middleware/whoami: add (#264)

Add a new middleware that tells you who you are; IP, port and transport
is echoed back.

Also some various cleanup and documentation improvements while at it:

* ResponseWriter: improve the documentation of these helper functions.
* And add an NextHandler for use in tests. Make chaos_test.go and
* whoam_test.go use it.

middleware/whoami/whoami.go

@@ -0,0 +1,50 @@
+package whoami
+
+import (
+	"net"
+	"strconv"
+
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+type Whoami struct {
+	Next middleware.Handler
+}
+
+func (wh Whoami) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+
+	a := new(dns.Msg)
+	a.SetReply(r)
+	a.Compress = true
+	a.Authoritative = true
+
+	ip := state.IP()
+	var rr dns.RR
+
+	switch state.Family() {
+	case 1:
+		rr = new(dns.A)
+		rr.(*dns.A).Hdr = dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass()}
+		rr.(*dns.A).A = net.ParseIP(ip).To4()
+	case 2:
+		rr = new(dns.AAAA)
+		rr.(*dns.AAAA).Hdr = dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass()}
+		rr.(*dns.AAAA).AAAA = net.ParseIP(ip)
+	}
+
+	srv := new(dns.SRV)
+	srv.Hdr = dns.RR_Header{Name: "_" + state.Proto() + "." + state.QName(), Rrtype: dns.TypeSRV, Class: state.QClass()}
+	port, _ := strconv.Atoi(state.Port())
+	srv.Port = uint16(port)
+
+	a.Extra = append(a.Extra, rr)
+	a.Extra = append(a.Extra, srv)
+
+	w.WriteMsg(a)
+	return 0, nil
+}