add example how to send NOTIFY message (#1007)

diff --git a/send_notify.py b/send_notify.py
new file mode 100755 (executable)
index 0000000..08a0894
--- /dev/null
+++ b/send_notify.py
@@ -0,0 +1,83 @@
+#!/usr/bin/python
+"""
+Minimalistic RFC 1996-like NOTIFY sender.
+"""
+import argparse
+import ipaddress
+import socket
+
+import dns.flags
+import dns.inet
+import dns.message
+import dns.name
+import dns.opcode
+import dns.query
+import dns.rrset
+import dns.rdataclass
+import dns.rdatatype
+
+
+def main():
+    """Also prints all inputs and intermediate values"""
+    parser = argparse.ArgumentParser(
+        description="Send DNS NOTIFY mesage via UDP, optionally with synthetized SOA RR "
+        "in ANSWER section. No checks. It's not RFC 1996 sect 3.6 compliant sender."
+    )
+    parser.add_argument("--source", type=ipaddress.ip_address, help="source IP address")
+    parser.add_argument("--port", type=int, help="target port", default=53)
+    parser.add_argument("target", type=ipaddress.ip_address, help="target IP address")
+    parser.add_argument("zone", type=dns.name.from_text)
+    parser.add_argument(
+        "serial",
+        nargs="?",
+        type=int,
+        help="optional serial - adds SOA RR into ANSWER section",
+    )
+    parser.add_argument(
+        "--rdclass",
+        default=dns.rdataclass.IN,
+        type=dns.rdataclass.from_text,
+        help="DNS class, defaults to IN",
+    )
+    args = parser.parse_args()
+    if args.source:
+        if dns.inet.af_for_address(str(args.target)) != dns.inet.af_for_address(
+            str(args.source)
+        ):
+            parser.error("address family for source and target must be the same")
+
+    print(args)
+    msg = construct_msg(args)
+    print(msg)
+    udp_send(msg, args)
+
+
+def construct_msg(args):
+    """if args.serial is specified it creates fake SOA RR with given serial"""
+    msg = dns.message.make_query(
+        args.zone, dns.rdatatype.SOA, rdclass=args.rdclass, flags=dns.flags.AA
+    )
+    msg.set_opcode(dns.opcode.NOTIFY)
+    if args.serial:
+        soa = dns.rrset.from_text_list(
+            name=args.zone,
+            ttl=0,
+            rdclass=args.rdclass,
+            rdtype=dns.rdatatype.SOA,
+            text_rdatas=[f". . {args.serial} 0 0 0 0"],
+        )
+        msg.answer.append(soa)
+    return msg
+
+
+def udp_send(msg, args):
+    """ignores checks prescribed by RFC 1996 sect 3.6"""
+    afam = dns.inet.af_for_address(str(args.target))
+    sock = socket.socket(afam, socket.SOCK_DGRAM)
+    if args.source:
+        sock.bind((str(args.source), 0))
+    dns.query.send_udp(sock, what=msg, destination=(str(args.target), args.port))
+
+
+if __name__ == "__main__":
+    main()