interfaces, cmd/tsshd: move interface lookup from tsshd to its own package

For reuse by derper, etc.

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

interfaces/interfaces.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package interfaces contains helpers for looking up system network interfaces.
+package interfaces
+
+import (
+	"net"
+	"strings"
+)
+
+// Tailscale returns the current machine's Tailscale interface, if any.
+// If none is found, all zero values are returned.
+// A non-nil error is only returned on a problem listing the system interfaces.
+func Tailscale() (net.IP, *net.Interface, error) {
+	ifs, err := net.Interfaces()
+	if err != nil {
+		return nil, nil, err
+	}
+	for _, iface := range ifs {
+		if !maybeTailscaleInterfaceName(iface.Name) {
+			continue
+		}
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue
+		}
+		for _, a := range addrs {
+			if ipnet, ok := a.(*net.IPNet); ok && IsTailscaleIP(ipnet.IP) {
+				return ipnet.IP, &iface, nil
+			}
+		}
+	}
+	return nil, nil, nil
+}
+
+// maybeTailscaleInterfaceName reports whether s is an interface
+// name that might be used by Tailscale.
+func maybeTailscaleInterfaceName(s string) bool {
+	return strings.HasPrefix(s, "wg") ||
+		strings.HasPrefix(s, "ts") ||
+		strings.HasPrefix(s, "tailscale")
+}
+
+// IsTailscaleIP reports whether ip is an IP in a range used by
+// Tailscale virtual network interfaces.
+func IsTailscaleIP(ip net.IP) bool {
+	return cgNAT.Contains(ip)
+}
+
+var cgNAT = func() *net.IPNet {
+	_, ipNet, err := net.ParseCIDR("100.64.0.0/10")
+	if err != nil {
+		panic(err)
+	}
+	return ipNet
+}()