cmd/tailscale, magicsock: add debug command to flip DERP homes

For testing netmap patchification server-side.

Updates #1909

Change-Id: Ib1d784bd97b8d4a31e48374b4567404aae5280cc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -139,6 +139,11 @@ var debugCmd = &ffcli.Command{
 			Exec:      localAPIAction("break-derp-conns"),
 			ShortHelp: "break any open DERP connections from the daemon",
 		},
+		{
+			Name:      "pick-new-derp",
+			Exec:      localAPIAction("pick-new-derp"),
+			ShortHelp: "switch to some other random DERP home region for a short time",
+		},
 		{
 			Name:      "force-netmap-update",
 			Exec:      localAPIAction("force-netmap-update"),

ipn/ipnlocal/local.go

@@ -2156,6 +2156,12 @@ func (b *LocalBackend) DebugForceNetmapUpdate() {
 	b.setNetMapLocked(nm)
 }
 
+// DebugPickNewDERP forwards to magicsock.Conn.DebugPickNewDERP.
+// See its docs.
+func (b *LocalBackend) DebugPickNewDERP() error {
+	return b.sys.MagicSock.Get().DebugPickNewDERP()
+}
+
 // send delivers n to the connected frontend and any API watchers from
 // LocalBackend.WatchNotifications (via the LocalAPI).
 //

ipn/localapi/localapi.go

@@ -566,6 +566,8 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 		if err == nil {
 			return
 		}
+	case "pick-new-derp":
+		err = h.b.DebugPickNewDERP()
 	case "":
 		err = fmt.Errorf("missing parameter 'action'")
 	default:

wgengine/magicsock/magicsock.go

@@ -2715,6 +2715,33 @@ func (c *Conn) getPinger() *ping.Pinger {
 	})
 }
 
+// DebugPickNewDERP picks a new DERP random home temporarily (even if just for
+// seconds) and reports it to control. It exists to test DERP home changes and
+// netmap deltas, etc. It serves no useful user purpose.
+func (c *Conn) DebugPickNewDERP() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	dm := c.derpMap
+	if dm == nil {
+		return errors.New("no derpmap")
+	}
+	if c.netInfoLast == nil {
+		return errors.New("no netinfo")
+	}
+	for _, r := range dm.Regions {
+		if r.RegionID == c.myDerp {
+			continue
+		}
+		c.logf("magicsock: [debug] switching derp home to random %v (%v)", r.RegionID, r.RegionCode)
+		go c.setNearestDERP(r.RegionID)
+		ni2 := c.netInfoLast.Clone()
+		ni2.PreferredDERP = r.RegionID
+		c.callNetInfoCallbackLocked(ni2)
+		return nil
+	}
+	return errors.New("too few regions")
+}
+
 // portableTrySetSocketBuffer sets SO_SNDBUF and SO_RECVBUF on pconn to socketBufferSize,
 // logging an error if it occurs.
 func portableTrySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {