class Net::SSH::Service::Forward

This class implements various port forwarding services for use by Net::SSH clients. The Forward class should never need to be instantiated directly; instead, it should be accessed via the singleton instance returned by Net::SSH::Connection::Session#forward:

ssh.forward.local(1234, "", 80)



The underlying connection service instance that the port-forwarding services employ.

Public Class Methods

new(session) click to toggle source

Instantiates a new Forward service instance atop the given connection service session. This will register new channel open handlers to handle the specialized channels that the SSH port forwarding protocols employ.

# File lib/net/ssh/service/forward.rb, line 24
def initialize(session)
  @session = session
  self.logger = session.logger
  @remote_forwarded_ports = {}
  @local_forwarded_ports = {}
  @agent_forwarded = false

  session.on_open_channel('forwarded-tcpip', &method(:forwarded_tcpip))
  session.on_open_channel('auth-agent', &method(:auth_agent_channel))
  session.on_open_channel('', &method(:auth_agent_channel))

Public Instance Methods

active_locals() click to toggle source

Returns a list of all active locally forwarded ports. The returned value is an array of arrays, where each element is a two-element tuple consisting of the local port and bind address corresponding to the forwarding port.

# File lib/net/ssh/service/forward.rb, line 118
def active_locals
active_remote_destinations() click to toggle source

Returns all active remote forwarded ports and where they forward to. The returned value is a hash from [<forwarding port on the local host>, <local forwarding address>] to [<port on the remote host>, <remote bind address>].

# File lib/net/ssh/service/forward.rb, line 219
def active_remote_destinations
  @remote_forwarded_ports.inject({}) do |result, (remote, local)|
    result[[local.port,]] = remote
active_remotes() click to toggle source

Returns all active forwarded remote ports. The returned value is an array of two-element tuples, where the first element is the port on the remote host and the second is the bind address.

# File lib/net/ssh/service/forward.rb, line 212
def active_remotes
agent(channel) click to toggle source

Enables SSH agent forwarding on the given channel. The forwarded agent will remain active even after the channel closes–the channel is only used as the transport for enabling the forwarded connection. You should never need to call this directly–it is called automatically the first time a session channel is opened, when the connection was created with :forward_agent set to true:

Net::SSH.start("", "me", :forward_agent => true) do |ssh|
  ssh.open_channel do |ch|
    # agent will be automatically forwarded by this point
# File lib/net/ssh/service/forward.rb, line 239
def agent(channel)
  return if @agent_forwarded
  @agent_forwarded = true

  channel.send_channel_request("") do |achannel, success|
    if success
      debug { "authentication agent forwarding is active" }
      achannel.send_channel_request("auth-agent-req") do |a2channel, success2|
        if success2
          debug { "authentication agent forwarding is active" }
          error { "could not establish forwarding of authentication agent" }
cancel_local(port, bind_address="") click to toggle source

Terminates an active local forwarded port. If no such forwarded port exists, this will raise an exception. Otherwise, the forwarded connection is terminated.

ssh.forward.cancel_local(1234, "")
# File lib/net/ssh/service/forward.rb, line 107
def cancel_local(port, bind_address="")
  socket = @local_forwarded_ports.delete([port, bind_address])
  socket.shutdown rescue nil
  socket.close rescue nil
cancel_remote(port, host="") click to toggle source

Requests that a remote forwarded port be cancelled. The remote forwarded port on the remote host, bound to the given address on the remote host, will be terminated, but not immediately. This method returns immediately after queueing the request to be sent to the server. If for some reason the port cannot be cancelled, an exception will be raised (asynchronously).

If you want to know when the connection has been cancelled, it will no longer be present in the active_remotes list. If you want to block until the port is no longer active, you could do something like this:

ssh.forward.cancel_remote(1234, "")
ssh.loop { ssh.forward.active_remotes.include?([1234, ""]) }
# File lib/net/ssh/service/forward.rb, line 199
def cancel_remote(port, host="")
  session.send_global_request("cancel-tcpip-forward", :string, host, :long, port) do |success, response|
    if success
      @remote_forwarded_ports.delete([port, host])
      raise Net::SSH::Exception, "could not cancel remote forward request on #{host}:#{port}"
local(*args) click to toggle source

Starts listening for connections on the local host, and forwards them to the specified remote host/port via the SSH connection. This method accepts either three or four arguments. When four arguments are given, they are:

  • the local address to bind to

  • the local port to listen on

  • the remote host to forward connections to

  • the port on the remote host to connect to

If three arguments are given, it is as if the local bind address is “”, and the rest are applied as above.

To request an ephemeral port on the remote server, provide 0 (zero) for the port number. In all cases, this method will return the port that has been assigned.

ssh.forward.local(1234, "", 80)
assigned_port = ssh.forward.local("", 0, "", 80)
# File lib/net/ssh/service/forward.rb, line 55
def local(*args)
  if args.length < 3 || args.length > 4
    raise ArgumentError, "expected 3 or 4 parameters, got #{args.length}"

  local_port_type = :long

  socket = begin
    if defined?(UNIXServer) and args.first.class == UNIXServer
      local_port_type = :string
      bind_address = ""
      bind_address = args.shift if args.first.is_a?(String) && args.first =~ /\D/
      local_port = args.shift.to_i
      local_port_type = :long, local_port)

  local_port = socket.addr[1] if local_port == 0 # ephemeral port was requested
  remote_host = args.shift
  remote_port = args.shift.to_i

  @local_forwarded_ports[[local_port, bind_address]] = socket

  session.listen_to(socket) do |server|
    client = server.accept
    debug { "received connection on #{socket}" }

    channel = session.open_channel("direct-tcpip", :string, remote_host, :long, remote_port, :string, bind_address, local_port_type, local_port) do |achannel| { "direct channel established" }

    prepare_client(client, channel, :local)

    channel.on_open_failed do |ch, code, description|
      channel.error { "could not establish direct channel: #{description} (#{code})" }

remote(port, host, remote_port, remote_host="") { |remote_port, remote_host| ... } click to toggle source

Requests that all connections on the given remote-port be forwarded via the local host to the given port/host. The last argument describes the bind address on the remote host, and defaults to

This method will return immediately, but the port will not actually be forwarded immediately. If the remote server is not able to begin the listener for this request, an exception will be raised asynchronously.

To request an ephemeral port on the remote server, provide 0 (zero) for the port number. The assigned port will show up in the # active_remotes list.

remote_host is interpreted by the server per RFC 4254, which has these special values:

  • “” means that connections are to be accepted on all protocol families supported by the SSH implementation.

  • “” means to listen on all IPv4 addresses.

  • “::” means to listen on all IPv6 addresses.

  • “localhost” means to listen on all protocol families supported by the SSH implementation on loopback addresses only ([RFC3330] and [RFC3513]).

  • “” and “::1” indicate listening on the loopback interfaces for IPv4 and IPv6, respectively.

You may pass a block that will be called when the the port forward request receives a response. This block will be passed the remote_port that was actually bound to, or nil if the binding failed. If the block returns :no_exception, the “failed binding” exception will not be thrown.

If you want to block until the port is active, you could do something like this:

got_remote_port = nil
remote(port, host, remote_port, remote_host) do |actual_remote_port|
  got_remote_port = actual_remote_port || :error
  :no_exception # will yield the exception on my own thread
session.loop { !got_remote_port }
if got_remote_port == :error
  raise Net::SSH::Exception, "remote forwarding request failed"
# File lib/net/ssh/service/forward.rb, line 165
def remote(port, host, remote_port, remote_host="")
  session.send_global_request("tcpip-forward", :string, remote_host, :long, remote_port) do |success, response|
    if success
      remote_port = response.read_long if remote_port == 0
      debug { "remote forward from remote #{remote_host}:#{remote_port} to #{host}:#{port} established" }
      @remote_forwarded_ports[[remote_port, remote_host]] =, port)
      yield remote_port, remote_host if block_given?
      instruction = if block_given?
        yield :error
      unless instruction == :no_exception
        error { "remote forwarding request failed" }
        raise Net::SSH::Exception, "remote forwarding request failed"
Also aliased as: remote_to
remote_to(port, host, remote_port, remote_host="")

an alias, for token backwards compatibility with the 1.x API

Alias for: remote