Enable TLS hostname verification by default

[EronWright]

Is your enhancement request related to a problem? Please describe.
Hostname verification is a critical aspect of TLS, especially when using the system-provided certificate bundle. TLS server authentication consists of two steps, a) verify the certificate is valid according to a trust chain (i.e. a certificate bundle), and b) verify that the certificate is applicable to the connection (i.e. hostname verification). See the article The Most Dangerous Code in the World for more details.

Without hostname verification, the client is willing to accept any certificate that is rooted in the certificate bundle; if you use the system bundle, any certificate from Let's Encrypt or other public issuer would suffice. For example, the client thinks it is connecting to pulsar.apache.org but accepts a certificate for domain-that-i-just-purchased.com.

Pulsar disables hostname verification by default, perhaps on the assumption that a private CA is being used and thus the set of valid certificates is tightly controlled. From the docs:

By default, Pulsar clients disable hostname verification, as it requires that each broker has a DNS record and a unique cert.

Describe the solution you'd like
The enhancement request is to enable hostname verification by default, at least when using the system cert bundle. Specifically, enable verification when both are true: a) an explicit setting has not been specified, and b) a certificate bundle has not been specified. Condition (b) is for backwards compatibility purposes.

[codelipenghui]

The issue had no activity for 30 days, mark with Stale label.

[tisonkun]

FYI @nodece is this issue still relevant?

[nodece]

This idea makes sense, but this change breaks user behavior.

[tisonkun]

Thanks for your explanation. Then moved to the discussion forum.

[michaeljmarshall]

This issue is still relevant. I have wanted to submit a PIP to do this, and to make sure that all configuration names for hostname verification are the same, but I haven't had a chance. I think Pulsar should move towards enabling hostname verification by default.