Add new plugin: external - resolve k8s ingress and LB address with external names (#2379)

* Add new plugin: external

This plugin works in conjunction with the kubernetes plugin and exports
ingress and LB addresses as DNS records. It bypasses backend.go and
backend_lookup.go flow because it is not needed.

README, tests are implemented. The tests only exercise the unit tests,
this has not been tested in any ci.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Rename to k8s_external

Signed-off-by: Miek Gieben <miek@miek.nl>

* go gen

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/k8s_external/apex.go

@@ -0,0 +1,110 @@
+package external
+
+import (
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// serveApex serves request that hit the zone' apex. A reply is written back to the client.
+func (e *External) serveApex(state request.Request) (int, error) {
+	m := new(dns.Msg)
+	m.SetReply(state.Req)
+	switch state.QType() {
+	case dns.TypeSOA:
+		m.Answer = []dns.RR{e.soa(state)}
+	case dns.TypeNS:
+		m.Answer = []dns.RR{e.ns(state)}
+
+		addr := e.externalAddrFunc(state)
+		for _, rr := range addr {
+			rr.Header().Ttl = e.ttl
+			rr.Header().Name = state.QName()
+			m.Extra = append(m.Extra, rr)
+		}
+	default:
+		m.Ns = []dns.RR{e.soa(state)}
+	}
+
+	state.W.WriteMsg(m)
+	return 0, nil
+}
+
+// serveSubApex serves requests that hit the zones fake 'dns' subdomain where our nameservers live.
+func (e *External) serveSubApex(state request.Request) (int, error) {
+	base, _ := dnsutil.TrimZone(state.Name(), state.Zone)
+
+	m := new(dns.Msg)
+	m.SetReply(state.Req)
+
+	// base is either dns. of ns1.dns (or another name), if it's longer return nxdomain
+	switch labels := dns.CountLabel(base); labels {
+	default:
+		m.SetRcode(m, dns.RcodeNameError)
+		m.Ns = []dns.RR{e.soa(state)}
+		state.W.WriteMsg(m)
+		return 0, nil
+	case 2:
+		nl, _ := dns.NextLabel(base, 0)
+		ns := base[:nl]
+		if ns != "ns1." {
+			// nxdomain
+			m.SetRcode(m, dns.RcodeNameError)
+			m.Ns = []dns.RR{e.soa(state)}
+			state.W.WriteMsg(m)
+			return 0, nil
+		}
+
+		addr := e.externalAddrFunc(state)
+		for _, rr := range addr {
+			rr.Header().Ttl = e.ttl
+			rr.Header().Name = state.QName()
+			switch state.QType() {
+			case dns.TypeA:
+				if rr.Header().Rrtype == dns.TypeA {
+					m.Answer = append(m.Answer, rr)
+				}
+			case dns.TypeAAAA:
+				if rr.Header().Rrtype == dns.TypeAAAA {
+					m.Answer = append(m.Answer, rr)
+				}
+			}
+		}
+
+		if len(m.Answer) == 0 {
+			m.Ns = []dns.RR{e.soa(state)}
+		}
+
+		state.W.WriteMsg(m)
+		return 0, nil
+
+	case 1:
+		// nodata for the dns empty non-terminal
+		m.Ns = []dns.RR{e.soa(state)}
+		state.W.WriteMsg(m)
+		return 0, nil
+	}
+}
+
+func (e *External) soa(state request.Request) *dns.SOA {
+	header := dns.RR_Header{Name: state.Zone, Rrtype: dns.TypeSOA, Ttl: e.ttl, Class: dns.ClassINET}
+
+	soa := &dns.SOA{Hdr: header,
+		Mbox:    dnsutil.Join(e.hostmaster, e.apex, state.Zone),
+		Ns:      dnsutil.Join("ns1", e.apex, state.Zone),
+		Serial:  12345, // Also dynamic?
+		Refresh: 7200,
+		Retry:   1800,
+		Expire:  86400,
+		Minttl:  e.ttl,
+	}
+	return soa
+}
+
+func (e *External) ns(state request.Request) *dns.NS {
+	header := dns.RR_Header{Name: state.Zone, Rrtype: dns.TypeNS, Ttl: e.ttl, Class: dns.ClassINET}
+	ns := &dns.NS{Hdr: header, Ns: dnsutil.Join("ns1", e.apex, state.Zone)}
+
+	return ns
+}