ipn/ipnlocal: add arpa suffixes to MagicDNS for reverse lookups.

This used to not be necessary, because MagicDNS always did full proxying.
But with split DNS, we need to know which names to route to our resolver,
otherwise reverse lookups break.

This captures the entire CGNAT range, as well as our Tailscale ULA.

Signed-off-by: David Anderson <danderson@tailscale.com>

ipn/ipnlocal/local.go

@@ -1806,7 +1806,19 @@ func magicDNSRootDomains(nm *netmap.NetworkMap) []dnsname.FQDN {
 			// TODO: propagate error
 			return nil
 		}
-		return []dnsname.FQDN{fqdn}
+		ret := []dnsname.FQDN{
+			fqdn,
+			dnsname.FQDN("0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa."),
+		}
+		for i := 64; i <= 127; i++ {
+			fqdn, err = dnsname.ToFQDN(fmt.Sprintf("%d.100.in-addr.arpa.", i))
+			if err != nil {
+				// TODO: propagate error
+				continue
+			}
+			ret = append(ret, fqdn)
+		}
+		return ret
 	}
 	return nil
 }