Question

dhcp server leases

Asked by: credog


We have a limited pool of ip addresses being served out from our redhat linux dhcp server.  Sometimes we seem to run out of addresses and some clients can not get an ip.  I thought I would decrease the lease time on the server so clients are required to renew more often.  When clients that have been turned off do not renew, the ip address would become available sooner.  Does this seem like it would help?  If a client (windows box or linux box) gets turned off when does the server know that the address is now available?  Do clients send a release message to the server or does the server just release it when the lease runs out?  Also, if you know that a client  has been shut down and the ip is available, is there anyway to force the server to place the ip back in the pool manually?

Thanks

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Asked On
2005-12-20 at 17:34:32ID21671369
Tags

dhcp

,

lease

Topic

Linux Networking

Participating Experts
3
Points
250
Comments
8

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Answers

 

by: jleviePosted on 2005-12-20 at 18:05:28ID: 15523602

If a client fails to renew its lease before expiration for any reason the DHCP server will return that IP to the free pool. Clients can release an IP at any time, though that is usually associated with negotiationg for a different one.

Who/what is imposing the limit on the pool and is the pool larger than the max client population?

For just about all cases now one will be using IP's from one of the RFC 1918 private networks for local clients. Accordingly there should be no reason to have a pool that's too small. However, windows clients aren't the best behaved network citizens and I've seen them take a different IP, rather that asking for or accepting the previous, sometimes when they reboot. Since one might reboot a windows box a number of times in a day that can eat up a DHCP pool that should be big enough (more IP's than possible clients).

 

by: credogPosted on 2005-12-20 at 20:43:02ID: 15524122

In our case we are using routable (not private space) ips on our dhcp server.  Until we re-vamp a few things we are limited by the  routable ip space we were assigned.  Are you aware of a way to put an ip back into the pool if you know the client has been turned off or otherwise is not using the ip?  Or do you just have to wait until the lease expires?  This gets me back to setting a shorter lease time to possibly put an ip back into the pool quicker?   Also, is the dhcpd.leases file the actually database that the deamon consults when giving out ips?  If so, could you possibly remove an ip stanza  that you know is no longer being used but the lease has not yet expired effectively putting it back in the pool?  I know this is not something you would want to do all the time, but if it is possible it would help when you really need an ip.

Thanks    

 

by: credogPosted on 2005-12-20 at 20:52:49ID: 15524159

Forgot to ask another question, is there any way on the  server to see if/what ips are available and in the pool?  Or do you just have to look in the dhcpd.leases file and figure it out?

 

by: ravenplPosted on 2005-12-20 at 23:51:55ID: 15524633

You have to look into dhcpd.leases and find last entry for particular IP.
>Are you aware of a way to put an ip back into the pool if you know the client has been turned off or otherwise is not using the ip?
No such way, except deleting the dhcpd.leases(or removing entries for that IP) and restarting dhcp server.

 

by: Cyclops3590Posted on 2005-12-21 at 07:46:03ID: 15527094

There are generally some tools installed with bind, although it is a different rpm file.  I use Mandriva with bind and my package name is bind-utils.

At any rate, I'd say just set the max lease time to a smaller number like you are thinking and see how it goes.  Remember that the clients will try to renew the lease at the 50% time to expiration and the 87.5% time to expiration levels.

 

by: Cyclops3590Posted on 2006-04-08 at 16:59:27ID: 16409596

No comment has been added to this question in more than 21 days, so it is now classified as abandoned.
I will leave the following recommendation for this question in the Cleanup topic area:
Split: jlevie & ravenpl & Cyclops3590

Any objections should be posted here in the next 4 days. After that time, the question will be closed.

Cyclops3590
EE Cleanup Volunteer

20120131-EE-VQP-002

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