wgengine/monitor: parse Linux netlink messages, ignore our own events

Fixes tailscale/corp#412 ("flood of link change events at start-up")

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

go.mod

@@ -12,6 +12,7 @@ require (
 	github.com/golang/groupcache v0.0.0-20200121045136-8c9f03a8e57e
 	github.com/google/go-cmp v0.4.0
 	github.com/goreleaser/nfpm v1.1.10
+	github.com/jsimonetti/rtnetlink v0.0.0-20200117123717-f846d4f6c1f4
 	github.com/klauspost/compress v1.10.10
 	github.com/kr/pty v1.1.1
 	github.com/mdlayher/netlink v1.1.0

wgengine/monitor/monitor.go

@@ -3,7 +3,8 @@
 // license that can be found in the LICENSE file.
 
 // Package monitor provides facilities for monitoring network
-// interface changes.
+// interface and route changes. It primarily exists to know when
+// portable devices move between different networks.
 package monitor
 
 import (
@@ -14,10 +15,10 @@
 )
 
 // message represents a message returned from an osMon.
-//
-// TODO: currently messages are being discarded, so the properties of
-// the message haven't been defined.
-type message interface{}
+type message interface {
+	// Ignore is whether we should ignore this message.
+	ignore() bool
+}
 
 // osMon is the interface that each operating system-specific
 // implementation of the link monitor must implement.
@@ -52,7 +53,8 @@ type Mon struct {
 // are propagated to the callback function.
 // The returned monitor is inactive until it's started by the Start method.
 func New(logf logger.Logf, callback ChangeFunc) (*Mon, error) {
-	om, err := newOSMon()
+	logf = logger.WithPrefix(logf, "monitor: ")
+	om, err := newOSMon(logf)
 	if err != nil {
 		return nil, err
 	}
@@ -100,7 +102,7 @@ func (m *Mon) Close() error {
 func (m *Mon) pump() {
 	defer m.goroutines.Done()
 	for {
-		_, err := m.om.Receive()
+		msg, err := m.om.Receive()
 		if err != nil {
 			select {
 			case <-m.stop:
@@ -108,10 +110,13 @@ func (m *Mon) pump() {
 			default:
 			}
 			// Keep retrying while we're not closed.
-			m.logf("Error receiving from connection: %v", err)
+			m.logf("error receiving from connection: %v", err)
 			time.Sleep(time.Second)
 			continue
 		}
+		if msg.ignore() {
+			continue
+		}
 		select {
 		case m.change <- struct{}{}:
 		case <-m.stop:

wgengine/monitor/monitor_freebsd.go

@@ -9,6 +9,8 @@
 	"fmt"
 	"net"
 	"strings"
+
+	"tailscale.com/types/logger"
 )
 
 // devdConn implements osMon using devd(8).
@@ -16,7 +18,7 @@ type devdConn struct {
 	conn net.Conn
 }
 
-func newOSMon() (osMon, error) {
+func newOSMon(logf logger.Logf) (osMon, error) {
 	conn, err := net.Dial("unixpacket", "/var/run/devd.seqpacket.pipe")
 	if err != nil {
 		return nil, fmt.Errorf("devd dial error: %v", err)
@@ -41,8 +43,14 @@ func (c *devdConn) Receive() (message, error) {
 		if !strings.Contains(msg, "system=IFNET") {
 			continue
 		}
-		// TODO(]|[): this is where the devd-specific message would
+		// TODO: this is where the devd-specific message would
 		// get converted into a "standard" event message and returned.
-		return nil, nil
+		return unspecifiedMessage{}, nil
 	}
 }
+
+// unspecifiedMessage is a minimal message implementation that should not
+// be ignored. TODO: make specific messages like monitor_linux.go.
+type unspecifiedMessage struct{}
+
+func (unspecifiedMessage) ignore() bool { return false }

wgengine/monitor/monitor_linux.go

@@ -8,10 +8,15 @@
 
 import (
 	"fmt"
+	"net"
 	"time"
 
+	"github.com/jsimonetti/rtnetlink"
 	"github.com/mdlayher/netlink"
 	"golang.org/x/sys/unix"
+	"inet.af/netaddr"
+	"tailscale.com/net/tsaddr"
+	"tailscale.com/types/logger"
 )
 
 // nlConn wraps a *netlink.Conn and returns a monitor.Message
@@ -20,10 +25,12 @@
 // on the type of event, this provides the capability of handling
 // each architecture-specific message in a generic fashion.
 type nlConn struct {
+	logf     logger.Logf
 	conn     *netlink.Conn
+	buffered []netlink.Message
 }
 
-func newOSMon() (osMon, error) {
+func newOSMon(logf logger.Logf) (osMon, error) {
 	conn, err := netlink.Dial(unix.NETLINK_ROUTE, &netlink.Config{
 		// IPv4 address and route changes. Routes get us most of the
 		// events of interest, but we need address as well to cover
@@ -35,7 +42,7 @@ func newOSMon() (osMon, error) {
 	if err != nil {
 		return nil, fmt.Errorf("dialing netlink socket: %v", err)
 	}
-	return &nlConn{conn}, nil
+	return &nlConn{logf: logf, conn: conn}, nil
 }
 
 func (c *nlConn) Close() error {
@@ -44,12 +51,73 @@ func (c *nlConn) Close() error {
 }
 
 func (c *nlConn) Receive() (message, error) {
-	// currently ignoring the message
-	_, err := c.conn.Receive()
+	if len(c.buffered) == 0 {
+		var err error
+		c.buffered, err = c.conn.Receive()
 		if err != nil {
 			return nil, err
 		}
-	// TODO(]|[): this is where the NetLink-specific message would
-	// get converted into a "standard" event message and returned.
+		if len(c.buffered) == 0 {
+			// Unexpected. Not seen in wild, but sleep defensively.
+			time.Sleep(time.Second)
 			return nil, nil
 		}
+	}
+	msg := c.buffered[0]
+	c.buffered = c.buffered[1:]
+
+	// See https://github.com/torvalds/linux/blob/master/include/uapi/linux/rtnetlink.h
+	// And https://man7.org/linux/man-pages/man7/rtnetlink.7.html
+	switch msg.Header.Type {
+	case unix.RTM_NEWADDR:
+		var rmsg rtnetlink.AddressMessage
+		if err := rmsg.UnmarshalBinary(msg.Data); err != nil {
+			c.logf("RTM_NEWADDR: failed to parse: %v", err)
+			return nil, nil
+		}
+		return &newAddrMessage{
+			Label: rmsg.Attributes.Label,
+			Addr:  netaddrIP(rmsg.Attributes.Local),
+		}, nil
+	case unix.RTM_NEWROUTE:
+		var rmsg rtnetlink.RouteMessage
+		if err := rmsg.UnmarshalBinary(msg.Data); err != nil {
+			c.logf("RTM_NEWROUTE: failed to parse: %v", err)
+			return nil, nil
+		}
+		return &newRouteMessage{
+			Table:   rmsg.Table,
+			Src:     netaddrIP(rmsg.Attributes.Src),
+			Dst:     netaddrIP(rmsg.Attributes.Dst),
+			Gateway: netaddrIP(rmsg.Attributes.Gateway),
+		}, nil
+	default:
+		c.logf("netlink msg %+v, %q", msg.Header, msg.Data)
+		return nil, nil
+	}
+}
+
+func netaddrIP(std net.IP) netaddr.IP {
+	ip, _ := netaddr.FromStdIP(std)
+	return ip
+}
+
+// newRouteMessage is a message for a new route being added.
+type newRouteMessage struct {
+	Src, Dst, Gateway netaddr.IP
+	Table             uint8
+}
+
+func (m *newRouteMessage) ignore() bool {
+	return m.Table == 88 || tsaddr.IsTailscaleIP(m.Dst)
+}
+
+// newAddrMessage is a message for a new address being added.
+type newAddrMessage struct {
+	Addr  netaddr.IP
+	Label string // netlink Label attribute (e.g. "tailscale0")
+}
+
+func (m *newAddrMessage) ignore() bool {
+	return tsaddr.IsTailscaleIP(m.Addr)
+}

wgengine/monitor/monitor_unsupported.go

@@ -6,4 +6,6 @@
 
 package monitor
 
-func newOSMon() (osMon, error) { return nil, nil }
+import "tailscale.com/types/logger"
+
+func newOSMon(logger.Logf) (osMon, error) { return nil, nil }