linux-net troubleshooting faq?

From: Robert Tagliarini (
Date: Thu Mar 02 2000 - 09:04:38 EST

Could someone please point me to some info that would help me figure out what
is wrong with my setup? Maybe there is a faq somewhere? Is there some network
debug logging that could be turned on that would tell my why forwarding isn't
Bob Tagliarini

Robert Tagliarini
Senior Engineer
eLutions (formerly TeCom)
702 N. Franklin St
Tampa, Florida 33602

---------- Forwarded Message ----------
Subject: ip_forward is 1 but forward does not work
Date: Tue, 29 Feb 2000 07:02:40 -0500
From: Robert Tagliarini <>

Please help me get ether/ppp forwarding to work on my setup. I have three
systems in my office running different versions of linux. My main workstation
is a compaq deskpro running redhat 6.1 (kernel version 2.2.12-20). It is
connected to an embedded target which is running uClinux via ppp on ttyS0. The
third system is an aptiva running redhat 5.2 (kernel version 2.0.36). The
compaq and the aptiva are connected to our company lan. I'm trying to make the
uClinux system visible to others on the company lan by having the compaq proxy
arp for it and forward packets to/from it.

            | |
        aptiva compaq

/proc/sys/net/ipv4/ip_forward is a 1 on the compaq and
/etc/sysconfig/network has

 # arp -n
Address HWtype HWaddress Flags Mask Iface ether 00:D0:BC:F0:87:58 C eth0 * * MP eth0
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface UH 0 0 0 ppp0 U 0 0 0 eth0 U 0 0 0 lo UG 0 0 0 eth0

I can telnet from the aptiva to the compaq (using either of the compaq's ip
addresses) and from there to the uClinux system. The proxy arp seems to be
working because a ping from the aptiva to the uClinux system gets an entry for placed in the aptiva's arp cache and it has the compaq's ethernet
address in it. Monitoring the serial link to the uClinux system shows me that
the ping is not being forwarded to the uClinux system.

One other possible clue (I've been thinking it's just a buggy tcpdump) is
tcpdump's odd behaviour on the compaq. When I run tcpdump on the compaq set to
display everything to/from the aptiva, it displays target ethernet addresses
of 0:0:0:0:0:1 when the aptiva pings,, or any other
tcpdump -n -i eth0 -e ether host 00:40:05:3a:66:6e
User level filter, protocol ALL, datagram packet socket
tcpdump: listening on eth0
07:41:11.794391 B 0:40:5:3a:66:6e Broadcast arp 60: arp who-has tell10.44.11.8
07:41:12.199692 < 0:40:5:3a:66:6e 0:0:0:0:0:1 ip 98: > ic
mp: echo request

Do I need to do something with chains to get this to work?

What can I look at on the compaq to determine why it is not forwarding?

Bob Tagliarini
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