If you have a situation where there is only one upstream that you need to proxy_pass, use a variable in the proxy_pass directive, e.g.

server {
  listen 80;
  set $target "http://target-host:3005";  # variable definition
  location / { proxy_pass $target; }
}

This will not error out if the target-host is not available (for example in some Docker setup).

The following example however will give you an error if the target-host can not be resolved:

upstream target {
    server target-host:3005;
}
 
server {
  listen 80;
 
  location / { proxy_pass $target; }
}

• nginx/1.22.1

• Nginx troubleshooting

• https://stackoverflow.com/questions/32845674/setup-nginx-not-to-crash-if-host-in-upstream-is-not-found