ICMP Ping being lost between kernel and the ping program.

From: Jesse Gordon
Date: Wed Nov 16 2005 - 15:23:13 EST

Greetings. This is my first post.

I'm having a weird intermittent problem with ping.

I'm pinging the WAN port of a cheap home DSL firewall (d-link di-604) and sometimes the ping program fails to get a response,
but if I run tcpdump I can see that the response is indeed coming back.

arping on the target IP always works.

We're a small wireless ISP with 40ish customers -- each of whom has a cheap dlink router at their home. We ping each of them regularly to be notified of a wireless loss of connection.
(The wireless is Trango running at 5.x Ghz or ~900Mhz -- not 802.11) and all the wireless and switches are just ethernet bridges -- so it's a flat network with no ip based routing.

Two different dlink routers cause this problem but not necisarly at the same time.

Two different Linux computers exhibit this problem, although again, not always at the same time. One seems to be more picky -- sometimes just the picky one has a problem, other times they both have the problem.

Also, I have not yet noticed this problem while pinging from a windows computer.

Picky one:Linux 2.4.26
Less picky one: Linux SMP

Anyhow, I don't know whether the dlinks are sometimes sending out sligly malformed packets, whether they are getting slightly damaged over the wireless network,
or whether Linux [the kernel] is having a problem, or if the ping program is having the problem. The facts just don't make sense for any of those.

I'll be glad to do all the normal stuff -- upgraded to latest kernel, (I did try upgrading to latest ping not too long ago no help - can try again), if anyone thinks that'll help me solve the mystery.
I can even capture the text description of or the raw contents of packets with tcpdump or maybe ethereal, if that will help.

I did one time capture with ethereal a packet when ping was working, then a packet when it wasn't and compared them side by side in two ethereal instances -- and aside from the fields that should have been different, everything looked the same.


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