all: wait for network up before running

If ctrld setup the interface correctly, the interface DNS is set to
ctrld listener address. At boot time, the ctrld is not up yet, so it
would break the processing Control D config fetching.

Fixing this by waiting for network up before doing the query.

cmd/ctrld/cli.go

@@ -103,6 +103,10 @@ func initCLI() {
 			if err := v.Unmarshal(&cfg); err != nil {
 				log.Fatalf("failed to unmarshal config: %v", err)
 			}
+			// Wait for network up.
+			if !netUp() {
+				log.Fatal("network is not up yet")
+			}
 			processCDFlags()
 			if err := ctrld.ValidateConfig(validator.New(), &cfg); err != nil {
 				log.Fatalf("invalid config: %v", err)

cmd/ctrld/net.go

@@ -1,20 +1,41 @@
 package main
 
 import (
+	"context"
+	"fmt"
 	"net"
 	"sync"
+	"time"
+
+	"tailscale.com/logtail/backoff"
 )
 
-const controldIPv6Test = "ipv6.controld.io"
+const (
+	controldIPv6Test = "ipv6.controld.io"
+	controldIPv4Test = "ipv4.controld.io"
+)
 
 var (
 	stackOnce          sync.Once
 	ipv6Enabled        bool
 	canListenIPv6Local bool
+	hasNetworkUp       bool
 )
 
 func probeStack() {
-	if _, err := net.Dial("tcp6", controldIPv6Test); err == nil {
+	logf := func(format string, args ...any) {
+		fmt.Printf(format, args...)
+	}
+	b := backoff.NewBackoff("probeStack", logf, time.Minute)
+	for {
+		if _, err := net.Dial("tcp", net.JoinHostPort(controldIPv4Test, "80")); err == nil {
+			hasNetworkUp = true
+			break
+		} else {
+			b.BackOff(context.Background(), err)
+		}
+	}
+	if _, err := net.Dial("tcp6", net.JoinHostPort(controldIPv6Test, "80")); err == nil {
 		ipv6Enabled = true
 	}
 	if ln, err := net.Listen("tcp6", "[::1]:53"); err == nil {
@@ -23,6 +44,11 @@ func probeStack() {
 	}
 }
 
+func netUp() bool {
+	stackOnce.Do(probeStack)
+	return hasNetworkUp
+}
+
 func supportsIPv6() bool {
 	stackOnce.Do(probeStack)
 	return ipv6Enabled