Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/etcd/stub_handler.go

@@ -0,0 +1,86 @@
+package etcd
+
+import (
+	"errors"
+	"log"
+
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Stub wraps an Etcd. We have this type so that it can have a ServeDNS method.
+type Stub struct {
+	*Etcd
+	Zone string // for what zone (and thus what nameservers are we called)
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (s Stub) ServeDNS(ctx context.Context, w dns.ResponseWriter, req *dns.Msg) (int, error) {
+	if hasStubEdns0(req) {
+		log.Printf("[WARNING] Forwarding cycle detected, refusing msg: %s", req.Question[0].Name)
+		return dns.RcodeRefused, errors.New("stub forward cycle")
+	}
+	req = addStubEdns0(req)
+	proxy, ok := (*s.Etcd.Stubmap)[s.Zone]
+	if !ok { // somebody made a mistake..
+		return dns.RcodeServerFailure, nil
+	}
+
+	state := request.Request{W: w, Req: req}
+	m, e := proxy.Forward(state)
+	if e != nil {
+		return dns.RcodeServerFailure, e
+	}
+	m.RecursionAvailable, m.Compress = true, true
+	state.SizeAndDo(m)
+	w.WriteMsg(m)
+	return dns.RcodeSuccess, nil
+}
+
+// hasStubEdns0 checks if the message is carrying our special edns0 zero option.
+func hasStubEdns0(m *dns.Msg) bool {
+	option := m.IsEdns0()
+	if option == nil {
+		return false
+	}
+	for _, o := range option.Option {
+		if o.Option() == ednsStubCode && len(o.(*dns.EDNS0_LOCAL).Data) == 1 &&
+			o.(*dns.EDNS0_LOCAL).Data[0] == 1 {
+			return true
+		}
+	}
+	return false
+}
+
+// addStubEdns0 adds our special option to the message's OPT record.
+func addStubEdns0(m *dns.Msg) *dns.Msg {
+	option := m.IsEdns0()
+	// Add a custom EDNS0 option to the packet, so we can detect loops when 2 stubs are forwarding to each other.
+	if option != nil {
+		option.Option = append(option.Option, &dns.EDNS0_LOCAL{Code: ednsStubCode, Data: []byte{1}})
+		return m
+	}
+
+	m.Extra = append(m.Extra, ednsStub)
+	return m
+}
+
+const (
+	ednsStubCode = dns.EDNS0LOCALSTART + 10
+	stubDomain   = "stub.dns"
+)
+
+var ednsStub = func() *dns.OPT {
+	o := new(dns.OPT)
+	o.Hdr.Name = "."
+	o.Hdr.Rrtype = dns.TypeOPT
+	o.SetUDPSize(4096)
+
+	e := new(dns.EDNS0_LOCAL)
+	e.Code = ednsStubCode
+	e.Data = []byte{1}
+	o.Option = append(o.Option, e)
+	return o
+}()