middleware/hosts for /etc/hosts parsing (#695)

* add hosts middleware

* forgot pointer receiver

* add appropriately modified hostsfile tests from golang repo

* remove test artifacts, separate hostsfile parsing from caching and opening, remove unused metrics references, move middleware up the chain

* refactored the logic for creating records and filtering ip address versions. also got PTR lookups working

* Add README.md. Modify config to be more concise. Add zones list to config. Filter PTR responses based on zones list.

* add Fallthrough and return correct dns response code otherwise

* Simplified Hostsfile to only store hosts in the zones we care about, and by ip version. Added handler tests and improved other tests.

* oops, goimports loaded a package from a different repo

middleware/hosts/setup.go

@@ -0,0 +1,88 @@
+package hosts
+
+import (
+	"log"
+	"os"
+	"path"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/middleware"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("hosts", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	h, err := hostsParse(c)
+	if err != nil {
+		return middleware.Error("hosts", err)
+	}
+
+	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
+		h.Next = next
+		return h
+	})
+
+	return nil
+}
+
+func hostsParse(c *caddy.Controller) (Hosts, error) {
+	var h = Hosts{
+		Hostsfile: &Hostsfile{path: "/etc/hosts"},
+	}
+	defer h.ReadHosts()
+
+	config := dnsserver.GetConfig(c)
+
+	for c.Next() {
+		if c.Val() == "hosts" { // hosts [FILE] [ZONES...]
+			args := c.RemainingArgs()
+			if len(args) >= 1 {
+				h.path = args[0]
+				args = args[1:]
+
+				if !path.IsAbs(h.path) && config.Root != "" {
+					h.path = path.Join(config.Root, h.path)
+				}
+				_, err := os.Stat(h.path)
+				if err != nil {
+					if os.IsNotExist(err) {
+						log.Printf("[WARNING] File does not exist: %s", h.path)
+					} else {
+						return h, c.Errf("unable to access hosts file '%s': %v", h.path, err)
+					}
+				}
+			}
+
+			origins := make([]string, len(c.ServerBlockKeys))
+			copy(origins, c.ServerBlockKeys)
+			if len(args) > 0 {
+				origins = args
+			}
+
+			for i := range origins {
+				origins[i] = middleware.Host(origins[i]).Normalize()
+			}
+			h.Origins = origins
+
+			for c.NextBlock() {
+				switch c.Val() {
+				case "fallthrough":
+					args := c.RemainingArgs()
+					if len(args) == 0 {
+						h.Fallthrough = true
+						continue
+					}
+					return h, c.ArgErr()
+				}
+			}
+		}
+	}
+	return h, nil
+}