502 Proxy Error and ISA Server 2004

It took me a bit to figure out why this ISA server was serving users with this error when they didn't ask for it: 

  • Error Code: 502 Proxy Error. Cannot complete this function. (1003)
  • IP Address:
  • Date: 21.9.2007 5:39:07
  • Server: isaserver
  • Source: proxy

This is what the users got in their browsers when they tried to surf to their website of choice that morning.



Basically there were three reasons this took me more than 5 minutes:
  • The problem was just too basic
  • Error didn't give any clue to the real problem
  • It was 7:30 a.m. and I was still half asleep :-)

I check the usual stuff for any hint what might be causing this. I checked the disk space and there was plenty of it. Nothing unusual in the Event Logs, ISA services were running fine and nothing unusual in ISA Server MMC.

Since there was almost nothing left to check, I check network cards. Here I noticed that external Network Interface Card (NIC) has been disabled. Once I enabled it, the problem went away and I was able to get another half an hour of sleep.

I wish I would get something like "Could not connect" instead of "Error Code 502 Proxy Error".

Note to self. Don't forget to check the basics. :-)

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