URI matching in Nginx

As a proxy server, URI matching is one of the most common problems in Nginx. Here are some cases that can be error-prone.

1. Automatic 301 redirect with a trailing slash

When URI points to a directory without a trailing slash, Nginx will stop processing and send a 301 redirect to the client. Also add a trailing slash on original URI.

location /help {
    root /var/www/html;
}

If help is directory under /var/www/html on local filesystem, a 301 redirect response with a trailing slash on original URI will be sent.

The request /help will be redirected to /help/.

2. Regular expression not checked

The ^~ modifier is used to specify prefix matching with regular expression. If the longest matching prefix location has the ^~ modifier then regular expressions are not checked.

location ^~ /images/ {
    [ configuration A ]
}

location ~* \.(gif|jpg|jpeg)$ {
    [ configuration B ]
}

The request /images/abc.jpg will match configuration A, the request /documents/abc.jpg will match configuration B.

3. Matching prefix replaced

When request is passed to proxied server using proxy_pass directive, in cases to change URI, rewrite is not the only option. Without rewrite, a URI specified in proxy_pass directive can have the same effect.

From Nginx docs on proxy_pass:

If the proxy_pass directive is specified with a URI, then when a request is passed to the server, the part of a normalized request URI matching the location is replaced by a URI specified in the directive.

location /name/ {
    proxy_pass http://127.0.0.1:2082/remote/;
}

Using the above configuration, the request /name/abc will be sent to proxied server and URI is changed to /remote/abc. As proxy server as Nginx is, this is preferable and intuitive to rewrite.