Honoring ETag of cached content

Marc Aymerich glicerinu at gmail.com
Sat Nov 2 01:55:09 UTC 2013

On Sat, Nov 2, 2013 at 12:16 AM, Maxim Dounin <mdounin at mdounin.ru> wrote:

> Hello!
> On Fri, Nov 01, 2013 at 10:29:06PM +0100, Marc Aymerich wrote:
> > Hi,
> > I'm using nginx proxy pass to cache content of our dynamic web
> application.
> >
> > In order to save some bandwidth our client application uses conditional
> > requests based on ETag. However nginx ignores the ETag of cached pages :(
> >
> > What would be required if I want to honor the ETag of cached pages? Is
> > something that can be achieve by means of configuration? or I would need
> to
> > write an nginx module in C?
> It is expected to just work in 1.3.3+.  If you don't see it
> working, you may want to check the version you are running.

Hi maxim,
yeap I'm running 1.4.1 but I never get a 304 :(
This is what a response from cache looks like

# wget --no-check https://[fdf5:5351:1dfd:0:0:0:0:2]/api/ -S --header
'If-None-Match: "83393a3900e4abce27212d7a27cae589"' -q
  HTTP/1.1 200 OK
  Server: nginx/1.4.1
  Date: Sat, 02 Nov 2013 01:40:24 GMT
  Content-Type: application/json
  Transfer-Encoding: chunked
  Connection: keep-alive
  ETag: "83393a3900e4abce27212d7a27cae589"
  Vary: Accept, Cookie, Accept-Encoding
  Expires: Sat, 02 Nov 2013 01:41:24 GMT
  Cache-Control: max-age=60

This is my nginx config for this cached location

location /api/ {
    proxy_redirect     off;

    proxy_ignore_headers Set-Cookie;
    proxy_set_header  Host             $host;
    proxy_set_header  X-Real-IP        $remote_addr;
    proxy_set_header  X-Forwarded-For  $proxy_add_x_forwarded_for;
    proxy_set_header  X-Forwarded-Protocol $scheme;

    proxy_cache          cache;
    proxy_cache_valid    1m;
    expires              1m;

    set $skip_cache 0;
    if ($request_method != GET) {
        set $skip_cache 1;
    if ($http_cookie) {
       set $skip_cache 1;
    if ($http_authorization) {
       set $skip_cache 1;
    proxy_cache_bypass  $skip_cache;

Maybe is there some conflicting configuration? I can not see it :( and
ETags work just fine if I request to my backend server

# wget --no-check -S --header 'If-None-Match:
"77e348fb6260a8dd90ca18c61f7cd472"' -q
  Server: nginx/1.4.1
  Date: Sat, 02 Nov 2013 01:47:30 GMT
  Content-Length: 0
  Connection: keep-alive

Notice the etag value from cached content is different from the fresh one,
even though the content is exactly the same, I presume this is nginx doing
its job of updating the ETag for some differences because of caching, but
still fails to reply a conditional request properly :(

Thanks for your help!!
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