DHCP Redundancy

Matt Jenkins matt at smarterbroadband.net
Wed Dec 1 22:14:21 UTC 2010

Would it be possible to have a distributed NFS directory and have many 
dhcp daemons read the same leases and configuration files? Does the DHCP 
Server re-read the leases file when it starts up so it knows about all 
existing leases?

On 11/30/2010 02:21 AM, Simon Hobson wrote:
> Matt Jenkins wrote:
>> So is it possible to maintain a central or distributed leases file 
>> for multiple (unknown quantity) of servers?
>> I ask because I am working on a design to change over all of my 
>> wireless clients to dhcp. With the wide spread nature of the network, 
>> ALL services are distributed so that any single point can fail and 
>> everything else stays active. This assumes that the point of failure 
>> will never recover. The system MUST be able to handle this 
>> automatically. I definitely do not have 2x the address space as 
>> others suggested. I kind of assumed that the dhcp servers maintained 
>> synchronised information regarding leases.
>> I estimate the need for 17 dhcp servers (right now) distributed 
>> across the system handling multiple /18's (in total) of address 
>> space. Can this be handled?
> Failover is only supported between two servers, but you can have 
> different pairings for different subnets. Eg, you can have A&B for one 
> subnet, A&C for another, A&D for another, C&E for another - whatever 
> you want.
> I think you can do this by pools, but that sounds like a recipe for 
> confusion myself !
> In your situation, a hub and spoke arrangement might make sense (it 
> depends to a certain extent on your network topology). Have one 
> central DHCP server that serves all networks, and a partner for each 
> network.
> Obviously the central server will need to be big enough to handle all 
> the traffic, but each satellite system can be much smaller.
> There was an interesting idea put up a while ago (that was in the 
> context of an ISP setup). In that suggestion, the remotes could keep 
> their lease database in a ram disk - good for performance on a low 
> spec machine. Should the remote site have a power failure, then on 
> startup, the DHCP server can load it's lease database from the central 
> server. Obviously, since the central machine is the only one with 
> non-volatile storage, then it will need to be reasonably reliable (not 
> too hard with server grade hardware).
> As already said, if you wish to do so, then you can script your own 
> checks to detect a partner failure, and automatically put the 
> remaining server into partner down state. What checks you do is up to 
> you - it's your network and only you will have the knowledge to 
> differentiate between failure modes and determine if "can't talk to 
> remote server" really means that it is either down or at least unable 
> to communicate with clients.

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