Solved: dhcp and client

sarahr202 · ‎05-09-2010

Hi every body.

I have two question

1) when dhcp release message is sent by client?

i was reading about Dhcp release message. dhcp message is sent by client to tell dhcp server to release the binding and the ip address is available to be re- assigned. The book does not say any thing if the client such as windows xp sends this message.

2) please consider the following:

Let say an hotel provides internet access to its guests. Let say 100 guests are assigned ip addresses by dhcp server to access internet. All these guests all of sudden unplug their laptops . Since dhcp server did not receive any dhcp relase messages, therefore it will maintain those bindings until their leases expires. Now we have a situation where these 100 ip addresses though not in use, can not be assigned as dhcp server still believe they are being used.

thanks and have a good day

david_casey · ‎05-09-2010

For part #2, I'm not sure I see your question but it's a good idea not to have too large of a lease-time in a situation like that. If the network admin began to receive reports that end-users were not obtaining an IP address they could either decrease the lease-time or increase the subnet size.

Dave

View solution in original post

Jon Marshall · ‎05-09-2010

sarahr202 wrote:

Hi every body.

I have two question

1) when dhcp release message is sent by client?

i was reading about Dhcp release message. dhcp message is sent by client to tell dhcp server to release the binding and the ip address is available to be re- assigned. The book does not say any thing if the client such as windows xp sends this message.

2) please consider the following:

Let say an hotel provides internet access to its guests. Let say 100 guests are assigned ip addresses by dhcp server to access internet. All these guests all of sudden unplug their laptops . Since dhcp server did not receive any dhcp relase messages, therefore it will maintain those bindings until their leases expires. Now we have a situation where these 100 ip addresses though not in use, can not be assigned as dhcp server still believe they are being used.

thanks and have a good day

Sarah

1) It depends. The RFC says that a client when it is gracefully shutdown should send a DHCP release message to the DHCP server but some OSs do and some don't. And the RFC also says that even if the client doesn't send a DHCP release this should not affect the correct operation of DHCP.

WIndows XP as far as i know does not send a DHCP release when rebooted. If the lease is still active when it reboots it sends a DHCP request. If the lease has expired it sends a DHCP discover.

2) Correct. So you need to -

a) make sure you have enough IP addresses in your pool

and

b) make the lease time short ie. in a corporate LAN a lease time of days is not unusual. In a hotel environment you would probably looking at less than an hour for a lease.

Jon

View solution in original post

david_casey · ‎05-09-2010

For part #2, I'm not sure I see your question but it's a good idea not to have too large of a lease-time in a situation like that. If the network admin began to receive reports that end-users were not obtaining an IP address they could either decrease the lease-time or increase the subnet size.

Dave

Jon Marshall · ‎05-09-2010

sarahr202 wrote:

Hi every body.

I have two question

1) when dhcp release message is sent by client?

i was reading about Dhcp release message. dhcp message is sent by client to tell dhcp server to release the binding and the ip address is available to be re- assigned. The book does not say any thing if the client such as windows xp sends this message.

2) please consider the following:

Let say an hotel provides internet access to its guests. Let say 100 guests are assigned ip addresses by dhcp server to access internet. All these guests all of sudden unplug their laptops . Since dhcp server did not receive any dhcp relase messages, therefore it will maintain those bindings until their leases expires. Now we have a situation where these 100 ip addresses though not in use, can not be assigned as dhcp server still believe they are being used.

thanks and have a good day

Sarah

1) It depends. The RFC says that a client when it is gracefully shutdown should send a DHCP release message to the DHCP server but some OSs do and some don't. And the RFC also says that even if the client doesn't send a DHCP release this should not affect the correct operation of DHCP.

WIndows XP as far as i know does not send a DHCP release when rebooted. If the lease is still active when it reboots it sends a DHCP request. If the lease has expired it sends a DHCP discover.

2) Correct. So you need to -

a) make sure you have enough IP addresses in your pool

and

b) make the lease time short ie. in a corporate LAN a lease time of days is not unusual. In a hotel environment you would probably looking at less than an hour for a lease.

Jon

sarahr202 · ‎05-16-2010

Thanks Jon and Dave.

sarahr202 · ‎05-16-2010

thanks Jon.

If you don't mind, i have few more questions.

1)Let say we have a client which was assigned ip address 199.199.199.2 by dhcp server, sends a dhcp release message upon shutdown.

Upon receiving that message, ip address 199.199.199.2 is returned to pool. Now dhcp has following addresses in its pool.

199.199.199.1

199.1991.199.2

After 30 minutes,the same client powers up and sends a request to dhcp server for ip address

Will the dhcp server assign 199.199.199.2 to client or it will assign 199.199.199.1 ?

I also read that when a client first powers up, it checks the lease of last ip address used if it is still valid, client then sends the dhcp request with the ip address it was assigned before to dhcp server.

Upon receiving the request, dhcp server first check if that ip address is available . if that ip address is available,it will assign that ip address to the client.

Now here is my problem. let say client sends a dhcp release message before it shuts down to dhcp server . We power up the client Now what will this client do? will it send dhcp request carrying the last ip address used provided that lease is still valid? Will client upon sending dhcp release message also erase the lease?

thanks a lot.

sarahr202 · ‎08-30-2010

I found the answer after a long search. Please see tcp/ip guide on line and check dhcp section.

Dhcp release message is only sent because of user action for e.g you want to relase ip address binding , you release the dhcp message .

I erroneously said in my earlier post when host shutdowns, it sends the dhcp release message. dhcp release message is sent as a result of user action.

When dhcp release message is sent, host loses the current ip binding and dhcp server also removes the binding and returns the address to the pool to be assigned. When host powers up, it will send dhcp discover message not dhcp request as there is no ip address binding currently on the host.

thanks and have a great day.