DNSResolver: Fix use of nameservers with ports

IPA DNS zone and forwardzone commands allow to use nameservers with ports
as "SERVER_IP port PORT_NUMBER". bind is supporting this syntax, but the
Resolver in dnspython that is used to verify the list of forwarders
(nameservers) is only allowing to have IP addresses in this list. With
dnspython version 2.20 there is a new validator in dns.resolver.BaseResolver
that ensures this.

Refs:
- https://bind9.readthedocs.io/en/v9_18_4/reference.html#zone-statement-grammar
- https://github.com/rthalley/dnspython/blob/master/dns/resolver.py#L1094

ipapython/dnsutil.DNSResolver derives from dns.resolver.Resolver. The setter
for nameservers has been overloaded in the DNSResolver class to split out
the port numbers into the nameserver_ports dict { SERVER_IP: PORT_NUMBER }.
After the setter for nameservers succeeded, nameserver_ports is set.
nameserver_ports is used in the resolve() method of dns.resolver.Resolver.

Additional tests have been added to verify that nameservers and also
nameserver_ports are properly set and also valid.

Fixes: https://pagure.io/freeipa/issue/9158

Signed-off-by: Thomas Woerner <twoerner@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipapython/dnsutil.py

@@ -144,6 +144,47 @@ class DNSResolver(dns.resolver.Resolver):
             nameservers.remove(ipv4_loopback)
         self.nameservers = nameservers
 
+    @dns.resolver.Resolver.nameservers.setter
+    def nameservers(self, nameservers):
+        """
+        *nameservers*, a ``list`` of nameservers with optional ports:
+            "SERVER_IP port PORT_NUMBER".
+
+        Overloads dns.resolver.Resolver.nameservers setter to split off ports
+        into nameserver_ports after setting nameservers successfully with the
+        setter in dns.resolver.Resolver.
+        """
+        # Get nameserver_ports if it is already set
+        if hasattr(self, "nameserver_ports"):
+            nameserver_ports = self.nameserver_ports
+        else:
+            nameserver_ports = {}
+
+        # Check nameserver items in list and split out converted port number
+        # into nameserver_ports: { nameserver: port }
+        if isinstance(nameservers, list):
+            _nameservers = []
+            for nameserver in nameservers:
+                splits = nameserver.split()
+                if len(splits) == 3 and splits[1] == "port":
+                    nameserver = splits[0]
+                    try:
+                        port = int(splits[2])
+                        if port < 0 or port > 65535:
+                            raise ValueError()
+                    except ValueError:
+                        raise ValueError(
+                            "invalid nameserver: %s is not a valid port" %
+                            splits[2])
+                    nameserver_ports[nameserver] = port
+                _nameservers.append(nameserver)
+            nameservers = _nameservers
+
+        # Call dns.resolver.Resolver.nameservers setter
+        dns.resolver.Resolver.nameservers.__set__(self, nameservers)
+        # Set nameserver_ports after successfull call to setter
+        self.nameserver_ports = nameserver_ports
+
 
 class DNSZoneAlreadyExists(dns.exception.DNSException):
     supp_kwargs = {'zone', 'ns'}