Add NSID plugin support for CoreDNS (#1273)

* Add NSID plugin support for CoreDNS

This fix adds NSID plugin support for CoreDNS, as was proposed
in 1256.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test cases for NSID plugin

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Generate code for NSID plugin

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Use hostname as the default (as with bind), and remove unneeded copy

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add README.md

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/nsid/nsid.go

@@ -0,0 +1,54 @@
+// Package nsid implements NSID protocol
+package nsid
+
+import (
+	"encoding/hex"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Nsid plugin
+type Nsid struct {
+	Next plugin.Handler
+	Data string
+}
+
+// ResponseWriter is a response writer that adds NSID response
+type ResponseWriter struct {
+	dns.ResponseWriter
+	Data string
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (n Nsid) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	if option := r.IsEdns0(); option != nil {
+		for _, o := range option.Option {
+			if _, ok := o.(*dns.EDNS0_NSID); ok {
+				nw := &ResponseWriter{ResponseWriter: w, Data: n.Data}
+				return plugin.NextOrFailure(n.Name(), n.Next, ctx, nw, r)
+
+			}
+		}
+	}
+	return plugin.NextOrFailure(n.Name(), n.Next, ctx, w, r)
+}
+
+// WriteMsg implements the dns.ResponseWriter interface.
+func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
+	if option := res.IsEdns0(); option != nil {
+		for _, o := range option.Option {
+			if e, ok := o.(*dns.EDNS0_NSID); ok {
+				e.Code = dns.EDNS0NSID
+				e.Nsid = hex.EncodeToString([]byte(w.Data))
+			}
+		}
+	}
+	returned := w.ResponseWriter.WriteMsg(res)
+	return returned
+}
+
+// Name implements the Handler interface.
+func (n Nsid) Name() string { return "nsid" }