update static leases

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Aug 22 07:14:07 UTC 2007

Richard Smits wrote:

>Can anyone please give me some information about this ? I am confused.
>The following scenario :
>ISC DHCP server with the folowing (relevant) options :
>ddns-hostname=                 host-decl-name;
>ddns-update-style              interim;
>ddns-updates                   on;
>zone domain.net.              {primary; key DHCP_UPDATER;}
>use-host-decl-names on;
>update-static-leases on;
>subnet netmask {
>   option domain-name             "domain.net";
>Folowing are the host statements......
>DDNS updates are working fine, but the records of old pc's are NOT
>removed. If I read the manual right, the "update-static-leases on" flag
>is the reason for this, because :
>It is not recommended because the DHCP server has no way
>to tell that the update has been done, and therefore will not delete the
>record when it is not in use.
>Now the big question : If I CHANGE an IP number or a Hostname , will the
>record be updated ???

Possibly. If the hostname is the same then the records should get 
replaced, but if the hostname is changed then NEW records will be 

>The record excists, so the server would be able to change it, right ?

Wrong. The way fixed host declarations work is to bypass much of the 
normal processing - including recording the lease in the leases file. 
It is through the leases file that the server records what DNS 
records it has created, just deleting (or modifying) a host 
declaration with fixed address merely removes all knowledge of it (or 
it's previous state) from the servers history.

You would probably be better off with reserved leases which is a new 
feature in 3.1.0. Here you set a lease to reserved (currently 
requires editing the leases file or using omapi) and then the lease 
is reserved for that client rather than getting recycled eventually 
for another client. But beyond this it has two important differences: 
the lease goes through the normal state processing (so dns creation & 
removal) and I believe all the class processing is done (so class 
specific options should work properly).

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