*

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

*

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.

*

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.

*

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

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()]

authentication_client_options() = 


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

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() = [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() |


    max_initial_idle_time_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. Similarly all writes to standard_output will be sent as data-events to the client. An OS shell client like the command 'ssh' will usually use stdin and stdout for the user interface.

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

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

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()} |


    {no_auth_needed, boolean()}

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()})

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()}

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.

For more information about timeouts, see the Timeouts section in the User's Guide Hardening chapter.

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.

For more information about timeouts, see the Timeouts section in the User's Guide Hardening chapter.

max_initial_idle_time_daemon_option() = 


    {max_initial_idle_time, timeout()}

Maximum time in milliseconds for the first channel start after completion of the authentication negotiation. Defaults to infinity.

For more information about timeouts, see the Timeouts section in the User's Guide Hardening chapter.

hardening_daemon_options() = 


    {max_sessions, integer() >= 1} |


    {max_channels, integer() >= 1} |


    {parallel_login, boolean()} |


    {minimal_remote_max_packet_size, integer() >= 1}

For more information about hardening, see the Hardening section in the User's Guide chapter.

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())}

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.

common_options() = [common_option()]

common_option() = 


    ssh_file:user_dir_common_option() |


    profile_common_option() |


    max_idle_time_common_option() |


    max_log_item_len_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.

For more information about timeouts, see the Timeouts section in the User's Guide Hardening chapter.

max_log_item_len_common_option() = 


    {max_log_item_len, limit_bytes()}

Sets a limit for the size of a logged item excluding a header. The unit is bytes and the value defaults to 500.

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:

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.

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 algorithm for modifications works like this:

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:

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).

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.