Re: [vserver] ping vs. IP address vs. MAC (address)

From: Roderick A. Anderson <>
Date: Fri 01 May 2009 - 23:58:19 BST
Message-ID: <>

Herbert Poetzl wrote:
> On Fri, May 01, 2009 at 11:02:21PM +0200, Dennis Roos wrote:
>> What you could do is clear the arp entry for the address on your
>> router/switch and generate some traffic (eg. ping) on the interface.
>> Mostly it takes 5 minutes for a router to learn about the new address,
>> as it caches the information for that amount of time.
> a trick often used by HA folks is to send an
> unsolicited or gratuitous ARP, with e.g.
> arping -U -I eth0 <guest-ip>
> this will update all directly connected routers
> with the new arp information

Thanks Herbert. I found lots of references to 'unsolicited or
gratuitous ARP' but didn't see (notice?) commands to do it. And since
arp is only three characters I've never hit TAB twice and noticed the
arping command.

I'll give it a try shortly.


> HTH,

I'm sure it will. :-)


> Herbert
>> Best regards,
>> Dennis Roos
>> Roderick A. Anderson wrote:
>>> I'm asking for a clue stick here.  First time I've run into this issue 
>>> ... hard.  Be a few times where it took a bit for outbound traffic to 
>>> get a route but can't remember running into this problem.
>>> The short question is: does the network/NIC _know_ which IP address it 
>>> is responding for when a ping request comes in or does it just respond 
>>> to the ping request at the MAC (hardware?) level?
>>> I have two hosts and each of their guests has its own IP address. I am 
>>> trying to move the guest's services from one host to the other.  The 
>>> new and old guests both use the same IP address.  So I shut down the 
>>> old guest and then start up the new guest on the other system.  
>>> Network traffic to the new guest, except pings, doesn't work.  
>>> Iptables, ip addr ls, ifconfig, etc., all appear to be good.
>>> I'm guessing the routing is sending the ping request to the old 
>>> guest's MAC.
>>> Thoughts, Ideas?
>>> \\||/
>>> Rod
Received on Fri May 1 23:58:33 2009
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