Customizable SPN Service Class for GSSAPI Authentication

[Barakat]

The JavaMail specifications do not explicitly define how the SPN service classes should be constructed when using GSSAPI authentication. This omission has led to hardcoded service classes, such as smtp/host@realm, across implementations, limiting compatibility with various Kerberos environments.

I propose updating the JavaMail specs to:

• Introduce a property, mail.smtp.sasl.gssapi.spnserviceclass, to allow customization of the service class.
• Specify that the default can remain smtp (or whatever the implementer was using) if no customization is provided.

References:

• Name Formats for Unique SPNs

Eclipse Angus's Implementation:

// https://github.com/eclipse-ee4j/angus-mail/blob/master/providers/smtp/src/main/java/org/eclipse/angus/mail/smtp/SMTPTransport.java

public class SMTPTransport extends Transport {
    private String name = "smtp";    // Name of this protocol
   ...
                saslAuthenticator = (SaslAuthenticator) c.newInstance(
                        new Object[]{
                                this,
                                name, // <-- hardcoded service name
                                session.getProperties(),
                                logger,
                                serviceHost
                        });
 ...
}

// https://github.com/eclipse-ee4j/angus-mail/blob/master/providers/smtp/src/main/java/org/eclipse/angus/mail/smtp/SMTPSaslAuthenticator.java

   public SMTPSaslAuthenticator(SMTPTransport pr, String name,
                                Properties props, MailLogger logger, String host) {
...
       this.name = name;
...
   }
...
           Map<String, ?> propsMap = (Map) props;
           sc = Sasl.createSaslClient(mechs, authzid, name, host, // <-- hardcoded service name
                   propsMap, cbh);
...