Load Balancing DHCP Servers

Simon Hobson dhcp1 at thehobsons.co.uk
Thu May 3 14:45:20 UTC 2007

Aggarwal Vivek-Q4997C wrote:

>There are 4 DHCP Servers and on router working as a relay agent for 4
>DHCP servers
>Now I want these 4 DHCP Servers to work in Load Balance Scenario means
>if first request comes it goes to 1st server, 2nd request to 2nd Server,
>3rd request to third and so on and if anyone server fails request is
>automatically forwarded to another server.
>Im using ISC DHCP 3.0 version and LINUX Operating system
>Is it possible? If yes how can I achieve it? What should be
>configuration of the servers and the relay agent?

No, that is not how DHCP works, and it makes no difference what 
server and relay agent you use.

Firstly, DHCP is a stateful function - the server that handles a 
query must know about the state of both the client making the query 
and other clients on the network. Consider if the relay agent sends a 
clients request to one server, then on the next boot/wake from 
sleep/whatever, sends the request to a different server. Unless the 
servers are synchronised then the second server knows nothing about 
the client and chaos ensues on your network.

With the ISC server you can run two (and only two) servers in a 
failover configuration. They will keep each other informed of what 
offers they have made so that both have the same view of the network 
at all times.

With care you can run more than one server without failover. 
Typically you either :
- Find a 'key' that can differentiate classes of clients so that one 
server only serves one set of clients while the other server ignores 
- Or you configure different pools on the servers so that although 
they serve the same subnet they serve non-overlapping pools and it 
will be non-deterministic which server a client takes a lease from.
In either case, if a server fails, the clients bound to it will 
eventually time out on their leases and will either lose networking 
or get a different address.

I suggest you read the man pages and look up failover, and also find 
a copy of The DHCP Handbook by Ralph Droms and Ted Lemon - this is 
considered to the 'the bible' of DHCP and is both very readable and 
very informative.

>Can anyone please send me the actual configuration for the above

You mean, leave you nothing to do except take the credit ?

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