Windows leases through a PC reset
Simon Hobson
dhcp1 at thehobsons.co.uk
Tue May 8 18:24:04 UTC 2007
Glenn Satchell wrote:
> >Basic question - RFC aside - does anyone have real world knowledge
>(personally
>conducted testing preferred) of whether a Windows machine will
>continue to use a
>valid ISC provided lease through a power down/power up sequence when the DHCP
>server(s) are unreachable ? Does the version of the Windows OS change the
>result ?
>I just tried this on my test network. Client is Win XP Home SP2. This
>should be very easy to re-create on a test network with a single dhcp
>server and however many clients you want to try.
>
>These are the snooped packets from the PC as it booted. Note the last
>two are from the correct IP address.
>
>OLD-BROADCAST -> BROADCAST DHCP/BOOTP DHCPREQUEST
>OLD-BROADCAST -> BROADCAST DHCP/BOOTP DHCPREQUEST
>OLD-BROADCAST -> BROADCAST DHCP/BOOTP DHCPREQUEST
>dhcp-14-249.uniq.com.au -> BROADCAST DHCP/BOOTP DHCPINFORM
>dhcp-14-249.uniq.com.au -> BROADCAST DHCP/BOOTP DHCPINFORM
>
>Running ipconfig /all on the PC after booting shows all the settings
>are still intact. Note that it is possible to configure dhcp to supply
>options that will cause Windows to release the lease when shutdown, but
>this is not the default behaviour in any release.
I've seen this before on the network at my last job - DHCP was down
for some reason and we didn't notice until the Mac users couldn't do
certain things.
I also suspect that the clients do more than just "can't reach a DHCP
server, carry on with last address" - I think they may well check to
see if the default router is the same (or something similar) to see
if they are actually still on the same network, and failing that then
they'll self-assign a link-local address. I think there's an RFC for
this.
As an aside a Windows PC is VERY sticky with it's address. Even if
you release the address, it will still include it as the requested
address in a future discover. It will even request the last static
assignment when you change from static to dynamic.
I discovered this when diagnosing a problem at a customers site. They
have a large campus with multiple buildings all linked by gigabit
ethernet. We had the network management set up a VLAN from the
customers office to the reception desk in one of the other buildings
- and the PC would NOT work, and I couldn't figure out why.
My PowerBook got an address straight away which made me even more
puzzled. Eventually by packet sniffing I figured that their DHCP
server (Windows) wasn't authoritative and so never NACKed the request
for a bad address (picked up from the previous network the PC was on)
that the PC kept asking for, so the PC just never stopped requesting
it ! In the end I statically configured the PC with a valid address
for the network, then switched it back to dynamic - hey presto, that
address now given by DHCP.
More information about the dhcp-users
mailing list