nameserver.go: use custom resolver to avoid update delays caused by dns caching

nameserver.go

@@ -1,17 +1,37 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"log"
 	"net"
 	"os/exec"
 	"strings"
+	"time"
 )
 
-func requiresRRUpdate(entry *RRConfig, addr net.IP) bool {
-	// TODO: use custom resolver to query authoritive nameserver instead of
-	// local resolver
-	addrs, err := net.LookupIP(entry.Recordname)
+const (
+	resolverTimeout = time.Millisecond * time.Duration(8000)
+)
+
+func getResolver(nameserver string) *net.Resolver {
+	return &net.Resolver{
+		// use go's built-in dns resolver
+		PreferGo: true,
+		// specify alternate dialer for go's dns resolver
+		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+			d := net.Dialer{
+				Timeout: resolverTimeout,
+			}
+			return d.DialContext(ctx, network, nameserver)
+		},
+	}
+}
+
+func requiresRRUpdate(record string, addr net.IP, resolver *net.Resolver) bool {
+	// TODO: the context could be replaced in order to allow cancelling the dns
+	// query if the user disconnects prematurely
+	addrs, err := resolver.LookupIP(context.Background(), "ip", record)
 	if err != nil {
 		log.Printf("dns lookup failed: %s", err)
 		// enforce update, it's better than not trying at all

web.go

@@ -124,7 +124,7 @@ func RequestHandler(cfg *Config) func(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
-		if !requiresRRUpdate(entry, ipaddr) {
+		if !requiresRRUpdate(entry.Recordname, ipaddr, getResolver(entry.Nameserver)) {
 			fmt.Fprintf(w, "nochg %s\n", ipaddr.String())
 			return
 		}