Re: how to adjust the number of DHCP require packets

[Jiri Popelka <jpopelka redhat com>]

On 09/14/2011 07:41 PM, Dan Williams wrote:
> 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;

I'm not sure this will help.
As I understand it the server just answers after more than 45 seconds 
(whew, that's really slow as I can tell)
so sending more requests don't speed that up. But you could try.
You can also try to set the 'timeout' variable by adding the line:

timeout 150;

From dhclient.conf man page:
"The  timeout  statement determines the amount of time that must pass 
between the time that the client begins
 to try to determine its address and the time that it decides that it's 
not going to be able to contact a server.
 By default, this timeout is sixty seconds."

> to /etc/dhcp/dhclient.conf.

I've been thinking that NM doesn't use /etc/dhcp/dhclient.conf and 
creates its own
/var/run/nm-dhclient-iface.conf which gets overwritten each time NM runs 
dhclient
so there's no way how to pass some options to NM started dhclient.
I'll be glad if you could educate me in this.

Thanks,
Jiri Popelka