> >I also have noticed windows doing that, not really sure the exact
mechanism
> >they use, maybe looking at the ARP.
>
>
> I have not seen this behavior under Windows. But then, I can't recall ever
> duplicating an IP address on a LAN I was managing (and I don't have enough
> non-essential hosts running here to do proper tests without disrupting the
> operation of the LAN). Could someone who has seen it please describe it a
> bit more exactly?
>
> Under what circumstances does it occur? For example, if WinXX host A has
IP
> address a.b.c.d, and WinXX host B tries to connect to the LAN using the
> same address, do *both* machines report an error right away? If not both,
> then which one? Does the version of Windows involved affect the answer?
> Does the use of a switch (rather than a hub) affect the result? What about
> a router doing proxy arp for one of the hosts?
My experience with NT 4 and Win2K has been that whichever machine has the IP
address first will continue to function without interruption should another
machine boot up with the same IP address. Yes, both machines report an error
right away. I just did some quick testing with two Win2K machines using a
hub (I'll try a switch later at work) and here were the configurations and
results:
1) My laptop (host A) running Win2K with a DHCP-assigned address of
192.168.0.17 was up and running.
2) I statically changed the IP address of a Win2K desktop (host B) to the
same address. Host B immediately popped up the message: "The static IP
address that was just configured is already in use on the network. Please
reconfigure a different IP address."
3) Checking the Event Log on Host B shows: "The system detected an address
conflict for IP address 192.168.0.17 with the system having network hardware
address 00:02:3F:34:3D:6A. The local interface has been disabled."
4) At the same time, host A displayed the following: "The system has
detected an IP address conflict with another system on the network. Network
operations on this system may be disrupted as a result. More details are
available in the system event log. Consult your network administrator
immediately to resolve the conflict." Host A continued to function on the
network without any problems.
I then tried rebooting Host B (leaving the same IP address). This time:
1) No message was displayed on Host B. Checking the Event Log showed the
same message: "The system detected an address conflict for IP address
192.168.0.17 with the system having network hardware address
00:02:3F:34:3D:6A. The local interface has been disabled." Doing an ipconfig
showed 0.0.0.0 for the IP address, so it was truly disabled.
2) Host A again displayed: "The system has detected an IP address conflict
with another system on the network. Network operations on this system may be
disrupted as a result. More details are available in the system event log.
Consult your network administrator
immediately to resolve the conflict" and it continued to function on the
network.
I then unplugged Host B from the hub and rebooted. Upon plugging the cable
into the hub:
1) The same message was displayed on Host A and it continued to function on
the network
2) No message was displayed on Host B again. Checking the Event Log showed a
different - and misleading - message: "The system detected that network
adapter \DEVICE\TCPIP_{E766D086-EB38-4BA2-8B02-68D202C66B14} was connected
to the network, and has initiated normal operation over the network
adapter." However, the machine didn't have an IP address, nor could it
function on the network.
> Does Windows-based (SMB) directory sharing seem to play any role in when
> this does and does not work?
Not to my knowledge. Both of my machines map the same drives. Host A never
lost connectivity to the drives and Host B was unable to connect to any.
> Will Windows host A or B (as appropriate) detect the duplication if the
> other device is not a Windows PC (a Mac, or a Linux host, or an embedded
> device like an ISDN router)?
Good question. I'll test this with Linux later. One thing I can say is that
I use a Linux box as an SMB server at home. If a Windows 2000 machine boots
up with the same IP address, it hijacks the IP, i.e., the Windows machine
gets the IP and I can no longer connect to my Linux SMB server.
BTW, I do believe that Win9x has the same built-in protection, but it's been
a while and I don't have any boxes running those "operating systems" (for
lack of a better term) to test them. I do remember that IP addresses could
be hijacked under Win 3.x but I'm not sure which IP stacks were affected by
this.
.
I'll try to do some more in depth testing later.
-sr
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