public Collection<String> getSubjectUsernameAttributes() throws GuacamoleException {
        return environment.getPropertyCollection(SSL_AUTH_SUBJECT_USERNAME_ATTRIBUTE);
    }
Returns a list of all attributes that may be used to represent a user's
username within their subject DN. If all attributes may be accepted,
null is returned.

@return
    A list of all attributes that may be used to represent a user's
    username within their subject DN, or null if any attribute may be
    used.

@throws GuacamoleException
    If the configured set of username attributes cannot be read.
extensions/guacamole-auth-sso/modules/guacamole-auth-sso-ssl/src/main/java/org/apache/guacamole/auth/ssl/conf/ConfigurationService.java