Sort and shuffle SRV record by priority and weight

On multiple occasions, SRV query answers were not properly sorted by
priority. Records with same priority weren't randomized and shuffled.
This caused FreeIPA to contact the same remote peer instead of
distributing the load across all available servers.

Two new helper functions now take care of SRV queries. sort_prio_weight()
sorts SRV and URI records. query_srv() combines SRV lookup with
sort_prio_weight().

Fixes: https://pagure.io/freeipa/issue/7475
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipapython/config.py

@@ -26,7 +26,6 @@ from copy import copy
 import socket
 import functools
 
-from dns import resolver, rdatatype
 from dns.exception import DNSException
 import dns.name
 # pylint: disable=import-error
@@ -36,6 +35,7 @@ from six.moves.urllib.parse import urlsplit
 
 from ipaplatform.paths import paths
 from ipapython.dn import DN
+from ipapython.dnsutil import query_srv
 from ipapython.ipautil import CheckedIPAddress, CheckedIPAddressLoopback
 
 
@@ -210,7 +210,7 @@ def __discover_config(discover_server = True):
             name = "_ldap._tcp." + domain
 
             try:
-                servers = resolver.query(name, rdatatype.SRV)
+                servers = query_srv(name)
             except DNSException:
                 # try cycling on domain components of FQDN
                 try:
@@ -225,7 +225,7 @@ def __discover_config(discover_server = True):
                         return False
                     name = "_ldap._tcp.%s" % domain
                     try:
-                        servers = resolver.query(name, rdatatype.SRV)
+                        servers = query_srv(name)
                         break
                     except DNSException:
                         pass
@@ -236,7 +236,7 @@ def __discover_config(discover_server = True):
             if not servers:
                 name = "_ldap._tcp.%s." % config.default_domain
                 try:
-                    servers = resolver.query(name, rdatatype.SRV)
+                    servers = query_srv(name)
                 except DNSException:
                     pass