middleware/httpproxy: Add (#439)

This PR adds a middleware that talks to dns.google.com over HTTPS,
meaning all your DNS traffic is encrypted when traversing your ISP and
the internet.

The `dns.google.com` address is re-resolved every 30 seconds.

middleware/httpproxy/proxy.go

@@ -0,0 +1,45 @@
+// Package httpproxy is middleware that proxies requests to a HTTPs server doing DNS.
+package httpproxy
+
+import (
+	"errors"
+	"time"
+
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+var errUnreachable = errors.New("unreachable backend")
+
+// Proxy represents a middleware instance that can proxy requests to HTTPS servers.
+type Proxy struct {
+	from string
+	e    Exchanger
+
+	Next middleware.Handler
+}
+
+// ServeDNS satisfies the middleware.Handler interface.
+func (p *Proxy) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	start := time.Now()
+	state := request.Request{W: w, Req: r}
+
+	reply, backendErr := p.e.Exchange(r)
+
+	if backendErr == nil {
+		state.SizeAndDo(reply)
+
+		w.WriteMsg(reply)
+		RequestDuration.WithLabelValues(p.from).Observe(float64(time.Since(start) / time.Millisecond))
+		return 0, nil
+	}
+	RequestDuration.WithLabelValues(p.from).Observe(float64(time.Since(start) / time.Millisecond))
+
+	return dns.RcodeServerFailure, errUnreachable
+}
+
+// Name implements the Handler interface.
+func (p Proxy) Name() string { return "httpproxy" }