feature/linkspeed: move cosmetic tstun netlink code out to modular feature

Part of making all netlink monitoring code optional.

Updates #17311 (how I got started down this path)
Updates #12614

Change-Id: Ic80d8a7a44dc261c4b8678b3c2241c3b3778370d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/linkspeed/doc.go

@@ -0,0 +1,6 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package linkspeed registers support for setting the TUN link speed on Linux,
+// to better integrate with system monitoring tools.
+package linkspeed

feature/linkspeed/linkspeed_linux.go

@@ -0,0 +1,70 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux && !android
+
+package linkspeed
+
+import (
+	"github.com/mdlayher/genetlink"
+	"github.com/mdlayher/netlink"
+	"github.com/tailscale/wireguard-go/tun"
+	"golang.org/x/sys/unix"
+	"tailscale.com/net/tstun"
+)
+
+func init() {
+	tstun.HookSetLinkAttrs.Set(setLinkAttrs)
+}
+
+// setLinkSpeed sets the advertised link speed of the TUN interface.
+func setLinkSpeed(iface tun.Device, mbps int) error {
+	name, err := iface.Name()
+	if err != nil {
+		return err
+	}
+
+	conn, err := genetlink.Dial(&netlink.Config{Strict: true})
+	if err != nil {
+		return err
+	}
+
+	defer conn.Close()
+
+	f, err := conn.GetFamily(unix.ETHTOOL_GENL_NAME)
+	if err != nil {
+		return err
+	}
+
+	ae := netlink.NewAttributeEncoder()
+	ae.Nested(unix.ETHTOOL_A_LINKMODES_HEADER, func(nae *netlink.AttributeEncoder) error {
+		nae.String(unix.ETHTOOL_A_HEADER_DEV_NAME, name)
+		return nil
+	})
+	ae.Uint32(unix.ETHTOOL_A_LINKMODES_SPEED, uint32(mbps))
+
+	b, err := ae.Encode()
+	if err != nil {
+		return err
+	}
+
+	_, err = conn.Execute(
+		genetlink.Message{
+			Header: genetlink.Header{
+				Command: unix.ETHTOOL_MSG_LINKMODES_SET,
+				Version: unix.ETHTOOL_GENL_VERSION,
+			},
+			Data: b,
+		},
+		f.ID,
+		netlink.Request|netlink.Acknowledge,
+	)
+	return err
+}
+
+// setLinkAttrs sets up link attributes that can be queried by external tools.
+// Its failure is non-fatal to interface bringup.
+func setLinkAttrs(iface tun.Device) error {
+	// By default the link speed is 10Mbps, which is easily exceeded and causes monitoring tools to complain (#3933).
+	return setLinkSpeed(iface, unix.SPEED_UNKNOWN)
+}