Software

Why you can't send an e-mail more than 500 miles

There's nothing worse than a user who reports a problem to his helpdesk along with a "description" of the cause--"description" here being a stand-in for "insane supposition that flies in the face of all logic and physics." This story reminds you that sometimes the insane user theory is correct: "We can't send [e-mail] more than 500 miles." It's totally true.

There's nothing worse than a user who reports a problem to his helpdesk along with a "description" of the cause—"description" here being a stand-in for "insane supposition that flies in the face of all logic and physics." This story reminds you that sometimes the insane user theory is correct: "We can't send [e-mail] more than 500 miles." It's totally true.

The details are actually rather involved and cite some specific quirks to Sendmail and SunOS—admins of both are encouraged to read the full recounting—but it breaks down to something like this:

A tech at a research university gets a call from the Chairman of the Statistics Department, who has noted that he can't send email to anyone more than 500 miles away. The tech is incredulous, but the story checks out: He, too, can send an e-mail to anyone within a rough 500-mile radius of campus, but not anywhere outside. Strange as it sounds, the insane user theory was correct...mostly.

Turns out, the tech couldn't reach anyone closer than 500 miles if the recipient's ISP was more than 500 miles away. Impossibly, physical distance had become a limiting factor, arbitrarily cutting off at about 500 miles.

A rare confluence of events lined up to make this happen. In a nutshell, the mail server had been reset to accept zero timeouts on connecting to a remote SMTP server. The university network was also 100 percent switched, so the mail server wouldn't have encountered a timeout of any kind until it hit the receiving mail server. The 500-mile limit was dictated by the speed of electrical transmission between the campus server and the receiving server.

A timeout of zero in Sendmail works out to about 3 milliseconds. Three light-milliseconds is about 558 miles, with a little less for electrical resistance in the line. Anything further away would timeout. Once the tech reset the default timeout, everything was fixed.

Insane, but true. Don't be so quick to discount those crazy user theories next time.

(Found via reddit.)

About

Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger — amusing and abusing IT pros. Read his full profile. You can a...

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