Insert delay in dhcp3-relay ?

Simon Hobson dhcp1 at
Wed May 4 14:54:25 UTC 2011

Benjamin wrote:

>I have three network (separated with router):
>Network A: DHCP server, a relay and my client
>Network B: interconnection netwok between A and C
>Network C: An other DHCP server
>I make this setup to test "fault tolerance" (sorry I'm french and I 
>don't know if you understand "fault tolerance" ...)
>So when my DHCP A is offline, the relay relaye my request to the 
>DHCP C . All is OK. but if I set back online my DHCP A, my client 
>still uses the DHCP C (because my XP client record the first IP 
>Adress and  when I do a release/renew, he request this old IP, so my 
>DHCP C send an 'ACK' for this IP)
>But if I can put a delay in the relay agent, my client request the 
>old IP adress, but the DHCP A will propose a new IP and the client 
>will accept it.

Adding a delay to the relay agent will NOT stop this.

For a client that already has a working lease, it will continue to 
renew the lease with the remote server. Delaying the packet a couple 
if seconds won't stop this.

You would have to block the request altogether, and then the client 
would start to broadcast renewal requests which the local server 
would pick up.

In practice, clients will normally take an initial lease from the 
first server to offer a valid offer. In a setup like yours, it is 
going to be normal for the fastest server to be the one on the local 

The question that comes up next is how you are doing the fault 
tolerance - yes I understand what you mean.

ISC DHCP server has a failover mode where two servers can co-ordinate 
their offers so as to share a common address space. If one fails, 
then the other can take over and run everything by itself.

As an alternative, you can simply run two independent servers with 
non-overlapping ranges. As long as each server has enough addresses 
to service all the clients then the worst case is a failure of one 
server which is longer than half the lease time* - at which point 
clients will start to switch addresses as their leases expire.

But, in both cases it shouldn't really matter which server a client 
is bound to.

* By default, clients renew their leases when they are half way 
through. So few (if any) client will have less than half a lease left 
and can continue without an active DHCP server for at least this long.

Simon Hobson

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