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/setup.go

@@ -0,0 +1,48 @@
+package nsid
+
+import (
+	"os"
+	"strings"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("nsid", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	nsid, err := nsidParse(c)
+	if err != nil {
+		return plugin.Error("nsid", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return Nsid{Next: next, Data: nsid}
+	})
+
+	return nil
+}
+
+func nsidParse(c *caddy.Controller) (string, error) {
+	// Use hostname as the default
+	nsid, err := os.Hostname()
+	if err != nil {
+		nsid = "localhost"
+	}
+	for c.Next() {
+		args := c.RemainingArgs()
+		if len(args) == 0 {
+			return nsid, nil
+		}
+		nsid = strings.Join(args, " ")
+		return nsid, nil
+	}
+	return nsid, nil
+}