implement intelligent updater (only sends ns update if IP really changed)

nsupdate/main/_tests/test_dnstools.py

@@ -4,13 +4,29 @@ Tests for dnstools module.
 
 import pytest
 
-from ..dnstools import (query_ns, parse_name, update_ns,
+from ..dnstools import (update, query_ns, parse_name, update_ns, SameIpError,
                         BASEDOMAIN, NONEXISTING_HOST,
                         WWW_HOST, WWW_IPV4_HOST, WWW_IPV4_IP, WWW_IPV6_HOST, WWW_IPV6_IP, )
 
 from dns.resolver import NXDOMAIN
 
 
+class TestIntelligentUpdater(object):
+    def test_double_update(self):
+        host, ip = 'test0.' + BASEDOMAIN, '1.2.3.4'
+        # make sure the host is not there
+        try:
+            update_ns(host, 'A', action='del')
+        except NXDOMAIN:
+            # it is ok if it was never there
+            pass
+        # first update with this IP, should work without issue:
+        update(host, ip)
+        with pytest.raises(SameIpError):
+            # trying to update again with same IP should raise
+            update(host, ip)
+
+
 class TestQuery(object):
     def test_queries_ok(self):
         assert query_ns(WWW_IPV4_HOST, 'A') == WWW_IPV4_IP  # v4 ONLY

nsupdate/main/dnstools.py

@@ -1,8 +1,10 @@
-"""
+s"""
 Misc. DNS related code: query, dynamic update, etc.
 """
 
 from django.conf import settings
+
+import dns.inet
 import dns.name
 import dns.resolver
 import dns.query
@@ -25,6 +27,32 @@ UPDATE_ALGO = dns.tsig.HMAC_SHA512
 UPDATE_KEY = settings.UPDATE_KEY
 
 
+class SameIpError(ValueError):
+    """
+    raised if an IP address is already present in DNS and and update was
+    requested, but is not needed.
+    """
+
+
+def update(fqdn, ipaddr, ttl=60):
+    af = dns.inet.af_for_address(ipaddr)
+    rdtype = 'A' if af == dns.inet.AF_INET else 'AAAA'
+    try:
+        current_ipaddr = query_ns(fqdn, rdtype)
+        # check if ip really changed
+        ok = ipaddr != current_ipaddr
+    except dns.resolver.NXDOMAIN:
+        # no dns entry yet, ok
+        ok = True
+    if ok:
+        # only send an update if the ip really changed as the update
+        # causes write I/O on the nameserver and also traffic to the
+        # dns slaves (they get a notify if we update the zone).
+        update_ns(fqdn, rdtype, ipaddr, action='upd', ttl=ttl)
+    else:
+        raise SameIpError
+
+
 def query_ns(qname, rdtype):
     """
     query a dns name from our master server