In Nginx reverse proxy configurations, proxy_set_header directives modify or add HTTP headers before forwarding requests to backend servers. While technically optional, these headers provide critical information flow between client, proxy, and application servers.

# Essential headers for proper reverse proxying
location / {
    # Preserves original client IP (crucial for logging/auth)
    proxy_set_header X-Real-IP $remote_addr;

    # Tracks full proxy chain for debugging
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

    # Maintains original Host header (vital for virtual hosting)
    proxy_set_header Host $http_host;

    # Custom header indicating Nginx proxy involvement
    proxy_set_header X-NginX-Proxy true;

    proxy_pass http://backend_app;
}

X-Real-IP: Without this, your Node.js app would only see Nginx's IP address. Essential for:

• Rate limiting
• Geo-based features
• Security logging

X-Forwarded-For: Builds a chain of proxy IPs when requests pass through multiple hops. Example format:

X-Forwarded-For: client_ip, proxy1_ip, proxy2_ip

Host Header: Critical when hosting multiple subdomains. Without it, your backend receives requests for http://localhost:3000 instead of http://api.yourdomain.com.

For WebSocket applications or custom auth flows:

location /ws/ {
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header X-Special-Auth $http_x_special_auth;
    proxy_pass http://websocket_backend;
}

While you can omit headers, benchmark tests show:

• With headers: 12% CPU overhead
• Without headers: 9% CPU overhead

The minimal performance gain rarely justifies losing critical request metadata.

Problem: App shows 127.0.0.1 for all users
Fix: Ensure X-Real-IP exists and your app reads it properly

Problem: Subdomains not routing correctly
Fix: Verify Host header preservation and server_name configuration

Always sanitize incoming headers in your Node.js application:

// In Express.js
app.enable('trust proxy'); // Required for X-Forwarded-*
app.use((req, res, next) => {
    if(!req.headers['x-nginx-proxy']) {
        return res.status(403).send('Direct access forbidden');
    }
    next();
});

When setting up Nginx as a reverse proxy for Node.js applications, the proxy_set_header directive plays a crucial role in preserving client information and ensuring proper application functionality. While proxy_pass is indeed the minimum requirement, these headers solve important real-world problems that emerge in production environments.

location / {
    # Preserves original client IP
    proxy_set_header X-Real-IP $remote_addr;
    
    # Maintains entire proxy chain information
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    
    # Ensures original Host header reaches backend
    proxy_set_header Host $http_host;
    
    # Custom header indicating Nginx proxy
    proxy_set_header X-NginX-Proxy true;
    
    proxy_pass http://otherapp/;
    proxy_redirect off;
}

X-Real-IP: Without this, your Node.js app would only see Nginx's IP address, breaking IP-based features like rate limiting or geo-location.

X-Forwarded-For: Critical when you have multiple proxy layers. The $proxy_add_x_forwarded_for variable intelligently appends to existing headers.

Host: Particularly important for virtual hosting. Node.js applications often need the original hostname for URL generation or multi-tenant setups.

Here's an enhanced version with WebSocket support and timeouts:

location / {
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header X-Forwarded-Proto $scheme;
    
    # Standard headers
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    
    proxy_pass http://node_backend;
    proxy_read_timeout 300s;
    proxy_connect_timeout 75s;
}

1. Overriding needed headers: Don't blindly overwrite existing X-Forwarded-For headers

2. Protocol mismatches: Always include X-Forwarded-Proto when using HTTPS

3. WebSocket issues: The Upgrade and Connection headers are essential for WS connections

For microservices architectures, consider adding:

proxy_set_header X-Request-ID $request_id;
proxy_set_header X-Service-Name frontend_gateway;

This enables better distributed tracing in your Node.js applications.