[K12OSN] Adding another server as backup.

Les Mikesell les at futuresource.com
Wed May 11 14:56:37 UTC 2005

On Wed, 2005-05-11 at 05:41, Martin Woolley wrote:

> How and why it all works is clearly related to the cleverness built into the 
> DHCP specification and to the time slice that each process gets from the Unix 
> kernel.  I vaguely remember reading that is there are two or more DHCP 
> servers dishing out addresses in the same i/p range, they keep track of 
> addresses given out by the other servers so that no duplicates occur.

I don't think I'd count on that.  More likely they try to allocate at
random but try to ping the address to be sure it is unused before giving
it out.  It would be safer to slice up the DHCP range and give each
server a non-overlapping set.  The servers should keep track of
previously assigned addresses and the matching ethernet MAC address
and try to give back the same IP to each client on the next request.
The load balancing should be determined by which server responds
first to the dhcp request.

> What I 
> think happens is that when a client does a DHCP request (a DHCP Discover is 
> put on the wire), my guess is that the machine that happens to have dhcpd as 
> the active process serves an i/p address (puts a DCHP Offer on the wire).  
> The client responds with a DHCP Request and when the DHCP Acknowledge is 
> broadcast, other DHCP servers take a note of the i/p address and delete it 
> from the pool. 

Actually they all may answer with an offer of an IP address - the client
takes the first one to reach it - or some clients (but not rebooted thin
clients) may remember the address they used previously and prefer that.
Your /var/log/messages files on the servers should show the

  Les Mikesell
    les at futuresource.com

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