very strange arp problem after ip move - icmp works udp doesn't

[Li, Qing]

You don't have the flowtable component enabled, do you ?

--Qing

________________________________________
From: owner-freebsd-net at freebsd.org [owner-freebsd-net at freebsd.org] on behalf of Steven Hartland [killing at multiplay.co.uk]
Sent: Friday, October 21, 2011 4:44 PM
To: freebsd-net at freebsd.org
Subject: very strange arp problem after ip move - icmp works udp doesn't

After a HD failure we moved an IP of one of our DNS
servers to some new hardware.

Now we are seeing some very strange behaviour on a number
of machines when talking to the ip which was moved.

Specifically it seems both icmp and tcp work just fine
but udp doesn't.

I've just done a trace from one such box and to my horror
the tcpdump shows icmp and udp traffic for the same
IP going to different mac's.

icmp is going to the right mac and is working fine
but udp is going to the wrong mac and isn't (as you
would expect)

In /var/log/messages there are arp messages
which show the ip moving from -> to the correct macs
but it seems like something somewhere is caching
the resolution.

arp -a also show's the correct result and arp -d -a doesn't
help.

So the question how on earth is udp resolving the mac
to something different than icmp and tcp?

To complicate matters even further some machines are
working intermittently and the traces show the udp
some times going to the right place and some times
not.

In all tests we're using just a dig with a specified
server.

The machines involved are all running 8.2-RELEASE on
amd64

Any help would be most appreciated as its causing chaos
due to the IP that moved being our DNS servers and hence
things are randomly stalling left right and center :(

    Regards
    Steve




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