Erlang -- Getting Started

SSH

User's Guide

Version 4.8.1

- User's Guide
- Reference Manual
- Release Notes
- PDF
- Top

- Expand All
- Contract All

### Chapters

- Introduction
  - Top of chapter
  - Scope and Purpose
  - Prerequisites
  - SSH Protocol Overview
  - Where to Find More Information
- Getting Started
  - Top of chapter
  - General Information
  - Using the Erlang ssh Terminal Client
  - Running an Erlang ssh Daemon
  - One-Time Execution
  - SFTP Server
  - SFTP Client
  - SFTP Client with TAR Compression
  - Creating a Subsystem
- Terminology
  - Top of chapter
  - General Information
  - The term "user"
- Configuring algorithms in SSH
  - Top of chapter
  - Introduction
  - Replacing the default set: preferred_algorithms
  - Modifying the default set: modify_algorithms

# 2 Getting Started

### 2.1 General Information

The following examples use the utility function ssh:start/0 to start all needed applications (crypto, public_key, and ssh). All examples are run in an Erlang shell, or in a bash shell, using openssh to illustrate how the ssh application can be used. The examples are run as the user otptest on a local network where the user is authorized to log in over ssh to the host tarlop.

If nothing else is stated, it is presumed that the otptest user has an entry in the authorized_keys file of tarlop (allowed to log in over ssh without entering a password). Also, tarlop is a known host in the known_hosts file of the user otptest. This means that host-verification can be done without user-interaction.

### 2.2 Using the Erlang ssh Terminal Client

The user otptest, which has bash as default shell, uses the ssh:shell/1 client to connect to the openssh daemon running on a host called tarlop:

    1> ssh:start().
    ok
    2> {ok, S} = ssh:shell("tarlop").
    otptest@tarlop:> pwd
    /home/otptest
    otptest@tarlop:> exit
    logout
    3>

### 2.3 Running an Erlang ssh Daemon

The system_dir option must be a directory containing a host key file and it defaults to /etc/ssh. For details, see Section Configuration Files in ssh(6).

Note

Normally, the /etc/ssh directory is only readable by root.

The option user_dir defaults to directory users ~/.ssh.

Step 1. To run the example without root privileges, generate new keys and host keys:

    $bash> ssh-keygen -t rsa -f /tmp/ssh_daemon/ssh_host_rsa_key
    [...]
    $bash> ssh-keygen -t rsa -f /tmp/otptest_user/.ssh/id_rsa
    [...]

Step 2. Create the file /tmp/otptest_user/.ssh/authorized_keys and add the content of /tmp/otptest_user/.ssh/id_rsa.pub.

Step 3. Start the Erlang ssh daemon:

    1> ssh:start().
    ok
    2> {ok, Sshd} = ssh:daemon(8989, [{system_dir, "/tmp/ssh_daemon"},
                                      {user_dir, "/tmp/otptest_user/.ssh"}]).
    {ok,<0.54.0>}
    3>

Step 4. Use the openssh client from a shell to connect to the Erlang ssh daemon:

    $bash> ssh tarlop -p 8989  -i /tmp/otptest_user/.ssh/id_rsa \
                      -o UserKnownHostsFile=/tmp/otptest_user/.ssh/known_hosts
    The authenticity of host 'tarlop' can't be established.
    RSA key fingerprint is 14:81:80:50:b1:1f:57:dd:93:a8:2d:2f:dd:90:ae:a8.
    Are you sure you want to continue connecting (yes/no)? yes
    Warning: Permanently added 'tarlop' (RSA) to the list of known hosts.
    Eshell V5.10  (abort with ^G)
    1>

There are two ways of shutting down an ssh daemon, see Step 5a and Step 5b.

Step 5a. Shut down the Erlang ssh daemon so that it stops the listener but leaves existing connections, started by the listener, operational:

    3> ssh:stop_listener(Sshd).
    ok
    4>

Step 5b. Shut down the Erlang ssh daemon so that it stops the listener and all connections started by the listener:

    3> ssh:stop_daemon(Sshd).
    ok
    4>

### 2.4 One-Time Execution

#### Erlang client contacting OS standard ssh server

In the following example, the Erlang shell is the client process that receives the channel replies as Erlang messages.

Do an one-time execution of a remote OS command ("pwd") over ssh to the ssh server of the OS at the host "tarlop":

    1> ssh:start().
    ok
    2> {ok, ConnectionRef} = ssh:connect("tarlop", 22, []).
    {ok,<0.57.0>}
    3> {ok, ChannelId} = ssh_connection:session_channel(ConnectionRef, infinity).
    {ok,0}
    4> success = ssh_connection:exec(ConnectionRef, ChannelId, "pwd", infinity).
    5> flush(). % Get all pending messages. NOTE: ordering may vary!
    Shell got {ssh_cm,<0.57.0>,{data,0,0,<<"/home/otptest\n">>}}
    Shell got {ssh_cm,<0.57.0>,{eof,0}}
    Shell got {ssh_cm,<0.57.0>,{exit_status,0,0}}
    Shell got {ssh_cm,<0.57.0>,{closed,0}}
    ok
    6> ssh:connection_info(ConnectionRef, channels).
    {channels,[]}
    7>

See ssh_connection and ssh_connection:exec/4 for finding documentation of the channel messages.

To collect the channel messages in a program, use receive...end instead of flush/1:

    5> receive
    5>     {ssh_cm, ConnectionRef, {data, ChannelId, Type, Result}} when Type == 0 ->
    5>         {ok,Result}
    5>     {ssh_cm, ConnectionRef, {data, ChannelId, Type, Result}} when Type == 1 ->
    5>         {error,Result}
    5> end.
    {ok,<<"/home/otptest\n">>}
    6>

Note that only the exec channel is closed after the one-time execution. The connection is still up and can handle previously opened channels. It is also possible to open a new channel:

    % try to open a new channel to check if the ConnectionRef is still open
    7> {ok, NewChannelId} = ssh_connection:session_channel(ConnectionRef, infinity).
    {ok,1}
    8>

To close the connection, call the function ssh:close(ConnectionRef). As an alternative, set the option {idle_time, 1} when opening the connection. This will cause the connection to be closed automaticaly when there are no channels open for the specified time period, in this case 1 ms.

#### OS standard client and Erlang daemon (server)

An Erlang SSH daemon could be called for one-time execution of a "command". The "command" must be as if entered into the erlang shell, that is a sequence of Erlang expressions ended by a period (.). Variables bound in that sequence will keep their bindings throughout the expression sequence. The bindings are disposed when the result is returned.

Here is an example of a suitable expression sequence:

    A=1, B=2, 3 == (A + B).

It evaluates to true if submitted to the Erlang daemon started in Step 3 above:

    $bash> ssh tarlop -p 8989 "A=1, B=2, 3 == (A + B)."
    true
    $bash>

The same example but now using the Erlang ssh client to contact the Erlang server:

    1> {ok, ConnectionRef} = ssh:connect("tarlop", 8989, []).
    {ok,<0.216.0>}
    2> {ok, ChannelId} = ssh_connection:session_channel(ConnectionRef, infinity).
    {ok,0}
    3> success = ssh_connection:exec(ConnectionRef, ChannelId,
                                     "A=1, B=2, 3 == (A + B).",
                                     infinity).
    success
    4> flush().
    Shell got {ssh_cm,<0.216.0>,{data,0,0,<<"true">>}}
    Shell got {ssh_cm,<0.216.0>,{exit_status,0,0}}
    Shell got {ssh_cm,<0.216.0>,{eof,0}}
    Shell got {ssh_cm,<0.216.0>,{closed,0}}
    ok
    5>

Note that Erlang shell specific functions and control sequences like for example h(). are not supported.

#### I/O from a function called in an Erlang ssh daemon

Output to stdout on the server side is also displayed as well as the resulting term from the function call:

    $bash> ssh tarlop -p 8989 'io:format("Hello!~n~nHow are ~p?~n",[you]).'
    Hello!

    How are you?
    ok
    $bash>

And similar for reading from stdin. As an example we use io:read/1 which displays the argument as a prompt on stdout, reads a term from stdin and returns it in an ok-tuple:

    $bash> ssh tarlop -p 8989 'io:read("write something: ").'
    write something: [a,b,c].
    {ok,[a,b,c]}
    $bash>

The same example but using the Erlang ssh client:

    Eshell V10.5.2  (abort with ^G)
    1> ssh:start().
    ok
    2> {ok, ConnectionRef} = ssh:connect(loopback, 8989, []).
    {ok,<0.92.0>}
    3> {ok, ChannelId} = ssh_connection:session_channel(ConnectionRef, infinity).
    {ok,0}
    4> success = ssh_connection:exec(ConnectionRef, ChannelId,
                                     "io:read(\"write something: \").",
                                     infinity).
    success
    5> flush().
    Shell got {ssh_cm,<0.92.0>,{data,0,0,<<"write something: ">>}}
    ok
    % All data is sent as binaries with string contents:
    6> ok = ssh_connection:send(ConnectionRef, ChannelId, <<"[a,b,c].">>).
    ok
    7> flush().
    ok
    %% Nothing is received, because the io:read/1
    %% requires the input line to end with a newline.

    %% Send a newline (it could have been included in the last send):
    8> ssh_connection:send(ConnectionRef, ChannelId, <<"\n">>).
    ok
    9> flush().
    Shell got {ssh_cm,<0.92.0>,{data,0,0,<<"{ok,[a,b,c]}">>}}
    Shell got {ssh_cm,<0.92.0>,{exit_status,0,0}}
    Shell got {ssh_cm,<0.92.0>,{eof,0}}
    Shell got {ssh_cm,<0.92.0>,{closed,0}}
    ok
    10>

#### Configuring the server's (daemon's) command execution

Every time a daemon is started, it enables one-time execution of commands as described in the previous section unless explicitly disabled.

There is often a need to configure some other exec evaluator to tailor the input language or restrict the possible functions to call. There are two ways of doing this which will be shown with examples below. See ssh:daemon/2,3 and exec_daemon_option()) for details.

Examples of the two ways to configure the exec evaluator:

- Disable one-time execution.
  To modify the daemon start example above to reject one-time execution requests, we change Step 3 by adding the option {exec, disabled} to:

      1> ssh:start().
      ok
      2> {ok, Sshd} = ssh:daemon(8989, [{system_dir, "/tmp/ssh_daemon"},
                                        {user_dir, "/tmp/otptest_user/.ssh"},
                                        {exec, disabled}
                                       ]).
      {ok,<0.54.0>}
      3>

  A call to that daemon will return the text "Prohibited." on stderr (depending on the client and OS), and the exit status 255:

      $bash> ssh tarlop -p 8989 "test."
      Prohibited.
      $bash> echo $?
      255
      $bash>

  And the Erlang client library also returns the text "Prohibited." on data type 1 instead of the normal 0 and exit status 255:

      2> {ok, ConnectionRef} = ssh:connect(loopback, 8989, []).
      {ok,<0.92.0>}
      3> {ok, ChannelId} = ssh_connection:session_channel(ConnectionRef, infinity).
      {ok,0}
      4> success = ssh_connection:exec(ConnectionRef, ChannelId, "test."
      success
      5> flush().
      Shell got {ssh_cm,<0.106.0>,{data,0,1,<<"Prohibited.">>}}
      Shell got {ssh_cm,<0.106.0>,{exit_status,0,255}}
      Shell got {ssh_cm,<0.106.0>,{eof,0}}
      Shell got {ssh_cm,<0.106.0>,{closed,0}}
      ok
      6>

- Install an alternative evaluator.
  Start the damon with a reference to a fun() that handles the evaluation:

      1> ssh:start().
      ok
      2> MyEvaluator = fun("1") -> {ok, some_value};
                          ("2") -> {ok, some_other_value};
                          ("3") -> {ok, V} = io:read("input erlang term>> "),
                                   {ok, V};
                          (Err) -> {error,{bad_input,Err}}
                       end.
      3> {ok, Sshd} = ssh:daemon(1234, [{system_dir, "/tmp/ssh_daemon"},
                                        {user_dir, "/tmp/otptest_user/.ssh"},
                                        {exec, {direct,MyEvaluator}}
                                       ]).
      {ok,<0.275.0>}
      4>

  and call it:

      $bash> ssh localhost -p 1234 1
      some_value
      $bash> ssh localhost -p 1234 2
      some_other_value
      # I/O works:
      $bash> ssh localhost -p 1234 3
      input erlang term>> abc.
      abc
      # Check that Erlang evaluation is disabled:
      $bash> ssh localhost -p 1234 1+ 2.
      **Error** {bad_input,"1+ 2."}
      $bash>

  Note that spaces are preserved and that no point (.) is needed at the end - that was required by the default evaluator.

  The error return in the Erlang client (The text as data type 1 and exit_status 255):

      2> {ok, ConnectionRef} = ssh:connect(loopback, 1234, []).
      {ok,<0.92.0>}
      3> {ok, ChannelId} = ssh_connection:session_channel(ConnectionRef, infinity).
      {ok,0}
      4> success = ssh_connection:exec(ConnectionRef, ChannelId, "1+ 2."
      success
      5> flush().
      Shell got {ssh_cm,<0.106.0>,{data,0,1,<<"**Error** {bad_input,\"1+ 2.\"}">>}}
      Shell got {ssh_cm,<0.106.0>,{exit_status,0,255}}
      Shell got {ssh_cm,<0.106.0>,{eof,0}}
      Shell got {ssh_cm,<0.106.0>,{closed,0}}
      ok
      6>

  The fun() in the exec option could take up to three arguments (Cmd, User and ClientAddress). See the exec_daemon_option() for the details.

Note

An old, discouraged and undocumented way of installing an alternative evaluator exists.

It still works, but lacks for example I/O possibility. It is because of that compatibility we need the {direct,...} construction.

### 2.5 SFTP Server

Start the Erlang ssh daemon with the SFTP subsystem:

    1> ssh:start().
    ok
    2> ssh:daemon(8989, [{system_dir, "/tmp/ssh_daemon"},
                         {user_dir, "/tmp/otptest_user/.ssh"},
                         {subsystems, [ssh_sftpd:subsystem_spec(
                                                [{cwd, "/tmp/sftp/example"}])
                                      ]}]).
    {ok,<0.54.0>}
    3>

Run the OpenSSH SFTP client:

    $bash> sftp -oPort=8989 -o IdentityFile=/tmp/otptest_user/.ssh/id_rsa \
                -o UserKnownHostsFile=/tmp/otptest_user/.ssh/known_hosts tarlop
    Connecting to tarlop...
    sftp> pwd
    Remote working directory: /tmp/sftp/example
    sftp>

### 2.6 SFTP Client

Fetch a file with the Erlang SFTP client:

    1> ssh:start().
    ok
    2> {ok, ChannelPid, Connection} = ssh_sftp:start_channel("tarlop", []).
    {ok,<0.57.0>,<0.51.0>}
    3> ssh_sftp:read_file(ChannelPid, "/home/otptest/test.txt").
    {ok,<<"This is a test file\n">>}

### 2.7 SFTP Client with TAR Compression

#### Basic example

This is an example of writing and then reading a tar file:

    {ok,HandleWrite} = ssh_sftp:open_tar(ChannelPid, ?tar_file_name, [write]),
    ok = erl_tar:add(HandleWrite, .... ),
    ok = erl_tar:add(HandleWrite, .... ),
    ...
    ok = erl_tar:add(HandleWrite, .... ),
    ok = erl_tar:close(HandleWrite),

    %% And for reading
    {ok,HandleRead} = ssh_sftp:open_tar(ChannelPid, ?tar_file_name, [read]),
    {ok,NameValueList} = erl_tar:extract(HandleRead,[memory]),
    ok = erl_tar:close(HandleRead),

#### Example with encryption

The previous Basic example can be extended with encryption and decryption as follows:

    %% First three parameters depending on which crypto type we select:
    Key = <<"This is a 256 bit key. abcdefghi">>,
    Ivec0 = crypto:strong_rand_bytes(16),
    DataSize = 1024,  % DataSize rem 16 = 0 for aes_cbc

    %% Initialization of the CryptoState, in this case it is the Ivector.
    InitFun = fun() -> {ok, Ivec0, DataSize} end,

    %% How to encrypt:
    EncryptFun =
        fun(PlainBin,Ivec) ->
            EncryptedBin = crypto:block_encrypt(aes_cbc256, Key, Ivec, PlainBin),
            {ok, EncryptedBin, crypto:next_iv(aes_cbc,EncryptedBin)}
        end,

    %% What to do with the very last block:
    CloseFun =
        fun(PlainBin, Ivec) ->
            EncryptedBin = crypto:block_encrypt(aes_cbc256, Key, Ivec,
                                                pad(16,PlainBin) %% Last chunk
                                               ),
           {ok, EncryptedBin}
        end,

    Cw = {InitFun,EncryptFun,CloseFun},
    {ok,HandleWrite} = ssh_sftp:open_tar(ChannelPid, ?tar_file_name, [write,{crypto,Cw}]),
    ok = erl_tar:add(HandleWrite, .... ),
    ok = erl_tar:add(HandleWrite, .... ),
    ...
    ok = erl_tar:add(HandleWrite, .... ),
    ok = erl_tar:close(HandleWrite),

    %% And for decryption (in this crypto example we could use the same InitFun
    %% as for encryption):
    DecryptFun =
        fun(EncryptedBin,Ivec) ->
            PlainBin = crypto:block_decrypt(aes_cbc256, Key, Ivec, EncryptedBin),
           {ok, PlainBin, crypto:next_iv(aes_cbc,EncryptedBin)}
        end,

    Cr = {InitFun,DecryptFun},
    {ok,HandleRead} = ssh_sftp:open_tar(ChannelPid, ?tar_file_name, [read,{crypto,Cw}]),
    {ok,NameValueList} = erl_tar:extract(HandleRead,[memory]),
    ok = erl_tar:close(HandleRead),

### 2.8 Creating a Subsystem

A small ssh subsystem that echoes N bytes can be implemented as shown in the following example:

    -module(ssh_echo_server).
    -behaviour(ssh_server_channel). % replaces ssh_daemon_channel
    -record(state, {
    	  n,
    	  id,
    	  cm
    	 }).
    -export([init/1, handle_msg/2, handle_ssh_msg/2, terminate/2]).

    init([N]) ->
        {ok, #state{n = N}}.

    handle_msg({ssh_channel_up, ChannelId, ConnectionManager}, State) ->
        {ok, State#state{id = ChannelId,
    		     cm = ConnectionManager}}.

    handle_ssh_msg({ssh_cm, CM, {data, ChannelId, 0, Data}}, #state{n = N} = State) ->
        M = N - size(Data),
        case M > 0 of
    	true ->
    	   ssh_connection:send(CM, ChannelId, Data),
    	   {ok, State#state{n = M}};
    	false ->
    	   <<SendData:N/binary, _/binary>> = Data,
               ssh_connection:send(CM, ChannelId, SendData),
               ssh_connection:send_eof(CM, ChannelId),
    	   {stop, ChannelId, State}
        end;
    handle_ssh_msg({ssh_cm, _ConnectionManager,
    		{data, _ChannelId, 1, Data}}, State) ->
        error_logger:format(standard_error, " ~p~n", [binary_to_list(Data)]),
        {ok, State};

    handle_ssh_msg({ssh_cm, _ConnectionManager, {eof, _ChannelId}}, State) ->
        {ok, State};

    handle_ssh_msg({ssh_cm, _, {signal, _, _}}, State) ->
        %% Ignore signals according to RFC 4254 section 6.9.
        {ok, State};

    handle_ssh_msg({ssh_cm, _, {exit_signal, ChannelId, _, _Error, _}},
    	       State) ->
        {stop, ChannelId,  State};

    handle_ssh_msg({ssh_cm, _, {exit_status, ChannelId, _Status}}, State) ->
        {stop, ChannelId, State}.

    terminate(_Reason, _State) ->
        ok.

The subsystem can be run on the host tarlop with the generated keys, as described in Section Running an Erlang ssh Daemon:

    1> ssh:start().
    ok
    2> ssh:daemon(8989, [{system_dir, "/tmp/ssh_daemon"},
                         {user_dir, "/tmp/otptest_user/.ssh"}
                         {subsystems, [{"echo_n", {ssh_echo_server, [10]}}]}]).
    {ok,<0.54.0>}
    3>

    1> ssh:start().
    ok
    2> {ok, ConnectionRef} = ssh:connect("tarlop", 8989,
                                        [{user_dir, "/tmp/otptest_user/.ssh"}]).
     {ok,<0.57.0>}
    3> {ok, ChannelId} = ssh_connection:session_channel(ConnectionRef, infinity).
    4> success = ssh_connection:subsystem(ConnectionRef, ChannelId, "echo_n", infinity).
    5> ok = ssh_connection:send(ConnectionRef, ChannelId, "0123456789", infinity).
    6> flush().
    {ssh_msg, <0.57.0>, {data, 0, 1, "0123456789"}}
    {ssh_msg, <0.57.0>, {eof, 0}}
    {ssh_msg, <0.57.0>, {closed, 0}}
    7> {error, closed} = ssh_connection:send(ConnectionRef, ChannelId, "10", infinity).

See also ssh_client_channel(3) (replaces ssh_channel(3)).