Work Life: Dueling DHCP Servers (DNS Errors, Unauthenticated Networks)

Heya!
  Today, I visited a school that was having issues with some of their computers in specific locations. Usually, when a district calls with something like that, we start in the room(s) where the trouble seems to be and work our way outward from there. In this case, there was an elementary lab in which all but one row of computers was throwing up some weird errors.

The machines are running Windows 7, and the network icon in the system tray would change from the standard computer to one with a yellow warning triangle. Then, a message would pop up stating that the computer was connected to the local network but was "unauthenticated." Other computers showed the normal computer icon but when they tried to get online, the browser would throw up messages about unresolved DNS issues, etc.

When the tech would check the IP address on any of the errant machines, the initial address would show the local network scheme. However, when the warning icon appeared, an ipconfig revealed the address had been changed to a 192.168 range.

That range, as many of you know, is where things like wifi routers like to live. So, I asked about switches, other network equipment, etc.  The tech had put in a new switch (new to them) that they had received from another district. Since the "new" switch was an HP Procurve, I thought maybe something was going on there. Plus, that had been the most recent change the tech could remember at the time.

I removed the switch and was about to put in another switch, the tech came in and told me that she remembered they were testing their new Kajeet-based MiFi hotspots at about the same time everything went wacko.

Instantly, we had our answer. You see, when they were testing the hotspots, they were using them in tethered mode, meaning they hooked up the device to the computer via USB cable and let the computer use that as the connection. Well, the hotspot is set up to hand out IP addresses (DHCP). Normally, this is not an issue - the computer gets an IP address and life goes on.

Here is the rub: If the computer in question is ALSO connected to the local network (via Ethernet cable, for example), the hotspot actually communicates down that wired line through the computer it is connected to. So... The hotspot was answering DHCP requests from certain computers on the network. Of course, I want to know why only those specific computers and not, say, computers spread throughout the district. Unfortunately, we may never know the answer because all of the trouble took place the day before I arrived and the hotspots were already disbursed.

So, in a nutshell (the TL;DR version, I suppose you kids would say these days): Erratic computer connectivity might be blamed on multiple DHCP devices on your network. Check for Wireless Routers, tethered hotspots, internet connection sharing, etc.