Subject: Re: [vserver] ping vs. IP address vs. MAC (address)
From: "Roderick A. Anderson" <raanders@cyber-office.net>
Date: Fri, 01 May 2009 09:39:21 -0700

John A. Sullivan III wrote:
> On Fri, 2009-05-01 at 08:54 -0700, Roderick A. Anderson wrote:
<snip />
> Hi, Rod.  I'm a bit confused (comes with being up all night integrating
> vserver and zimbra!) but did you mean all traffic to the new guest works
> except ping or all traffic fails except ping?

I feel your pain.  Pulled far too many all nighters and some times 
several in a row.

Ping works, other stuff doesn't.

> The NIC itself only knows about MAC addresses. If the MAC address
> matches, it passes it up the stack to the IP layer.  However, the  issue
> may have more to do with the ping initiator than the receiver.
> 
> Since you ping an IP address but need to put a packet on the wire with a
> MAC address, the initiator issues an ARP (Address Resolution Protocol)
> request asking which MAC address is associated with the desired IP
> address.  This information is cached although the cache expiration is
> pretty short usually.

Like I said in my other reply I'm suspecting the router between the two 
hosts has a long lived arp cache.

> If the pinging station is still using the old ARP information, it will
> send the packets to the wrong MAC address.  You can check your ARP cache
> with something like arp -n to see where your ping initiator thinks the
> IP address lives. 

I'll have to get a few more systems involved.  My workstation at home is 
behind a firewall/router so I only see its arp entry.  :-(

Get on a third system, and have it start a script/command line that 
pings the IP address then checks the arp table.  Get on the two hosts 
and shut the old guest down watch the third systems script, then on 
other host start the new guest.

Thanks for the input from all of you -- Martin, Michael, and John.  It 
was a long day yesterday by the time I gave up on this and haven't 
started on it this morning.


\\||/
Rod
-- 
> Hope this helps - John