proxy_pass does not form correct URI when declared with $scheme variable

[Sergei Kudimov]

proxy_pass does not form correct URI when declared with $scheme variable:

server {
    listen 80;
    listen 443 ssl http2;

    server_name frontend.com;

    include conf.d/ssl.conf;

    location / {
        proxy_pass              $scheme://127.0.0.1/hu/;
        # proxy_pass              http://127.0.0.1/hu/;
        proxy_set_header        Host    backend.com;
        include conf.d/proxy-params.conf;
    }
}

Test request from x.x.x.x host:
curl http://frontend.com/css/style.css

Server logs following lines:

backend.com 127.0.0.1 - - [12/Apr/2017...] "GET /hu/ HTTP/1.0" 200 13995 "-" "curl/7.35.0" "-"
frontend.com x.x.x.x - - [12/Apr/2017...] "GET /css/style.css HTTP/1.1" 200 14023 "-" "curl/7.35.0" "-"

As we see request for /css/style.css was proxied as /hu/, which is incorrect. According to documentation expected form is /hu/css/style.css.

When I have replaced $scheme:// with explicit http:// it began to work properly:

backend.com 127.0.0.1 - - [12/Apr/2017...] "GET /hu/css/style.css HTTP/1.0" 404 328 "-" "curl/7.35.0" "-"
frontend.com x.x.x.x - - [12/Apr/2017...] "GET /css/style.css HTTP/1.1" 404 356 "-" "curl/7.35.0" "-"

[Ruslan Ermilov]

Try proxy_pass $scheme://127.0.0.1/hu$request_uri;.

[Maxim Dounin]

Quoting the proxy_pass directive description:

A server name, its port and the passed URI can also be specified using variables:

proxy_pass http://$host$uri;

When using variables, you have to specify _full URI_ to be passed to an upstream server. This might be somewhat counterintuitive, but this is how it works and expected to work.