middleware/{file, auto}: resolve external CNAMEs

Do the same thing as in etcd and give the option of externally resolving
CNAME. This is needed when CoreDNS is a proxy as well is serving zones.

middleware/auto/setup.go

@@ -2,6 +2,7 @@ package auto
 
 import (
 	"log"
+	"net"
 	"os"
 	"path"
 	"regexp"
@@ -12,6 +13,7 @@ import (
 	"github.com/miekg/coredns/middleware"
 	"github.com/miekg/coredns/middleware/file"
 	"github.com/miekg/coredns/middleware/metrics"
+	"github.com/miekg/coredns/middleware/proxy"
 
 	"github.com/mholt/caddy"
 )
@@ -142,6 +144,19 @@ func autoParse(c *caddy.Controller) (Auto, error) {
 				case "no_reload":
 					a.loader.noReload = true
 
+				case "upstream":
+					args := c.RemainingArgs()
+					if len(args) == 0 {
+						return a, false, c.ArgErr()
+					}
+					for i := 0; i < len(args); i++ {
+						h, p, e := net.SplitHostPort(args[i])
+						if e != nil && p == "" {
+							args[i] = h + ":53"
+						}
+					}
+					a.loader.Proxy = proxy.New(args)
+
 				default:
 					t, _, e := file.TransferParse(c, false)
 					if e != nil {