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Re: ICMP "Host unreachable" - more info


On Tue, 03 Aug 2004 09:16:41 -0400
Jason Opperisano <opie_(_at_)_817west_(_dot_)_com> wrote:

> ICMP Host Unreachables from a router mean that the router did not get
> a reply to it's ARP request on the segment attached to the server for
> the server's MAC address.
> Verify this on the router with:
>         tcpdump -n -nn -p -i <SERVER SEGMENT IF> arp
> My guess is that you'll see a bunch of:
>         arp who-has <SERVER IP> tell <ROUTER IP>
> Without seeing:
>         arp reply <SERVER IP> is-at <SERVER MAC>

Reasonable suggestion, but no. Captures on both the server interface and
the server-side router interface show no ARP's occuring. This is, in
some sense, what I'd expect - as my first captures show (not sure you
saw my original post), there is successful IP traffic to the server less
than a second before the router issues the ICMP's. Checking the router's
ARP tables (via "arp -na") immediately before the workstation boots also
verifies that the server has an entry.

So, the ARP tables on the router are correct, no ARP's are occuring on
the wire, and yet the router returns ICMP Host Unreachable. I must be
missing something here...

Todd Pytel