Broadband (Cable, DSL, Satellite, Fiber Optics)/DHCP Request

Advertisement


Question
QUESTION: Dear Sir,

I am allocating IP addr with DHCP Server to my clients with 300Sec a leased time...

I understood renewal time = 50% * leased time, hence After 150 sec i am expecting
request from clients to sever for renewal....

But when i captured traffic by  wireshark, DHCP request interval from client is not
uniform and changing from 117sec to 147sec

Why ? What could be the reason??? I am not able to send snap shots & wireshark
capture by attachment...

Regards
M Srinivasa Rao

ANSWER: All the DHCP clients I've ever worked with have problems with leases shorter than 24 hours.  Often they will not request a release at all, or they will popup periodic network error messages to the user even if they are able to successfully get a re-lease.

Nothing in the DHCP standard prohibits a client from requesting a renewal anytime during the lease.  For example you might issue a number and the client might go into suspension or hibernation multiple times during the life of the lease.  The DHCP server must honor those requests.  So, the standard allows the clients to issue a renewal when they feel like doing it.

Note that a DHCP request that is not a renewal request is a broadcast request not a unicast request.  Excessively short DHCP lease times increase broadcasting since some clients will just issue broadcasts.  It is not considered good network practice to use DHCP lease times shorter than a day.

---------- FOLLOW-UP ----------

QUESTION: I was shocked to know that type of behavior of DHCP client ...
I have not seen in any blog....

But why it is like so ???
Then can i say it is bug???

I want to share a screen shots and .pcap  file on this ... How can i do it??

Regards

ANSWER: It is not a bug for the client to request a re-lease any time SHORTER the max lease.  If your lease is 300 seconds the client is allowed to request a re-lease any time from immediately after getting the IP number up to 300 seconds.

The other behavior I mentioned is a bug, but you didn't say that you observed it.  But, as I said, it's not good network practice to use 300 second lease times.

---------- FOLLOW-UP ----------

QUESTION: With the help of netstat the way we can see tcp-socket connection status like fin_sent, established, time_wait, close_wait etc...

in similar manner "netstat -un | grep :67" command i am not giving states like bind, renew, rebind states... WHY??????????????? Can u share some idea on this??????

If netstat is not the right way, what is the command at client and server side?

Regards

Answer
You need to run the DHCP server in debug mode to see what is happening.

I would suggest netstat -n -p udp however

Also, the socket is going to be created then deleted for an instant.  I don't know if you will be able to see it with netstat unless your very lucky to hit it at the right instant.

on Linux, the dhcp client function is run by a client program.  You can run that in debug mode also.

Broadband (Cable, DSL, Satellite, Fiber Optics)

All Answers


Answers by Expert:


Ask Experts

Volunteer


Ted Mittelstaedt

Expertise

I will answer questions on the following kinds of High Speed Internet connectivity; ISDN, Frame Relay, dedicated T1, higher speeeds such as DS3/OC3, wireless such as 802.11 and DSL. No questions on dialup V90 or Cable access. I can also answer questions on many types of Cisco routers, and a number of other brands of business routers as well.

Experience

I am the technical manager of Internet Partners Inc. an ISP.

Publications
I've written a book titled "The FreeBSD Corporate Networker's Guide" published by Addison Wesley

©2016 About.com. All rights reserved.