internal/controld: check if ipv4 is available before connect to API

Updates #53

internal/net/net.go

@@ -0,0 +1,86 @@
+package net
+
+import (
+	"context"
+	"net"
+	"sync"
+	"time"
+
+	"tailscale.com/logtail/backoff"
+)
+
+const (
+	controldIPv6Test = "ipv6.controld.io"
+	controldIPv4Test = "ipv4.controld.io"
+	bootstrapDNS     = "76.76.2.0:53"
+)
+
+var Dialer = &net.Dialer{
+	Resolver: &net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+			d := net.Dialer{
+				Timeout: 10 * time.Second,
+			}
+			return d.DialContext(ctx, "udp", bootstrapDNS)
+		},
+	},
+}
+
+var (
+	stackOnce          sync.Once
+	ipv4Enabled        bool
+	ipv6Enabled        bool
+	canListenIPv6Local bool
+	hasNetworkUp       bool
+)
+
+func probeStack() {
+	b := backoff.NewBackoff("probeStack", func(format string, args ...any) {}, time.Minute)
+	for {
+		if _, err := Dialer.Dial("udp", net.JoinHostPort(bootstrapDNS, "53")); err == nil {
+			hasNetworkUp = true
+			break
+		} else {
+			b.BackOff(context.Background(), err)
+		}
+	}
+	if _, err := Dialer.Dial("tcp4", net.JoinHostPort(controldIPv4Test, "80")); err == nil {
+		ipv4Enabled = true
+	}
+	if _, err := Dialer.Dial("tcp6", net.JoinHostPort(controldIPv6Test, "80")); err == nil {
+		ipv6Enabled = true
+	}
+	if ln, err := net.Listen("tcp6", "[::1]:53"); err == nil {
+		ln.Close()
+		canListenIPv6Local = true
+	}
+}
+
+func Up() bool {
+	stackOnce.Do(probeStack)
+	return hasNetworkUp
+}
+
+func SupportsIPv4() bool {
+	stackOnce.Do(probeStack)
+	return ipv4Enabled
+}
+
+func SupportsIPv6() bool {
+	stackOnce.Do(probeStack)
+	return ipv6Enabled
+}
+
+func SupportsIPv6ListenLocal() bool {
+	stackOnce.Do(probeStack)
+	return canListenIPv6Local
+}
+
+// IsIPv6 checks if the provided IP is v6.
+//
+//lint:ignore U1000 use in os_windows.go
+func IsIPv6(ip string) bool {
+	parsedIP := net.ParseIP(ip)
+	return parsedIP != nil && parsedIP.To4() == nil && parsedIP.To16() != nil
+}