Fetch DNS listen address from daemon

instead of assuming it's on the Docker bridge

Also move DNS address validation into NewDNSServer(),
so we only do it once.

nameserver/dns.go

@@ -90,7 +90,7 @@ type DNSServer struct {
 	ns      *Nameserver
 	domain  string
 	ttl     uint32
-	address string
+	address string // listen address and port, e.g. 1.2.3.4:53
 
 	servers   []*dns.Server
 	upstream  Upstream
@@ -99,6 +99,10 @@ type DNSServer struct {
 }
 
 func NewDNSServer(ns *Nameserver, domain, address string, upstream Upstream, ttl uint32, clientTimeout time.Duration) (*DNSServer, error) {
+	if _, _, err := net.SplitHostPort(address); err != nil {
+		return nil, fmt.Errorf("invalid DNS listen address %q: %v", address, err)
+	}
+
 	s := &DNSServer{
 		ns:        ns,
 		domain:    dns.Fqdn(domain),

nameserver/http.go

@@ -3,6 +3,7 @@ package nameserver
 import (
 	"encoding/json"
 	"fmt"
+	"net"
 	"net/http"
 
 	"github.com/gorilla/mux"
@@ -84,3 +85,13 @@ func (n *Nameserver) HandleHTTP(router *mux.Router, dockerCli *docker.Client) {
 		}
 	})
 }
+
+func (d *DNSServer) HandleHTTP(router *mux.Router) {
+	router.Methods("GET").Path("/dns-address-port").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, d.address)
+	})
+	router.Methods("GET").Path("/dns-address").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		host, _, _ := net.SplitHostPort(d.address) // address is validated in NewDNSServer()
+		fmt.Fprint(w, host)
+	})
+}