[AIRFLOW-2698] Simplify Kerberos code (apache#3563)

Some functions were not used. On top of that, the
`principal_from_username` function was getting the wrong config value
("security" instead of "kerberos").

Since the results were only used by `kerberos.checkPassword`, and the
function can cope with needing a realm in the `username` when `realm` is
provided, we removed the `principal_from_username` function altogether.

airflow/contrib/auth/backends/kerberos_auth.py

@@ -19,6 +19,7 @@
 
 import logging
 import flask_login
+from airflow.exceptions import AirflowConfigException
 from flask_login import current_user
 from flask import flash
 from wtforms import Form, PasswordField, StringField
@@ -56,7 +57,13 @@ def authenticate(username, password):
             utils.get_fqdn()
         )
         realm = configuration.conf.get("kerberos", "default_realm")
-        user_principal = utils.principal_from_username(username)
+
+        try:
+            user_realm = configuration.conf.get("security", "default_realm")
+        except AirflowConfigException:
+            user_realm = realm
+
+        user_principal = utils.principal_from_username(username, user_realm)
 
         try:
             # this is pykerberos specific, verify = True is needed to prevent KDC spoofing
@@ -66,7 +73,8 @@ def authenticate(username, password):
                 raise AuthenticationError()
         except kerberos.KrbError as e:
             logging.error(
-                'Password validation for principal %s failed %s', user_principal, e)
+                'Password validation for user '
+                '%s in realm %s failed %s', user_principal, realm, e)
             raise AuthenticationError(e)
 
         return

airflow/security/utils.py

@@ -22,21 +22,6 @@
 
 from airflow.utils.net import get_hostname
 
-# Pattern to replace with hostname
-HOSTNAME_PATTERN = '_HOST'
-
-
-def get_kerberos_principal(principal, host):
-    components = get_components(principal)
-    if not components or len(components) != 3 or components[1] != HOSTNAME_PATTERN:
-        return principal
-    else:
-        if not host:
-            raise IOError("Can't replace %s pattern "
-                          "since host is null." % HOSTNAME_PATTERN)
-        return replace_hostname_pattern(components, host)
-
-
 def get_components(principal):
     """
     get_components(principal) -> (short name, instance (FQDN), realm)
@@ -51,33 +36,27 @@ def get_components(principal):
 def replace_hostname_pattern(components, host=None):
     fqdn = host
     if not fqdn or fqdn == '0.0.0.0':
-        fqdn = get_localhost_name()
+        fqdn = get_hostname()
     return '%s/%s@%s' % (components[0], fqdn.lower(), components[2])
 
 
-def get_localhost_name():
-    return get_hostname()
-
-
 def get_fqdn(hostname_or_ip=None):
     # Get hostname
     try:
         if hostname_or_ip:
             fqdn = socket.gethostbyaddr(hostname_or_ip)[0]
+            if fqdn == 'localhost':
+                fqdn = get_hostname()
         else:
-            fqdn = get_localhost_name()
+            fqdn = get_hostname()
     except IOError:
         fqdn = hostname_or_ip
 
-    if fqdn == 'localhost':
-        fqdn = get_localhost_name()
-
     return fqdn
 
 
-def principal_from_username(username):
-    realm = conf.get("security", "default_realm")
-    if '@' not in username and realm:
+def principal_from_username(username, realm):
+    if ('@' not in username) and realm:
         username = "{}@{}".format(username, realm)
 
     return username