SSL/TLS Key Log Feature - Developer Guide

Overview

ProxySQL implements SSL/TLS key logging to enable decryption of encrypted traffic for debugging purposes. This feature writes TLS secrets to a file in the NSS Key Log Format, which can be used by tools like Wireshark to decrypt and analyze TLS traffic.

PR Reference: #4236 - "Added support for SSLKEYLOGFILE"

Variable Naming Convention

ProxySQL variables belong to modules. This is important for understanding how variables are referenced:

Code Location: include/proxysql_admin.h - the variable is defined as char* ssl_keylog_file within the admin variables struct.

SQL Registration: lib/ProxySQL_Admin.cpp - registered as "ssl_keylog_file" in the admin_variables array.

When users set this variable via SQL, they must use the module prefix:

SET admin-ssl_keylog_file = '/path/to/file.txt';

Architecture

Component Diagram

┌─────────────────────────────────────────────────────────────────────┐
│                         ProxySQL Process                            │
│                                                                      │
│  ┌────────────────┐         ┌─────────────────────────────────┐   │
│  | ProxySQL_Admin |────────>|  Global Variables               │   │
│  |                │         |  .ssl_keylog_file = "/path/..."  │   │
│  └────────────────┘         └─────────────────────────────────┘   │
│           │                                                          │
│           | SET admin-ssl_keylog_file='/path/keylog.txt'           │
│           v                                                          │
│  ┌──────────────────────────────────────────────────────────────┐ │
│  |            proxysql_sslkeylog module                          │ │
│  │                                                               │ │
│  │  ┌──────────────┐  ┌──────────────────┐  ┌───────────────┐ │ │
│  │  | keylog_init  |  | keylog_open     |  | keylog_close  │ │ │
│  │  └──────────────┘  └──────────────────┘  └───────────────┘ │ │
│  │                                                               │ │
│  │  ┌──────────────────────────────────────────────────────────┐│ │
│  │  | proxysql_keylog_attach_callback(SSL_CTX*)                ││ │
│  │  |                                                          ││ │
│  │  |   SSL_CTX_set_keylog_callback(ctx, write_line_callback)  ││ │
│  │  └──────────────────────────────────────────────────────────┘│ │
│  └──────────────────────────────────────────────────────────────┘ │
│                              │                                      │
│                              │ Callback invoked by OpenSSL         │
│                              v                                      │
│  ┌──────────────────────────────────────────────────────────────┐ │
│  │ proxysql_keylog_write_line_callback(ssl, line)               │ │
│  │                                                              │ │
│  │   - Validate line length                                     │ │
│  │   - Acquire read lock (rwlock)                               │ │
│  │   - Write to keylog file                                    │ │
│  │   - Release lock                                            │ │
│  └──────────────────────────────────────────────────────────────┘ │
│                              │                                      │
│                              v                                      │
│  ┌──────────────────────────────────────────────────────────────┐ │
│  │           keylog_file_fp (FILE*)                             │ │
│  │           "/var/log/proxysql/sslkeys.txt"                    │ │
│  │                                                              │ │
│  │   CLIENT_RANDOM 3a4b5c... <48-byte-secret>                   │ │
│  │   CLIENT_HANDSHAKE_TRAFFIC_SECRET 3a4b... <32-byte-secret>   │ │
│  │   SERVER_HANDSHAKE_TRAFFIC_SECRET 3a4b... <32-byte-secret>   │ │
│  └──────────────────────────────────────────────────────────────┘ │
│                                                                      │
└──────────────────────────────────────────────────────────────────────┘

Thread Safety Model

The keylog subsystem uses a pthread read-write lock for concurrent access.

Key Points:

• Multiple threads can write to the keylog file simultaneously during TLS handshakes
• File rotation (open/close) acquires exclusive lock
• Double-checked locking in write_line_callback() for performance

NSS Key Log Format

File Structure

Each line in the keylog file has the following format:

<LABEL> <ClientRandom> <Secret>\n

Label Types

Example Keylog File

# TLS 1.2 connection
CLIENT_RANDOM 3a4b5c6d7e8f... 48_byte_master_secret_here...

# TLS 1.3 connection
CLIENT_HANDSHAKE_TRAFFIC_SECRET 3a4b5c6d7e8f... 32_byte_secret_here...
SERVER_HANDSHAKE_TRAFFIC_SECRET 3a4b5c6d7e8f... 32_byte_secret_here...
CLIENT_TRAFFIC_SECRET_0 3a4b5c6d7e8f... 32_byte_secret_here...
SERVER_TRAFFIC_SECRET_0 3a4b5c6d7e8f... 32_byte_secret_here...

API Reference

Public Functions

proxysql_keylog_init()

void proxysql_keylog_init();

Purpose: Initialize the keylog subsystem

Called from: proxysql_global.cpp during startup

Thread-safety: Safe (single-threaded initialization only)

proxysql_keylog_open(const char* keylog_file)

bool proxysql_keylog_open(const char* keylog_file);

Purpose: Open/create the keylog file

Parameters:

• keylog_file: Path to the keylog file

Returns: true on success, false on failure

Behavior:

• Opens file in append mode with line buffering (4096 bytes)
• Closes existing file if open (supports rotation)
• Acquires write lock for thread safety

Called from:

• ProxySQL_Admin::set_variable() when ssl_keylog_file is set
• ProxySQL_Admin::flush_logs() for log rotation

proxysql_keylog_close(bool lock = true)

void proxysql_keylog_close(bool lock = true);

Purpose: Close the keylog file

Parameters:

• lock: If true, acquires write lock before closing

Behavior:

• Closes the file if open
• Sets keylog_file_fp to NULL

proxysql_keylog_attach_callback(SSL_CTX* ssl_ctx)

void proxysql_keylog_attach_callback(SSL_CTX* ssl_ctx);

Purpose: Attach keylog callback to an SSL context

Parameters:

• ssl_ctx: The SSL context to attach to

Behavior:

• Idempotent: only attaches if no callback already registered
• Uses SSL_CTX_set_keylog_callback() (OpenSSL 1.1.1+)

Called from:

• MySQL_Session::handler() for frontend MySQL connections
• PgSQL_Session::handler() for frontend PostgreSQL connections

proxysql_keylog_write_line_callback(const SSL* ssl, const char* line)

void proxysql_keylog_write_line_callback(const SSL* ssl, const char* line);

Purpose: OpenSSL callback invoked during TLS handshake

Parameters:

• ssl: The SSL connection (unused)
• line: The keylog line generated by OpenSSL (no newline)

Behavior:

• Validates line length (max 254 bytes)
• Ensures newline termination
• Acquires read lock and writes to file
• Uses double-checked locking for performance

Called by: OpenSSL automatically during TLS handshake

Configuration Variable

ssl_keylog_file (internal name)

Module: Admin

Internal Name: ssl_keylog_file

SQL Interface Name: admin-ssl_keylog_file

Type: String (path)

Default: "" (empty string = disabled)

Runtime Configurable: Yes

Path Resolution:

• Absolute path (starts with /): used as-is
• Relative path: resolved relative to ProxySQL data directory
• Empty string: disables key logging

SQL Usage:

-- Enable key logging
SET admin-ssl_keylog_file = '/var/log/proxysql/sslkeys.txt';

-- Disable key logging
SET admin-ssl_keylog_file = '';

-- Apply to runtime
LOAD ADMIN VARIABLES TO RUNTIME;

Config File Usage:

admin_variables=
{
    ssl_keylog_file='/var/log/proxysql/sslkeys.txt'
}

Integration Points

1. Variable Definition

File: include/proxysql_admin.h

struct {
    // ...
    char* ssl_keylog_file;  // Path to keylog file
} variables;

2. Variable Registration

File: lib/ProxySQL_Admin.cpp

{ (char *)"ssl_keylog_file", ... }

3. Runtime Variable Setter

File: lib/ProxySQL_Admin.cpp:set_variable()

if (!strcasecmp(name, "ssl_keylog_file")) {
    // Handle absolute vs relative paths
    // Open file or validate path
    // Set GloVars.global.ssl_keylog_enabled
}

4. Log Rotation

File: lib/ProxySQL_Admin.cpp:flush_logs()

void ProxySQL_Admin::flush_logs() {
    // ...
    proxysql_keylog_close();  // Close current file
    proxysql_keylog_open(ssl_keylog_file);  // Reopen
}

5. SSL Context Integration

Files:

• lib/MySQL_Session.cpp
• lib/PgSQL_Session.cpp

proxysql_keylog_attach_callback(GloVars.get_SSL_ctx());

Security Considerations

WARNING: Critical Security Implications

The keylog file contains cryptographic secrets that can decrypt ALL TLS traffic:

1. What the file contains:

   • TLS master secrets (TLS 1.2)
   • Traffic encryption keys (TLS 1.3)
   • Enough to decrypt past, present, and future connections

2. Attack scenarios if compromised:

   • Decryption of captured network traffic, exposing all plaintext data, including credentials, queries, and results.

3. Recommended safeguards:

   • File permissions: 0600 (owner read/write only)
   • Directory permissions: 0700 (owner access only)
   • Enable only for debugging: Disable in production
   • Rotate regularly: PROXYSQL FLUSH LOGS
   • Secure deletion: Shred file before deletion

Code Review Checklist

When reviewing changes to this module:

• File permissions are set correctly
• Path validation prevents directory traversal
• Lock acquisition is correct (rdlock vs wrlock)
• Double-checked locking is properly implemented
• No sensitive data in logs/error messages
• Proper cleanup on error conditions

Testing

Unit Test Structure

File: test/tap/tests/test_auth_methods-t.cpp

void ssl_keylog_callback(SSL*, const char* line) {
    // Verify line format
    // Verify file contents
}

// Test callback registration
mysql_options(proxy, MARIADB_OPT_SSL_KEYLOG_CALLBACK, ...);

Manual Testing

1. Enable key logging:

mysql -h 127.0.0.1 -P 6032 -u admin -padmin -e "SET admin-ssl_keylog_file='/tmp/keylog.txt'; LOAD ADMIN VARIABLES TO RUNTIME;"

2. Create TLS connection:

mysql -h 127.0.0.1 -P 6033 -u user -ppass --ssl

3. Verify keylog file:

cat /tmp/keylog.txt
# Should see CLIENT_RANDOM or TRAFFIC_SECRET lines

4. Configure Wireshark:
   • Edit → Preferences → Protocols → TLS
   • Set "(Pre)-Master-Secret log filename" to /tmp/keylog.txt
   • Decrypt TLS traffic in Wireshark

Performance Considerations

1. Line buffering: 4096 bytes minimizes syscalls
2. Read-write lock: Allows concurrent writes during handshakes
3. Double-checked locking: Avoids lock acquisition when disabled
4. File mode: Append mode minimizes disk seeks

References

• NSS Key Log Format
• Wireshark TLS Decryption
• OpenSSL SSL_CTX_set_keylog_callback
• PR #4236