net/tstun: set link speed to SPEED_UNKNOWN

Fixes #3933.

Signed-off-by: Dmytro Shynkevych <dm.shynk@gmail.com>

net/tstun/linkattrs_linux.go

@@ -0,0 +1,64 @@
+// Copyright (c) 2022 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 tstun
+
+import (
+	"github.com/mdlayher/genetlink"
+	"github.com/mdlayher/netlink"
+	"golang.org/x/sys/unix"
+	"golang.zx2c4.com/wireguard/tun"
+)
+
+// 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)
+}

net/tstun/linkattrs_notlinux.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2022 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.
+
+//go:build !linux
+// +build !linux
+
+package tstun
+
+import "golang.zx2c4.com/wireguard/tun"
+
+func setLinkAttrs(iface tun.Device) error {
+	return nil
+}

net/tstun/tun.go

@@ -69,6 +69,9 @@ func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
 		dev.Close()
 		return nil, "", err
 	}
+	if err := setLinkAttrs(dev); err != nil {
+		logf("setting link attributes: %v", err)
+	}
 	name, err := interfaceName(dev)
 	if err != nil {
 		dev.Close()