi there 

I'm trying to set up policy routing in a simple scenario without further
success. My linux router is connected to two different cable ISPs. One
of these links (hereafter named A) permits traffic to SMTP servers while
the other (link B) doesn't. 

What I am trying to set up is: all traffic should flow through link B
and only SMTP traffic through link A. 

What I am doing: 

Marking all packets from my intranet with source port within the range
1024 to 65535 and with destination port 25 with mark 0x3. I am also
marking packets from our internal SMTP server with source port 25 to any
other port with the same mark. Marks are being made in the OUTPUT chain
(using iptables) as follows: 

iptables -t mangle -A OUTPUT -p tcp --sport 1024:65535 --dport 25 -j
MARK --set-mark 3 

iptables  -t mangle -A OUTPUT -p tcp -s <MY SMTP SERVER> --sport 25 -j
MARK --set-mark 3 

Then, I have created a new routing table named SMTP-ROUTE with default
gateway being the default gateway for link A as follows: 

ip ro add default via <A's default GW> table SMTP-ROUTE 

To complete the setup, I've added a rule stating that all packets marked
with 0x3 should use SMTP-ROUTE with this command: 

ip ru add fwmark 3 table SMTP-ROUTE 

Yes, I've flushed the routing cache with: 

ip ro flush cache 

Default route in main routing table is B's gateway. 

I don't know why, but SMTP traffic keeps going through B's gateway,
instead of A's, as expected. 

I've sniffed the network and, in fact, the packets are trying to go out
through B's gateway.

Can anyone please point what I am doing wrong?

Thank you in advance 
-- 
Sellaro

Agente Livre - Linux Community (www.agentelivre.org)

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