static reservation from dynamic pool

Simon Hobson dhcp1 at
Wed Oct 13 20:43:29 UTC 2010

Top posting fixed ...

Adam Moffett wrote:

>>       The answer is yes.  It can cause problems.  The IP address can be given
>>to another system other than the one in the fixed address one.  You have to
>>break up you pool by having different ranges.

>  I wonder I haven't had a problem yet.
>Once the host "foo" has the lease and keeps renewing it, I assume 
>any old host requesting an IP address isn't going to get the IP that 
>"foo" already has.
>Is the danger that when "foo" is offline, someone else can get the IP?

The address can be assigned to another host even if host "foo" is 
online ! Host declarations do NOT go through the normal lease 
processing, and so the server doesn't actually keep any record of 
having given the address to "foo". therefore, it can, and will, at 
some point select that address to give to another client.

By default, the server will do a ping check to see if the address if 
free - but many devices now default to not answering pings because 
some people believe that it somehow enhances security. If the device 
doesn't answer, then the address will be offered to another client.

The client itself may well, and should, check to see if the address 
is in use - and decline it if it is. However, if the client declines 
the offer, the server will keep offering it over and over again.

Obviously, if "foo" is down at the time then another device will be 
offered, and accept, the address. Once "foo" comes back online, it 
too will be offered the address - which it will probably decline. 
Since it's configured to only get that one address, "foo" will then 
simply not get onto the network.

Two solutions :

1) As already stated, split your range so that the fixed address is 
not within any range statement. This is known functionality and a 
known requirement. Eg :
pool {
   range ;
   range ;

2) In newer versions, you can create a reserved lease instead of 
using a host statement with fixed address. This lease is then 
"locked" to a specific client, but otherwise it goes through the 
normal lifecycle - including appearing in the leases file and 
undergoing dynamic DNS.

Simon Hobson

Visit for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

More information about the dhcp-users mailing list