Arthur Donkers
2002-Aug-31 19:03 UTC
Splitting locally generated traffic based on destination port
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 -- /* Disclaimer : you hire my skills, not my opinions, those are mine ! */ /* email : arthur@reseau.nl Security ''Me ? I''m not me ! I''m just a */ /* phone : (+31) 50 549 2701 is not a computer simulation of me'' */ /* URL http://www.reseau.nl dirty word Red Dwarf, First Episode */ _______________________________________________ LARTC mailing list / LARTC@mailman.ds9a.nl http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/
Miernik
2002-Aug-31 19:46 UTC
Re: Splitting locally generated traffic based on destination port
On Sat, 31 Aug 2002, Arthur Donkers wrote:> 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).Try: echo "0" > /proc/sys/net/ipv4/conf/eth0/rp_filter If it works, to make it permanent, include this line in your /etc/sysctl.conf file: net/ipv4/conf/eth0/rp_filter = 0 -- Miernik _____________________________________ / / tel.: +48603070983 / / mailto:miernik@ctnet.pl __________________/___/ ICQ UIN: 4004001 _______________________________________________ LARTC mailing list / LARTC@mailman.ds9a.nl http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/
Sebastian Bleikamp
2002-Aug-31 19:47 UTC
Re: Splitting locally generated traffic based on destination port
Arthur Donkers wrote:> 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).Hello Arthur, I´m not a professional in routing, but I think I´ve read something about the routing mechanism: Packet ---> Table PREROUTING -+-> Table FORWARD -+-> Table POSTROUTING | | (and then out) +-> Table INCOMING | | Table OUTGOING -+ The problem is, that the routing decision is done AFTER processing the forward table, but BEFORE the postrouting table (NAT). So it is possible to modify the packets in POSTROUTING that way, that they will go out via device A, but have the ip of the other device B. If the "answer" packet comes in at A (but are addressed to B), the address will be checked and device A drops (/does not listen to) them. ("Hey, that packet is not for me !"). Possible unclean variant to fix that would be to put device A in the so called "promisc" (promiscueing ?) mode, where all packets are processed. Clean variant is _not_ to do NAT by the filter. Make the _routing_ decision depending on the filter. And NAT is done afterwards based on the routing. Hope I told not too much wrong, Sebastian -=> Sebastian Bleikamp -=> EMail: <Sebastian.Bleikamp@web.de> -=> Phone: +49-172-6545394 _______________________________________________ LARTC mailing list / LARTC@mailman.ds9a.nl http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/
Arthur Donkers
2002-Aug-31 20:41 UTC
Re: Splitting locally generated traffic based on destination port
On Sat, Aug 31, 2002 at 09:46:31PM +0200, Miernik wrote:> On Sat, 31 Aug 2002, Arthur Donkers wrote: > > > 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). > > Try: > > echo "0" > /proc/sys/net/ipv4/conf/eth0/rp_filter > > If it works, to make it permanent, include this line in your > /etc/sysctl.conf file: > > net/ipv4/conf/eth0/rp_filter = 0thanks ! this works ! grtz, Arthur> > -- > Miernik _____________________________________ > / / > tel.: +48603070983 / / mailto:miernik@ctnet.pl > __________________/___/ ICQ UIN: 4004001-- /* Disclaimer : you hire my skills, not my opinions, those are mine ! */ /* email : arthur@reseau.nl Security ''Me ? I''m not me ! I''m just a */ /* phone : (+31) 50 549 2701 is not a computer simulation of me'' */ /* URL http://www.reseau.nl dirty word Red Dwarf, First Episode */ _______________________________________________ LARTC mailing list / LARTC@mailman.ds9a.nl http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/