Disabling URL decoding in nginx proxy

When I browse to this URL: http://localhost:8080/foo/%5B-%5D server (nc -l 8080) receives it as-is:

GET /foo/%5B-%5D HTTP/1.1

However when I proxy this application via nginx (1.1.19):

location /foo {
        proxy_pass    http://localhost:8080/foo;
}

The same request routed through nginx port is forwarded with path decoded:

GET /foo/[-] HTTP/1.1

Decoded square brackets in the GET path are causing the errors in the target server (HTTP Status 400 - Illegal character in path...) as they arrive un-escaped.

Is there a way to disable URL decoding or encode it back so that the target server gets the exact same path when routed through nginx? Some clever URL rewrite rule?

Quoting Valentin V. Bartenev (who should get the full credit for this answer):

A quote from documentation:

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

• If proxy_pass is specified without URI, a request URI is passed to the server in the same form as sent by a client when processing an original request

The correct configuration in your case would be:

location /foo {
   proxy_pass http://localhost:8080;
}

Note that URL decoding, commonly known as $uri "normalisation" within the documentation of nginx, happens before the backend IFF:

• either any URI is specified within proxy_pass itself, even if just the trailing slash all by itself,

• or, URI is changed during the processing, e.g., through rewrite.

Both conditions are explicitly documented at http://nginx.org/r/proxy_pass (emphasis mine):

• 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

• If proxy_pass is specified without a URI, the request URI is passed to the server in the same form as sent by a client when the original request is processed, or the full normalized request URI is passed when processing the changed URI

The solution is to either omit the URI as in OPs case, or, indeed, use a clever rewrite rule:

# map `/foo` to `/foo`:
location /foo {
    proxy_pass  http://localhost:8080;  # no URI -- not even just a slash
}

# map `/foo` to `/bar`:
location /foo {
    rewrite  ^  $request_uri;            # get original URI
    rewrite  ^/foo(/.*)  /bar$1  break;  # drop /foo, put /bar
    return 400;   # if the second rewrite won't match
    proxy_pass    http://localhost:8080$uri;
}

You can see it live in a related Stack Overflow answer, including control group.