[ale] Networking problem with 3 nics

[Joe Steele]

A few questions:

1)  I presume you are running tcpdump from the box that is giving you 
trouble -- correct?

2)  On the assumption that the answer to 1) is yes -- are the tcpdump 
results the same if you use the '-p' option (i.e., without the 
NIC in promiscuous mode)?

3)  Using tcpdump with the -e option, does the destination MAC 
address match the NIC's MAC address? (otherwise the packet will be 
dropped, except when the NIC is in promiscuous mode)

4)  And for the really obvious, does tcpdump (using the -n option) 
show a destination IP address that matches the NIC's IP address?  
(obviously, if this doesn't match, the packet will be dropped)

Your answers will probably, be 'yes', but it will at least eliminate 
a couple of (far-fetched) reasons for dropping packets.

--Joe

-----Original Message-----
From:	Chuck Huber [SMTP:chuck at cehuber.org]
Sent:	Wednesday, November 06, 2002 5:12 AM
To:	ale at ale.org
Subject:	[ale] Networking problem with 3 nics


I'm having a real problem adding a DSL line to my gateway.
It appears that packets are making it in to the system,
however, they're not making all the way up the stack.

Here's the scenario:
    From an unrelated host on the Internet, if I ping my
    DSL address, tcpdump shows the packets coming in and
    ifconfig shows them being received, however, no echo
    reply is produce.

    Also, if I ping the unrelated host from here, tcpdump
    shows the echo requests going out, and the echo replies
    coming back, but ping shows no responses.

At first I thought this was a firewall problem - something
somewhere is too tight.  That suspicion was quickly debunked
when I flushed the iptables to no avail.

Here's the setup:
    2.4.9-31 kernel (RH 7.2)
    eth0: external to cable modem
    eth1: internal LAN (192.168.x.x)
    eth2: external to DSL modem

Everything going out the cable modem works fine.  This problem
exists only on eth2.

Ideas would be more than welcome.

Thanks,
    - Chuck


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