IP hash for session persistence in Nginx - Commands & Configuration

Introduction

When you have multiple servers handling user requests, you want each user to keep talking to the same server during their visit. IP hash helps by sending requests from the same user IP to the same server, so their session stays consistent.

When you run a website with multiple backend servers and want users to keep their session on one server.

When you have a shopping cart that must stay on the same server during checkout.

When you want to avoid losing user data by switching servers mid-session.

When you want a simple way to balance load but keep user sessions sticky.

When you cannot use cookies or other session tracking methods.

Config File - nginx.conf

nginx.conf

http {
    upstream myapp {
        ip_hash;
        server 192.168.1.101;
        server 192.168.1.102;
        server 192.168.1.103;
    }

    server {
        listen 80;

        location / {
            proxy_pass http://myapp;
        }
    }
}

This configuration defines an upstream group named myapp with three backend servers.

The ip_hash; directive ensures requests from the same client IP go to the same server.

The server block listens on port 80 and forwards requests to the myapp upstream group.

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 the nginx service to apply the new configuration with IP hash load balancing.

Terminal

systemctl restart nginx

Expected Output

No output (command runs silently)

Send a simple HTTP request to verify nginx is responding and proxying requests.

Terminal

curl -I http://localhost

Expected Output

HTTP/1.1 200 OK

Server: nginx

Date: Wed, 01 Jan 2025 12:00:00 GMT

Content-Type: text/html

Content-Length: 612

Connection: keep-alive

Key Concept

If you remember nothing else from this pattern, remember: ip_hash sends requests from the same client IP to the same backend server to keep sessions sticky.

Common Mistakes

Not including the ip_hash directive inside the upstream block.

Without ip_hash, nginx will load balance without session persistence, causing user sessions to break.

Always add ip_hash; inside the upstream block to enable session persistence by client IP.

Forgetting to test nginx configuration before restarting.

A syntax error can cause nginx to fail restarting, making the site unavailable.

Run nginx -t to verify configuration syntax before restarting the service.

Summary

Define an upstream group with backend servers and add ip_hash; to enable session persistence by client IP.

Test nginx configuration syntax with nginx -t before restarting.

Restart nginx to apply the new configuration and verify with a simple HTTP request.

Practice

1. What is the main purpose of using ip_hash in an Nginx upstream block?

easy

A. To ensure requests from the same client IP go to the same backend server

B. To encrypt traffic between Nginx and backend servers

C. To balance load evenly regardless of client IP

D. To cache static files on the client side

Solution

1. Step 1: Understand the role of ip_hash

   The ip_hash directive in Nginx is used to route requests from the same client IP address to the same backend server.

2. Step 2: Identify the purpose in session persistence

   This helps keep user sessions sticky, meaning the user stays connected to the same server, which is important for apps storing session data locally.

3. Final Answer:

   To ensure requests from the same client IP go to the same backend server -> Option A

4. Quick Check:

   IP hash = session stickiness [OK]

Hint: ip_hash keeps same IP on same server for session stickiness [OK]

Common Mistakes:

• Confusing ip_hash with encryption
• Thinking ip_hash balances load evenly
• Assuming ip_hash caches files

2. Which of the following is the correct way to enable IP hash in an Nginx configuration for upstream servers?

easy

A. upstream backend { hash_ip; server 192.168.1.10; server 192.168.1.11; }

B. upstream backend { iphash; server 192.168.1.10; server 192.168.1.11; }

C. upstream backend { ip_hash; server 192.168.1.10; server 192.168.1.11; }

D. upstream backend { ip_hash = on; server 192.168.1.10; server 192.168.1.11; }

Solution

1. Step 1: Recall the exact directive syntax

   The correct directive to enable IP hash in Nginx is ip_hash; placed inside the upstream block without any assignment or spelling changes.

2. Step 2: Check each option for syntax correctness

   upstream backend { ip_hash; server 192.168.1.10; server 192.168.1.11; } uses ip_hash; correctly. Options A and C have wrong directive names. upstream backend { ip_hash = on; server 192.168.1.10; server 192.168.1.11; } uses an invalid assignment syntax.

3. Final Answer:

   upstream backend { ip_hash; server 192.168.1.10; server 192.168.1.11; } -> Option C

4. Quick Check:

   Correct directive is ip_hash; [OK]

Hint: Use exact directive 'ip_hash;' inside upstream block [OK]

Common Mistakes:

• Misspelling ip_hash as iphash or hash_ip
• Adding = on which is invalid
• Placing ip_hash outside upstream block

3. Given this Nginx upstream configuration with ip_hash; enabled, what will happen if two requests come from the same client IP?

medium

A. Both requests will be sent to different backend servers randomly

B. Both requests will be sent to the same backend server

C. Requests will be load balanced based on server load

D. Requests will be rejected due to IP conflict

Solution

1. Step 1: Understand ip_hash behavior on requests

   With ip_hash; enabled, Nginx uses the client's IP address to decide which backend server to send the request to, ensuring the same IP always maps to the same server.

2. Step 2: Apply this to two requests from the same IP

   Since both requests come from the same IP, they will be routed to the same backend server for session persistence.

3. Final Answer:

   Both requests will be sent to the same backend server -> Option B

4. Quick Check:

   Same IP = same server [OK]

Hint: Same IP means same backend server with ip_hash [OK]

Common Mistakes:

• Assuming random distribution despite ip_hash
• Thinking load balancing ignores ip_hash
• Believing requests get rejected on IP reuse

4. You added ip_hash; inside your upstream block, but session stickiness is not working. Which of these is the most likely cause?

medium

A. You placed ip_hash; inside the server block instead of upstream

B. You forgot to restart Nginx after adding ip_hash;

C. Your backend servers do not support IP hashing

D. You used ip_hash on; instead of ip_hash;

Solution

1. Step 1: Verify correct placement of ip_hash;

   The ip_hash; directive must be inside the upstream block, not inside individual server blocks.

2. Step 2: Analyze other options

   Restarting Nginx is necessary but usually not the main cause. Backend servers don't affect ip_hash behavior. Using ip_hash on; is invalid syntax.

3. Final Answer:

   You placed ip_hash; inside the server block instead of upstream -> Option A

4. Quick Check:

   ip_hash must be inside upstream block [OK]

Hint: ip_hash belongs inside upstream, not server block [OK]

Common Mistakes:

• Putting ip_hash in server block
• Using wrong syntax like ip_hash on;
• Blaming backend servers for ip_hash issues

5. You have three backend servers and want to use IP hash for session persistence. However, one server is down. What will Nginx do with requests from an IP previously mapped to the down server?

hard

A. Nginx will randomly distribute requests ignoring IP hash

B. Nginx will keep sending requests to the down server causing errors

C. Nginx will reject the requests with a 503 error

D. Nginx will send requests to a different available backend server

Solution

1. Step 1: Understand ip_hash behavior with unavailable servers

   When a backend server is down, Nginx detects it as unavailable and will not send requests to it, even if IP hash maps to it.

2. Step 2: Determine fallback behavior

   Nginx will route requests to another available backend server to keep service running, though session stickiness is temporarily lost for that IP.

3. Final Answer:

   Nginx will send requests to a different available backend server -> Option D

4. Quick Check:

   Down server skipped, requests go to available servers [OK]

Hint: Down servers skipped; requests go to other servers [OK]

Common Mistakes:

• Assuming Nginx keeps sending to down server
• Thinking requests get rejected automatically
• Believing ip_hash ignores server health