Generic Ssh Channel Behavior

Ssh services are implemented as channels that are multiplexed over an ssh connection and communicates via the ssh connection protocol. This module provides a callback API that takes care of generic channel aspects such as flow control and close messages and lets the callback functions take care of the service specific parts.


Type definitions that are used more than once in this module and/or abstractions to indicate the intended use of the data type:

boolean() = true | false

string() = list of ASCII characters

timeout() = infinity | integer() - in milliseconds.

ssh_connection_ref() - opaque to the user returned by ssh:connect/3 or sent to a ssh channel process

ssh_channel_id() = integer()

ssh_data_type_code() = 1 ("stderr") | 0 ("normal") are currently valid values see RFC 4254 section 5.2.


call(ChannelRef, Msg) ->

call(ChannelRef, Msg, Timeout) -> Reply | {error, Reason}

  • ChannelRef = pid()
  • As returned by start_link/4
  • Msg = term()
  • Timeout = timeout()
  • Reply = term()
  • Reason = closed | timeout

Makes a synchronous call to the channel process by sending a message and waiting until a reply arrives or a timeout occurs. The channel will call CallbackModule:handle_call/3 to handle the message. If the channel process does not exist {error, closed} is returned.

cast(ChannelRef, Msg) -> ok

  • ChannelRef = pid()
  • As returned by start_link/4
  • Msg = term()

Sends an asynchronous message to the channel process and returns ok immediately, ignoring if the destination node or channel process does not exist. The channel will call CallbackModule:handle_cast/2 to handle the message.

enter_loop(State) -> _

  • State = term() - as returned by ssh_channel:init/1

Makes an existing process into a ssh_channel process. Does not return, instead the calling process will enter the ssh_channel process receive loop and become a ssh_channel process. The process must have been started using one of the start functions in proc_lib, see proc_lib(3). The user is responsible for any initialization of the process and needs to call ssh_channel:init/1.

init(Options) -> {ok, State} | {ok, State, Timeout} | {stop, Reason}

  • Options = [{Option, Value}]

The following options must be present:

{channel_cb, atom()}
The module that implements the channel behavior.
{init_args(), list()}
The list of arguments to the callback modules init function.
{cm, connection_ref()}
Reference to the ssh connection.
{channel_id, channel_id()}
Id of the ssh channel.


This function is normally not called by the user, it is only needed if for some reason the channel process needs to be started with help of proc_lib instead calling ssh_channel:start/4 or ssh_channel:start_link/4

reply(Client, Reply) -> _

  • Client - opaque to the user, see explanation below
  • Reply = term()

This function can be used by a channel to explicitly send a reply to a client that called call/[2,3] when the reply cannot be defined in the return value of CallbackModule:handle_call/3.

Client must be the From argument provided to the callback function handle_call/3. Reply is an arbitrary term, which will be given back to the client as the return value of ssh_channel:call/[2,3].

start(SshConnection, ChannelId, ChannelCb, CbInitArgs) ->

start_link(SshConnection, ChannelId, ChannelCb, CbInitArgs) -> {ok, ChannelRef} | {error, Reason}

  • SshConnection = ssh_connection_ref()
  • ChannelId = ssh_channel_id()
  • As returned by ssh_connection:session_channel/[2,4]
  • ChannelCb = atom()
  • The name of the module implementing the service specific parts of the channel.
  • CbInitArgs = [term()]
  • Argument list for the init function in the callback module.
  • ChannelRef = pid()

Starts a processes that handles a ssh channel. Will be called internally by the ssh daemon or explicitly by the ssh client implementations. A channel process traps exit signals by default.


The functions init/1, terminate/2, handle_ssh_msg/2 and handle_msg/2 are the functions that are required to provide the implementation for a server side channel, such as a ssh subsystem channel that can be plugged into the erlang ssh daemon see ssh:daemon/[2, 3]. The handle_call/3, handle_cast/2 code_change/3 and enter_loop/1 functions are only relevant when implementing a client side channel.


If an integer timeout value is provided in a return value of one of the callback functions, a timeout will occur unless a message is received within Timeout milliseconds. A timeout is represented by the atom timeout which should be handled by the handle_msg/2 callback function. The atom infinity can be used to wait indefinitely, this is the default value.


CallbackModule:code_change(OldVsn, State, Extra) -> {ok, NewState}

  • Converts process state when code is changed.

This function is called by a client side channel when it should update its internal state during a release upgrade/downgrade, i.e. when the instruction {update,Module,Change,...} where Change={advanced,Extra} is given in the appup file. See OTP Design Principles for more information. Any new connection will benefit from a server side upgrade but already started connections on the server side will not be affected.


If there are long lived ssh connections and more than one upgrade in a short time this may cause the old connections to fail as only two versions of the code may be loaded simultaneously.

In the case of an upgrade, OldVsn is Vsn, and in the case of a downgrade, OldVsn is {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old version of the callback module Module. If no such attribute is defined, the version is the checksum of the BEAM file.

State is the internal state of the channel.

Extra is passed as-is from the {advanced,Extra} part of the update instruction.

The function should return the updated internal state.

CallbackModule:init(Args) -> {ok, State} | {ok, State, Timeout} | {stop, Reason}

  • Args = term()
  • Last argument to ssh_channel:start_link/4.
  • State = term()
  • Timeout = timeout()
  • Reason = term()

Makes necessary initializations and returns the initial channel state if the initializations succeed.

For more detailed information on timeouts see the section CALLBACK TIMEOUTS.

CallbackModule:handle_call(Msg, From, State) -> Result

  • Msg = term()
  • From = opaque to the user should be used as argument to ssh_channel:reply/2
  • State = term()
  • Result = {reply, Reply, NewState} | {reply, Reply, NewState, Timeout} | {noreply, NewState} | {noreply , NewState, Timeout} | {stop, Reason, Reply, NewState} | {stop, Reason, NewState}
  • Reply = term() - will be the return value of ssh_channel:call/[2,3]
  • Timeout = timeout()
  • NewState = term() - a possible updated version of State
  • Reason = term()

Handles messages sent by calling ssh_channel:call/[2,3]

For more detailed information on timeouts see the section CALLBACK TIMEOUTS.

CallbackModule:handle_cast(Msg, State) -> Result

  • Msg = term()
  • State = term()
  • Result = {noreply, NewState} | {noreply, NewState, Timeout} | {stop, Reason, NewState}
  • NewState = term() - a possible updated version of State
  • Timeout = timeout()
  • Reason = term()

Handles messages sent by calling ssh_channel:cast/2

For more detailed information on timeouts see the section CALLBACK TIMEOUTS.

CallbackModule:handle_msg(Msg, State) -> {ok, State} | {stop, ChannelId, State}

  • Msg = timeout | term()
  • State = term()

Handle other messages than ssh connection protocol, call or cast messages sent to the channel.

Possible erlang 'EXIT'-messages should be handled by this function and all channels should handle the following message.

{ssh_channel_up, ssh_channel_id(), ssh_connection_ref()}
This is the first messages that will be received by the channel, it is sent just before the ssh_channel:init/1 function returns successfully. This is especially useful if the server wants to send a message to the client without first receiving a message from the client. If the message is not useful for your particular problem just ignore it by immediately returning {ok, State}.

CallbackModule:handle_ssh_msg(Msg, State) -> {ok, State} | {stop, ssh_channel_id(), State}

  • Msg = {ssh_cm, ssh_connection_ref(), SshMsg}
  • SshMsg = tuple() - see message list below
  • State = term()

Handles ssh connection protocol messages that may need service specific attention.

All channels should handle the following messages. For channels implementing subsystems the handle_ssh_msg-callback will not be called for any other messages.

{ssh_cm, ssh_connection_ref(), {data, ssh_channel_id(), ssh_data_type_code(), binary() = Data}}
Data has arrived on the channel. When the callback for this message returns the channel behavior will adjust the ssh flow control window.
{ssh_cm, ssh_connection_ref(), {eof, ssh_channel_id()}}
Indicteas that the other side will not send any more data.
{ssh_cm, ssh_connection_ref(), {signal, ssh_channel_id(), ssh_signal()}}
A signal can be delivered to the remote process/service using the following message. Some systems may not implement signals, in which case they should ignore this message.
{ssh_cm, ssh_connection_ref(), {exit_signal, ssh_channel_id(), string() = exit_signal, string() = ErrorMsg, string() = LanguageString}}
A remote execution may terminate violently due to a signal then this message may be received. For details on valid string values see RFC 4254 section 6.10
{ssh_cm, ssh_connection_ref(), {exit_status, ssh_channel_id(), integer() = ExitStatus}}
When the command running at the other end terminates, the following message can be sent to return the exit status of the command. A zero 'exit_status' usually means that the command terminated successfully.

Channels implementing a shell and command execution on the server side should also handle the following messages.

{ssh_cm, ssh_connection_ref(), {env, ssh_channel_id(), boolean() = WantReply, string() = Var, string() = Value}}
Environment variables may be passed to the shell/command to be started later. Note that before the callback returns it should call the function ssh_connection:reply_request/4 with the boolean value of WantReply as the second argument.
{ssh_cm, ConnectionRef, {exec, ssh_channel_id(), boolean() = WantReply, string() = Cmd}}
This message will request that the server start the execution of the given command. Note that before the callback returns it should call the function ssh_connection:reply_request/4 with the boolean value of WantReply as the second argument.
{ssh_cm, ssh_connection_ref(), {pty, ssh_channel_id(), boolean() = WantReply, {string() = Terminal, integer() = CharWidth, integer() = RowHeight, integer() = PixelWidth, integer() = PixelHight, [{atom() | integer() = Opcode, integer() = Value}] = TerminalModes}}}
A pseudo-terminal has been requested for the session. Terminal is the value of the TERM environment variable value (e.g., vt100). Zero dimension parameters must be ignored. The character/row dimensions override the pixel dimensions (when nonzero). Pixel dimensions refer to the drawable area of the window. The Opcode in the TerminalModes list is the mnemonic name, represented as an lowercase erlang atom, defined in RFC 4254 section 8, or the opcode if the mnemonic name is not listed in the RFC. Example OP code: 53, mnemonic name ECHO erlang atom: echo. Note that before the callback returns it should call the function ssh_connection:reply_request/4 with the boolean value of WantReply as the second argument.
{ssh_cm, ConnectionRef, {shell, boolean() = WantReply}}
This message will request that the user's default shell be started at the other end. Note that before the callback returns it should call the function ssh_connection:reply_request/4 with the value of WantReply as the second argument.
{ssh_cm, ssh_connection_ref(), {window_change, ssh_channel_id(), integer() = CharWidth, integer() = RowHeight, integer() = PixWidth, integer() = PixHeight}}
When the window (terminal) size changes on the client side, it MAY send a message to the other side to inform it of the new dimensions.

The following message is completely taken care of by the ssh channel behavior

{ssh_cm, ssh_connection_ref(), {closed, ssh_channel_id()}}
The channel behavior will send a close message to the other side if such a message has not already been sent and then terminate the channel with reason normal.

CallbackModule:terminate(Reason, State) -> _

  • Reason = term()
  • State = term()

This function is called by a channel process when it is about to terminate. Before this function is called ssh_connection:close/2 will be called if it has not been called earlier. This function should be the opposite of CallbackModule:init/1 and do any necessary cleaning up. When it returns, the channel process terminates with reason Reason. The return value is ignored.

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