NATed packets only enter the default routing table

[richard lucassen <mailinglists@lucassen.org>]

(copy from lartc mailinglist)

I set up this config:

 +------+
-+ ISP1 +--+
 +------+  |  +-------+
           +--+ linux |
 +------+  |  +-------+
-+ ISP2 +--+
 +------+

No problem. Standard setup with two ISP's. Both routed subnets. Default
gateway is ISP1. No magic here.

Now I put a server behind the Linux box. I want the server to be
reachable on an /extra/ IP in the routed subnet of ISP2.

 +------+
-+ ISP1 +--+
 +------+  |  +-------+  +-----------------+
           +--+ linux +--+ server 10.0.0.2 |
 +------+  |  +-------+  +-----------------+
-+ ISP2 +--+
 +------+

router ISP2: 1.2.3.1/24
dev ISP2: eth1
Linux box eth1: 1.2.3.2/24
external ip ISP2 for server 10.0.0.2: 1.2.3.3

arp -s 1.2.3.3 aa:bb:cc:dd:ee:ff pub
ip route add 1.2.3.3 via 10.0.0.2
iptables -t nat -A PREROUTING -i eth1 -d 1.2.3.3 -j DNAT --to 10.0.0.2

When pinging 1.2.3.3, the packets get in through eth1 (ok), but the
replies are following the default routing table through eth0 (wrong)

Even a

ip rule add from 1.2.3.3 lookup table_eth1

doesn't change this behaviour. It is working ok when I add the address
1.2.3.3 directly to eth1 (without NAT):

ip a a 1.2.3.3 dev eth1

Why is this?

R.


-- 
___________________________________________________________________
It is better to remain silent and be thought a fool, than to speak
aloud and remove all doubt.

+------------------------------------------------------------------+
| Richard Lucassen, Utrecht                                        |
| Public key and email address:                                    |
| http://www.lucassen.org/mail-pubkey.html                         |
+------------------------------------------------------------------+