> I see that httrack uses if-modified-since attribute
> to check if the file is modified or not. But for
> dynamic pages (aspx) this attribute will always
> return the date time when the page is requested. 

And thus, the dynamic page should use the "Etag" header field to handle
If-None-Match requests, using the page's entropy (variables, internal or
external, that will be hashed to produce an unique identifier)

Unfortunately most servers are just unable to serve Etags, and tools like
httrack can not efficiently update the corresponding dynamic pages.