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/setup.go

@@ -0,0 +1,96 @@
+package httpproxy
+
+import (
+	"fmt"
+
+	"github.com/miekg/coredns/core/dnsserver"
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/mholt/caddy"
+	"github.com/mholt/caddy/caddyfile"
+)
+
+func init() {
+	caddy.RegisterPlugin("httpproxy", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	p, err := httpproxyParse(c)
+	if err != nil {
+		return middleware.Error("httpproxy", err)
+	}
+
+	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
+		p.Next = next
+		return p
+	})
+
+	c.OnStartup(func() error {
+		OnStartupMetrics()
+		e := p.e.OnStartup()
+		if e != nil {
+			return middleware.Error("httpproxy", e)
+		}
+		return nil
+	})
+	c.OnShutdown(func() error {
+		e := p.e.OnShutdown()
+		if e != nil {
+			return middleware.Error("httpproxy", e)
+		}
+		return nil
+	})
+
+	return nil
+}
+
+func httpproxyParse(c *caddy.Controller) (*Proxy, error) {
+	var p = &Proxy{}
+
+	for c.Next() {
+		if !c.Args(&p.from) {
+			return p, c.ArgErr()
+		}
+		to := c.RemainingArgs()
+		if len(to) != 1 {
+			return p, c.ArgErr()
+		}
+		switch to[0] {
+		case "dns.google.com":
+			p.e = newGoogle()
+			u, _ := newSimpleUpstream([]string{"8.8.8.8:53", "8.8.4.4:53"})
+			p.e.SetUpstream(u)
+		default:
+			return p, fmt.Errorf("unknown http proxy %q", to[0])
+		}
+
+		for c.NextBlock() {
+			if err := parseBlock(&c.Dispenser, p); err != nil {
+				return p, err
+			}
+		}
+	}
+
+	return p, nil
+}
+
+func parseBlock(c *caddyfile.Dispenser, p *Proxy) error {
+	switch c.Val() {
+	case "upstream":
+		upstreams := c.RemainingArgs()
+		if len(upstreams) == 0 {
+			return c.ArgErr()
+		}
+		u, err := newSimpleUpstream(upstreams)
+		if err != nil {
+			return err
+		}
+		p.e.SetUpstream(u)
+	default:
+		return c.Errf("unknown property '%s'", c.Val())
+	}
+	return nil
+}