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NAME

       ssh - Main API of the ssh application

DESCRIPTION

       This is the interface module for the SSH application. The Secure Shell (SSH) Protocol is a
       protocol for secure remote login and  other  secure  network  services  over  an  insecure
       network.  See  ssh(7)  for  details  of  supported  RFCs, versions, algorithms and unicode
       handling.

       With the SSH application it is possible to start clients  and to start daemons  (servers).

       Clients are started with  connect/2,  connect/3  or  connect/4.  They  open  an  encrypted
       connection  on  top  of TCP/IP. In that encrypted connection one or more channels could be
       opened with ssh_connection:session_channel/2,4.

       Each channel is an isolated  "pipe"  between  a  client-side  process  and  a  server-side
       process.  Those  process  pairs  could  handle for example file transfers (sftp) or remote
       command execution (shell, exec and/or cli). If a custom shell is implemented, the user  of
       the  client  could  execute  the  special  commands  remotely.  Note  that the user is not
       necessarily a human but probably a system interfacing the SSH app.

       A  server-side  subssystem  (channel)   server   is   requested   by   the   client   with
       ssh_connection:subsystem/4.

       A  server  (daemon)  is  started  with  daemon/1,  daemon/2  or daemon/3. Possible channel
       handlers (subsystems) are declared with the subsystem option when the daemon is started.

       To just run a shell on a remote machine, there are functions that bundles the needed three
       steps  needed  into  one:  shell/1,2,3.  Similarily,  to just open an sftp (file transfer)
       connection to a remote machine, the simplest way is to use ssh_sftp:start_channel/1,2,3.

       To write your own client channel handler, use the behaviour ssh_client_channel. For server
       channel handlers use ssh_server_channel behaviour (replaces ssh_daemon_channel).

       Both  clients  and daemons accepts options that controls the exact behaviour. Some options
       are common to both. The three sets are called Client Options, Daemon  Options  and  Common
       Options.

       The descriptions of the options uses the Erlang Type Language with explaining text.

   Note:
       The User's Guide has examples and a Getting Started section.

KEYS AND FILES

       A number of objects must be present for the SSH application to work. Those objects are per
       default stored in files. The default names, paths and file formats are  the  same  as  for
       OpenSSH.  Keys could be generated with the ssh-keygen program from OpenSSH. See the User's
       Guide.

       The paths could easily be changed by options: user_dir and system_dir.

       A completly different storage could be interfaced by writing call-back modules  using  the
       behaviours  ssh_client_key_api  and/or  ssh_server_key_api. A callback module is installed
       with the option key_cb to the client and/or the daemon.

   Daemons
       The keys are by default stored in files:

         * Mandatory: one or more Host key(s) , both private and public. Default is to store them
           in the directory /etc/ssh in the files

           * ssh_host_dsa_key and ssh_host_dsa_key.pub

           * ssh_host_rsa_key and ssh_host_rsa_key.pub

           * ssh_host_ecdsa_key and ssh_host_ecdsa_key.pub

           The host keys directory could be changed with the option system_dir.

         * Optional:  one  or more User's public key  in case of publickey authorization. Default
           is to store them concatenated in the file  .ssh/authorized_keys  in  the  user's  home
           directory.

           The user keys directory could be changed with the option user_dir.

   Clients
       The  keys  and some other data are by default stored in files in the directory .ssh in the
       user's home directory.

       The directory could be changed with the option user_dir.

         * Optional: a list of Host public key(s)  for previously connected hosts. This  list  is
           handled  by the SSH application without any need of user assistance. The default is to
           store them in the file known_hosts.

           The host_accepting_client_options() are associated with this list of keys.

         * Optional: one or more User's private key(s)  in case of publickey  authorization.  The
           default files are

           * id_dsa and id_dsa.pub

           * id_rsa and id_rsa.pub

           * id_ecdsa and id_ecdsa.pub

DATA TYPES

   Client Options
       client_options() = [client_option()]

       client_option() =
           ssh_file:pubkey_passphrase_client_options() |
           host_accepting_client_options() |
           authentication_client_options() |
           diffie_hellman_group_exchange_client_option() |
           connect_timeout_client_option() |
           recv_ext_info_client_option() |
           opaque_client_options() |
           gen_tcp:connect_option() |
           common_option()

              Options  for  clients.  The  individual  options  are further explained below or by
              following the hyperlinks.

              Note that not every gen_tcp:connect_option() is accepted. See set_sock_opts/2 for a
              list of prohibited options.

              Also  note that setting a gen_tcp:connect_option() could change the socket in a way
              that impacts the ssh client's behaviour negatively. You use it on your own risk.

       host_accepting_client_options() =
           {silently_accept_hosts, accept_hosts()} |
           {user_interaction, boolean()} |
           {save_accepted_host, boolean()} |
           {quiet_mode, boolean()}

       accept_hosts() =
           boolean() |
           accept_callback() |
           {HashAlgoSpec :: fp_digest_alg(), accept_callback()}

       fp_digest_alg() = md5 | crypto:sha1() | crypto:sha2()

       accept_callback() =
           fun((PeerName :: string(), fingerprint()) -> boolean()) |
           fun((PeerName :: string(),
                Port :: inet:port_number(),
                fingerprint()) ->
                   boolean())

       fingerprint() = string() | [string()]

                silently_accept_hosts:
                  This option guides the connect function on how to act when the connected server
                  presents  a Host Key that the client has not seen before. The default is to ask
                  the user with a question on stdio of whether to accept or reject the  new  Host
                  Key.  See  the  option user_dir for specifying the path to the file known_hosts
                  where previously accepted Host Keys are recorded. See also  the  option  key_cb
                  for the general way to handle keys.

                  The option can be given in three different forms as seen above:

                  * The  value  is  a  boolean().  The value true will make the client accept any
                    unknown Host Key without any user interaction. The value false preserves  the
                    default behaviour of asking the user on stdio.

                  * An  accept_callback()  will  be called and the boolean return value true will
                    make the client accept the Host Key. A return value of false  will  make  the
                    client  to reject the Host Key and as a result the connection will be closed.
                    The arguments to the fun are:

                    * PeerName - a string with the name or address of the remote host.

                    * FingerPrint    -    the    fingerprint    of    the     Host     Key     as
                      public_key:ssh_hostkey_fingerprint/1 calculates it.

                  * A  tuple  {HashAlgoSpec,  accept_callback}.  The HashAlgoSpec specifies which
                    hash algorithm shall be used to calculate the fingerprint used in the call of
                    the  accept_callback(). The HashALgoSpec is either an atom or a list of atoms
                    as the first argument in public_key:ssh_hostkey_fingerprint/2.  If  it  is  a
                    list   of   hash   algorithm   names,   the   FingerPrint   argument  in  the
                    accept_callback() will be a list of fingerprints in the  same  order  as  the
                    corresponding name in the HashAlgoSpec list.

                user_interaction:
                  If  false, disables the client to connect to the server if any user interaction
                  is needed, such as accepting the server to be added to the known_hosts file, or
                  supplying a password.

                  Even if user interaction is allowed it can be suppressed by other options, such
                  as silently_accept_hosts and password. However, those options  are  not  always
                  desirable to use from a security point of view.

                  Defaults to true.

                save_accepted_host:
                  If true, the client saves an accepted host key to avoid the accept question the
                  next time the same host is connected. If the option key_cb is not present,  the
                  key is saved in the file "known_hosts". See option user_dir for the location of
                  that file.

                  If false, the key is not saved and the key will still be unknown  at  the  next
                  access of the same host.

                  Defaults to true

                quiet_mode:
                  If true, the client does not print anything on authorization.

                  Defaults to false

       authentication_client_options() =
           {user, string()} | {password, string()}

                user:
                  Provides  the  username.  If  this  option  is  not  given,  ssh reads from the
                  environment (LOGNAME or USER on UNIX, USERNAME on Windows).

                password:
                  Provides a password for password authentication. If this option is  not  given,
                  the  user  is  asked  for  a password, if the password authentication method is
                  attempted.

       diffie_hellman_group_exchange_client_option() =
           {dh_gex_limits,
            {Min :: integer() >= 1,
             I :: integer() >= 1,
             Max :: integer() >= 1}}

              Sets the three diffie-hellman-group-exchange parameters that guides  the  connected
              server  in  choosing  a  group.  See RFC 4419 for the details. The default value is
              {1024, 6144, 8192}.

       connect_timeout_client_option() = {connect_timeout, timeout()}

              Sets a timeout on the transport layer connect time. For  gen_tcp  the  time  is  in
              milli-seconds and the default value is infinity.

              See the parameter Timeout in connect/4 for a timeout of the negotiation phase.

       recv_ext_info_client_option() = {recv_ext_info, boolean()}

              Make the client tell the server that the client accepts extension negotiation, that
              is, include ext-info-c in the kexinit message sent. See RFC 8308  for  details  and
              ssh(7) for a list of currently implemented extensions.

              Default value is true which is compatible with other implementations not supporting
              ext-info.

   Daemon Options (Server Options)
       daemon_options() = [daemon_option()]

       daemon_option() =
           subsystem_daemon_option() |
           shell_daemon_option() |
           exec_daemon_option() |
           ssh_cli_daemon_option() |
           tcpip_tunnel_out_daemon_option() |
           tcpip_tunnel_in_daemon_option() |
           authentication_daemon_options() |
           diffie_hellman_group_exchange_daemon_option() |
           negotiation_timeout_daemon_option() |
           hello_timeout_daemon_option() |
           hardening_daemon_options() |
           callbacks_daemon_options() |
           send_ext_info_daemon_option() |
           opaque_daemon_options() |
           gen_tcp:listen_option() |
           common_option()

              Options for daemons. The individual options  are  further  explained  below  or  by
              following the hyperlinks.

              Note  that not every gen_tcp:listen_option() is accepted. See set_sock_opts/2 for a
              list of prohibited options.

              Also note that setting a gen_tcp:listen_option() could change the socket in  a  way
              that impacts the ssh deamon's behaviour negatively. You use it on your own risk.

       subsystem_daemon_option() = {subsystems, subsystem_specs()}

       subsystem_specs() = [subsystem_spec()]

       subsystem_spec() = {Name :: string(), mod_args()}

              Defines a subsystem in the daemon.

              The  subsystem_name  is  the  name that a client requests to start with for example
              ssh_connection:subsystem/4.

              The channel_callback is the module that implements the ssh_server_channel (replaces
              ssh_daemon_channel)  behaviour  in the daemon. See the section Creating a Subsystem
              in the User's Guide for more information and an example.

              If the subsystems option is not present, the value of  ssh_sftpd:subsystem_spec([])
              is  used.  This enables the sftp subsystem by default. The option can be set to the
              empty list if you do not want the daemon to run any subsystems.

       shell_daemon_option() = {shell, shell_spec()}

       shell_spec() = mod_fun_args() | shell_fun() | disabled

       shell_fun() = 'shell_fun/1'() | 'shell_fun/2'()

       'shell_fun/1'() = fun((User :: string()) -> pid())

       'shell_fun/2'() =
           fun((User :: string(), PeerAddr :: inet:ip_address()) -> pid())

              Defines the read-eval-print loop used in a daemon when a shell is requested by  the
              client. The default is to use the Erlang shell: {shell, start, []}

              See  the  option  exec-option  for  a description of how the daemon executes shell-
              requests and exec-requests depending on the shell- and exec-options.

       exec_daemon_option() = {exec, exec_spec()}

       exec_spec() =
           {direct, exec_fun()} | disabled | deprecated_exec_opt()

       exec_fun() = 'exec_fun/1'() | 'exec_fun/2'() | 'exec_fun/3'()

       'exec_fun/1'() = fun((Cmd :: string()) -> exec_result())

       'exec_fun/2'() =
           fun((Cmd :: string(), User :: string()) -> exec_result())

       'exec_fun/3'() =
           fun((Cmd :: string(),
                User :: string(),
                ClientAddr :: ip_port()) ->
                   exec_result())

       exec_result() =
           {ok, Result :: term()} | {error, Reason :: term()}

              This option changes how the daemon executes exec-requests from clients. The term in
              the  return  value is formatted to a string if it is a non-string type. No trailing
              newline is added in the ok-case.

              See the User's Guide section on One-Time Execution for examples.

              Error texts are returned on channel-type 1 which usually is piped to stderr on  e.g
              Linux systems. Texts from a successful execution are returned on channel-type 0 and
              will in similar manner be piped to stdout. The exit-status code is  set  to  0  for
              success  and 255 for errors. The exact results presented on the client side depends
              on the client and the client's operating system.

              In case of the {direct, exec_fun()} variant or no exec-option  at  all,  all  reads
              from standard_input will be from the received data-events of type 0. Those are sent
              by the client. Similarily all writes to standard_output will be sent as data-events
              to  the client. An OS shell client like the command 'ssh' will usally use stdin and
              stdout for the user interface.

              The option cooperates with the daemon-option shell in the following way:

                1. If neither the exec-option nor the shell-option is present::
                  The default Erlang evaluator is used both for  exec  and  shell  requests.  The
                  result is returned to the client.

                2.  If  the  exec_spec's  value  is  disabled (the shell-option may or may not be
                present)::
                  No exec-requests are executed but shell-requests are not affected, they  follow
                  the shell_spec's value.

                3. If the exec-option is present and the exec_spec value =/= disabled (the shell-
                option may or may not be present)::
                  The exec_spec fun() is called with the same number of parameters as  the  arity
                  of  the  fun,  and the result is returned to the client. Shell-requests are not
                  affected, they follow the shell_spec's value.

                4. If the exec-option is absent, and the shell-option is present with the default
                Erlang shell as the shell_spec's value::
                  The  default  Erlang  evaluator  is  used both for exec and shell requests. The
                  result is returned to the client.

                5. If the exec-option is absent, and the shell-option is  present  with  a  value
                that is neither the default Erlang shell nor the value disabled::
                  The  exec-request  is  not  evaluated  and  an error message is returned to the
                  client. Shell-requests are executed according to the value of the shell_spec.

                6. If the exec-option is absent, and the shell_spec's value is disabled::
                  Exec requests are executed by the default shell,  but  shell-requests  are  not
                  executed.

              If  a custom CLI is installed (see the option ssh_cli) the rules above are replaced
              by thoose implied by the custom CLI.

          Note:
              The exec-option has existed for a long time but has not previously been documented.
              The  old  definition  and  behaviour  are  retained but obey the rules 1-6 above if
              conflicting. The old and undocumented style should not be used in new programs.

       deprecated_exec_opt() = function() | mod_fun_args()

              Old-style exec specification that are kept for compatibility,  but  should  not  be
              used in new programs

       ssh_cli_daemon_option() = {ssh_cli, mod_args() | no_cli}

              Provides your own CLI implementation in a daemon.

              It  is a channel callback module that implements a shell and command execution. The
              shell's read-eval-print loop can be customized, using the option shell. This  means
              less  work  than  implementing an own CLI channel. If ssh_cli is set to no_cli, the
              CLI channels like shell and exec are  disabled  and  only  subsystem  channels  are
              allowed.

       authentication_daemon_options() =
           ssh_file:system_dir_daemon_option() |
           {auth_method_kb_interactive_data, prompt_texts()} |
           {user_passwords, [{UserName :: string(), Pwd :: string()}]} |
           {pk_check_user, boolean()} |
           {password, string()} |
           {pwdfun, pwdfun_2() | pwdfun_4()}

       prompt_texts() =
           kb_int_tuple() | kb_int_fun_3() | kb_int_fun_4()

       kb_int_tuple() =
           {Name :: string(),
            Instruction :: string(),
            Prompt :: string(),
            Echo :: boolean()}

       kb_int_fun_3() =
           fun((Peer :: ip_port(), User :: string(), Service :: string()) ->
                   kb_int_tuple())

       kb_int_fun_4() =
           fun((Peer :: ip_port(),
                User :: string(),
                Service :: string(),
                State :: any()) ->
                   kb_int_tuple())

       pwdfun_2() =
           fun((User :: string(), Password :: string() | pubkey) ->
                   boolean())

       pwdfun_4() =
           fun((User :: string(),
                Password :: string() | pubkey,
                PeerAddress :: ip_port(),
                State :: any()) ->
                   boolean() |
                   disconnect |
                   {boolean(), NewState :: any()})

                auth_method_kb_interactive_data:
                  Sets  the  text strings that the daemon sends to the client for presentation to
                  the user when using keyboard-interactive authentication.

                  If the fun/3 or fun/4 is used, it is  called  when  the  actual  authentication
                  occurs and may therefore return dynamic data like time, remote ip etc.

                  The parameter Echo guides the client about need to hide the password.

                  The  default  value is: {auth_method_kb_interactive_data, {"SSH server", "Enter
                  password for \""++User++"\"", "password: ", false}>

                user_passwords:
                  Provides passwords for password authentication. The  passwords  are  used  when
                  someone  tries  to  connect  to  the  server and public key user-authentication
                  fails. The option provides a list of  valid  usernames  and  the  corresponding
                  passwords.

                pk_check_user:
                  Enables  checking of the client's user name in the server when doing public key
                  authentication. It is disabled by default.

                  The term "user" is used differently in OpenSSH and SSH in Erlang/OTP: see  more
                  in the User's Guide.

                  If  the option is enabled, and no pwdfun is present, the user name must present
                  in the user_passwords for the check to succeed but the value of the password is
                  not checked.

                  In  case  of a pwdfun checking the user, the atom pubkey is put in the password
                  argument.

                password:
                  Provides a global password that authenticates any user.

            Warning:
                Intended to facilitate testing.

                From a security perspective this option makes the server very vulnerable.

                pwdfun with pwdfun_4():
                  Provides a function for password validation. This could  used  for  calling  an
                  external system or handeling passwords stored as hash values.

                  This fun can also be used to make delays in authentication tries for example by
                  calling timer:sleep/1.

                  To facilitate for instance counting of failed tries, the State  variable  could
                  be used. This state is per connection only. The first time the pwdfun is called
                  for a connection, the State variable has the value undefined.

                  The fun should return:

                  * true if the user and password is valid

                  * false if the user or password is invalid

                  * disconnect if a SSH_MSG_DISCONNECT message should  be  sent  immediately.  It
                    will be followed by a close of the underlying tcp connection.

                  * {true, NewState:any()} if the user and password is valid

                  * {false, NewState:any()} if the user or password is invalid

                  A  third  usage  is to block login attempts from a missbehaving peer. The State
                  described above can be used for this. The return value disconnect is useful for
                  this.

                  In  case  of  the  pk_check_user is set, the atom pubkey is put in the password
                  argument when validating a public key login. The pwdfun is then responsible  to
                  check that the user name is valid.

                pwdfun with pwdfun_2():
                  Provides  a function for password validation. This function is called with user
                  and password as strings, and returns:

                  * true if the user and password is valid

                  * false if the user or password is invalid

                  In case of the pk_check_user is set, the atom pubkey is  put  in  the  password
                  argument  when validating a public key login. The pwdfun is then responsible to
                  check that the user name is valid.

                  This variant is kept for compatibility.

       diffie_hellman_group_exchange_daemon_option() =
           {dh_gex_groups,
            [explicit_group()] |
            explicit_group_file() |
            ssh_moduli_file()} |
           {dh_gex_limits, {Min :: integer() >= 1, Max :: integer() >= 1}}

       explicit_group() =
           {Size :: integer() >= 1,
            G :: integer() >= 1,
            P :: integer() >= 1}

       explicit_group_file() = {file, string()}

       ssh_moduli_file() = {ssh_moduli_file, string()}

                dh_gex_groups:
                  Defines the groups the  server  may  choose  among  when  diffie-hellman-group-
                  exchange  is  negotiated.  See RFC 4419 for details. The three variants of this
                  option are:

                  {Size=integer(),G=integer(),P=integer()}:
                    The groups are given explicitly in this list. There may be  several  elements
                    with  the  same  Size. In such a case, the server will choose one randomly in
                    the negotiated Size.

                  {file,filename()}:
                    The     file      must      have      one      or      more      three-tuples
                    {Size=integer(),G=integer(),P=integer()}  terminated  by  a  dot. The file is
                    read when the daemon starts.

                  {ssh_moduli_file,filename()}:
                    The file must be in ssh-keygen moduli file format. The file is read when  the
                    daemon starts.

                  The default list is fetched from the public_key application.

                dh_gex_limits:
                  Limits  what  a client can ask for in diffie-hellman-group-exchange. The limits
                  will be {MaxUsed =  min(MaxClient,Max),  MinUsed  =  max(MinClient,Min)}  where
                  MaxClient and MinClient are the values proposed by a connecting client.

                  The default value is {0,infinity}.

                  If MaxUsed < MinUsed in a key exchange, it will fail with a disconnect.

                  See RFC 4419 for the function of the Max and Min values.

       hello_timeout_daemon_option() = {hello_timeout, timeout()}

              Maximum time in milliseconds for the first part of the ssh session setup, the hello
              message exchange. Defaults to 30000 ms (30 seconds). If the client  fails  to  send
              the first message within this time, the connection is closed.

       negotiation_timeout_daemon_option() =
           {negotiation_timeout, timeout()}

              Maximum time in milliseconds for the authentication negotiation. Defaults to 120000
              ms (2 minutes). If the client fails to log in within this time, the  connection  is
              closed.

       hardening_daemon_options() =
           {max_sessions, integer() >= 1} |
           {max_channels, integer() >= 1} |
           {parallel_login, boolean()} |
           {minimal_remote_max_packet_size, integer() >= 1}

                max_sessions:
                  The  maximum  number of simultaneous sessions that are accepted at any time for
                  this daemon. This includes sessions that are being authorized. Thus, if set  to
                  N,  and  N clients have connected but not started the login process, connection
                  attempt N+1 is aborted. If N connections are authenticated and still logged in,
                  no more logins are accepted until one of the existing ones log out.

                  The  counter  is per listening port. Thus, if two daemons are started, one with
                  {max_sessions,N} and the other with {max_sessions,M}, in total N+M  connections
                  are accepted for the whole ssh application.

                  Notice that if parallel_login is false, only one client at a time can be in the
                  authentication phase.

                  By default, this option is not set. This means that the number is not limited.

                max_channels:
                  The maximum number of channels with active remote subsystem that  are  accepted
                  for each connection to this daemon

                  By default, this option is not set. This means that the number is not limited.

                parallel_login:
                  If  set  to  false (the default value), only one login is handled at a time. If
                  set to true, an unlimited number of login attempts are allowed simultaneously.

                  If the max_sessions option is set to N and parallel_login is set to  true,  the
                  maximum  number  of  simultaneous login attempts at any time is limited to N-K,
                  where K is the number of authenticated connections present at this daemon.

            Warning:
                Do not enable parallel_logins without protecting the server by other  means,  for
                example,  by the max_sessions option or a firewall configuration. If set to true,
                there is no protection against DOS attacks.

                minimal_remote_max_packet_size:
                  The least maximum packet size that the  daemon  will  accept  in  channel  open
                  requests from the client. The default value is 0.

       callbacks_daemon_options() =
           {failfun,
            fun((User :: string(),
                 PeerAddress :: inet:ip_address(),
                 Reason :: term()) ->
                    term())} |
           {connectfun,
            fun((User :: string(),
                 PeerAddress :: inet:ip_address(),
                 Method :: string()) ->
                    term())}

                connectfun:
                  Provides  a  fun to implement your own logging when a user authenticates to the
                  server.

                failfun:
                  Provides a fun to implement your own logging when a user fails to authenticate.

       send_ext_info_daemon_option() = {send_ext_info, boolean()}

              Make the server  (daemon)  tell  the  client  that  the  server  accepts  extension
              negotiation,  that is, include ext-info-s in the kexinit message sent. See RFC 8308
              for details and ssh(7) for a list of currently implemented extensions.

              Default value is true which is compatible with other implementations not supporting
              ext-info.

       tcpip_tunnel_in_daemon_option() = {tcpip_tunnel_in, boolean()}

              Enables (true) or disables (false) the possibility to tunnel a TCP/IP connection in
              to a server. Disabled per default.

       tcpip_tunnel_out_daemon_option() =
           {tcpip_tunnel_out, boolean()}

              Enables (true) or disables (false) the possibility to tunnel  a  TCP/IP  connection
              out of a server. Disabled per default.

   Options common to clients and daemons
       common_options() = [common_option()]

       common_option() =
           ssh_file:user_dir_common_option() |
           profile_common_option() |
           max_idle_time_common_option() |
           key_cb_common_option() |
           disconnectfun_common_option() |
           unexpectedfun_common_option() |
           ssh_msg_debug_fun_common_option() |
           rekey_limit_common_option() |
           id_string_common_option() |
           pref_public_key_algs_common_option() |
           preferred_algorithms_common_option() |
           modify_algorithms_common_option() |
           auth_methods_common_option() |
           inet_common_option() |
           fd_common_option()

              The  options  above  can be used both in clients and in daemons (servers). They are
              further explained below.

       profile_common_option() = {profile, atom()}

              Used together with ip-address and port to uniquely identify a ssh daemon. This  can
              be  useful  in  a  virtualized environment, where there can be more that one server
              that has the same ip-address and port. If this property is not explicitly  set,  it
              is assumed that the the ip-address and port uniquely identifies the SSH daemon.

       max_idle_time_common_option() = {idle_time, timeout()}

              Sets  a  time-out  on a connection when no channels are open. Defaults to infinity.
              The unit is milliseconds.

              The timeout is not active until channels are started, so it does not limit the time
              from the connection creation to the first channel opening.

       rekey_limit_common_option() =
           {rekey_limit,
            Bytes ::
                limit_bytes() |
                {Minutes :: limit_time(), Bytes :: limit_bytes()}}

       limit_bytes() = integer() >= 0 | infinity

       limit_time() = integer() >= 1 | infinity

              Sets  the  limit when rekeying is to be initiated. Both the max time and max amount
              of data could be configured:

                * {Minutes, Bytes} initiate rekeying when any of the limits are reached.

                * Bytes initiate rekeying when Bytes number  of  bytes  are  transferred,  or  at
                  latest after one hour.

              When  a  rekeying  is  done,  both  the  timer  and the byte counter are restarted.
              Defaults to one hour and one GByte.

              If Minutes is set to infinity, no rekeying will ever occur due to that max time has
              passed.  Setting  Bytes to infinity will inhibit rekeying after a certain amount of
              data has been transferred. If the option value is set to {infinity,  infinity},  no
              rekeying  will be initiated. Note that rekeying initiated by the peer will still be
              performed.

       key_cb_common_option() =
           {key_cb,
            Module :: atom() | {Module :: atom(), Opts :: [term()]}}

              Module implementing the behaviour ssh_client_key_api and/or ssh_server_key_api. Can
              be  used to customize the handling of public keys. If callback options are provided
              along with the module name, they are made available to the callback module via  the
              options passed to it under the key 'key_cb_private'.

              The Opts defaults to [] when only the Module is specified.

              The  default  value  of  this  option  is  {ssh_file,  []}. See also the manpage of
              ssh_file.

              A call to the call-back function F will be

                     Module:F(..., [{key_cb_private,Opts}|UserOptions])

              where ... are arguments to F as in  ssh_client_key_api  and/or  ssh_server_key_api.
              The UserOptions are the options given to ssh:connect, ssh:shell or ssh:daemon.

       pref_public_key_algs_common_option() =
           {pref_public_key_algs, [pubkey_alg()]}

              List of user (client) public key algorithms to try to use.

              The   default   value   is   the   public_key   entry   in  the  list  returned  by
              ssh:default_algorithms/0.

              If there is no public key of a specified type available, the corresponding entry is
              ignored.  Note  that the available set is dependent on the underlying cryptolib and
              current user's public keys.

              See also the option user_dir for specifying the path to the user's keys.

       disconnectfun_common_option() =
           {disconnectfun, fun((Reason :: term()) -> void | any())}

              Provides a fun to implement your own logging or other handling at disconnects.

       unexpectedfun_common_option() =
           {unexpectedfun,
            fun((Message :: term(), {Host :: term(), Port :: term()}) ->
                    report | skip)}

              Provides a fun to implement your own logging or other  action  when  an  unexpected
              message  arrives.  If the fun returns report the usual info report is issued but if
              skip is returned no report is generated.

       ssh_msg_debug_fun_common_option() =
           {ssh_msg_debug_fun,
            fun((ssh:connection_ref(),
                 AlwaysDisplay :: boolean(),
                 Msg :: binary(),
                 LanguageTag :: binary()) ->
                    any())}

              Provide a fun to implement your own logging of the SSH message  SSH_MSG_DEBUG.  The
              last  three  parameters  are  from  the  message,  see  RFC 4253, section 11.3. The
              connection_ref() is the reference to the connection on which the  message  arrived.
              The return value from the fun is not checked.

              The  default  behaviour  is  ignore the message. To get a printout for each message
              with AlwaysDisplay = true, use for  example  {ssh_msg_debug_fun,  fun(_,true,M,_)->
              io:format("DEBUG: ~p~n", [M]) end}

       id_string_common_option() =
           {id_string,
            string() |
            random |
            {random, Nmin :: integer() >= 1, Nmax :: integer() >= 1}}

              The  string  the  daemon  will  present to a connecting peer initially. The default
              value is "Erlang/VSN" where VSN is the ssh application version number.

              The value random will cause a random  string  to  be  created  at  each  connection
              attempt.  This  is to make it a bit more difficult for a malicious peer to find the
              ssh software brand and version.

              The value {random, Nmin, Nmax} will  make  a  random  string  with  at  least  Nmin
              characters and at most Nmax characters.

       preferred_algorithms_common_option() =
           {preferred_algorithms, algs_list()}

       algs_list() = [alg_entry()]

       alg_entry() =
           {kex, [kex_alg()]} |
           {public_key, [pubkey_alg()]} |
           {cipher, double_algs(cipher_alg())} |
           {mac, double_algs(mac_alg())} |
           {compression, double_algs(compression_alg())}

       kex_alg() =
           'diffie-hellman-group-exchange-sha1' |
           'diffie-hellman-group-exchange-sha256' |
           'diffie-hellman-group1-sha1' | 'diffie-hellman-group14-sha1' |
           'diffie-hellman-group14-sha256' |
           'diffie-hellman-group16-sha512' |
           'diffie-hellman-group18-sha512' | 'curve25519-sha256' |
           'curve25519-sha256@libssh.org' | 'curve448-sha512' |
           'ecdh-sha2-nistp256' | 'ecdh-sha2-nistp384' |
           'ecdh-sha2-nistp521'

       pubkey_alg() =
           'ecdsa-sha2-nistp256' | 'ecdsa-sha2-nistp384' |
           'ecdsa-sha2-nistp521' | 'ssh-ed25519' | 'ssh-ed448' |
           'rsa-sha2-256' | 'rsa-sha2-512' | 'ssh-dss' | 'ssh-rsa'

       cipher_alg() =
           '3des-cbc' | 'AEAD_AES_128_GCM' | 'AEAD_AES_256_GCM' |
           'aes128-cbc' | 'aes128-ctr' | 'aes128-gcm@openssh.com' |
           'aes192-ctr' | 'aes192-cbc' | 'aes256-cbc' | 'aes256-ctr' |
           'aes256-gcm@openssh.com' | 'chacha20-poly1305@openssh.com'

       mac_alg() =
           'AEAD_AES_128_GCM' | 'AEAD_AES_256_GCM' | 'hmac-sha1' |
           'hmac-sha1-etm@openssh.com' | 'hmac-sha1-96' |
           'hmac-sha2-256' | 'hmac-sha2-512' |
           'hmac-sha2-256-etm@openssh.com' |
           'hmac-sha2-512-etm@openssh.com'

       compression_alg() = none | zlib | 'zlib@openssh.com'

       double_algs(AlgType) =
           [{client2server, [AlgType]} | {server2client, [AlgType]}] |
           [AlgType]

              List of algorithms to use in the algorithm negotiation. The default algs_list() can
              be obtained from default_algorithms/0.

              If an alg_entry() is missing in the algs_list(), the default value is used for that
              entry.

              Here is an example of this option:

                     {preferred_algorithms,
                     [{public_key,['ssh-rsa','ssh-dss']},
                     {cipher,[{client2server,['aes128-ctr']},
                        {server2client,['aes128-cbc','3des-cbc']}]},
                     {mac,['hmac-sha2-256','hmac-sha1']},
                     {compression,[none,zlib]}
                     ]
                     }

              The example specifies different algorithms in the two directions (client2server and
              server2client),  for  cipher  but  specifies  the  same  algorithms  for  mac   and
              compression  in  both directions. The kex (key exchange) is implicit but public_key
              is set explicitly.

              For background and more examples see the User's Guide.

              If an algorithm name occurs more than once in a list, the behaviour  is  undefined.
              The tags in the property lists are also assumed to occur at most one time.

          Warning:
              Changing  the  values  can  make a connection less secure. Do not change unless you
              know exactly what you are doing. If you do not understand the values then  you  are
              not supposed to change them.

       modify_algorithms_common_option() =
           {modify_algorithms, modify_algs_list()}

       modify_algs_list() =
           [{append, algs_list()} |
            {prepend, algs_list()} |
            {rm, algs_list()}]

              Modifies  the  list  of  algorithms  to  use  in  the  algorithm  negotiation.  The
              modifications are applied after the option preferred_algorithms  (if  existing)  is
              applied.

              The algoritm for modifications works like this:

                * Input  is  the  modify_algs_list()  and a set of algorithms A obtained from the
                  preferred_algorithms    option    if    existing,    or    else    from     the
                  ssh:default_algorithms/0.

                * The head of the modify_algs_list() modifies A giving the result A'.

                  The possible modifications are:

                  * Append  or  prepend  supported  but  not  enabled algorithm(s) to the list of
                    algorithms. If the wanted algorithms already are in  A  they  will  first  be
                    removed and then appended or prepended,

                  * Remove (rm) one or more algorithms from A.

                * Repeat  the  modification  step  with  the  tail  of modify_algs_list() and the
                  resulting A'.

              If an unsupported algorithm is in  the  modify_algs_list(),  it  will  be  silently
              ignored

              If there are more than one modify_algorithms options, the result is undefined.

              Here is an example of this option:

                     {modify_algorithms,
                     [{prepend, [{kex, ['diffie-hellman-group1-sha1']}],
                     {rm,      [{compression, [none]}]}
                     ]
                     }

              The example specifies that:

                * the  old key exchange algorithm 'diffie-hellman-group1-sha1' should be the main
                  alternative. It will be the main alternative since it is prepened to the list

                * The compression algorithm none (= no compression) is removed so compression  is
                  enforced

              For background and more examples see the User's Guide.

       inet_common_option() = {inet, inet | inet6}

              IP version to use when the host address is specified as any.

       auth_methods_common_option() = {auth_methods, string()}

              Comma-separated string that determines which authentication methods that the client
              shall support and in which order they are tried. Defaults  to  "publickey,keyboard-
              interactive,password"

              Note that the client is free to use any order and to exclude methods.

       fd_common_option() = {fd, gen_tcp:socket()}

              Allows  an  existing  file-descriptor  to  be  used  (passed  on  to  the transport
              protocol).

   Other data types
       host() = string() | inet:ip_address() | loopback

       ip_port() = {inet:ip_address(), inet:port_number()}

       mod_args() = {Module :: atom(), Args :: list()}

       mod_fun_args() =
           {Module :: atom(), Function :: atom(), Args :: list()}

       open_socket() = gen_tcp:socket()

              The socket is supposed to be result of a gen_tcp:connect or a  gen_tcp:accept.  The
              socket must be in passive mode (that is, opened with the option {active,false}).

       daemon_ref()

              Opaque data type representing a daemon.

              Returned by the functions daemon/1,2,3.

       connection_ref()

              Opaque data type representing a connection between a client and a server (daemon).

              Returned by the functions connect/2,3,4 and ssh_sftp:start_channel/2,3.

       channel_id()

              Opaque data type representing a channel inside a connection.

              Returned by the functions ssh_connection:session_channel/2,4.

       connection_info_tuple() =
           {client_version, version()} |
           {server_version, version()} |
           {user, string()} |
           {peer, {inet:hostname(), ip_port()}} |
           {sockname, ip_port()} |
           {options, client_options()} |
           {algorithms, conn_info_algs()} |
           {channels, conn_info_channels()}

       version() = {protocol_version(), software_version()}

       protocol_version() =
           {Major :: integer() >= 1, Minor :: integer() >= 0}

       software_version() = string()

       conn_info_algs() =
           [{kex, kex_alg()} |
            {hkey, pubkey_alg()} |
            {encrypt, cipher_alg()} |
            {decrypt, cipher_alg()} |
            {send_mac, mac_alg()} |
            {recv_mac, mac_alg()} |
            {compress, compression_alg()} |
            {decompress, compression_alg()} |
            {send_ext_info, boolean()} |
            {recv_ext_info, boolean()}]

       conn_info_channels() = [proplists:proplist()]

              Return values from the connection_info/1 and connection_info/2 functions.

              In  the  option  info  tuple  are  only  the options included that differs from the
              default values.

       daemon_info_tuple() =
           {port, inet:port_number()} |
           {ip, inet:ip_address()} |
           {profile, atom()} |
           {options, daemon_options()}

              Return values from the daemon_info/1 and daemon_info/2 functions.

              In the option info tuple are only  the  options  included  that  differs  from  the
              default values.

       opaque_client_options()

       opaque_daemon_options()

       opaque_common_options()

              Opaque types that define experimental options that are not to be used in products.

EXPORTS

       close(ConnectionRef) -> ok | {error, term()}

              Types:

                 ConnectionRef = connection_ref()

              Closes an SSH connection.

       connect(Host, Port, Options) -> Result
       connect(Host, Port, Options, NegotiationTimeout) -> Result
       connect(TcpSocket, Options) -> Result
       connect(TcpSocket, Options, NegotiationTimeout) -> Result

              Types:

                 Host = host()
                 Port = inet:port_number()
                 Options = client_options()
                 TcpSocket = open_socket()
                 NegotiationTimeout = timeout()
                 Result = {ok, connection_ref()} | {error, term()}

              Connects to an SSH server at the Host on Port.

              As  an  alternative,  an already open TCP socket could be passed to the function in
              TcpSocket. The SSH initiation and negotiation will be initiated on  that  one  with
              the SSH that should be at the other end.

              No  channel is started. This is done by calling  ssh_connection:session_channel/[2,
              4].

              The NegotiationTimeout is in milli-seconds. The  default  value  is  infinity.  For
              connection timeout, use the option connect_timeout.

       connection_info(ConnectionRef) -> InfoTupleList

       connection_info(ConnectionRef, Key :: ItemList | Item) ->
                          InfoTupleList | InfoTuple

              Types:

                 ConnectionRef = connection_ref()
                 ItemList = [Item]
                 Item =
                     client_version | server_version | user | peer | sockname |
                     options | algorithms | sockname
                 InfoTupleList = [InfoTuple]
                 InfoTuple = connection_info_tuple()

              Returns information about a connection intended for e.g debugging or logging.

              When the Key is a single Item, the result is a single InfoTuple

       set_sock_opts(ConnectionRef, SocketOptions) ->
                        ok | {error, inet:posix()}

              Types:

                 ConnectionRef = connection_ref()
                 SocketOptions = [gen_tcp:option()]

              Sets tcp socket options on the tcp-socket below an ssh connection.

              This   function   calls   the  inet:setopts/2,  read  that  documentation  and  for
              gen_tcp:option().

              All gen_tcp socket options except

                * active

                * deliver

                * mode and

                * packet

              are allowed. The excluded options are reserved by the SSH application.

          Warning:
              This is an extremly dangerous function. You use it on your own risk.

              Some options are OS and OS version dependent. Do not use it unless  you  know  what
              effect your option values will have on an TCP stream.

              Some values may destroy the functionality of the SSH protocol.

       get_sock_opts(ConnectionRef, SocketGetOptions) ->
                        ok | {error, inet:posix()}

              Types:

                 ConnectionRef = connection_ref()
                 SocketGetOptions = [gen_tcp:option_name()]

              Get tcp socket option values of the tcp-socket below an ssh connection.

              This function calls the inet:getopts/2, read that documentation.

       daemon(Port | TcpSocket) -> Result
       daemon(Port | TcpSocket, Options) -> Result
       daemon(HostAddress, Port, Options) -> Result

              Types:

                 Port = integer()
                 TcpSocket = open_socket()
                 Options = daemon_options()
                 HostAddress = host() | any
                 Result = {ok, daemon_ref()} | {error, atom()}

              Starts  a server listening for SSH connections on the given port. If the Port is 0,
              a random free port is selected. See daemon_info/1 about how to  find  the  selected
              port number.

              As  an  alternative,  an already open TCP socket could be passed to the function in
              TcpSocket. The SSH initiation and negotiation will be initiated on that one when an
              SSH starts at the other end of the TCP socket.

              For a description of the options, see Daemon Options.

              Please  note  that  by  historical  reasons  both  the HostAddress argument and the
              gen_tcp connect_option() {ip,Address} set the listening address. This is  a  source
              of possible inconsistent settings.

              The rules for handling the two address passing options are:

                * if  HostAddress  is an IP-address, that IP-address is the listening address. An
                  'ip'-option will be discarded if present.

                * if HostAddress is the atom loopback, the listening address is loopback  and  an
                  loopback  address will be choosen by the underlying layers. An 'ip'-option will
                  be discarded if present.

                * if HostAddress is the atom any and no 'ip'-option  is  present,  the  listening
                  address is any and the socket will listen to all addresses

                * if  HostAddress  is any and an 'ip'-option is present, the listening address is
                  set to the value of the 'ip'-option

       daemon_info(DaemonRef) ->
                      {ok, InfoTupleList} | {error, bad_daemon_ref}

       daemon_info(DaemonRef, Key :: ItemList | Item) ->
                      InfoTupleList | InfoTuple | {error, bad_daemon_ref}

              Types:

                 DaemonRef = daemon_ref()
                 ItemList = [Item]
                 Item = ip | port | profile | options
                 InfoTupleList = [InfoTuple]
                 InfoTuple = daemon_info_tuple()

              Returns information about a daemon intended for e.g debugging or logging.

              When the Key is a single Item, the result is a single InfoTuple

              Note  that  daemon_info/1  and  daemon_info/2  returns  different  types   due   to
              compatibility reasons.

       default_algorithms() -> algs_list()

              Returns  a key-value list, where the keys are the different types of algorithms and
              the values are the algorithms themselves.

              See the User's Guide for an example.

       shell(Host | TcpSocket) -> Result
       shell(Host | TcpSocket, Options) -> Result
       shell(Host, Port, Options) -> Result

              Types:

                 Host = host()
                 TcpSocket = open_socket()
                 Port = inet:port_number()
                 Options = client_options()
                 Result = ok | {error, Reason::term()}

              Connects to an SSH server  at  Host  and  Port  (defaults  to  22)  and  starts  an
              interactive shell on that remote host.

              As  an  alternative,  an already open TCP socket could be passed to the function in
              TcpSocket. The SSH initiation and negotiation will be initiated  on  that  one  and
              finaly a shell will be started on the host at the other end of the TCP socket.

              For a description of the options, see Client Options.

              The  function  waits  for user input, and does not return until the remote shell is
              ended (that is, exit from the shell).

       start() -> ok | {error, term()}

       start(Type) -> ok | {error, term()}

              Types:

                 Type = permanent | transient | temporary

              Utility function that starts the applications crypto, public_key, and ssh.  Default
              type  is  temporary. For more information, see the application(3erl) manual page in
              Kernel.

       stop() -> ok | {error, term()}

              Stops the ssh application. For more information, see the  application(3erl)  manual
              page in Kernel.

       stop_daemon(DaemonRef :: daemon_ref()) -> ok

       stop_daemon(Address :: inet:ip_address(),
                   Port :: inet:port_number()) ->
                      ok

       stop_daemon(Address :: any | inet:ip_address(),
                   Port :: inet:port_number(),
                   Profile :: atom()) ->
                      ok

              Stops the listener and all connections started by the listener.

       stop_listener(SysSup :: daemon_ref()) -> ok

       stop_listener(Address :: inet:ip_address(),
                     Port :: inet:port_number()) ->
                        ok

       stop_listener(Address :: any | inet:ip_address(),
                     Port :: inet:port_number(),
                     Profile :: term()) ->
                        ok

              Stops  the  listener,  but  leaves  existing  connections  started  by the listener
              operational.

       tcpip_tunnel_from_server(ConnectionRef, ListenHost, ListenPort,
                                ConnectToHost, ConnectToPort) ->
                                   {ok, TrueListenPort} | {error, term()}

       tcpip_tunnel_from_server(ConnectionRef, ListenHost, ListenPort,
                                ConnectToHost, ConnectToPort, Timeout) ->
                                   {ok, TrueListenPort} | {error, term()}

              Types:

                 ConnectionRef = connection_ref()
                 ListenHost = host()
                 ListenPort = inet:port_number()
                 ConnectToHost = host()
                 ConnectToPort = inet:port_number()
                 Timeout = timeout()
                 TrueListenPort = inet:port_number()

              Asks the remote server of ConnectionRef to listen  to  ListenHost:ListenPort.  When
              someone  connects that address, the connection is forwarded in an encrypted channel
              from the server to the client. The client (that is, at the  node  that  calls  this
              function) then connects to ConnectToHost:ConnectToPort.

              The  returned  TrueListenPort  is  the  port that is listened to. It is the same as
              ListenPort, except when ListenPort = 0. In that case a free port is selected by the
              underlying OS.

              Note  that  in  case of an Erlang/OTP SSH server (daemon) as peer, that server must
              have been started with the option tcpip_tunnel_out to allow the connection.

       tcpip_tunnel_to_server(ConnectionRef, ListenHost, ListenPort,
                              ConnectToHost, ConnectToPort) ->
                                 {ok, TrueListenPort} | {error, term()}

       tcpip_tunnel_to_server(ConnectionRef, ListenHost, ListenPort,
                              ConnectToHost, ConnectToPort, Timeout) ->
                                 {ok, TrueListenPort} | {error, term()}

              Types:

                 ConnectionRef = connection_ref()
                 ListenHost = host()
                 ListenPort = inet:port_number()
                 ConnectToHost = host()
                 ConnectToPort = inet:port_number()
                 Timeout = timeout()
                 TrueListenPort = inet:port_number()

              Tells the local client to listen to ListenHost:ListenPort. When someone connects to
              that  address,  the  connection  is  forwarded  in an encrypted channel to the peer
              server of ConnectionRef. That server then connects to ConnectToHost:ConnectToPort.

              The returned TrueListenPort is the port that is listened to.  It  is  the  same  as
              ListenPort, except when ListenPort = 0. In that case a free port is selected by the
              underlying OS.

              Note that in case of an Erlang/OTP SSH server (daemon) as peer,  that  server  must
              have been started with the option tcpip_tunnel_in to allow the connection.