: Should 304 Not Modified responses include the "Last Modified" header? I've been using the most excellent http://redbot.org tool for testing have HTTP headers on my site correct (its custom code

@Kevin317

Posted in: #Apache2 #Http #HttpHeaders #Proxy

I've been using the most excellent redbot.org tool for testing have HTTP headers on my site correct (its custom code in PHP serving dynamic content - wanting to ensure it cached where possible - served by Apache2)

One of the tests says:

The If-Modified-Since response is missing required headers

HTTP requires 304 Not Modified responses to have certain headers, if they are also present in a normal (e.g., 200 OK response).
This response is missing the following headers: last-modified.
This can affect cache operation; because the headers are missing, caches might remove them from their cached copies.


... so resonse doesnt include the Last-Modified header. However the code does try to send it. Investigating further, it seems that Apache uses a whitelist of HTTP headers, it will allow on 304 responses...

if (r->status == HTTP_NOT_MODIFIED) {
apr_table_do((int (*)(void *, const char *, const char *)) form_header_field,
(void *) &h, r->headers_out,
"Connection",
"Keep-Alive",
"ETag",
"Content-Location",
"Expires",
"Cache-Control",
"Vary",
"Warning",
"WWW-Authenticate",
"Proxy-Authenticate",
"Set-Cookie",
"Set-Cookie2",
NULL);
}
else {
send_all_header_fields(&h, r);
}


This is found around line 1281 in modules/http/http_filters.c of the Apache HTTPD 2.2.22 source code.

.... "Last-Modified" is not on that list.

So the question is - which is wrong, redbot, or Apache?
www.w3.org/Protocols/rfc2616/rfc2616-sec10.html doesn't seem to specify if the Last-Modified should be included. (it does say the Etag should be - and that is allowed in Apaches list)

Just incase its useful, this is the test case: redbot.org/?uri=http%3A%2F%2Fwww.geograph.org.uk%2Fhelp%2Fsitemap - my code does include the Last-Modified header - its just that redbot never gets it (nor any custom X-.. headers).

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