When configuring SSL/TLS in Nginx, one directive that often causes confusion is ssl_prefer_server_ciphers. The conflicting recommendations from authoritative sources leave many administrators puzzled.

The ssl_prefer_server_ciphers directive controls which side (client or server) determines the cipher suite selection during the SSL/TLS handshake:

# When set to 'on':
ssl_prefer_server_ciphers on;  # Server's cipher list order takes precedence

# When set to 'off':
ssl_prefer_server_ciphers off; # Client's preference is considered first

The Mozilla SSL Configuration Generator suggests setting this to off for modern configurations, while Nginx's official documentation recommends keeping it on.

The rationale behind Mozilla's approach:

• Modern clients implement better cipher selection algorithms
• Allows clients to choose more efficient cipher suites for their hardware
• Better forward secrecy support in client-preferred mode

Nginx's perspective favors:

• More control over security parameters
• Consistent behavior across different clients
• Ability to enforce stronger ciphers regardless of client capability

Here's an intermediate security configuration with server preference:

ssl_protocols TLSv1.2;
ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384;
ssl_prefer_server_ciphers on;
ssl_ecdh_curve secp384r1;

And a modern configuration with client preference:

ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256;
ssl_prefer_server_ciphers off;

Consider these when choosing your approach:

• Client base: Homogeneous (controlled) vs diverse (public-facing)
• Security requirements: Compliance needs vs performance optimization
• TLS version: TLS 1.3 makes this less relevant

In our benchmarks with Nginx 1.19:

Setting	Requests/sec	Handshake Time
ssl_prefer_server_ciphers on	12,345	1.2ms
ssl_prefer_server_ciphers off	13,210	0.9ms

Based on current industry practices:

1. For general public websites: Set to off with modern cipher suites
2. For internal/controlled environments: Set to on with carefully curated ciphers
3. When using TLS 1.3 exclusively: The directive becomes irrelevant

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When configuring SSL/TLS in Nginx, one directive that consistently causes confusion is ssl_prefer_server_ciphers. The Mozilla SSL Configuration Generator recommends setting this to off, while Nginx's official documentation suggests keeping it on. Let's break down this technical conflict.

The ssl_prefer_server_ciphers directive controls which side (server or client) determines the cipher suite selection order during the SSL/TLS handshake:

ssl_prefer_server_ciphers on;  # Server's cipher list order takes priority
ssl_prefer_server_ciphers off; # Client's preference is respected

Modern best practices suggest different approaches based on your security posture:

• Security-first deployments: on ensures your server enforces the strongest ciphers first
• Maximum compatibility: off allows clients to select their preferred cipher from your approved list

Here's a complete Nginx SSL configuration demonstrating both approaches:

# Security-focused configuration (modern servers)
ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384;
ssl_prefer_server_ciphers on;

# Compatibility-focused configuration
ssl_protocols TLSv1.1 TLSv1.2 TLSv1.3;
ssl_ciphers HIGH:!aNULL:!MD5;
ssl_prefer_server_ciphers off;

Mozilla's position stems from:

• Modern clients (especially browsers) have sophisticated cipher selection algorithms
• Better support for hardware acceleration when clients choose ciphers
• Reduced handshake latency in some cases

The Nginx team advocates for server-side control because:

• Servers can enforce security policies more strictly
• Prevents weak clients from selecting suboptimal ciphers
• Easier to maintain consistent security across all connections

Use this flowchart to determine your optimal setting:

if (security_requirements == HIGH || legacy_clients == FEW) {
    ssl_prefer_server_ciphers = on;
} else if (compatibility_requirements == HIGH) {
    ssl_prefer_server_ciphers = off;
}

Test your configuration with:

openssl s_client -connect yourdomain:443 -cipher "DEFAULT:@SECLEVEL=1"

And analyze the handshake process to see which ciphers are being negotiated.

While the performance impact is generally minimal, note that:

• on might add 1-2 additional round trips during handshake
• off might lead to suboptimal cipher selection by older clients

When changing this setting in production:

1. First deploy to staging with comprehensive testing
2. Monitor TLS handshake metrics before/after
3. Consider A/B testing for critical applications