Get only 200 responses when "if_modified_since" is set to "off"

[rjorel@…]

Hello,

These last days, I tried to understand how web servers like Nginx deal with cache control of resources like JS, CSS or font files. So I learned some things about If-Modified-Since and If-None-Match headers that allows to determine if a resource must be re-sent or not.

I thought that reading source code should be the best solution to understand how etags are generated and how Nginx uses them to generate 304 responses. But I was surprised to read that in src/http/modules/ngx_http_not_modified_filter_module.c:

        if (r->headers_in.if_modified_since
            && ngx_http_test_if_modified(r))
        {
            return ngx_http_next_header_filter(r);
        }

        if (r->headers_in.if_none_match
            && !ngx_http_test_if_match(r, r->headers_in.if_none_match, 1))
        {
            return ngx_http_next_header_filter(r);
        }

whereas I see on MDN web docs and RFC 7232 that If-None-Match takes precedence over If-Modified-Since if web server supports it.

I supposed it's an optimization because etags seems to be generated from last modified times (if I well understood ngx_http_set_etag function in src/http/ngx_http_core_module.c), so checking last modified times before etags is almost the same thing.

So, to be sure that resource validation via etags was done after last modified times, I deactivate If-Modified-Since check with directive if_modified_since off;. After that, I no longer got any 304 responses but only 200 ones. I thought that etag check didn't work, but reading ngx_http_test_if_modified function gave me a clue:

static ngx_uint_t
ngx_http_test_if_modified(ngx_http_request_t *r)
{
    time_t                     ims;
    ngx_http_core_loc_conf_t  *clcf;

    if (r->headers_out.last_modified_time == (time_t) -1) {
        return 1;
    }

    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

    if (clcf->if_modified_since == NGX_HTTP_IMS_OFF) {
        return 1;
    }

    // ...

By default, Nginx sends Last-Modified header, so browsers send back If-Modified-Since one. But as I set to off the check, I think ngx_http_test_if_modified stops at the 2e if.

To be sure, I removed If-Modified-Since header using cURL tool, and I also found how to remove Last-Modified header from response. I correctly got 304 responses using these 2 methods separately.

I suppose you're aware of that, but as I cannot be sure I think it's a good idea to report the "issue" to you.

I just have two questions to be sure I well understood:

1. Am I correct in thinking that the check order is an optimization because etags are generated from timestamps (I suppose checking timestamps is faster than checking etags) ?

2. Is this behavior when deactivating If-Modified-Since check normal? Because thinking to remove Last-Modified header from response is not obvious to guess here, if we want to keep etag check only.

Regards.
Raphael Jorel