WebSocket proxying in Nginx - Commands & Configuration

Introduction

WebSocket proxying lets Nginx forward WebSocket connections from clients to backend servers. This solves the problem of handling real-time communication through a reverse proxy.

When you want to serve a chat app through Nginx that uses WebSocket for live messages.

When your backend server supports WebSocket but clients connect through Nginx for security.

When you need to load balance WebSocket connections across multiple backend servers.

When you want to add SSL termination at Nginx for WebSocket traffic.

When you want to keep WebSocket connections alive and stable through a proxy.

Config File - nginx.conf

nginx.conf

events {
    worker_connections 1024;
}

http {
    map $http_upgrade $connection_upgrade {
        default upgrade;
        '' close;
    }

    server {
        listen 8080;

        location /ws/ {
            proxy_pass http://localhost:9000/;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection $connection_upgrade;
            proxy_set_header Host $host;
            proxy_read_timeout 86400;
        }
    }
}

This config sets up Nginx to listen on port 8080 and forward WebSocket requests from the /ws/ path to a backend server on port 9000.

The map block creates a variable $connection_upgrade that is upgrade if the client requests an upgrade, otherwise close.

The proxy_set_header directives ensure the WebSocket handshake headers are correctly passed.

proxy_read_timeout is set high to keep connections alive.

Commands

Check the Nginx configuration file syntax to make sure there are no errors before restarting.

Terminal

nginx -t

Expected Output

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok

nginx: configuration file /etc/nginx/nginx.conf test is successful

Restart Nginx to apply the new configuration for WebSocket proxying.

Terminal

systemctl restart nginx

Expected Output

No output (command runs silently)

Test the WebSocket proxy by sending an HTTP request with WebSocket upgrade headers to Nginx.

Terminal

curl -i -N -H "Connection: Upgrade" -H "Upgrade: websocket" http://localhost:8080/ws/

Expected Output

HTTP/1.1 101 Switching Protocols

Upgrade: websocket

Connection: Upgrade

Sec-WebSocket-Accept: <calculated_key>

Key Concept

If you remember nothing else from this pattern, remember: proxy_set_header Upgrade and Connection must be set correctly to enable WebSocket proxying through Nginx.

Common Mistakes

Not setting proxy_http_version to 1.1

WebSocket requires HTTP/1.1 for the upgrade handshake; without this, the connection upgrade fails.

Always set proxy_http_version 1.1 in the location block handling WebSocket.

Forgetting to set proxy_set_header Connection with the correct value

The Connection header controls if the connection upgrades or closes; wrong value breaks WebSocket handshake.

Use a map to set $connection_upgrade variable and set proxy_set_header Connection $connection_upgrade.

Not increasing proxy_read_timeout

WebSocket connections stay open long; default timeout may close them prematurely.

Set proxy_read_timeout to a high value like 86400 seconds to keep connections alive.

Summary

Configure Nginx to listen on a port and forward WebSocket requests to the backend.

Use proxy_set_header Upgrade and Connection with correct values to enable the WebSocket handshake.

Test the configuration syntax, restart Nginx, and verify the WebSocket upgrade response.

Practice

1. What is the main reason to use proxy_set_header Upgrade $http_upgrade; in nginx when proxying WebSocket connections?

easy

A. To tell nginx to allow the connection to switch protocols to WebSocket

B. To set the client's IP address in the header

C. To enable gzip compression for WebSocket data

D. To disable caching for WebSocket requests

Solution

1. Step 1: Understand the Upgrade header role

   The Upgrade header tells the server to switch from HTTP to another protocol, here WebSocket.

2. Step 2: Recognize nginx's proxy_set_header usage

   Setting proxy_set_header Upgrade $http_upgrade; passes the client's Upgrade header to the backend, enabling protocol switching.

3. Final Answer:

   To tell nginx to allow the connection to switch protocols to WebSocket -> Option A

4. Quick Check:

   Upgrade header enables protocol switch = A [OK]

Hint: Upgrade header signals WebSocket protocol switch [OK]

Common Mistakes:

• Confusing Upgrade with client IP headers
• Thinking it enables compression
• Assuming it disables caching

2. Which of the following nginx configuration snippets correctly enables WebSocket proxying?

easy

A. proxy_http_version 1.0; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "upgrade";

B. proxy_http_version 1.1; proxy_set_header Upgrade "websocket"; proxy_set_header Connection "close";

C. proxy_http_version 2.0; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "keep-alive";

D. proxy_http_version 1.1; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "upgrade";

Solution

1. Step 1: Identify required HTTP version for WebSocket

   WebSocket proxying requires HTTP/1.1 because HTTP/1.0 does not support the Upgrade header properly.

2. Step 2: Check correct headers for WebSocket

   The Upgrade header must be passed as $http_upgrade and Connection header must be "upgrade" to maintain the protocol switch.

3. Final Answer:

   proxy_http_version 1.1; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "upgrade"; -> Option D

4. Quick Check:

   HTTP/1.1 + Upgrade + Connection upgrade = B [OK]

Hint: Use HTTP/1.1 and set Upgrade and Connection headers correctly [OK]

Common Mistakes:

• Using HTTP/1.0 instead of 1.1
• Setting Connection to keep-alive or close
• Hardcoding Upgrade header instead of using $http_upgrade

3. Given this nginx config snippet inside a location block:

proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_pass http://backend_server;

What will happen if a client tries to establish a WebSocket connection through nginx?

medium

A. The connection will be cached by nginx, causing stale data.

B. The WebSocket connection will be successfully proxied to the backend server.

C. The connection will be proxied but without protocol upgrade, causing failure.

D. The connection will fail because HTTP/2 is required for WebSocket.

Solution

1. Step 1: Analyze HTTP version and headers

   The config uses HTTP/1.1 and sets Upgrade and Connection headers correctly for WebSocket proxying.

2. Step 2: Understand nginx behavior with this config

   nginx will forward the WebSocket upgrade request to the backend, enabling a successful WebSocket connection.

3. Final Answer:

   The WebSocket connection will be successfully proxied to the backend server. -> Option B

4. Quick Check:

   Correct HTTP version and headers = successful proxy [OK]

Hint: HTTP/1.1 + Upgrade + Connection upgrade = success [OK]

Common Mistakes:

• Thinking HTTP/2 is required for WebSocket
• Assuming protocol upgrade is automatic without headers
• Believing nginx caches WebSocket connections

4. You have this nginx config for WebSocket proxying:

proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "keep-alive";
proxy_pass http://backend;

Why might WebSocket connections fail with this setup?

medium

A. Because Connection header should be "upgrade" not "keep-alive" for WebSocket

B. Because proxy_http_version should be 1.0 for WebSocket

C. Because Upgrade header should be hardcoded to "websocket"

D. Because proxy_pass URL is missing a trailing slash

Solution

1. Step 1: Check Connection header value

   For WebSocket proxying, Connection header must be "upgrade" to signal protocol switch.

2. Step 2: Identify impact of incorrect Connection header

   Using "keep-alive" prevents the protocol upgrade, causing WebSocket connection failure.

3. Final Answer:

   Because Connection header should be "upgrade" not "keep-alive" for WebSocket -> Option A

4. Quick Check:

   Connection header must be upgrade for WebSocket [OK]

Hint: Connection header must be "upgrade" for WebSocket [OK]

Common Mistakes:

• Using keep-alive instead of upgrade
• Setting HTTP version to 1.0
• Hardcoding Upgrade header instead of using $http_upgrade

5. You want to proxy WebSocket connections through nginx to two backend servers for load balancing. Which configuration snippet correctly supports WebSocket proxying with load balancing?

hard

A. upstream ws_backend {
server backend1.example.com;
server backend2.example.com;
}
location /ws/ {
proxy_pass http://ws_backend;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "close";
}

B. upstream ws_backend {
server backend1.example.com;
server backend2.example.com;
}
location /ws/ {
proxy_pass http://ws_backend;
proxy_http_version 1.0;
proxy_set_header Upgrade websocket;
proxy_set_header Connection "keep-alive";
}

C. upstream ws_backend {
server backend1.example.com;
server backend2.example.com;
}
location /ws/ {
proxy_pass http://ws_backend;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
}

D. upstream ws_backend {
server backend1.example.com;
server backend2.example.com;
}
location /ws/ {
proxy_pass http://ws_backend;
proxy_http_version 2.0;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
}

Solution

1. Step 1: Confirm HTTP version and headers for WebSocket

   HTTP/1.1 is required, and headers Upgrade and Connection must be set to allow protocol upgrade.

2. Step 2: Check load balancing setup and header correctness

   upstream ws_backend { server backend1.example.com; server backend2.example.com; } location /ws/ { proxy_pass http://ws_backend; proxy_http_version 1.1; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "upgrade"; } uses correct HTTP version, headers, and defines upstream servers for load balancing properly.

3. Final Answer:

   upstream ws_backend { server backend1.example.com; server backend2.example.com; } location /ws/ { proxy_pass http://ws_backend; proxy_http_version 1.1; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "upgrade"; } -> Option C

4. Quick Check:

   HTTP/1.1 + Upgrade + Connection upgrade + upstream = A [OK]

Hint: Use HTTP/1.1, Upgrade and Connection headers plus upstream for load balancing [OK]

Common Mistakes:

• Using HTTP/1.0 or 2.0 instead of 1.1
• Setting Connection to keep-alive or close
• Hardcoding Upgrade header instead of using $http_upgrade