How to know when to send a 304 Not Modified response

Question

I'm writing a resource handling method where I control access to various files, and I'd like to be able to make use of the browser's cache. My question is two-fold:

1. Which are the definitive HTTP headers that I need to check in order to know for sure whether I should send a 304 response, and what am I looking for when I do check them?

2. Additionally, are there any headers that I need to send when I initially send the file (like 'Last-Modified') as a 200 response?

Some psuedo-code would probably be the most useful answer.

---

What about the cache-control header? Can the various possible values of that affect what you send to the client (namely max-age) or should only if-modified-since be obeyed?

Answer 1

Here's how I implemented it. The code has been working for a bit more than a year and with multiple browsers, so I think it's pretty reliable. This is based on RFC 2616 and by observing what and when the various browsers were sending.

Here's the pseudocode:

server_etag = gen_etag_for_this_file(myfile)
etag_from_browser = get_header("Etag")

if etag_from_browser does not exist:
    etag_from_browser = get_header("If-None-Match")
if the browser has quoted the etag:
    strip the quotes (e.g. "foo" --> foo)

set server_etag into http header

if etag_from_browser matches server_etag
    send 304 return code to browser

Here's a snippet of my server logic that handles this.

/* the client should set either Etag or If-None-Match */
/* some clients quote the parm, strip quotes if so    */
mketag(etag, &sb);

etagin = apr_table_get(r->headers_in, "Etag");
if (etagin == NULL)
    etagin = apr_table_get(r->headers_in, "If-None-Match");
if (etag != NULL && etag[0] == '"') {
    int sl; 
    sl = strlen(etag);
    memmove(etag, etag+1, sl+1);
    etag[sl-2] = 0;
    logit(2,"etag=:%s:",etag);
}   
... 
apr_table_add(r->headers_out, "ETag", etag);
... 
if (etagin != NULL && strcmp(etagin, etag) == 0) {
    /* if the etag matches, we return a 304 */
    rc = HTTP_NOT_MODIFIED;
}   

If you want some help with etag generation post another question and I'll dig out some code that does that as well. HTH!