Nginx proxy cache expires too soon

Question

I'm using nginx 1.9.9 as a proxy for Gravatar (drops the waiting time for an avatar from 300ms to 15ms ). However, the cache expires in just a few minutes while I specified it to expire in 7 days. In just a few minutes I receive the header X-Proxy-Cache: EXPIRED.

The configuration is as follows:

proxy_cache_path /var/www/gravatar-cache levels=1:2 keys_zone=gravatar:8m inactive=7d use_temp_path=off max_size=1000m;
proxy_cache_key "$scheme$request_method$host$request_uri";

server {
    listen 443 ssl http2;
    server_name [hidden];

    access_log off;

    ssl_certificate [hidden];
    ssl_certificate_key [hidden];

    location /avatar/ {
        expires 365d;
        proxy_cache gravatar;
        proxy_cache_valid any 7d;
        proxy_pass https://secure.gravatar.com;
        proxy_ignore_headers Cache-Control;
        proxy_hide_header link;
        proxy_hide_header source-age;
        proxy_hide_header content-disposition;
        proxy_hide_header via;
        proxy_hide_header x-varnish;
        proxy_hide_header accept-ranges;
        proxy_hide_header last-modified;
        add_header Cache-Control public;
        add_header X-Proxy-Cache $upstream_cache_status;
    }
}

Is there something wrong with the configuration?

Thanks!

Answer 1

$ curl -vv https://en.gravatar.com/userimage/...jpeg -o /dev/null 2>&1 | egrep 'Expires|Cache-Control'
< Expires: Wed, 23 Dec 2015 20:17:02 GMT
< Cache-Control: max-age=300

I think it respects the Expires header.