DNSSEC validation change for reverse lookups in RFC-1918 ranges and friends.

The large public DNS services seem not to return proof-of-nonexistence
for DS records at the start of RFC-1918 in-addr.arpa domains and the their
IPv6 equivalents. 10.in-addr.arpa, 168.192.in-addr.arpa etc.

Since dnsmasq already has an option which instructs it not bother
upstream servers with pointless queries about these address ranges,
namely --bogus-priv, we extend that to enable behaviour which allows
dnsmasq to assume that insecure NXDOMAIN replies for these domains
are expected and to assume that the domains are legitimately unsigned.

This behaviour only matters when some address range is directed to
another upstream server using --rev-server. In that case it allows
replies from that server to pass DNSSEC validation. Without such a
server configured, queries are never sent upstream so they are never
validated and the new behaviour is moot.

src/dnssec.c

@@ -1020,7 +1020,16 @@ int dnssec_validate_ds(time_t now, struct dns_header *header, size_t plen, char
     {
       if (STAT_ISEQUAL(rc, STAT_INSECURE))
 	{
-	  if (lookup_domain(name, F_DOMAINSRV, NULL, NULL))
+	  if (option_bool(OPT_BOGUSPRIV) &&
+	      (flags = in_arpa_name_2_addr(name, &a)) &&
+	      ((flags == F_IPV6 && private_net6(&a.addr6, 0)) || (flags == F_IPV4 && private_net(a.addr4, 0))))
+	    {
+	      my_syslog(LOG_INFO, _("Insecure reply received for DS %s, assuming that's OK for a RFC-1918 address."), name);
+	      neganswer = 1;
+	      nons = 0; /* If we're faking a DS, fake one with an NS. */
+	      neg_ttl = DNSSEC_ASSUMED_DS_TTL;
+	    }
+	  else if (lookup_domain(name, F_DOMAINSRV, NULL, NULL))
 	    {
 	      my_syslog(LOG_INFO, _("Insecure reply received for DS %s, assuming non-DNSSEC domain-specific server."), name);
 	      neganswer = 1;