ssh

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

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

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.

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}.

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.

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(6) for a list of currently implemented extensions.

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

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

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.

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

This option changes how the daemon execute 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 but in the error case.

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

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

1. If the exec-option is present (the shell-option may or may not be present):

The exec-option fun is called with the same number of parameters as the arity of the fun, and the result is returned to the client.

2. If the exec-option is absent, but a shell-option is present with the default Erlang shell:

The default Erlang evaluator is used and the result is returned to the client.

3. If the exec-option is absent, but a shell-option is present that is not the default Erlang shell:

The exec-request is not evaluated and an error message is returned to the client.

4. If neither the exec-option nor the shell-option is present:

The default Erlang evaluator is used and the result is returned to the client.

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

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 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.

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.

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

This variant is kept for compatibility.

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.

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.

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.

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.

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(6) for a list of currently implemented extensions.

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

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

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.

Sets a time-out on a connection when no channels are active. Defaults to 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.

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.

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.

Provides a fun to implement your own logging when the peer disconnects.

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.

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}

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.

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.

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.

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

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.

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

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}).

Opaque data type representing a daemon.

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

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.

Opaque data type representing a channel inside a connection.

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

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