I’m facing an issue where Nginx fails to start or reload due to an error:

nginx: [emerg] host not found in upstream "golang-test-server1" in /etc/nginx/http.d/golang-test-server1.conf:6  
nginx: configuration file /etc/nginx/nginx.conf test failed  

Here’s my current configuration:

server {
    listen 80;
    server_name *.golang-test-server1.localhost;

    location / {
        proxy_pass http://golang-test-server1;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_connect_timeout 60s;
        proxy_read_timeout 3600s;
        proxy_send_timeout 3600s;
    }

    error_page 502 503 504 /custom_50x.html;

    location = /custom_50x.html {
        root /usr/share/nginx/html;
        internal;
    }
}

What I Want

Even if the upstream (golang-test-server1) is unavailable or unresolvable during Nginx startup, I want:

1. Nginx to start successfully because other services depend on it.
2. A custom error page to be displayed when the backend is down.

What I Tried

1. Using a Resolver
   I added resolver 127.0.0.11 in the http block (since I’m using Docker), but Nginx still fails to start with the same error.

2. Upstream Block with Backup
   I defined an upstream block like this:

   upstream golang_test_backend {
       server golang-test-server1;
       server 127.0.0.1:9999 backup;
   }
   

   And updated proxy_pass to http://golang_test_backend, but Nginx still doesn’t start if golang-test-server1 is unresolvable.

3. Custom Error Pages
   I configured proxy_intercept_errors on; with error_page directives, but the issue is Nginx won’t even start if the upstream isn’t resolvable.

Environment Details

• Nginx version: 1.x.x
• OS: Alpine Linux
• Nginx configuration directory: /etc/nginx/http.d/
• Backend service: Dynamically assigned (Dockerized service).