Table of Contents
(SSH client) is a program for logging into
a remote machine and for executing commands on a remote machine. It is intended
to replace rlogin and rsh, and provide secure encrypted communications
between two untrusted hosts over an insecure network. X11 connections and
arbitrary TCP/IP ports can also be forwarded over the secure channel.
connects and logs into the specified (with optional name). The user must
prove his/her identity to the remote machine using one of several methods
depending on the protocol version used. If is specified, is executed
on the remote host instead of a login shell. First, if the machine the
user logs in from is listed in or on the remote machine, and the user
names are the same on both sides, the user is immediately permitted to
log in. Second, if or exists in the user’s home directory on the remote
machine and contains a line containing the name of the client machine and
the name of the user on that machine, the user is permitted to log in. This
form of authentication alone is normally not allowed by the server because
it is not secure. The second authentication method is the or method combined
with RSA-based host authentication. It means that if the login would be permitted
by or and if additionally the server can verify the client’s host key
(see and in the section), only then is login permitted. This authentication
method closes security holes due to IP spoofing, DNS spoofing and routing
spoofing. [Note to the administrator: and the rlogin/rsh protocol in general,
are inherently insecure and should be disabled if security is desired.]
As a third authentication method, supports RSA based authentication. The
scheme is based on public-key cryptography: there are cryptosystems where
encryption and decryption are done using separate keys, and it is not possible
to derive the decryption key from the encryption key. RSA is one such system.
The idea is that each user creates a public/private key pair for authentication
purposes. The server knows the public key, and only the user knows the private
key. The file lists the public keys that are permitted for logging in.
When the user logs in, the program tells the server which key pair it
would like to use for authentication. The server checks if this key is permitted,
and if so, sends the user (actually the program running on behalf of the
user) a challenge, a random number, encrypted by the user’s public key. The
challenge can only be decrypted using the proper private key. The user’s
client then decrypts the challenge using the private key, proving that
he/she knows the private key but without disclosing it to the server.
implements the RSA authentication protocol automatically. The user creates
his/her RSA key pair by running This stores the private key in and stores
the public key in in the user’s home directory. The user should then copy
the to in his/her home directory on the remote machine (the file corresponds
to the conventional file, and has one key per line, though the lines can
be very long). After this, the user can log in without giving the password.
RSA authentication is much more secure than authentication. The most convenient
way to use RSA authentication may be with an authentication agent. See
for more information. If other authentication methods fail, prompts the
user for a password. The password is sent to the remote host for checking;
however, since all communications are encrypted, the password cannot be
seen by someone listening on the network. When a user connects using protocol
version 2, similar authentication methods are available. Using the default
values for the client will try to authenticate first using the hostbased
method; if this method fails, public key authentication is attempted, and
finally if this method fails, keyboard-interactive and password authentication
are tried. The public key method is similar to RSA authentication described
in the previous section and allows the RSA or DSA algorithm to be used:
The client uses his private key, or to sign the session identifier and
sends the result to the server. The server checks whether the matching public
key is listed in and grants access if both the key is found and the signature
is correct. The session identifier is derived from a shared Diffie-Hellman
value and is only known to the client and the server. If public key authentication
fails or is not available, a password can be sent encrypted to the remote
host to prove the user’s identity. Additionally, supports hostbased or
challenge response authentication. Protocol 2 provides additional mechanisms
for confidentiality (the traffic is encrypted using 3DES, Blowfish, CAST128
or Arcfour) and integrity (hmac-md5, hmac-sha1). Note that protocol 1 lacks
a strong mechanism for ensuring the integrity of the connection. When the
user’s identity has been accepted by the server, the server either executes
the given command, or logs into the machine and gives the user a normal
shell on the remote machine. All communication with the remote command or
shell will be automatically encrypted. If a pseudo-terminal has been allocated
(normal login session), the user may use the escape characters noted below.
If no pseudo-tty has been allocated, the session is transparent and can
be used to reliably transfer binary data. On most systems, setting the escape
character to will also make the session transparent even if a tty is used.
The session terminates when the command or shell on the remote machine
exits and all X11 and TCP/IP connections have been closed. The exit status
of the remote program is returned as the exit status of When a pseudo-terminal
has been requested, supports a number of functions through the use of
an escape character. A single tilde character can be sent as or by following
the tilde by a character other than those described below. The escape character
must always follow a newline to be interpreted as special. The escape character
can be changed in configuration files using the configuration directive
or on the command line by the option. The supported escapes (assuming
the default are: Disconnect. Background List forwarded connections.
Background at logout when waiting for forwarded connection / X11 sessions
to terminate. Display a list of escape characters. Send a BREAK to the
remote system (only useful for SSH protocol version 2 and if the peer supports
it). Open command line (only useful for adding port forwardings using the
and options). Request rekeying of the connection (only useful for SSH
protocol version 2 and if the peer supports it). If the variable is set
to (or see the description of the and options described later) and the
user is using X11 (the environment variable is set), the connection to
the X11 display is automatically forwarded to the remote side in such a
way that any X11 programs started from the shell (or command) will go through
the encrypted channel, and the connection to the real X server will be
made from the local machine. The user should not manually set Forwarding
of X11 connections can be configured on the command line or in configuration
files. The value set by will point to the server machine, but with a
display number greater than zero. This is normal, and happens because creates
a X server on the server machine for forwarding the connections over the
encrypted channel. will also automatically set up Xauthority data on the
server machine. For this purpose, it will generate a random authorization
cookie, store it in Xauthority on the server, and verify that any forwarded
connections carry this cookie and replace it by the real cookie when the
connection is opened. The real authentication cookie is never sent to the
server machine (and no cookies are sent in the plain). If the variable
is set to (or see the description of the and options described later)
and the user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side. Forwarding of arbitrary
TCP/IP connections over the secure channel can be specified either on the
command line or in a configuration file. One possible application of TCP/IP
forwarding is a secure connection to an electronic purse; another is going
through firewalls. automatically maintains and checks a database containing
identifications for all hosts it has ever been used with. Host keys are
stored in in the user’s home directory. Additionally, the file is automatically
checked for known hosts. Any new hosts are automatically added to the user’s
file. If a host’s identification ever changes, warns about this and disables
password authentication to prevent a trojan horse from getting the user’s
password. Another purpose of this mechanism is to prevent man-in-the-middle
attacks which could otherwise be used to circumvent the encryption. The
option can be used to prevent logins to machines whose host key is not
known or has changed. The options are as follows: Forces to try protocol
version 1 only. Forces to try protocol version 2 only. Forces to use
IPv4 addresses only. Forces to use IPv6 addresses only. Enables forwarding
of the authentication agent connection. This can also be specified on a
per-host basis in a configuration file. Agent forwarding should be enabled
with caution. Users with the ability to bypass file permissions on the remote
host (for the agent’s Unix-domain socket) can access the local agent through
the forwarded connection. An attacker cannot obtain key material from the
agent, however they can perform operations on the keys that enable them
to authenticate using the identities loaded into the agent. Disables forwarding
of the authentication agent connection. Specify the interface to transmit
from on machines with multiple interfaces or aliased addresses. Requests
compression of all data (including stdin, stdout, stderr, and data for
forwarded X11 and TCP/IP connections). The compression algorithm is the
same used by and the can be controlled by the option for protocol version
1. Compression is desirable on modem lines and other slow connections, but
will only slow down things on fast networks. The default value can be set
on a host-by-host basis in the configuration files; see the option. Selects
the cipher to use for encrypting the session. is used by default. It is
believed to be secure. (triple-des) is an encrypt-decrypt-encrypt triple with
three different keys. is a fast block cipher; it appears very secure and
is much faster than is only supported in the client for interoperability
with legacy protocol 1 implementations that do not support the cipher.
Its use is strongly discouraged due to cryptographic weaknesses. Additionally,
for protocol version 2 a comma-separated list of ciphers can be specified
in order of preference. See for more information. Specifies a local application-level
port forwarding. This works by allocating a socket to listen to on the
local side, and whenever a connection is made to this port, the connection
is forwarded over the secure channel, and the application protocol is then
used to determine where to connect to from the remote machine. Currently
the SOCKS4 and SOCKS5 protocols are supported, and will act as a SOCKS
server. Only root can forward privileged ports. Dynamic port forwardings
can also be specified in the configuration file. Sets the escape character
for sessions with a pty (default: The escape character is only recognized
at the beginning of a line. The escape character followed by a dot closes
the connection; followed by control-Z suspends the connection; and followed
by itself sends the escape character once. Setting the character to disables
any escapes and makes the session fully transparent. Specifies an alternative
per-user configuration file. If a configuration file is given on the command
line, the system-wide configuration file will be ignored. The default for
the per-user configuration file is Requests to go to background just
before command execution. This is useful if is going to ask for passwords
or passphrases, but the user wants it in the background. This implies The
recommended way to start X11 programs at a remote site is with something
like Allows remote hosts to connect to local forwarded ports. Specifies
which smartcard device to use. The argument is the device should use to
communicate with a smartcard used for storing the user’s private RSA key.
Selects a file from which the identity (private key) for RSA or DSA authentication
is read. The default is for protocol version 1, and and for protocol
version 2. Identity files may also be specified on a per-host basis in the
configuration file. It is possible to have multiple options (and multiple
identities specified in configuration files). Disables forwarding (delegation)
of GSSAPI credentials to the server. Specifies that the given port
on the local (client) host is to be forwarded to the given host and port
on the remote side. This works by allocating a socket to listen to on the
local side, and whenever a connection is made to this port, the connection
is forwarded over the secure channel, and a connection is made to port
from the remote machine. Port forwardings can also be specified in the
configuration file. Only root can forward privileged ports. IPv6 addresses
can be specified with an alternative syntax: Specifies the user
to log in as on the remote machine. This also may be specified on a per-host
basis in the configuration file. Additionally, for protocol version 2 a
comma-separated list of MAC (message authentication code) algorithms can
be specified in order of preference. See the keyword for more information.
Do not execute a remote command. This is useful for just forwarding ports
(protocol version 2 only). Redirects stdin from (actually, prevents reading
from stdin). This must be used when is run in the background. A common trick
is to use this to run X11 programs on a remote machine. For example, will
start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically
forwarded over an encrypted channel. The program will be put in the background.
(This does not work if needs to ask for a password or passphrase; see
also the option.) Can be used to give options in the format used in the
configuration file. This is useful for specifying options for which there
is no separate command-line flag. For full details of the options listed
below, and their possible values, see
Port to connect to on the remote host. This can
be specified on a per-host basis in the configuration file. Quiet mode. Causes
all warning and diagnostic messages to be suppressed. Only fatal errors
are displayed. If a second is given then even fatal errors are suppressed.
Specifies that the given port on the remote (server) host is to be
forwarded to the given host and port on the local side. This works by allocating
a socket to listen to on the remote side, and whenever a connection is
made to this port, the connection is forwarded over the secure channel,
and a connection is made to port from the local machine. Port forwardings
can also be specified in the configuration file. Privileged ports can be
forwarded only when logging in as root on the remote machine. IPv6 addresses
can be specified with an alternative syntax: May be used to request
invocation of a subsystem on the remote system. Subsystems are a feature
of the SSH2 protocol which facilitate the use of SSH as a secure transport
for other applications (eg. The subsystem is specified as the remote command.
Disable pseudo-tty allocation. Force pseudo-tty allocation. This can be used
to execute arbitrary screen-based programs on a remote machine, which can
be very useful, e.g., when implementing menu services. Multiple options force
tty allocation, even if has no local tty. Display the version number and
exit. Verbose mode. Causes to print debugging messages about its progress.
This is helpful in debugging connection, authentication, and configuration
problems. Multiple options increase the verbosity. The maximum is 3. Enables
X11 forwarding. This can also be specified on a per-host basis in a configuration
file. X11 forwarding should be enabled with caution. Users with the ability
to bypass file permissions on the remote host (for the user’s X authorization
database) can access the local X11 display through the forwarded connection.
An attacker may then be able to perform activities such as keystroke monitoring.
Disables X11 forwarding. Enables trusted X11 forwarding.
may additionally obtain configuration data from a per-user configuration
file and a system-wide configuration file. The file format and configuration
options are described in
will normally set the following environment
variables: The variable indicates the location of the X11 server. It
is automatically set by to point to a value of the form where hostname
indicates the host where the shell runs, and n is an integer Ge 1. uses
this special value to forward X11 connections over the secure channel. The
user should normally not set explicitly, as that will render the X11 connection
insecure (and will require the user to manually copy any required authorization
cookies). Set to the path of the user’s home directory. Synonym for set
for compatibility with systems that use this variable. Set to the path
of the user’s mailbox. Set to the default as specified when compiling
If needs a passphrase, it will read the passphrase from the current terminal
if it was run from a terminal. If does not have a terminal associated with
it but and are set, it will execute the program specified by and open
an X11 window to read the passphrase. This is particularly useful when calling
from a or related script. (Note that on some machines it may be necessary
to redirect the input from to make this work.) Identifies the path of
a unix-domain socket used to communicate with the agent. Identifies the
client and server ends of the connection. The variable contains four space-separated
values: client ip-address, client port number, server ip-address and server
port number. The variable contains the original command line if a forced
command is executed. It can be used to extract the original arguments. This
is set to the name of the tty (path to the device) associated with the
current shell or command. If the current session has no tty, this variable
is not set. The timezone variable is set to indicate the present timezone
if it was set when the daemon was started (i.e., the daemon passes the value
on to new connections). Set to the name of the user logging in. Additionally,
reads and adds lines of the format to the environment if the file exists
and if users are allowed to change their environment. For more information,
see the option in
Records host keys for all hosts the user has
logged into that are not in See Contains the authentication identity
of the user. They are for protocol 1 RSA, protocol 2 DSA, and protocol 2
RSA, respectively. These files contain sensitive data and should be readable
by the user but not accessible by others (read/write/execute). Note that
ignores a private key file if it is accessible by others. It is possible
to specify a passphrase when generating the key; the passphrase will be
used to encrypt the sensitive part of this file using 3DES. Contains the
public key for authentication (public part of the identity file in human-readable
form). The contents of the file should be added to the file on all machines
where the user wishes to log in using protocol version 1 RSA authentication.
The contents of the and file should be added to on all machines where
the user wishes to log in using protocol version 2 DSA/RSA authentication.
These files are not sensitive and can (but need not) be readable by anyone.
These files are never used automatically and are not necessary; they are
only provided for the convenience of the user. This is the per-user configuration
file. The file format and configuration options are described in Lists
the public keys (RSA/DSA) that can be used for logging in as this user.
The format of this file is described in the manual page. In the simplest
form the format is the same as the identity files. This file is not highly
sensitive, but the recommended permissions are read/write for the user,
and not accessible by others. Systemwide list of known host keys. This file
should be prepared by the system administrator to contain the public host
keys of all machines in the organization. This file should be world-readable.
This file contains public keys, one per line, in the following format (fields
separated by spaces): system name, public key and optional comment field.
When different names are used for the same machine, all such names should
be listed, separated by commas. The format is described in the manual page.
The canonical system name (as returned by name servers) is used by to
verify the client host when logging in; other names are needed because
does not convert the user-supplied name to a canonical name before checking
the key, because someone with access to the name servers would then be
able to fool host authentication. Systemwide configuration file. The file
format and configuration options are described in These three files contain
the private parts of the host keys and are used for and If the protocol
version 1 method is used, must be setuid root, since the host key is
readable only by root. For protocol version 2, uses to access the host
keys for This eliminates the requirement that be setuid root when that
authentication method is used. By default is not setuid root. This file
is used in authentication to list the host/user pairs that are permitted
to log in. (Note that this file is also used by rlogin and rsh, which makes
using this file insecure.) Each line of the file contains a host name (in
the canonical form returned by name servers), and then a user name on that
host, separated by a space. On some machines this file may need to be world-readable
if the user’s home directory is on a NFS partition, because reads it as
root. Additionally, this file must be owned by the user, and must not have
write permissions for anyone else. The recommended permission for most machines
is read/write for the user, and not accessible by others. Note that by
default will be installed so that it requires successful RSA host authentication
before permitting authentication. If the server machine does not have the
client’s host key in it can be stored in The easiest way to do this is
to connect back to the client from the server machine using ssh; this will
automatically add the host key to This file is used exactly the same
way as The purpose for having this file is to be able to use rhosts authentication
with without permitting login with or This file is used during authentication.
It contains canonical hosts names, one per line (the full format is described
in the manual page). If the client host is found in this file, login is
automatically permitted provided client and server user names are the same.
Additionally, successful RSA host authentication is normally required. This
file should only be writable by root. This file is processed exactly as
This file may be useful to permit logins using but not using rsh/rlogin.
Commands in this file are executed by when the user logs in just before
the user’s shell (or command) is started. See the manual page for more information.
Commands in this file are executed by when the user logs in just before
the user’s shell (or command) is started. See the manual page for more information.
Contains additional definitions for environment variables, see section
above.
exits with the exit status of the remote command or
with 255 if an error occurred.
OpenSSH
is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and
Dug Song removed many bugs, re-added newer features and created OpenSSH.
Markus Friedl contributed the support for SSH protocol versions 1.5 and
2.0.
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