[LARTC] OUTPUT chain marking after or before routing?

["Catalin Borcea" <catalin@electricant.ambra.ro>]

Hello,
I have a Linux box with 3 network adapters:
eth0 : IP:10.200.0.1/24
eth1/ppp0: IP:80.97.105.98
eth2 : IP:192.168.1.100/24

I want that all the Internet traffic goes to the eth2 interface except the
smtp traffic that I want to go to the ppp0 interface. The main routing table
is:
172.16.20.1 dev ppp0  proto kernel  scope link  src 80.97.105.98
192.168.1.0/24 dev eth2  scope link
10.200.0.0/24 dev eth0  scope link
192.168.254.0/24 dev eth1  scope link
127.0.0.0/8 dev lo  scope link
default via 192.168.1.1 dev eth2


I decided to use netfilter to mark the packets that leave the gateway from
and to the smtp port. I do this in the OUTPUT chain of the mangle table. So,
according to the docs, the marking will occur before routing for locally
generated packets:

$IT -t mangle -A OUTPUT -p tcp --dport smtp -j MARK --set-mark 2
$IT -t mangle -A OUTPUT -p tcp --sport smtp -j MARK --set-mark 2

Then I define a new routing table (named "smtp") and a rule to redirect smtp
packets to this table. The output of "ip rule ls" is:
# 0:      from all lookup local
# 32765:  from all fwmark        2 lookup smtp
# 32766:  from all lookup main
# 32767:  from all lookup 253

In table "smtp" I defined a default route by the dev ppp0. The output of "ip
route ls table smtp" is:
# default dev ppp0

When I try to connect to a smtp port somewhere in the Internet, tcpdump show
me that these packets go to the eth2 interface (the main table default
route). I don't know where is my mistake but it seems that the marking in
the OUTPUT chain occurs AFTER and not BEFORE routing. Is this a correct
behaviour? How can I solve my problem? Please help!

TIA
- catalin -


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