class_docstr: Implementation of ZAP authentication for zmq connections.
  
  Note:
  
  - libzmq provides four levels of security: default NULL (which the Authenticator does
    not see), and authenticated NULL, PLAIN, CURVE, and GSSAPI, which the Authenticator can see.
  - until you add policies, all incoming NULL connections are allowed.
    (classic ZeroMQ behavior), and all PLAIN and CURVE connections are denied.
  - GSSAPI requires no configuration.
docstr: Nothing to do for GSSAPI, which has already been handled by an external service.
docstr_tok: ["Nothing","to","do","for","GSSAPI",",","which","has","already","been","handled","by","an","external","service","."]
code: def _authenticate_gssapi(self, domain, principal):
      """Nothing to do for GSSAPI, which has already been handled by an external service."""
      self.log.debug(
          "ALLOWED (GSSAPI) domain=%s principal=%s", domain, principal
      )
      return True, b"OK"
  
imports: import logging
  import zmq
  from zmq.utils import z85
  from zmq.utils.strtypes import bytes, unicode, b, u
  from zmq.error import _check_version
  from .certs import load_certificates
label_desc: Use image node_id 14 for calling the Authenticator obj's underlying member method code with example usage: obj._authenticate_gssapi(domain, principal) and returns: True,  bytes