middleware/proxy: absorb httpproxy (#481)

* middleware/proxy: absorb httpproxy

Move the httproxy into proxy. This adds and Exchanger interface which
is used to exchange the messages with the upstream.

The https_google upstream will re-resolve itself and update the upstream
hosts used every 300s.

* Remove and add TODO

middleware/proxy/google_rr.go

@@ -0,0 +1,90 @@
+package proxy
+
+import (
+	"fmt"
+
+	"github.com/miekg/dns"
+)
+
+// toMsg converts a googleMsg into the dns message. The returned RR is the comment disquised as a TXT record.
+func toMsg(g *googleMsg) (*dns.Msg, dns.RR, error) {
+	m := new(dns.Msg)
+	m.Response = true
+	m.Rcode = g.Status
+	m.Truncated = g.TC
+	m.RecursionDesired = g.RD
+	m.RecursionAvailable = g.RA
+	m.AuthenticatedData = g.AD
+	m.CheckingDisabled = g.CD
+
+	m.Question = make([]dns.Question, 1)
+	m.Answer = make([]dns.RR, len(g.Answer))
+	m.Ns = make([]dns.RR, len(g.Authority))
+	m.Extra = make([]dns.RR, len(g.Additional))
+
+	m.Question[0] = dns.Question{Name: g.Question[0].Name, Qtype: g.Question[0].Type, Qclass: dns.ClassINET}
+
+	var err error
+	for i := 0; i < len(m.Answer); i++ {
+		m.Answer[i], err = toRR(g.Answer[i])
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+	for i := 0; i < len(m.Ns); i++ {
+		m.Ns[i], err = toRR(g.Authority[i])
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+	for i := 0; i < len(m.Extra); i++ {
+		m.Extra[i], err = toRR(g.Additional[i])
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+
+	txt, _ := dns.NewRR(". 0 CH TXT " + g.Comment)
+	return m, txt, nil
+}
+
+// toRR transforms a "google" RR to a dns.RR.
+func toRR(g googleRR) (dns.RR, error) {
+	typ, ok := dns.TypeToString[g.Type]
+	if !ok {
+		return nil, fmt.Errorf("failed to convert type %q", g.Type)
+	}
+
+	str := fmt.Sprintf("%s %d %s %s", g.Name, g.TTL, typ, g.Data)
+	rr, err := dns.NewRR(str)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse %q: %s", str, err)
+	}
+	return rr, nil
+}
+
+// googleRR represents a dns.RR in another form.
+type googleRR struct {
+	Name string
+	Type uint16
+	TTL  uint32
+	Data string
+}
+
+// googleMsg is a JSON representation of the dns.Msg.
+type googleMsg struct {
+	Status   int
+	TC       bool
+	RD       bool
+	RA       bool
+	AD       bool
+	CD       bool
+	Question []struct {
+		Name string
+		Type uint16
+	}
+	Answer     []googleRR
+	Authority  []googleRR
+	Additional []googleRR
+	Comment    string
+}