allow ttl 0 (#2348)

plugin/kubernetes/README.md

@@ -92,7 +92,7 @@ kubernetes [ZONES...] {
   will resolve External Services against itself. **ADDRESS** can be an IP, an IP:port, or a path
   to a file structured like resolv.conf.
 * `ttl` allows you to set a custom TTL for responses. The default (and minimum allowed) is
-  5 seconds, while the maximum is capped at 3600 seconds.
+  0 seconds, while the maximum is capped at 3600 seconds. Setting TTL to 0 will prevent records from being cached.
 * `noendpoints` will turn off the serving of endpoint records by disabling the watch on endpoints.
   All endpoint queries and headless service queries will result in an NXDOMAIN.
 * `transfer` enables zone transfers. It may be specified multiples times. `To` signals the direction

plugin/kubernetes/setup.go

@@ -247,8 +247,8 @@ func ParseStanza(c *caddy.Controller) (*Kubernetes, error) {
 			if err != nil {
 				return nil, err
 			}
-			if t < 5 || t > 3600 {
-				return nil, c.Errf("ttl must be in range [5, 3600]: %d", t)
+			if t < 0 || t > 3600 {
+				return nil, c.Errf("ttl must be in range [0, 3600]: %d", t)
 			}
 			k8s.ttl = uint32(t)
 		case "transfer":