how to force new leases (not renewals)

Jeff Haran jharan at Brocade.COM
Tue Jan 26 18:23:08 UTC 2010


> -----Original Message-----
> From: dhcp-users-bounces+jharan=brocade.com at lists.isc.org 
> [mailto:dhcp-users-bounces+jharan=brocade.com at lists.isc.org] 
> On Behalf Of Simon Hobson
> Sent: Tuesday, January 26, 2010 12:07 AM
> To: dhcp-users at lists.isc.org
> Subject: Re: how to force new leases (not renewals)
> 
> Justin DeMaris wrote:
> >One very unconventional trick we have used to push out lease changes 
> >is to do shut/no shut on the switch port interface. This only works 
> >for devices wired directly to the switch with no active 
> >amplification (hub, etc) in between but it makes windows and linux 
> >do a full new lease since it feels like it got unplugged
> 
> Does it still do that ? Some modern clients do tricks like checking 
> if the MAC address of the default router is still the same to 
> determine if they are on the same network after bringing up an 
> interface.
> 
> Back to the original issue, I'm sure it's been reported before - 
> WIndows clients changing their address but not recognising any 
> changed options.

Though sometimes a little obstinance on the part of the client is a good thing.

Over the Xmas break I upgraded the Suse Linux distro I use on my home PC. After doing so I found network access through my long time ISP (Comcast cable) to be quite flakey. It turned out to be caused by Comcast's DHCP servers offering leases with MTU options of (IIRC) 576 bytes rather than the normal 1500. When I configured the client to ignore the MTU from the server and use 1500 instead, everything worked fine.

Presumably the Windows clients that most of Comcast's customers use ignore bogus MTUs like this by default.

Way to go, Bill...

Jeff Haran



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