Question

I am trying to proxy a request to different targets depending on an environment variable. My approach was to put the target url into the custom variable $target and give this to proxy_pass.

But using a variable with proxy_pass doesn’t seem to work. This simple config leads to a “502 Bad Gateway” response from nginx.

server {
  listen   8080;
  server_name  myhost.example.com;
  access_log  /var/log/nginx/myhost.access.log;
  location /proxy {
    set $target http://proxytarget.example.com;
    proxy_pass $target;
  }
}

The same config without the variable works:

server {
  listen   8080;
  server_name  myhost.example.com;
  access_log  /var/log/nginx/myhost.access.log;
  location /proxy {
    proxy_pass http://proxytarget.example.com;
  }
}

Is it really not possible to use proxy_pass this way or am I just doing something wrong?

Answer 1

I’ve recently stumbled upon this need myself and have found that in order to use variables in a proxy_pass destination you need to set a resolver as your error.log would most probably contain something like no resolver defined to resolve ...

The solution in my case was to setup the following using a local DNS for DNS resolution:

location ~ /proxy/(.*) {
    resolver 127.0.0.1 [::1];
    proxy_pass http://$1;
}

In your case this should work:

location /proxy {
    resolver 127.0.0.1 [::1];
    set $target http://proxytarget.example.com;
    proxy_pass $target;
}

For resolver 127.0.0.1 to work, you need to install bind9 locally.
For Debian/Ubuntu:

sudo apt-get install bind9

More information on nginx and dynamic proxy_passing here: http://www.nginx-discovery.com/2011/05/day-51-proxypass-and-resolver.html

Edit:
Replaced the previous public DNS with a local one for security issues.