CIDR query routing (#1159)

* core: allow all CIDR ranges in zone specifications

Allow (e.g.) a v4 reverse on a /17. If a zone is specified in such a
way a FilterFunc is set in the config. This filter is checked against
incoming queries.

For all other queries this adds a 'x != nil' check which will not impact
performace too much. Benchmark function is added as well to check for
this as wel.

Add multiple tests in tests/server_reverse_test.go.

Benchmark shows in the non-reverse case this hardly impact the speed:

~~~
classless:
pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1431 ns/op	      16 B/op	       1 allocs/op

pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1429 ns/op	      16 B/op	       1 allocs/op

master:
pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1412 ns/op	      16 B/op	       1 allocs/op

pkg: github.com/coredns/coredns/core/dnsserver
BenchmarkCoreServeDNS-4   	 1000000	      1429 ns/op	      16 B/op	       1 allocs/op
~~~

* README.md updates

1 files changed, 29 insertions, 1 deletions

diff --git a/plugin/normalize_test.go b/plugin/normalize_test.go
index 3eb9c5231..e81d32629 100644
--- a/plugin/normalize_test.go
+++ b/plugin/normalize_test.go
@@ -70,7 +70,7 @@ func TestNameNormalize(t *testing.T) {
 
 func TestHostNormalize(t *testing.T) {
 	hosts := []string{".:53", ".", "example.org:53", "example.org.", "example.org.:53", "example.org.",
-		"10.0.0.0/8:53", "10.in-addr.arpa.", "10.0.0.0/9", "10.0.0.0/9.",
+		"10.0.0.0/8:53", "10.in-addr.arpa.", "10.0.0.0/9", "10.in-addr.arpa.",
 		"dns://example.org", "example.org."}
 
 	for i := 0; i < len(hosts); i += 2 {
@@ -82,3 +82,31 @@ func TestHostNormalize(t *testing.T) {
 		}
 	}
 }
+
+func TestSplitHostPortReverse(t *testing.T) {
+	tests := map[string]int{
+		"example.org.": 0,
+		"10.0.0.0/9":   32 - 9,
+		"10.0.0.0/8":   32 - 8,
+		"10.0.0.0/17":  32 - 17,
+		"10.0.0.0/0":   32 - 0,
+		"10.0.0.0/64":  0,
+		"10.0.0.0":     0,
+		"10.0.0":       0,
+		"2003::1/65":   128 - 65,
+	}
+	for in, expect := range tests {
+		_, _, n, err := SplitHostPort(in)
+		if err != nil {
+			t.Errorf("Expected no error, got %q for %s", in, err)
+		}
+		if n == nil {
+			continue
+		}
+		ones, bits := n.Mask.Size()
+		got := bits - ones
+		if got != expect {
+			t.Errorf("Expected %d, got %d for %s", expect, got, in)
+		}
+	}
+}