remote: introduce notation for reverse port forwarding

An upcoming enhancement will add reverse port forwarding support (client
sharing its ports to the server) to complement the existing port
forwarding (server sharing its ports to the client). As a first step,
introduce notation for specifying a remote for reverse port forwarding;
i.e. "R:<local-interface>:<local-port>:<remote-host>:<remote-port>".

At this stage, reverse port forwarding remotes are recognized but never
actually created. A subsequent change will flesh out the functionality.

client/client.go

@@ -133,6 +133,9 @@ func (c *Client) Start(ctx context.Context) error {
 	}
 	//prepare proxies
 	for i, r := range c.config.shared.Remotes {
+		if r.Reverse {
+			continue
+		}
 		proxy := chshare.NewTCPProxy(c.Logger, func() ssh.Conn { return c.sshConn }, i, r)
 		if err := proxy.Start(ctx); err != nil {
 			return err

server/handler.go

@@ -98,6 +98,9 @@ func (s *Server) handleWebsocket(w http.ResponseWriter, req *http.Request) {
 	//access to the desired remotes
 	if user != nil {
 		for _, r := range c.Remotes {
+			if r.Reverse {
+				continue
+			}
 			addr := r.RemoteHost + ":" + r.RemotePort
 			if !user.HasAccess(addr) {
 				failed(s.Errorf("access to '%s' denied", addr))

share/remote.go

@@ -20,20 +20,31 @@ import (
 
 type Remote struct {
 	LocalHost, LocalPort, RemoteHost, RemotePort string
-	Socks                                        bool
+	Socks, Reverse                               bool
 }
 
 func DecodeRemote(s string) (*Remote, error) {
+	const revPrefix = "R:"
+	reverse := false
+	if strings.HasPrefix(s, revPrefix) {
+		s = s[len(revPrefix):]
+		reverse = true
+	}
 	parts := strings.Split(s, ":")
 	if len(parts) <= 0 || len(parts) >= 5 {
 		return nil, errors.New("Invalid remote")
 	}
-	r := &Remote{}
+	r := &Remote{Reverse: reverse}
 	//TODO fix up hacky decode
 	for i := len(parts) - 1; i >= 0; i-- {
 		p := parts[i]
 		//last part "socks"?
 		if i == len(parts)-1 && p == "socks" {
+			if reverse {
+				// TODO allow reverse+socks by having client
+				// automatically start local SOCKS5 server
+				return nil, errors.New("'socks' incompatible with reverse port forwarding")
+			}
 			r.Socks = true
 			continue
 		}
@@ -95,7 +106,11 @@ func isHost(s string) bool {
 
 //implement Stringer
 func (r *Remote) String() string {
-	return r.LocalHost + ":" + r.LocalPort + "=>" + r.Remote()
+	var tag string
+	if r.Reverse {
+		tag = " [reverse]"
+	}
+	return r.LocalHost + ":" + r.LocalPort + "=>" + r.Remote() + tag
 }
 
 func (r *Remote) Remote() string {