dhcp-users Digest V2 #80

Glenn Satchell Glenn.Satchell at uniq.com.au
Sun Apr 1 13:44:52 UTC 2007

>Date: Fri, 30 Mar 2007 17:11:54 -0400
>From: "DOYLE FOSTER" <DFOSTER at salliemae.com>
>Simon Hobson wrote:
>>  Now that's interesting. The CLIENT has requested a different address and as 
it's available it's been acked. I see also that failover is in use here, are you 
sure that both servers are fully in sync ?
>How do I check to insure that my server are in sync, and are kept in sync?  I 
understood that the 2 server in failover mode would split the leases and should 
not hand out a different address to the same machine.  
>So are you suggesting that the machine gets an offer from each server, and one 
time accepts one and another time the other?   My workstation for example get 
the same address on every reboot, but I'm not using the PXE boot process.  
Hi Doyle

When the client sends out a DHCPDISCOVER (this is broadcast, so dhcp
relays forward it to the servers if necessary) both servers will see
the request and send a DHCPOFFER. The client usually waits a short
period for offers to arrive and then selects one, sending a DHCPREQUEST
to that server. The server then responds with a DHCPACK.

Some time later the client decides to renew the lease. This time is
sends a packet directly to the server that issued the lease. That
server responds. Note that the client does not talk to any other
servers at this time.

Now suppose that the dhcp server fails for some reason. The client
tries to contact it to renew the lease, but get no answer. It keeps
trying, until eventually the lease expires. Then it goes back to the
original state and broadcasts a DHCPDISCOVER. The other server sees
this and makes an offer.

Some clients are written so that they remember the previous address
they had, and will prefer to accept an offer containing that address.
Your client may work this way. Also if you reboot a client, it doesn't
necessarily release the IP address that it had, so if the lease is
still valid, then it is legitimate for it to keep using it after
booting. Different clients check this in different ways.


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