[client] Detect default route changes in the Linux network monitor (#8216)

The Linux network monitor skipped every route update whose Dst was not
nil, treating a nil Dst as the default route. Since the bump to
vishvananda/netlink v1.3 (v0.43.0) the default route arrives as an
explicit 0.0.0.0/0 or ::/0, so every default route change was filtered
out and the monitor never fired on Linux, no matter how the underlay
moved.

Treat both a nil Dst and a zero-length prefix as the default route.

client/internal/networkmonitor/check_change_linux.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"net"
 	"syscall"
 
 	log "github.com/sirupsen/logrus"
@@ -35,7 +36,7 @@ func checkChange(ctx context.Context, nexthopv4, nexthopv6 systemops.Nexthop) er
 		// handle route changes
 		case route := <-routeChan:
 			// default route and main table
-			if route.Dst != nil || route.Table != syscall.RT_TABLE_MAIN {
+			if !isDefaultRoute(route.Dst) || route.Table != syscall.RT_TABLE_MAIN {
 				continue
 			}
 			switch route.Type {
@@ -52,3 +53,13 @@ func checkChange(ctx context.Context, nexthopv4, nexthopv6 systemops.Nexthop) er
 		}
 	}
 }
+
+// isDefaultRoute reports whether dst is a default route. netlink v1.3 reports
+// the default as 0.0.0.0/0 or ::/0, older versions as a nil Dst.
+func isDefaultRoute(dst *net.IPNet) bool {
+	if dst == nil {
+		return true
+	}
+	ones, _ := dst.Mask.Size()
+	return ones == 0
+}