Understanding the 'Failed to Send SSL Close Message' Error in Network Programming

What does the error message 'Failed to send SSL Close message' mean and how can it be resolved?

// Example code for SSL connection closure
SSL_CTX *ctx = SSL_CTX_new(TLS_method());
SSL *ssl = SSL_new(ctx);
if (SSL_shutdown(ssl) == 0) {
    SSL_shutdown(ssl); // Attempting to send close message again
}

The 'Failed to send SSL Close message' error typically occurs when an SSL/TLS connection has not been properly shut down. This can happen due to interrupted connections, misconfigured servers, or underlying protocol issues. Understanding the potential causes and solutions can help developers and network administrators troubleshoot the problem effectively.

// Sample code for properly closing an SSL connection
if (SSL_shutdown(ssl) == 1) {
    // Successfully sent SSL close message
} else {
    // Handle the error
}

Causes

• The connection was interrupted unexpectedly before a proper closure sequence could be completed.
• Incorrect SSL/TLS version configurations between client and server.
• Firewall or network issues disrupting the SSL handshake or termination process.
• Using an improper method for closing an SSL connection.

Solutions

• Ensure that your SSL connections are being closed using the correct shutdown procedures.
• Verify that both client and server are using compatible SSL/TLS protocols and versions.
• Check firewall settings to ensure they are not interfering with SSL/TLS traffic.
• Review logs for more detailed error messages that can indicate configuration problems or other issues.

Mistake: Not checking SSL_shutdown return values.

Solution: Always check the return value of SSL_shutdown to confirm successful closure.

Mistake: Failing to clean up SSL resources after use.

Solution: Structuring your code to always free up SSL pointers using SSL_free and properly shutting down the connection.