core: add more transports (#574)

* core: add listening for other protocols

Allow CoreDNS to listen for TLS request coming over port 853. This can
be enabled with `tls://` in the config file.

Implement listening for grps:// as well.

a Corefile like:

~~~
. tls://.:1853 {
    whoami
    tls
}
~~~

Means we listen on 1853 for tls requests, the `tls` config item allows
configuration for TLS parameters. We *might* be tempted to use Caddy's
Let's Encrypt implementation here.

* Refactor coredns/grpc into CoreDNS

This makes gRPC a first class citizen in CoreDNS. Add defines as being
just another server.

* some cleanups

* unexport the servers

* Move protobuf dir

* Hook up TLS properly

* Fix test

* listen for TLS as well. README updates

* disable test, fix package

* fix test

* Fix tests

* Fix remaining test

* Some tests

* Make the test work

* Add grpc test from #580

* fix crash

* Fix tests

* Close conn

* README cleanups

* README

* link RFC

middleware/tls/tls.go

@@ -0,0 +1,37 @@
+package tls
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/middleware"
+	"github.com/coredns/coredns/middleware/pkg/tls"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("tls", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	config := dnsserver.GetConfig(c)
+
+	if config.TLSConfig != nil {
+		return middleware.Error("tls", c.Errf("TLS already configured for this server instance"))
+	}
+
+	for c.Next() {
+		args := c.RemainingArgs()
+		if len(args) != 3 {
+			return middleware.Error("tls", c.ArgErr())
+		}
+		tls, err := tls.NewTLSConfig(args[0], args[1], args[2])
+		if err != nil {
+			return middleware.Error("tls", c.ArgErr())
+		}
+		config.TLSConfig = tls
+	}
+	return nil
+}