DNS Resolver: support both dnspython before and after 2.0.0

`dnspython` 2.0.0 has many changes and several deprecations like:

```
> dns.resolver.resolve() has been added, allowing control of whether
search lists are used. dns.resolver.query() is retained for backwards
compatibility, but deprecated. The default for search list behavior can
be set at in the resolver object with the use_search_by_default
parameter. The default is False.

> dns.resolver.resolve_address() has been added, allowing easy
address-to-name lookups.
```

The new class `DNSResolver`:
- provides the compatibility layer
- defaults the previous behavior (the search list configured in the
  system's resolver configuration is used for relative names)
- defaults lifetime to 15sec (determines the number of seconds
  to spend trying to get an answer to the question)

The compatibility shim was developed by Stanislav Levin for FreeIPA and
adopted for Samba by Alexander Bokovoy.

Signed-off-by: Stanislav Levin <slev@altlinux.org>
Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>

1 files changed, 68 insertions, 0 deletions

diff --git a/python/samba/dnsresolver.py b/python/samba/dnsresolver.py
new file mode 100644
index 00000000000..a627555a855
--- /dev/null
+++ b/python/samba/dnsresolver.py
@@ -0,0 +1,68 @@
+# Samba wrapper for DNS resolvers
+#
+# Copyright (C) Stanislav Levin <slev@altlinux.org>
+# Copyright (C) Alexander Bokovoy <ab@samba.org>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+import dns.resolver
+import dns.rdatatype
+import dns.reversename
+
+class DNSResolver(dns.resolver.Resolver):
+    """DNS stub resolver compatible with both dnspython < 2.0.0
+    and dnspython >= 2.0.0.
+
+    Set `use_search_by_default` attribute to `True`, which
+    determines the default for whether the search list configured
+    in the system's resolver configuration is used for relative
+    names, and whether the resolver's domain may be added to relative
+    names.
+
+    Increase the default lifetime which determines the number of seconds
+    to spend trying to get an answer to the question. dnspython 2.0.0
+    changes this to 5sec, while the previous one was 30sec.
+    """
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.reset_defaults()
+        self.resolve = getattr(super(), "resolve", self.query)
+        self.resolve_address = getattr(
+            super(),
+            "resolve_address",
+            self._resolve_address
+        )
+
+    def reset_defaults(self):
+        self.use_search_by_default = True
+        # the default is 5sec
+        self.lifetime = 15
+
+    def reset(self):
+        super().reset()
+        self.reset_defaults()
+
+    def _resolve_address(self, ip_address, *args, **kwargs):
+        """Query nameservers for PTR records.
+
+        :param ip_address: IPv4 or IPv6 address
+        :type ip_address: str
+        """
+        return self.resolve(
+            dns.reversename.from_address(ip_address),
+            rdtype=dns.rdatatype.PTR,
+            *args,
+            **kwargs,
+        )