cmd/xdpderper: add autodetection for default interface name

This makes deployment easier in hetrogenous environments.

Updates ENG-4274

Signed-off-by: James Tucker <james@tailscale.com>

net/netutil/default_interface_portable.go

@@ -0,0 +1,64 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netutil
+
+import (
+	"errors"
+	"net"
+	"net/netip"
+)
+
+// DefaultInterfacePortable looks up the current default interface using a portable lookup method that
+// works on most systems with a BSD style socket interface.
+//
+// Returns the interface name and IP address of the default route interface.
+//
+// If the default cannot be determined, an error is returned.
+// Requires that there is a route on the system servicing UDP IPv4.
+func DefaultInterfacePortable() (string, netip.Addr, error) {
+	// Note: UDP dial just performs a connect(2), and doesn't actually send a packet.
+	c, err := net.Dial("udp4", "8.8.8.8:53")
+	if err != nil {
+		return "", netip.Addr{}, err
+	}
+	laddr := c.LocalAddr().(*net.UDPAddr)
+	c.Close()
+
+	ifs, err := net.Interfaces()
+	if err != nil {
+		return "", netip.Addr{}, err
+	}
+
+	var (
+		iface *net.Interface
+		ipnet *net.IPNet
+	)
+	for _, ifc := range ifs {
+		addrs, err := ifc.Addrs()
+		if err != nil {
+			return "", netip.Addr{}, err
+		}
+		for _, addr := range addrs {
+			if ipn, ok := addr.(*net.IPNet); ok {
+				if ipn.Contains(laddr.IP) {
+					if ipnet == nil {
+						ipnet = ipn
+						iface = &ifc
+					} else {
+						newSize, _ := ipn.Mask.Size()
+						oldSize, _ := ipnet.Mask.Size()
+						if newSize > oldSize {
+							ipnet = ipn
+							iface = &ifc
+						}
+					}
+				}
+			}
+		}
+	}
+	if iface == nil {
+		return "", netip.Addr{}, errors.New("no default interface")
+	}
+	return iface.Name, laddr.AddrPort().Addr(), nil
+}