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, 10 insertions, 10 deletions

diff --git a/core/dnsserver/server_test.go b/core/dnsserver/server_test.go
index 16235fd7c..75f5b98f7 100644
--- a/core/dnsserver/server_test.go
+++ b/core/dnsserver/server_test.go
@@ -18,7 +18,7 @@ func (tp testPlugin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.
 
 func (tp testPlugin) Name() string { return "testplugin" }
 
-func testConfig(transport string) *Config {
+func testConfig(transport string, p plugin.Handler) *Config {
 	c := &Config{
 		Zone:       "example.com.",
 		Transport:  transport,
@@ -27,31 +27,31 @@ func testConfig(transport string) *Config {
 		Debug:      false,
 	}
 
-	c.AddPlugin(func(next plugin.Handler) plugin.Handler { return testPlugin{} })
+	c.AddPlugin(func(next plugin.Handler) plugin.Handler { return p })
 	return c
 }
 
 func TestNewServer(t *testing.T) {
-	_, err := NewServer("127.0.0.1:53", []*Config{testConfig("dns")})
+	_, err := NewServer("127.0.0.1:53", []*Config{testConfig("dns", testPlugin{})})
 	if err != nil {
-		t.Errorf("Expected no error for NewServer, got %s.", err)
+		t.Errorf("Expected no error for NewServer, got %s", err)
 	}
 
-	_, err = NewServergRPC("127.0.0.1:53", []*Config{testConfig("grpc")})
+	_, err = NewServergRPC("127.0.0.1:53", []*Config{testConfig("grpc", testPlugin{})})
 	if err != nil {
-		t.Errorf("Expected no error for NewServergRPC, got %s.", err)
+		t.Errorf("Expected no error for NewServergRPC, got %s", err)
 	}
 
-	_, err = NewServerTLS("127.0.0.1:53", []*Config{testConfig("tls")})
+	_, err = NewServerTLS("127.0.0.1:53", []*Config{testConfig("tls", testPlugin{})})
 	if err != nil {
-		t.Errorf("Expected no error for NewServerTLS, got %s.", err)
+		t.Errorf("Expected no error for NewServerTLS, got %s", err)
 	}
 }
 
 func BenchmarkCoreServeDNS(b *testing.B) {
-	s, err := NewServer("127.0.0.1:53", []*Config{testConfig("dns")})
+	s, err := NewServer("127.0.0.1:53", []*Config{testConfig("dns", testPlugin{})})
 	if err != nil {
-		b.Errorf("Expected no error for NewServer, got %s.", err)
+		b.Errorf("Expected no error for NewServer, got %s", err)
 	}
 
 	ctx := context.TODO()