prober: perform DERP bandwidth probes over TUN device to mimic real client

Updates tailscale/corp#24635

Co-authored-by: Mario Minardi <mario@tailscale.com>
Signed-off-by: Percy Wegmann <percy@tailscale.com>

prober/tun_darwin.go

@@ -0,0 +1,35 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build darwin
+
+package prober
+
+import (
+	"fmt"
+	"net/netip"
+	"os/exec"
+
+	"go4.org/netipx"
+)
+
+const tunName = "utun"
+
+func configureTUN(addr netip.Prefix, tunname string) error {
+	cmd := exec.Command("ifconfig", tunname, "inet", addr.String(), addr.Addr().String())
+	res, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to add address: %w (%s)", err, string(res))
+	}
+
+	net := netipx.PrefixIPNet(addr)
+	nip := net.IP.Mask(net.Mask)
+	nstr := fmt.Sprintf("%v/%d", nip, addr.Bits())
+	cmd = exec.Command("route", "-q", "-n", "add", "-inet", nstr, "-iface", addr.Addr().String())
+	res, err = cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to add route: %w (%s)", err, string(res))
+	}
+
+	return nil
+}