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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. Similarly, 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 completely 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 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 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() |
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:
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()} |
{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()})
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.
Warning:
Note that this is very insecure due to the plain-text passwords; it is intended
for test purposes. Use the pwdfun option to handle the password checking instead.
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 handling 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.
no_auth_needed:
If true, a client is authenticated without any need of providing any password
or key.
This option is only intended for very special applications due to the high risk
of accepting any connecting client.
The default value is false.
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.
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.
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() |
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:
* {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 algorithm 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 or the
value of the connect_timeout option, if present. 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 extremely 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 chosen 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_replace_options(DaemonRef, NewUserOptions) ->
{ok, daemon_ref()} | {error, term()}
Types:
DaemonRef = daemon_ref()
NewUserOptions = daemon_options()
Replaces the options in a running daemon with the options in NewUserOptions. Only
connections established after this call are affected, already established
connections are not.
Note:
In the final phase of this function, the listening process is restarted. Therfore a
connection attempt to the daemon in this final phase could fail.
The handling of Erlang configurations is described in the User's Guide; see
chapters Configuration in SSH and Configuring algorithms in SSH.
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
finally 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.
hostkey_fingerprint(HostKey) -> string()
hostkey_fingerprint(DigestType, HostKey) -> string()
hostkey_fingerprint([DigestType], HostKey) -> [string()]
Types:
HostKey = public_key:public_key()
DigestType = public_key:digest_type()
Calculates a ssh fingerprint from a public host key as openssh does.
The algorithm in hostkey_fingerprint/1 is md5 to be compatible with older ssh-
keygen commands. The string from the second variant is prepended by the algorithm
name in uppercase as in newer ssh-keygen commands.
Examples:
2> ssh:hostkey_fingerprint(Key).
"f5:64:a6:c1:5a:cb:9f:0a:10:46:a2:5c:3e:2f:57:84"
3> ssh:hostkey_fingerprint(md5,Key).
"MD5:f5:64:a6:c1:5a:cb:9f:0a:10:46:a2:5c:3e:2f:57:84"
4> ssh:hostkey_fingerprint(sha,Key).
"SHA1:bSLY/C4QXLDL/Iwmhyg0PGW9UbY"
5> ssh:hostkey_fingerprint(sha256,Key).
"SHA256:aZGXhabfbf4oxglxltItWeHU7ub3Dc31NcNw2cMJePQ"
6> ssh:hostkey_fingerprint([sha,sha256],Key).
["SHA1:bSLY/C4QXLDL/Iwmhyg0PGW9UbY",
"SHA256:aZGXhabfbf4oxglxltItWeHU7ub3Dc31NcNw2cMJePQ"]