Add description of TLS unsupported_record_type X errors

[acammack]

This probably belongs in some sort of operations document besides the (already quite long) existing site/ssl.md page.

When connecting an Erlang TLS client or server to an un-encrypted server or client, instead of saying that the other side is probably when un-encrypted when the handshake is invalid, it generates stacktrace and a SASL error in the RabbitMQ logs like:

TLS server: In state hello at tls_record.erl:564 generated SERVER ALERT: Fatal - Unexpected Message
 - {unsupported_record_type,65}

Errors from an outbound TLS connection generate CLIENT ALERT messages instead of SERVER ALERT above, but the format is otherwise the same. Packet capturing is the best way to diagnose which ports are not behaving as expected, but there is also a clue in the error message itself. The number given for the unsupported_record_type is the decimal value of the first byte received over the connection, which for a TLS connection would be message ("record") type identifier. For the example above, 65 is the ASCII letter A, which is the first byte of the AMQP protocol, indicating that the incoming connection is likely an AMQP client that does not have TLS enabled. Another common code is 71 for the ASCII letter G, indicating a likely HTTP GET request.

These sorts of errors are somewhat expected on internet-facing RabbitMQ installations due to prevalence of internet scanning tools that inconsistently use TLS even on common TLS ports. If none of the expected clients are having issues connecting, the errors can be ignored and do not reflect an issue with the RabbitMQ server itself (though you may want to block noisy IP addresses).

[michaelklishin]

We have a guide on Troubleshooting TLS, so these would be a good fit there.

[gsmith-sas]

In my environment (Kubernetes running RabbitMQ version 3.10), these log messages are emitted with a log level of "notice" which is not one of the documented logging levels. I'm not sure if that's an issue with the log message itself (should it be one of the documented levels?) or the documentation (should 'notice' be a documented level?). Given the explanation above, should we consider "notice" as comparable to "debug"?

[michaelklishin]

Consider using GitHub Discussions for questions.

Those messages are logged by Erlang's TLS implementation, not RabbitMQ. RabbitMQ itself very rarely if ever logs notice messages.

notice sits between warning and info on the list of log severity levels supported by the runtime logger.

[danielschnetler]

When connecting to a TLS endpoint from non TLS enabled client I also received this error. Try to do something like this if you are using a C# client:

var factory = new ConnectionFactory() { 
    HostName = "hostname", 
    UserName="user", 
    Password="password", 
    VirtualHost="/",
    Port = 5671,
    Ssl = new SslOption
            {
                Enabled = true,
                ServerName = "hostname"
            }
 };

[garry-t]

same for me, have no idea what does it mean and why ssl management doesnt work