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_linux.go

@@ -0,0 +1,36 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux
+
+package prober
+
+import (
+	"fmt"
+	"net/netip"
+
+	"github.com/tailscale/netlink"
+	"go4.org/netipx"
+)
+
+const tunName = "derpprobe"
+
+func configureTUN(addr netip.Prefix, tunname string) error {
+	link, err := netlink.LinkByName(tunname)
+	if err != nil {
+		return fmt.Errorf("failed to look up link %q: %w", tunname, err)
+	}
+
+	// We need to bring the TUN device up before assigning an address. This
+	// allows the OS to automatically create a route for it. Otherwise, we'd
+	// have to manually create the route.
+	if err := netlink.LinkSetUp(link); err != nil {
+		return fmt.Errorf("failed to bring tun %q up: %w", tunname, err)
+	}
+
+	if err := netlink.AddrReplace(link, &netlink.Addr{IPNet: netipx.PrefixIPNet(addr)}); err != nil {
+		return fmt.Errorf("failed to add address: %w", err)
+	}
+
+	return nil
+}