Re: how to adjust the number of DHCP require packets

[Dan Williams <dcbw redhat com>]

On Wed, 2011-09-14 at 10:46 +0800, cute walker wrote:
> Hello.
> I have a problem and need your help .
> On my laptop it runs Ubuntu 10.04 , gnome 2.30.2 and NM 0.8 . I have
> enjoying them for more than one year. But nowadays I have a problem.
> My laptop can not access the campus' wifi network for most time I try.
> After communication with the network administrator, we find the
> problem happens in the process of IP obtainment via DHCP. By capturing
> packets using wireshark, we find that the DHCP server is slow and my
> laptop  need 4 or more DHCP require packets before it can get a DHCP
> offer packet. But, after 3 DHCP require packets, my laptop will stop
> trying and deassociates the link on layer 2( IEEE802.11) . 
> 
> Of course, the network administrator has his work to speed up the DHCP
> server. But before he does, I still can not access the network.
> 
> Now my idea is to adjust the number of DHCP require packets while
> tring to access, namely, setting it bigger enough to get the DHCP
> offer packet. ( In fact , we find the Windows keep sending DHCP
> require packets  for many times, so Windows do not suffer this problem
> ). But I don't know how to do that.
> 
> So, can anyone of you give me a simple way to adjust the parameter?

The actual number of packets sent is determined by dhclient itself; it
uses a randomized backoff algorithm to ensure it doesn't flood the DHCP
server with requests.  The entire process is bounded by a 45-second
timer, because if your DHCP server really doesn't respond within 45
seconds, there's something really really wrong with the network or the
DHCP server, and that should be fixed.  Bumping this timeout to 60
seconds or so has been discussed before and maybe we'll do that.  But in
any case, there is one dhclient variable that you could change, and
that's the 'initial-interval'.  Setting this to 1 makes dhclient
immediately emit the second request, instead of waiting a random period
of time.  The theory is that if you have 100 machines that all turn on
at the same time (computer lab) and all 100 machines immediately emit
DHCP requests, that's effectively a DDoS against the DHCP server.  You
can modify this behavior by adding the line:

initial-interval 1;

to /etc/dhcp/dhclient.conf.

Dan