Dhcpd failover with fixed IP only : thoughts ?

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Sep 9 11:22:41 UTC 2013

tom wrote:
>> Fixed IP and dhcpd, I assume you mean you use host statements with
>> fixed-address only? [..]
>Yes. Although not exactly because I use a pool containing only one IP 
>because dhcpd 4.1.1P1 does not support host statement based on option82. 
>Reference: http://www.miquels.cistron.nl/isc-dhcpd/

Same thing applies. Because each server only has one address it can give to the client, and that address can only be given to that client, there is no need toi synchronise servers. Providing your admin process keeps the configs in sync, the client will get the same address regardless fo which server it asks. In effect, teh synchronisation has been done in advance at the same time as allocating addresses.

>> Fixed-address is a special case within the server in that it doesn't get
>> handled like a dynamic lease, so each server can hand out the same address
>> and it will all "just work".
>Great! Can you provide more information about "doesn't get handled like 
>a dynamic lease" ? I am asking mainly because I do use host host {} but 
>a 'fake' pool.

Doesn't apply to you, but if you use a host statement with fixed-address clause, then a dynamic lease record isn't created.
With a dynamic lease (which yours are, even though you are "bypassing the dynamic alocation), when a client comes along, several things happen.
The server picks an address to offer to the client, and records that in a lease record.
After recording it, it offers it to a client.
Assuming the client responds and wants the address, then the server :
Records the fact that the lease has been accepted with a fresh lease record.
Confirms to the client that it's OK.

If the client goes away, then eventually the lease expires and that fact is recorded in a new lease record (and various other internal stuff).

With host declarations & fixed addresses, the process is much simpler: Client asks for lease, server offers it the fixed address, client accepts offer, server confirms it. There are no lease records created - either in memory or on disk. There isn't really any need - the information is there in the config file.

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