behaviour of location + proxy_pass with a variable does not match behaviour with hardcoded value

[allan-simon@…]

Here's a minimal case to reproduce

server {
  listen 80;
  location /myservice/ {
    proxy_pass http://127.0.0.1:8080/;
  }
}

with a nc -l 8080 acting as proxified proxy

the following command

curl ​http://localhost/myservice/foo/far

show on netcat side, as expected

GET /foo/bar HTTP/1.0
Host: 127.0.0.1:8080
Connection: close
User-Agent: curl/7.43.0
Accept: */*

now if i use a variable to store my proxified service's domain like this

server {
  listen 80;
  set $MY_SERVICE "127.0.0.1:8080";
  location /myservice/ {
    proxy_pass http://$MY_SERVICE/;
  }
}

now regardless of the URI, the proxified value will always be

GET / HTTP/1.0
Host: 127.0.0.1:8080
Connection: close
User-Agent: curl/7.43.0
Accept: */*

notice now the GET has become / , though it should be GET /foo/bar

[Maxim Dounin]

Using variables in proxy_pass implies, that the URI specified is a full one. Quoting ​the documentation:

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

proxy_pass http://$host$uri;

This behaviour is not very intuitive, though it's how it works. Variables in proxy_pass were introduced

If you want nginx to use URI of the original request instead of the one specified in the proxy_pass, avoid specifying URI at all (note no trailing slash):

proxy_pass http://$host;

If you want to pass some changed URI, you can do so by using rewrite ... break in the same location.