SSL protocol and cipher configuration in Nginx - Commands & Configuration

Introduction

Websites need to protect data sent between users and servers. SSL protocols and ciphers help encrypt this data to keep it safe from hackers.

When you want to secure your website with HTTPS to protect user information.

When you need to disable old insecure SSL protocols to prevent attacks.

When you want to specify which encryption methods (ciphers) your server accepts for better security.

When you want to improve your website's security score on tools like SSL Labs.

When you want to ensure compatibility with modern browsers while blocking weak encryption.

Config File - nginx.conf

nginx.conf

server {
    listen 443 ssl;
    server_name example.com;

    ssl_certificate /etc/ssl/certs/example.com.crt;
    ssl_certificate_key /etc/ssl/private/example.com.key;

    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256';
    ssl_prefer_server_ciphers on;

    location / {
        root /var/www/html;
        index index.html;
    }
}

listen 443 ssl; tells Nginx to listen on port 443 with SSL enabled.

ssl_certificate and ssl_certificate_key specify the paths to your SSL certificate and private key files.

ssl_protocols sets which SSL/TLS versions are allowed; here only TLS 1.2 and 1.3 are enabled for security.

ssl_ciphers lists strong encryption methods your server accepts.

ssl_prefer_server_ciphers on; makes the server choose the best cipher from the list instead of the client's choice.

Commands

This command tests the Nginx configuration file for syntax errors before applying changes.

Terminal

sudo 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

This command reloads Nginx to apply the new SSL protocol and cipher settings without stopping the server.

Terminal

sudo systemctl reload nginx

Expected Output

No output (command runs silently)

This command tries to connect to the server using TLS 1.1, which should fail because it is disabled.

Terminal

openssl s_client -connect example.com:443 -tls1_1

Expected Output

CONNECTED(00000003)

140735213994560:error:1409442E:SSL routines:ssl3_read_bytes:tlsv1 alert protocol version:s3_pkt.c:1493:SSL alert number 70

---

no peer certificate available

---

No client certificate CA names sent

---

SSL handshake has read 7 bytes and written 301 bytes

---

New, (NONE), Cipher is (NONE)

Secure Renegotiation IS NOT supported

---

This command connects to the server using TLS 1.2, which should succeed showing the certificate details.

Terminal

openssl s_client -connect example.com:443 -tls1_2

Expected Output

CONNECTED(00000003)

depth=2 C = US, O = Let's Encrypt, CN = R3

verify return:1

depth=1 C = US, O = Let's Encrypt, CN = R3

verify return:1

depth=0 CN = example.com

verify return:1

---

Certificate chain

 0 s:CN = example.com

   i:C = US, O = Let's Encrypt, CN = R3

---

Server certificate

-----BEGIN CERTIFICATE-----

MIIF... (certificate content) ...

-----END CERTIFICATE-----

---

SSL handshake has read 3456 bytes and written 456 bytes

---

New, TLSv1.2, Cipher is ECDHE-RSA-AES256-GCM-SHA384

Secure Renegotiation IS supported

---

Key Concept

If you remember nothing else from this pattern, remember: always enable only modern TLS protocols and strong ciphers to keep your website secure and compatible.

Common Mistakes

Leaving old SSL protocols like SSLv3 or TLS 1.0 enabled.

These old protocols have known security flaws that attackers can exploit.

Explicitly set ssl_protocols to only TLSv1.2 and TLSv1.3 in your Nginx config.

Using weak or default cipher suites without specifying strong ones.

Weak ciphers can allow attackers to decrypt or tamper with data.

Specify a strong list of ciphers in ssl_ciphers and enable ssl_prefer_server_ciphers.

Not testing the configuration with nginx -t before reloading.

Syntax errors can cause Nginx to fail to reload, leading to downtime.

Always run sudo nginx -t and fix errors before reloading.

Summary

Configure ssl_protocols in nginx.conf to allow only TLS 1.2 and 1.3 for security.

Specify strong ssl_ciphers and enable ssl_prefer_server_ciphers to control encryption methods.

Test the configuration with 'nginx -t' and reload Nginx to apply changes safely.

Use openssl s_client to verify which SSL/TLS versions your server accepts.

Practice

1. What is the purpose of the ssl_protocols directive in an nginx configuration?

easy

A. To specify which SSL/TLS protocol versions are allowed for secure connections

B. To define the encryption algorithms used for data compression

C. To set the maximum size of SSL certificates

D. To enable HTTP/2 support over SSL

Solution

1. Step 1: Understand the role of ssl_protocols

   The ssl_protocols directive controls which SSL/TLS versions nginx will accept for secure connections.

2. Step 2: Compare options with this role

   Only To specify which SSL/TLS protocol versions are allowed for secure connections correctly describes this purpose; others mention unrelated features like compression or HTTP/2.

3. Final Answer:

   To specify which SSL/TLS protocol versions are allowed for secure connections -> Option A

4. Quick Check:

   ssl_protocols = Allowed SSL/TLS versions [OK]

Hint: ssl_protocols controls TLS versions allowed [OK]

Common Mistakes:

• Confusing ssl_protocols with cipher settings
• Thinking ssl_protocols sets certificate size
• Assuming ssl_protocols enables HTTP/2

2. Which of the following is the correct syntax to enable only TLSv1.2 and TLSv1.3 in nginx?

easy

A. ssl_protocols tlsv12 tlsv13;

B. ssl_protocols tls1.2 tls1.3;

C. ssl_protocols TLSv1.2 TLSv1.3;

D. ssl_protocols TLS1_2 TLS1_3;

Solution

1. Step 1: Recall correct nginx ssl_protocols syntax

   The correct syntax uses capitalized TLS versions separated by spaces, ending with a semicolon, e.g., ssl_protocols TLSv1.2 TLSv1.3;.

2. Step 2: Check each option's format

   ssl_protocols TLSv1.2 TLSv1.3; matches the correct syntax exactly. Options A, B, and C use incorrect casing or underscores, which nginx does not accept.

3. Final Answer:

   ssl_protocols TLSv1.2 TLSv1.3; -> Option C

4. Quick Check:

   Correct syntax = ssl_protocols TLSv1.2 TLSv1.3; [OK]

Hint: Use exact case: TLSv1.2 TLSv1.3 with spaces [OK]

Common Mistakes:

• Using lowercase letters for TLS versions
• Replacing dots with underscores
• Missing semicolon at the end

3. Given this nginx snippet:

ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers HIGH:!aNULL:!MD5;
ssl_prefer_server_ciphers on;

What effect does ssl_prefer_server_ciphers on; have during SSL handshake?

medium

A. Client chooses the cipher from its list ignoring server preference

B. SSL handshake fails if client and server ciphers differ

C. Cipher selection is random between client and server lists

D. Server forces selection of its preferred cipher from the client's list

Solution

1. Step 1: Understand ssl_prefer_server_ciphers directive

   When set to on, nginx forces the server's cipher preference to override the client's during SSL handshake.

2. Step 2: Analyze options

   Server forces selection of its preferred cipher from the client's list correctly states the server enforces its cipher choice from the client's offered list. Client chooses the cipher from its list ignoring server preference is the opposite. Cipher selection is random between client and server lists is incorrect as selection is not random. SSL handshake fails if client and server ciphers differ is false; handshake does not fail just because of differing preferences.

3. Final Answer:

   Server forces selection of its preferred cipher from the client's list -> Option D

4. Quick Check:

   ssl_prefer_server_ciphers on = Server cipher priority [OK]

Hint: ssl_prefer_server_ciphers on means server picks cipher [OK]

Common Mistakes:

• Thinking client always chooses cipher
• Assuming random cipher selection
• Believing handshake fails on cipher mismatch

4. You configured nginx with:

ssl_protocols TLSv1.2 TLSv1.3
ssl_ciphers HIGH:!aNULL:!MD5
ssl_prefer_server_ciphers on;

But nginx fails to start. What is the likely cause?

medium

A. Missing semicolons at the end of directives

B. Using unsupported TLS versions

C. Incorrect cipher string syntax

D. ssl_prefer_server_ciphers must be off

Solution

1. Step 1: Check syntax of nginx directives

   Each directive must end with a semicolon. The given config lines miss semicolons at the end.

2. Step 2: Validate other options

   TLS versions and cipher strings are valid. ssl_prefer_server_ciphers can be on. So the syntax error (missing semicolons) causes nginx to fail.

3. Final Answer:

   Missing semicolons at the end of directives -> Option A

4. Quick Check:

   Missing semicolons cause config errors [OK]

Hint: Always end nginx directives with semicolon [OK]

Common Mistakes:

• Ignoring semicolon syntax errors
• Assuming TLS versions are unsupported
• Thinking ssl_prefer_server_ciphers must be off

5. You want to configure nginx to only allow TLSv1.3 and use strong ciphers, but also ensure clients that do not support TLSv1.3 can still connect securely using TLSv1.2. Which configuration snippet achieves this best?

hard

A. ssl_protocols TLSv1.3;
ssl_ciphers TLS_AES_256_GCM_SHA384;
ssl_prefer_server_ciphers on;

B. ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers TLS_AES_256_GCM_SHA384:HIGH:!aNULL;
ssl_prefer_server_ciphers on;

C. ssl_protocols TLSv1.2;
ssl_ciphers HIGH:!aNULL:!MD5;
ssl_prefer_server_ciphers off;

D. ssl_protocols TLSv1.3 TLSv1.1;
ssl_ciphers HIGH:!aNULL;
ssl_prefer_server_ciphers on;

Solution

1. Step 1: Identify required TLS versions

   We want to allow TLSv1.3 and TLSv1.2 to support clients without TLSv1.3.

2. Step 2: Check cipher configuration

   ssl_protocols TLSv1.2 TLSv1.3; ssl_ciphers TLS_AES_256_GCM_SHA384:HIGH:!aNULL; ssl_prefer_server_ciphers on; includes TLS_AES_256_GCM_SHA384 (a strong TLSv1.3 cipher) plus HIGH strength ciphers for TLSv1.2, with server cipher preference enabled.

3. Step 3: Validate other options

   ssl_protocols TLSv1.3; ssl_ciphers TLS_AES_256_GCM_SHA384; ssl_prefer_server_ciphers on; excludes TLSv1.2, blocking older clients. ssl_protocols TLSv1.2; ssl_ciphers HIGH:!aNULL:!MD5; ssl_prefer_server_ciphers off; excludes TLSv1.3. ssl_protocols TLSv1.3 TLSv1.1; ssl_ciphers HIGH:!aNULL; ssl_prefer_server_ciphers on; includes insecure TLSv1.1, which is deprecated.

4. Final Answer:

   ssl_protocols TLSv1.2 TLSv1.3; ssl_ciphers TLS_AES_256_GCM_SHA384:HIGH:!aNULL; ssl_prefer_server_ciphers on; -> Option B

5. Quick Check:

   Allow TLSv1.2 and TLSv1.3 with strong ciphers = ssl_protocols TLSv1.2 TLSv1.3; ssl_ciphers TLS_AES_256_GCM_SHA384:HIGH:!aNULL; ssl_prefer_server_ciphers on; [OK]

Hint: Include TLSv1.2 and TLSv1.3 with strong ciphers [OK]

Common Mistakes:

• Excluding TLSv1.2 and blocking older clients
• Including deprecated TLS versions like TLSv1.1
• Not combining strong ciphers for both TLS versions