CompSci teacher sets lab task: Accidentally breaking the university

Hey! Teacher! Leave our network alone!

Who, Me? At the start of working week, it can sometimes feel like you’re just another brick in the wall and the next five days will require you to carry weight for others. To ease you into the mucky business of exchanging your labor for currency, The Register therefore uses each Monday to offer a fresh instalment of Who, Me? It’s the column in which you admit to escaping your errors and emerging unscathed.

This week, meet a reader we’ll Regomize as “Don” who is now a college professor, but early in his teaching career found himself running a networking course in a dedicated lab that had a Windows PC for every student. Don had his students run Linux instead.

“The lab had its own switch in an open rackmount at the back of the room. That connected to a router which served the building floor that we were on,” Don told Who, Me?

One of Don’s lessons aimed to educate about Dynamic Host Configuration Protocol (DHCP), the protocol that automatically assigns IP addresses to devices on a network. DHCP works its magic with a server that detects client devices and hooks them up.

Don demonstrated DHCP by having all his students disconnect from the campus network. He then fired up a DHCP server in the lab and assigned it a 192.168/16 subnet – that’s the range of IPv4 addresses that aren’t routable on the open internet. Choosing that address range should have meant that his DHCP server didn’t bother anyone outside the lab.

About 15 minutes into the exercise, a member of the university’s tech team purposefully strode into the lab, found to the switch, and “energetically disconnected its backbone link.”

The university techie then glared at Don, who recognized malice and frustration in the look, before exiting the lab.

Don was flustered for a moment but then remembered the university policy of turning off computers when they weren’t being used.

That mean PCs around the building were often turned on, and when they powered up they’d go looking for a DHCP server.

Don had inadvertently made his server the machine most likely to respond first. But as it was built to keep traffic in the lab, folks in the rest of the university who used it could not reach the internet as they desired.

Unbeknown to Don, his DHCP server had caused a torrent of calls to the campus helpdesk. Which was why they tech staffer had stared daggers in his direction.

Don doesn’t know how the uni tech team figured out he was the culprit within minutes of his server booting up, but he’s impressed they did.

And the next time he taught this exercise, he did it with virtual machines on student PCs.

“That we got the machines of the day to run four or more VMs at the same time is rather surprising,” he mused. “Maybe it was a testament to Linux's lighter memory footprint?”

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