Trouble with apt-get and Squid

I recently started having trouble with APT transactions on my Kubuntu desktop. “apt-get update” would fail for some source entries with the error “The HTTP server sent an invalid reply header”. I thought it was something specific to (K)Ubuntu, but when I had the exact problem on my NSLU2 running Debian I figured the problem must be elsewhere…

I’d recently updated the machine that provides the transparent web proxy function for the network; one of the updates took Squid up to version 3.0 (from 2.6). This was the first thing I was suspicious of.

There’s an option in Squid that controls how it handles an “If-Modified-Since” request from a client. The default is for Squid to respond based on the age of the item in the cache, not based on the real item on the source web page. The comments in the Squid config file indicate that some clients use an IMS when requesting a reload — looks like APT is one of those clients.

Setting this option to “on” (from the default of “off”) in squid.conf fixed the issue for me:

refresh_all_ims on