[LARTC] Splitting locally generated traffic based on destination port

From: Arthur Donkers <arthur@reseau.nl>
To: lartc@vger.kernel.org
Subject: [LARTC] Splitting locally generated traffic based on destination port
Date: Sat, 31 Aug 2002 19:03:48 +0000	[thread overview]
Message-ID: <marc-lartc-103082070321370@msgid-missing> (raw)

Hi All,

I have a Linux box (2.4.19, iptables 1.2.6a) that has two Ethernet
interfaces. Eth0 is connected to a cisco which is connected to the
Internet via a leasedline. Eth1 is connected to an ADSL connection.
Both interfaces have a fixed IP address, and the default route points
over the ADSL connection (because this is used for browsing, ftp and
such).

However, the Eth0 has the 'official' IP address and should be used for
send e-mail and some ssh traffic.

I already defined a policy to return incoming traffic over the interface
it was received, so both interface can be contacted via the Internet.

My question now is how to define a policy so specific traffic, generated
locally by sendmail and ssh, uses eth0 as its interface in stead of eth1
which is used for the default route ?

I already mark outgoing traffic in the mangle table thus

iptables -A OUTPUT -t mangle -p tcp --dport 25 -j MARK --set-mark 1

I have to SNAT outgoing traffic as well:

iptables -A POSTROUTING -t nat -m mark --mark 1 -j SNAT --to-source <eth0>

I have added a table mail.out to /etc/iproute2/rt_tables and defined rules
like this:

ip rule add fwmark 1 table mail.out
ip route add default via <gw on eth0 network> dev eth0 src <eth0>

And when I do this:

telnet <mailhost> 25

I indeed see a SYN packet coming out of eth0 to <mailhost>, and a SYN/ACK
packet comes back. The problem is that it appears that the Linux box does
not see this SYN/ACK packet and resends the SYN packet after a few
seconds, which again is answered by a SYN/ACK and this goes on a few
times. No other packet filters are active on the Linux box (yet).

So how do I get this setup to work ? Am I heading for the wrong direction
with the policy or is there something else missing ?

thnx in advance,

Arthur Donkers

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