PYTHON-2192 - Use krb5's canonicalization algorithm

.evergreen/run-enterprise-auth-tests.sh

@@ -18,6 +18,7 @@ if [ ${PLATFORM} != "Java" ]; then
     if [ "Windows_NT" = "$OS" ]; then
         echo "Setting GSSAPI_PASS"
         export GSSAPI_PASS=${SASL_PASS}
+        export GSSAPI_CANONICALIZE="true"
     else
         # BUILD-3830
         touch ${PROJECT_DIRECTORY}/.evergreen/krb5.conf.empty

pymongo/auth.py

@@ -375,6 +375,20 @@ def _auth_key(nonce, username, password):
     return _unicode(md5hash.hexdigest())
 
 
+def _canonicalize_hostname(hostname):
+    """Canonicalize hostname following MIT-krb5 behavior."""
+    # https://github.com/krb5/krb5/blob/d406afa363554097ac48646a29249c04f498c88e/src/util/k5test.py#L505-L520
+    af, socktype, proto, canonname, sockaddr = socket.getaddrinfo(
+        hostname, None, 0, 0, socket.IPPROTO_TCP, socket.AI_CANONNAME)[0]
+
+    try:
+        name = socket.getnameinfo(sockaddr, socket.NI_NAMEREQD)
+    except socket.gaierror:
+        return canonname.lower()
+
+    return name[0].lower()
+
+
 def _authenticate_gssapi(credentials, sock_info):
     """Authenticate using GSSAPI.
     """
@@ -390,7 +404,7 @@ def _authenticate_gssapi(credentials, sock_info):
         # the security context. See RFC 4752, Section 3.1, first paragraph.
         host = sock_info.address[0]
         if props.canonicalize_host_name:
-            host = socket.getfqdn(host)
+            host = _canonicalize_hostname(host)
         service = props.service_name + '@' + host
         if props.service_realm is not None:
             service = service + '@' + props.service_realm