plugin/kubernetes: Add upstream @self and loop count (#1484)

* add upstream @self and loop count

* 1st round of feedback

* allow argless upstream

* update test

* readmes

* feedback

plugin/pkg/upstream/upstream.go

@@ -0,0 +1,55 @@
+// Package upstream abstracts a upstream lookups so that plugins
+// can handle them in an unified way.
+package upstream
+
+import (
+	"github.com/miekg/dns"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+	"github.com/coredns/coredns/plugin/pkg/nonwriter"
+	"github.com/coredns/coredns/plugin/proxy"
+	"github.com/coredns/coredns/request"
+)
+
+// Upstream is used to resolve CNAME targets
+type Upstream struct {
+	self    bool
+	Forward *proxy.Proxy
+}
+
+// NewUpstream creates a new Upstream for given destination(s)
+func NewUpstream(dests []string) (Upstream, error) {
+	u := Upstream{}
+	if len(dests) == 0 {
+		u.self = true
+		return u, nil
+	}
+	u.self = false
+	ups, err := dnsutil.ParseHostPortOrFile(dests...)
+	if err != nil {
+		return u, err
+	}
+	p := proxy.NewLookup(ups)
+	u.Forward = &p
+	return u, nil
+}
+
+// Lookup routes lookups to Self or Forward
+func (u Upstream) Lookup(state request.Request, name string, typ uint16) (*dns.Msg, error) {
+	if u.self {
+		// lookup via self
+		req := new(dns.Msg)
+		req.SetQuestion(name, typ)
+		state.SizeAndDo(req)
+		nw := nonwriter.New(state.W)
+		state2 := request.Request{W: nw, Req: req}
+		server := state.Context.Value(dnsserver.Key{}).(*dnsserver.Server)
+		server.ServeDNS(state.Context, state2.W, req)
+		return nw.Msg, nil
+	}
+	if u.Forward != nil {
+		return u.Forward.Lookup(state, name, typ)
+	}
+	return &dns.Msg{}, nil
+}