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.go

@@ -0,0 +1,82 @@
+package etcd
+
+import (
+	"log"
+	"net"
+	"strconv"
+	"time"
+
+	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+	"github.com/coredns/coredns/plugin/proxy"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// UpdateStubZones checks etcd for an update on the stubzones.
+func (e *Etcd) UpdateStubZones() {
+	go func() {
+		for {
+			e.updateStubZones()
+			time.Sleep(15 * time.Second)
+		}
+	}()
+}
+
+// Look in .../dns/stub/<zone>/xx for msg.Services. Loop through them
+// extract <zone> and add them as forwarders (ip:port-combos) for
+// the stub zones. Only numeric (i.e. IP address) hosts are used.
+// Only the first zone configured on e is used for the lookup.
+func (e *Etcd) updateStubZones() {
+	zone := e.Zones[0]
+
+	fakeState := request.Request{W: nil, Req: new(dns.Msg)}
+	fakeState.Req.SetQuestion(stubDomain+"."+zone, dns.TypeA)
+
+	services, err := e.Records(fakeState, false)
+	if err != nil {
+		return
+	}
+
+	stubmap := make(map[string]proxy.Proxy)
+	// track the nameservers on a per domain basis, but allow a list on the domain.
+	nameservers := map[string][]string{}
+
+Services:
+	for _, serv := range services {
+		if serv.Port == 0 {
+			serv.Port = 53
+		}
+		ip := net.ParseIP(serv.Host)
+		if ip == nil {
+			log.Printf("[WARNING] Non IP address stub nameserver: %s", serv.Host)
+			continue
+		}
+
+		domain := msg.Domain(serv.Key)
+		labels := dns.SplitDomainName(domain)
+
+		// If the remaining name equals any of the zones we have, we ignore it.
+		for _, z := range e.Zones {
+			// Chop of left most label, because that is used as the nameserver place holder
+			// and drop the right most labels that belong to zone.
+			// We must *also* chop of dns.stub. which means cutting two more labels.
+			domain = dnsutil.Join(labels[1 : len(labels)-dns.CountLabel(z)-2])
+			if domain == z {
+				log.Printf("[WARNING] Skipping nameserver for domain we are authoritative for: %s", domain)
+				continue Services
+			}
+		}
+		nameservers[domain] = append(nameservers[domain], net.JoinHostPort(serv.Host, strconv.Itoa(serv.Port)))
+	}
+
+	for domain, nss := range nameservers {
+		stubmap[domain] = proxy.NewLookup(nss)
+	}
+	// atomic swap (at least that's what we hope it is)
+	if len(stubmap) > 0 {
+		e.Stubmap = &stubmap
+	}
+	return
+}