Allow disabling of TLS 1.0

[jbwdevries]

At the moment you can only pick a certain TLS version, or allow all TLS versions. It would be better if that could be done for TLS as well, since it's vulnerable to BEAST.

I think this is done in net.c, mosquitto__tls_server_ctx. There it picks one *_server_method based on the settings.

It would be useful if there was also a way to have mosquitto pass SSL_OP_NO_TLSv1 via ssl_options slightly below that. Possibly the option could be comma separated; if so, the code would use SSLv23_server_method and then use the related SSL_OP_NO_SSL* and SSL_OP_NO_TLS* for the ones not in the list.

[ralight]

What's the downside to just disabling TLS v1.0?

[jbwdevries]

I don't have a problem to just disable it, but there may be users who have clients that still support only TLS v1.0 or lower.

[oej]

In many projects, we've disabled TLS v1.0 as a default in the code, removed all versions of SSL from the code and still have the option to enable TLS v1.0 in the configuration file.

[toast-uz]

I agree, but I guess disabling TLS v1.0 should be cautious. Because, IoT use cases are different from web browsing. There might be many devices cannot be upgraded to use TLS v1.1 or later. For mosquitto, changing the default behavior causes larger impact than for web servers.

[ralight]

I think this can be closed now as TLS 1.0 is not available in version 1.6.