NGINX proxy_pass not caching content - caching

Question

I'm having issues getting NGINX to cache thumbnails that I'm pulling from Dropbox using the proxy_pass command. On the same server that NGINX is running I run the following command multiple times
wget --server-response --spider http://localhost:8181/1/thumbnails/auto/test.jpg?access_token=123
and get the exact same response with X-Cache: MISS every time
HTTP/1.1 200 OK
Server: nginx/1.1.19
Date: Wed, 25 Mar 2015 20:05:36 GMT
Content-Type: image/jpeg
Content-Length: 1691
Connection: keep-alive
pragma: no-cache
cache-control: no-cache
X-Robots-Tag: noindex, nofollow, noimageindex
X-Cache: MISS
Here's my meat of my nginx.conf file .. any ideas on what I'm doing wrong here?
## Proxy Server Caching
proxy_cache_path /data/nginx/cache keys_zone=STATIC:10m max_size=1g;
## Proxy Server Setting
server {
listen *:8181;
proxy_cache STATIC;
proxy_cache_key "$request_uri";
proxy_cache_use_stale error timeout invalid_header updating
http_500 http_502 http_503 http_504;
location ~ ^/(.*) {
set $dropbox_api 'api-content.dropbox.com';
set $url '$1';
resolver 8.8.8.8;
proxy_set_header Host $dropbox_api;
proxy_cache STATIC;
proxy_cache_key "$request_uri";
proxy_cache_use_stale error timeout invalid_header updating
http_500 http_502 http_503 http_504;
add_header X-Cache $upstream_cache_status;
proxy_pass https://$dropbox_api/$url$is_args$args;
}
##Error Handling
error_page 500 502 503 504 404 /error/;
location = /error/ {
default_type text/html;
}
}

Answer 1

Turns out that thumbnail requests returned from Dropbox include the header
Cache-Control: no-cache
and Nginx will adhere to these headers unless they are explicitly ignored which can be done by simply using the following config line that will ignore any caching control.
proxy_ignore_headers X-Accel-Expires Expires Cache-Control;
We also had issues placing the "proxy_ignore_headers" option in different areas within the nginx.conf file. Finally after much playing around we got it to work by explicitly setting it in the "location" block. The full snippet of the config file can be found below
## Proxy Server Caching
proxy_cache_path /data/nginx/cache levels=1:2 keys_zone=STATIC:50m inactive=2h max_size=2g;
## Proxy Server Setting
server {
listen *:8181;
location ~ ^/(.*) {
set $dropbox_api 'api-content.dropbox.com';
set $url '$1';
resolver 8.8.8.8;
proxy_set_header Host $dropbox_api;
proxy_hide_header x-dropbox-thumbcachehit;
proxy_hide_header x-dropbox-metadata;
proxy_hide_header x-server-response-time;
proxy_hide_header x-dropbox-request-id;
proxy_hide_header cache-control;
proxy_hide_header expires;
add_header cache-control "private";
add_header x-cache $upstream_cache_status; # HIT / MISS / BYPASS / EXPIRED
proxy_cache STATIC;
proxy_cache_valid 200 1d;
proxy_cache_use_stale error timeout invalid_header updating
http_500 http_502 http_503 http_504;
proxy_ignore_headers X-Accel-Expires Expires Cache-Control;
proxy_pass https://$dropbox_api/$url$is_args$args;
}
}

Answer 2

In order to cache the proxy response the request between Nginx and origin should be cookie-less:
proxy_hide_header Set-Cookie;
proxy_ignore_headers Set-Cookie;
See full configuration with invalidation methods: https://gist.github.com/mikhailov/9639593

Answer 3

If above answers didn't solved your issue, try this:
proxy_cache_valid 200 2d;
(or whatever amount of time and whatever response code you want)
add this where you are using or activating your proxy_cache <keys_zone_name>.
Apparently for me as soon as I remove proxy_cache_valid parameter caching status doesn't show up. Also documentation doesn't says that this is required field. Let me know if this works for you. So we might update documentation.
I expect expect proxy_cache get started page should show that you need 3 at least these parameters to get started: proxy_cache_path, proxy_cache and proxy_cache_valid