DHCP failover and fixed-address

Phil Mayers p.mayers at imperial.ac.uk
Mon Mar 3 10:16:52 UTC 2008

Sebastian Hagedorn wrote:
> Hi,
> we're currently using ISC dhcpd 3.0.3 in a failover setup. From previous 
> discussions I think I know (but am uncertain) that 3.0.3 does not 
> support host statements with fixed-address in failover mode. However, it 

As you've seen, it doesn't need to. Because fixed-addresses are, well, 
fixed, an arbitrarily large number of DHCP servers can OFFER/ACK them 
and the client will take the first one it gets (time-wise).

> seems to work (somewhat). We have many lines like this:
> host ap1200-ustb-ort-1                  {hardware ethernet 
> 00:0c:30:60:09:09;fixed-address;}
> They are in both our server's configs. Both servers are configured as 
> helper-addresses on our Cisco routers. Both servers get and serve the 
> requests. I don't think there's any harm in that, but I wanted to make 
> sure ...

We've been doing that for years without failover enabled and it works fine.

> Also, ISTR that some newer version of dhcpd properly supports such 
> scenarios, but I haven't been able to find anything in the release 
> notes. Am I confused?

I'm not sure how it could support them any better, but I guess there 
might be optimisations.

> Thanks, Sebastian

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