`gzip_static always` doesn't use index.html.gz unless index.html exists

[foolip@…]

My config:

server {
  listen 443 ssl http2 default_server;
  ssl_certificate /etc/letsencrypt/live/html.foolip.org/fullchain.pem;
  ssl_certificate_key /etc/letsencrypt/live/html.foolip.org/privkey.pem;

  add_header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload";

  server_name html.foolip.org;

  root /var/www/html.foolip.org;

  gzip_static always;
}

(This might not match what's currently on html.foolip.org.)

With this config, if /var/www/html.foolip.org/index.html.gz exists but index.html does not, ​https://html.foolip.org/ is 403 forbidden. Creating an empty index.html makes it work, serving the content from index.html.gz and ignoring the content of index.html, as expected with the gzip_static always config.

Note that the problem does not apply when requesting ​https://html.foolip.org/index.html or something like ​https://html.foolip.org/foo.html if only foo.html.gz exists on disk.

Just a guess, but the fact that ​https://html.foolip.org/ is 403 instead of 404 is significant, perhaps gzip_static hooks into a "file not found" phase that's different from what happens if no index file is found. Note that adding "index index.html" to the config doesn't change this.

[Maxim Dounin]

The problem is that requests to / are first handled by the ​index module, and 403 is be returned as there are no index file. The gzip_static module only works when returning a particular file, and no file is returned in this case. See ​How nginx processes a request for an introduction on request processing in nginx.

For the gzip_static module to work you have to somehow instruct nginx that a request to / should return /index.html. Normally this is done by index module due to an index file, and most trivial solution would be to create an empty index.html as you did in your tests. Alternatively, you can configure ​rewrite to change requests from / to /index.html.

[Maxim Dounin]

See also #1620, #1621, #1622.

[zijung@…]

Replying to mdounin:

The problem is that requests to / are first handled by the ​index module, and 403 is be returned as there are no index file. The gzip_static module only works when returning a particular file, and no file is returned in this case. See ​How nginx processes a request for an introduction on request processing in nginx.

For the gzip_static module to work you have to somehow instruct nginx that a request to / should return /index.html. Normally this is done by index module due to an index file, and most trivial solution would be to create an empty index.html as you did in your tests. Alternatively, you can configure ​rewrite to change requests from / to /index.html.

But even if index.html exists, index.html.gz will not be used.
The module returns NGX_DECLINED if the last character of URI is '/'.
This is a part of the module ​source (line 102):

if (r->uri.data[r->uri.len - 1] == '/') {
    return NGX_DECLINED;
}

[Maxim Dounin]

Replying to zijung@…:

But even if index.html exists, index.html.gz will not be used.

This is not true. You can try it yourself to find out.

The module returns NGX_DECLINED if the last character of URI is '/'.

Please refer to ​How nginx processes a request to better understand how nginx works.