> my tests seem to suggest that the If-Modified-Since 
> is seen by the cache of the httrack and not the 
proxy 
> which in this case is squid. Is there a work around?
What do you mean? Squid should leave the If-Modified-
Since untouched - and it is (should) be the client 
(httrack) job to handle the rest. Try to activate the 
extended debug mode (Options/Experts only/Activate 
debug mode) and check the hts-ioinfo.txt file 
(warning: recheck this option each time you want to 
get the log, as the checkbox is not saved among with 
other options) - the requests/responses should show 
some interesting things.