When working with NGINX reverse proxy configurations, you'll often encounter directives like:

proxy_pass_header Server;

This directive controls whether NGINX passes the Server HTTP header from upstream servers to clients. By default, NGINX filters out this header to prevent exposing backend server information.

In security-sensitive environments, hiding server information is considered a best practice to:

• Prevent attackers from identifying specific server versions with known vulnerabilities
• Maintain a unified frontend appearance regardless of backend infrastructure
• Simplify debugging by standardizing headers across environments

You might want to enable passing the Server header when:

# Case 1: Debugging backend responses
location /debug/ {
    proxy_pass http://backend;
    proxy_pass_header Server;
    proxy_pass_header X-Backend-Status;
}

# Case 2: API gateways needing original headers
location /api/ {
    proxy_pass http://api_cluster;
    proxy_pass_header Server;
    proxy_pass_header X-Api-Version;
}

If you choose to pass the Server header, consider these alternatives:

# Option 1: Override with custom value
proxy_set_header Server "Custom Server Name";

# Option 2: Completely remove the header
proxy_hide_header Server;

Here's a complete NGINX configuration snippet demonstrating proper usage:

server {
    listen 80;
    server_name example.com;
    
    location / {
        proxy_pass http://backend;
        proxy_pass_header Server;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        
        # Cache settings
        proxy_cache my_cache;
        proxy_cache_valid 200 302 10m;
    }
}

While the directive itself has negligible performance impact, consider that:

• Larger headers increase bandwidth usage
• Backend server information might help attackers optimize their exploits
• Debugging becomes easier with more header information available

For more granular control over headers:

# Pass only specific headers from upstream
map $upstream_http_server $backend_server {
    default "";
    "~*" $upstream_http_server;
}

server {
    # ...
    add_header X-Backend-Server $backend_server;
}

When working with NGINX as a reverse proxy, the proxy_pass_header Server; directive plays a crucial role in HTTP header manipulation. By default, NGINX strips the Server header from upstream server responses as a security measure. This directive explicitly allows the Server header from the proxied server to pass through to the client.

Here's a typical configuration example where you might use this directive:

location / {
    proxy_pass http://backend;
    proxy_pass_header Server;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
}

The primary use cases for enabling the Server header include:

• Debugging purposes during development to identify which backend server handled the request
• Maintaining API compatibility when client applications expect specific Server headers
• Load balancer configurations where you need to preserve original server information

While the Server header can be useful, be aware that:

# For production environments, consider masking server information:
location / {
    proxy_pass http://backend;
    proxy_pass_header Server;
    proxy_hide_header Server;
    proxy_set_header Server "Custom-Server";
}

This configuration allows you to inspect the original Server header internally while presenting a generic value to clients.

If you need more control over headers, consider these NGINX directives:

proxy_set_header - Allows setting custom headers
proxy_hide_header - Hides specific headers from clients
more_clear_headers - Clears multiple headers at once