url rewriting - Setup nginx not to crash if host in upstream is not found

Question

We have several rails apps under common domain in Docker, and we use nginx to direct requests to specific apps.

our_dev_server.com/foo # proxies to foo app
our_dev_server.com/bar # proxies to bar

Config looks like this:

upstream foo {
  server foo:3000;
}

upstream bar {
  server bar:3000;
}

# and about 10 more...

server {
  listen *:80 default_server;

  server_name our_dev_server.com;

  location /foo {
      # this is specific to asset management in rails dev
      rewrite ^/foo/assets(/.*)$ /assets/$1 break;
      rewrite ^/foo(/.*)$ /foo/$1 break;
      proxy_pass http://foo;
  }

  location /bar {
      rewrite ^/bar/assets(/.*)$ /assets/$1 break;
      rewrite ^/bar(/.*)$ /bar/$1 break;
      proxy_pass http://bar;
  }

  # and about 10 more...
}

If one of these apps is not started then nginx fails and stops:

host not found in upstream "bar:3000" in /etc/nginx/conf.d/nginx.conf:6

We don't need them all to be up but nginx fails otherwise. How to make nginx ignore failed upstreams?

Answer 1

1. If you can use a static IP then just use that, it'll startup and just return 503's if it doesn't respond.

2. Use the resolver directive to point to something that can resolve the host, regardless if it's currently up or not.

3. Resolve it at the location level, if you can't do the above (this will allow Nginx to start/run):

    location /foo {
      resolver 127.0.0.1 valid=30s;
      # or some other DNS (your company's internal DNS server)
      #resolver 8.8.8.8 valid=30s;
      set $upstream_foo foo;
      proxy_pass http://$upstream_foo:80;
    }
   
    location /bar {
      resolver 127.0.0.1 valid=30s;
      # or some other DNS (your company's internal DNS server)
      #resolver 8.8.8.8 valid=30s;
      set $upstream_bar foo;
      proxy_pass http://$upstream_bar:80;
    }