Re: arp ?

[Greg Troxel <gdt%lexort.com@localhost>]

Patrick Welche <prlw1%cam.ac.uk@localhost> writes:

> Given two NetBSD-9.99.2/amd64 boxen on the same local network, I ssh in from
> client to server. On server the server, the arp entry for the client is
> "incomplete". If I try then to ssh from the server back to the client, I
> have "Host is down". I'm puzzled, as obviously I am typing that command
> over the network. No firewalls are involved in this setup.

Your ssh may be over v6 if you used "foo.local", which runs ndp, not arp
-- and has different failure modes :-)

Presumably these are real ethernet on a switch.

Check the IPv4 prefixes that they have the same net part and the same
netmask.

Check the addresses on every other interface on each machine to make
sure you don't have the same or overlappign network prefix reused.

Run tcpdump on each and watch the arp traffic.

Run netstat -i and look at error counters.

run netstat -s to a file, test, do it again, diff.  Understand the
counters that increaseed.  (hard, but illuminating for odd issues)

Use -v in ssh to see what it's doing, and maybe numeric addresses, so
that it doesn't use v6 behind your back (which is usually fine, but not
helpful when debugging).