Re: [LARTC] linux ip forwarding problem

[Nelson Castillo <nelsoneci@gmail.com>]

On 6/17/05, Dariusz Dwornikowski <tdi@pozman.pl> wrote:
> On Fri, 17 Jun 2005 13:14:23 -0400 (EDT)
> "Ji Li" <ji.li3@hp.com> wrote:
> 
> > Hi,
> > I have three linux machines, and I want to let one of them forward packets
> > betwen the other two. The forwarding node has two ethernet cards,
> > connecting the two two machines respectively. However, when I ping between
> > the two end points, the forwarding node can receive the ping requests at
> > its eth0, but it never forwards them to its eth1. So is the reverse
> > direction.
> >
> > The forwarding node is Redhat 7.2, kernel 2.4.7-10. The two end points are
> > FC3, 2.6.9-1.667smp.
> >
> > What we have done to enable IP forwarding on the RH7.2 node are:
> > (1) In /etc/sysconfig/network, add "FORWARD_IPV4=yes"
> > (2) "echo 1 > /proc/sys/net/ipv4/ip_forward".
> > (3) Change "net.ipv4.ip_forward=1" in /etc/sysctl.conf.
> > (4)
> > "echo 0 > /proc/sys/net/ipv4/conf/eth0/rp_filter"
> > "echo 0 > /proc/sys/net/ipv4/conf/eth1/rp_filter"
> > (5) We tried "iptables -F" to flush the rules, but ip forwarding still
> > doesn' work, so we add some rules as follows. We run "iptables" to
> > configure firewall to enable IP forwarding.
> > iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT
> > iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
> route add net comp1-net gw comp1-ip
> route add net comp2-net gw comp2-ip

The last 2 lines are strange... I think you don't need them.

In comp1, you need:
route add -host comp2-ip gw reachable-router-ip-from-comp1

in comp2, you need
route add -host comp1-ip gw reachable-router-ip-from-comp2

You should also use tcpdump and test with pings. Example:

 # tcpdump -n -i eth0 icmp

It will allow you to debug common issues, for instance, when the
icmp echo request reaches its destination, but the host cannot reply
because a route is missing.

I assume the forwarding host is not the default route of
comp1 nor comp2.

Regards,
Nelson.-

-- 
Homepage : http://geocities.com/arhuaco

The first principle is that you must not fool yourself
and you are the easiest person to fool.
     -- Richard Feynman.
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