Start of netcheck package & including network state in Hostinfo.

* adds new packet "netcheck" to do the checking of UDP, IPv6, and
  nearest DERP server, and the Report type for all that (and more
  in the future, probably pulling in danderson's natprobe)
* new tailcfg.NetInfo type
* cmd/tailscale netcheck subcommand (tentative name, likely to
  change/move) to print out the netcheck.Report.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

interfaces/interfaces.go

@@ -35,6 +35,42 @@ func Tailscale() (net.IP, *net.Interface, error) {
 	return nil, nil, nil
 }
 
+// HaveIPv6GlobalAddress reports whether the machine appears to have a
+// global scope unicast IPv6 address.
+//
+// It only returns an error if there's a problem querying the system
+// interfaces.
+func HaveIPv6GlobalAddress() (bool, error) {
+	ifs, err := net.Interfaces()
+	if err != nil {
+		return false, err
+	}
+	for _, iface := range ifs {
+		if isLoopbackInterfaceName(iface.Name) {
+			continue
+		}
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue
+		}
+		for _, a := range addrs {
+			ipnet, ok := a.(*net.IPNet)
+			if !ok {
+				continue
+			}
+			if ipnet.IP.To4() != nil || !ipnet.IP.IsGlobalUnicast() {
+				continue
+			}
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+func isLoopbackInterfaceName(s string) bool {
+	return strings.HasPrefix(s, "lo")
+}
+
 // maybeTailscaleInterfaceName reports whether s is an interface
 // name that might be used by Tailscale.
 func maybeTailscaleInterfaceName(s string) bool {