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/README.md

@@ -13,6 +13,8 @@ zonefile. New zones or changed zone are automatically picked up from disk.
 ~~~
 auto [ZONES...] {
     directory DIR [REGEXP ORIGIN_TEMPLATE [TIMEOUT]]
+    no_reload
+    upstream ADDRESS...
 }
 ~~~
 
@@ -26,6 +28,10 @@ are used.
   name `db.example.com`, the extracted origin will be `example.com`. **TIMEOUT** specifies how often
   CoreDNS should scan the directory, the default is every 60 seconds. This value is in seconds.
   The minimum value is 1 second.
+* `no_reload` by default CoreDNS will reload a zone from disk whenever it detects a change to the
+  file. This option disables that behavior.
+* `upstream` defines upstream resolvers to be used resolve external names found (think CNAMEs)
+  pointing to external names.
 
 All directives from the *file* middleware are supported. Note that *auto* will load all zones found,
 even though the directive might only receive queries for a specific zone. I.e:

middleware/auto/auto.go

@@ -9,6 +9,7 @@ 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/miekg/coredns/request"
 
 	"github.com/miekg/dns"
@@ -33,6 +34,7 @@ type (
 		// In the future this should be something like ZoneMeta that contains all this stuff.
 		transferTo []string
 		noReload   bool
+		Proxy      proxy.Proxy // Proxy for looking up names during the resolution process
 
 		duration time.Duration
 	}

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 {