Raphaël Huck wrote:
As I understand it, the client should try an arping before using the
address it has been given by the server.
The interesting question is whether WinXP actually implements this
arping. If Windows follows the specification on this, there should be
no point in the server using arping.
I'm going to check if WinXP does the arping check.
DHCP client computers running Windows 2000 or Windows XP that obtain an IP
address use a gratuitous ARP request to perform client-based conflict
detection before completing configuration and use of a server offered IP
address. If the DHCP client detects a conflict, it will send a DHCP decline
message (DHCPDECLINE) to the server.
So it seems that Windows XP does an ARP check before using an IP address.
I'll check that with Wireshark on Monday.
But the other problem with a SOHO including a DHCP server is that a SOHO
is often rebooted (because the user changed settings which require a
reboot, because of a power failure,...).
So each time it reboots, the lease file is cleared.
Which is a bug in the SOHO. Assuming by SOHO you mean WRT-54G-class
stuff, then you might like to consider moving to DD-WRT. I worked with
them to add hooks into dnsmasq so that the lease file can be stored in
the non-volatile RAM and not trashed on a reboot. You might also like to
consider asking the developers of your current firmware to implement the
same thing.
Could you give me pointers on how to do that?
Set the (rather misnamed) --leasefile-ro flag. That actually stops
dnsmasq from using a lease file at all. Instead it relies a script which
gets run at start-up and whenever a lease changes, to maintain the lease
database. Dnsmasq needs to be pointed at this script using the
--dhcp-script option.
Using this mechanism, it is possible to store the lease database in any
storage system (a SQL database, for instance). The dnsmasq distribution
has a sample script (in contrib/wrt/lease_update.sh) which uses the
"nvram" command to keep the lease database in the router's non-voltile
memory. This is exactly what you need.
Is it always safe to reload an old lease file on reboot?
Yes. On a system which doesn't maintain system time over a reboot,
dnsmasq needs to be compiled with HAVE_BROKEN_RTC set to cope with its
idea of time changing over a reboot. This likely to be the case on a
WRT-class router.
I have the issue only in this case: the SOHO reboots, the WinXP PC
(firewall enabled) has 192.168.1.20, the Linux PC also has 192.168.1.20,
and in the lease file of dnsmasq, 192.168.1.20 corresponds to the MAC
address of the WinXP PC.
That does look like WinXP might be broken: I'd be interested in the
results of your tests.
Again, I'll have to check with Wireshark on Monday. Why do you think it's
WinXP the problem?
As far as I can see, the only order of events which could get to the
situation you see is:
Linux box has lease on 192.168.1.20
router reboots (and clears leasefile)
Windows box takes a lease and gets 192.168.1.20
If the ARP check had worked during the windows box lease-aquisition, it
would have seen the Linux box on 192.168.1.20, and taken an alternative
address.
Cheers,
Simon.
