Add hostname canonicalization helper to k5test.py

To facilitate fallback tests, add a canonicalize_hostname() function
to k5test.py which works similarly to krb5_expand_hostname().  Use it
in t_gssapi.py for the recently-added acceptor name fallback test.

src/tests/gssapi/t_gssapi.py

@@ -8,7 +8,7 @@
     realm.run(['./t_iov', '-s', 'p:' + realm.host_princ])
     realm.run(['./t_pcontok', 'p:' + realm.host_princ])
 
-realm = K5Realm(krb5_conf={'libdefaults': {'rdns': 'false'}})
+realm = K5Realm()
 
 # Test gss_add_cred().
 realm.run(['./t_add_cred'])
@@ -62,13 +62,8 @@
           expected_msg=' not found in keytab')
 
 # If possible, test with an acceptor name requiring fallback to match
-# against a keytab entry.  Forward-canonicalize the hostname, relying
-# on the rdns=false realm setting.
-try:
-    ai = socket.getaddrinfo(hostname, None, 0, 0, 0, socket.AI_CANONNAME)
-    (family, socktype, proto, canonname, sockaddr) = ai[0]
-except socket.gaierror:
-    canonname = hostname
+# against a keytab entry.
+canonname = canonicalize_hostname(hostname)
 if canonname != hostname:
     os.rename(realm.keytab, realm.keytab + '.save')
     canonprinc = 'host/' + canonname

src/util/k5test.py

@@ -155,6 +155,10 @@
 * password(name): Return a weakly random password based on name.  The
   password will be consistent across calls with the same name.
 
+* canonicalize_hostname(name, rdns=True): Return the DNS
+  canonicalization of name, optionally using reverse DNS.  On error,
+  return name converted to lowercase.
+
 * stop_daemon(proc): Stop a daemon process started with
   realm.start_server() or realm.start_in_inetd().  Only necessary if
   the port needs to be reused; daemon processes will be stopped
@@ -458,6 +462,24 @@ def password(name):
     return name + str(os.getpid())
 
 
+def canonicalize_hostname(name, rdns=True):
+    """Canonicalize name using DNS, optionally with reverse DNS."""
+    try:
+        ai = socket.getaddrinfo(name, None, 0, 0, 0, socket.AI_CANONNAME)
+    except socket.gaierror as e:
+        return name.lower()
+    (family, socktype, proto, canonname, sockaddr) = ai[0]
+
+    if not rdns:
+        return canonname.lower()
+
+    try:
+        rname = socket.getnameinfo(sockaddr, socket.NI_NAMEREQD)
+    except socket.gaierror:
+        return canonname.lower()
+    return rname[0].lower()
+
+
 # Exit handler which ensures processes are cleaned up and, on failure,
 # prints messages to help developers debug the problem.
 def _onexit():