derp: add frameClosePeer to move around clients within a region

For various reasons (mostly during rollouts or config changes on our
side), nodes may end up connecting to a fallback DERP node in a
region, rather than the primary one we tell them about in the DERP
map.

Connecting to the "wrong" node is fine, but it's in our best interest
for all nodes in a domain to connect to the same node, to reduce
intra-region packet forwarding.

This adds a privileged frame type used by the control system that can
kick off a client connection when they're connected to the wrong node
in a region. Then they hopefully reconnect immediately to the correct
location. (If not, we can leave them alone and stop closing them.)

Updates tailscale/corp#372

derp/derp.go

@@ -95,6 +95,12 @@ const (
 	// framePeerPresent for all connected nodes, and then a stream of
 	// framePeerPresent & framePeerGone has peers connect and disconnect.
 	frameWatchConns = frameType(0x10)
+
+	// frameClosePeer is a privileged frame type (requires the
+	// mesh key for now) that closes the provided peer's
+	// connection. (To be used for cluster load balancing
+	// purposes, when clients end up on a non-ideal node)
+	frameClosePeer = frameType(0x11) // 32B pub key of peer to close.
 )
 
 var bin = binary.BigEndian

derp/derp_client.go

@@ -269,7 +269,7 @@ func (c *Client) NotePreferred(preferred bool) (err error) {
 }
 
 // WatchConnectionChanges sends a request to subscribe to the peer's connection list.
-// It's a fatal error if the client wasn't created with NewMeshClient.
+// It's a fatal error if the client wasn't created using MeshKey.
 func (c *Client) WatchConnectionChanges() error {
 	c.wmu.Lock()
 	defer c.wmu.Unlock()
@@ -279,6 +279,14 @@ func (c *Client) WatchConnectionChanges() error {
 	return c.bw.Flush()
 }
 
+// ClosePeer asks the server to close target's TCP connection.
+// It's a fatal error if the client wasn't created using MeshKey.
+func (c *Client) ClosePeer(target key.Public) error {
+	c.wmu.Lock()
+	defer c.wmu.Unlock()
+	return writeFrame(c.bw, frameClosePeer, target[:])
+}
+
 // ReceivedMessage represents a type returned by Client.Recv. Unless
 // otherwise documented, the returned message aliases the byte slice
 // provided to Recv and thus the message is only as good as that

derp/derp_server.go

@@ -420,6 +420,8 @@ func (c *sclient) run(ctx context.Context) error {
 			err = c.handleFrameForwardPacket(ft, fl)
 		case frameWatchConns:
 			err = c.handleFrameWatchConns(ft, fl)
+		case frameClosePeer:
+			err = c.handleFrameClosePeer(ft, fl)
 		default:
 			err = c.handleUnknownFrame(ft, fl)
 		}
@@ -457,6 +459,32 @@ func (c *sclient) handleFrameWatchConns(ft frameType, fl uint32) error {
 	return nil
 }
 
+func (c *sclient) handleFrameClosePeer(ft frameType, fl uint32) error {
+	if fl != keyLen {
+		return fmt.Errorf("handleFrameClosePeer wrong size")
+	}
+	if !c.canMesh {
+		return fmt.Errorf("insufficient permissions")
+	}
+	var targetKey key.Public
+	if _, err := io.ReadFull(c.br, targetKey[:]); err != nil {
+		return err
+	}
+	s := c.s
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if target, ok := s.clients[targetKey]; ok {
+		c.logf("frameClosePeer closing peer %x", targetKey)
+		go target.nc.Close()
+	} else {
+		c.logf("frameClosePeer failed to find peer %x", targetKey)
+	}
+
+	return nil
+}
+
 // handleFrameForwardPacket reads a "forward packet" frame from the client
 // (which must be a trusted client, a peer in our mesh).
 func (c *sclient) handleFrameForwardPacket(ft frameType, fl uint32) error {

derp/derphttp/derphttp_client.go

@@ -518,6 +518,10 @@ func (c *Client) NotePreferred(v bool) {
 	}
 }
 
+// WatchConnectionChanges sends a request to subscribe to
+// notifications about clients connecting & disconnecting.
+//
+// Only trusted connections (using MeshKey) are allowed to use this.
 func (c *Client) WatchConnectionChanges() error {
 	client, _, err := c.connect(context.TODO(), "derphttp.Client.WatchConnectionChanges")
 	if err != nil {
@@ -530,6 +534,21 @@ func (c *Client) WatchConnectionChanges() error {
 	return err
 }
 
+// ClosePeer asks the server to close target's TCP connection.
+//
+// Only trusted connections (using MeshKey) are allowed to use this.
+func (c *Client) ClosePeer(target key.Public) error {
+	client, _, err := c.connect(context.TODO(), "derphttp.Client.ClosePeer")
+	if err != nil {
+		return err
+	}
+	err = client.ClosePeer(target)
+	if err != nil {
+		c.closeForReconnect(client)
+	}
+	return err
+}
+
 // Recv reads a message from c. The returned message may alias memory from Client.
 // The message should only be used until the next Client call.
 func (c *Client) Recv() (derp.ReceivedMessage, error) {