[Synopsis] [Description] [Options] [Configuration Files] [Environment] [Files] [Authors] [See also]

ssh - secure shell client (remote login program)


ssh [-l login_name] hostname [command]

ssh [-a] [-c idea|blowfish|des|3des|arcfour|tss|none] [-e escape_char] [-i identity_file] [-l login_name] [-n] [-o option] [-p port] [-q] [-t] [-v] [-x] [-C] [-L port:host:hostport] [-R port:host:hostport] hostname [command]


Ssh (Secure Shell) a program for logging into a remote machine and for executing commands in 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.

Ssh connects and logs into the specified hostname. The user must prove his/her identity to the remote machine using one of several methods.

First, if the machine the user logs in from is listed in %ETC%/hosts.equiv or %ETC%/shosts.equiv on the remote machine, and the user names are the same on both sides, the user is immediately permitted to log in. Second, if .rhosts or .shosts 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 (and primary) authentication method is the rhosts or hosts.equiv method combined with RSA-based host authentication. It means that if the login would be permitted by .rhosts, .shosts, %ETC%/hosts.equiv, or %ETC%/shosts.equiv, and additionally it can verify the client's host key (see %HOME%/.ssh/known_hosts and %ETC%/ssh_known_hosts in the Files section), only then login is permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing and routing spoofing. [Note to the administrator: %ETC%/hosts.equiv, .rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.]

As a third authentication method, ssh supports RSA based authentication. The scheme is based on public-key cryptography: these 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 %HOME%/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh 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 ssh 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.

Ssh implements the RSA authentication protocol automatically. The user creates his/her RSA key pair by running ssh-keygen. This stores the private key in .ssh/identity and the public key in .ssh/identity.pub in the user's home directory. The user then copies the identity.pub to the remote machine and appends it to the file .ssh/authorized_keys in her home directory (the authorized_keys file corresponds to the conventional .rhosts 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 rhosts authentication.

The most convenient way to use RSA authentication may be with an authentication agent. See ssh-agent for more information.

If other authentication methods fail, ssh 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 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 can disconnect with "~.". All forwarded connections can be listed with "~#". All available escapes can be listed with "~?". A single tilde character can be sent as "~~" (or by following the tilde by a character other than those described above). The escape character must always follow a newline to be interpreted as special. The escape character can be changed in configuration files or on the command line.

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 ``none'' will also make the session transparent even if a tty is used.

The session terminates when the command or shell in on the remote machine exists and all X11 and TCP/IP connections have been closed. The exit status of the remote program is returned as the exit status of ssh.

If the user is using X11 (the DISPLAY environment variable is set on the local machine), the connection to the X11 display is automatically forwarded to the remote side in such a way that any X11 programs started from the remote 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 DISPLAY. Forwarding of X11 connections can be configured on the command line or in configuration files.

The DISPLAY value set by ssh on the remote machine will point to the server machine, but with a display number greater than zero. This is normal, and happens because ssh creates a "proxy" X server on the server machine for forwarding the connections over the encrypted channel.

Ssh 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 user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side unless disabled on command line or in a configuration file.

Forwarding of arbitrary TCP/IP connections over the secure channel can be specified either on 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 trough firewalls.

Ssh automatically maintains and checks a database containing RSA-based identifications for all hosts it has ever been used with. The database is stored in the file .ssh/known_hosts in the user's home directory. Additionally, the file %ETC%/ssh_known_hosts is automatically checked for known hosts. Any new hosts are automatically added to the user's file. If a host's identification ever changes, ssh 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 StrictHostKeyChecking option (see below) can be used to prevent logins to machines whose host key is not known or has changed.

Considerable work has been put into making ssh secure. However, if you find a security problem, please report it immediately to <ssh-bugs@cs.hut.fi>.

The current OS/2 version of ssh is based on version 1.2.16. There is a newer version 1.2.17 available, the main difference is that it uses a different mechanism for passing data to and from the authentication agent ssh-agent. This mechanism is based on local sockets and I could not yet get it working under OS/2. Furthermore, this mechanism was deemed to be less safe than the mechanism currently implemented in the OS/2 version. Quote from the original ssh-agent manual page: On some machines, an alternative method is used. A unix-domain socket is created and the name of this socket is stored in the SSH_AUTHENTICATION_SOCKET environment variable. The socket is made accessible only to the current user. This method is easily abused by root or another instance of the same user.

Since ssh originates in the Unix environment certain "unicisms" also surface in the OS/2 version.


Disables forwarding of the authentication agent connection. This may also be specified on a perhost basis in the configuration file.
-c idea|des|3des|blowfish|arcfour|tss|none
Selects the cipher to use for encrypting the session. idea is used by default. It is believed to be secure. des is the data encryption standard, but is breakable by governments, large corporations, and major criminal organizations. 3des (triple-des) is encrypt-decrypt-encrypt triple with three different keys. It is presumably more secure than DES. It is used as default if both sites do not support IDEA. blowfish is an encryption algorithm invented by Bruce Schneier. It uses 128 bit keys. arcfour is an algorithm published in the Usenet News in 1995. This algorithm is believed to be equivalent with the RC4 cipher from RSA Data Security (RC4 is a trademark of RSA Data Security). This is the fastest algorithm currently supported. TSS is a fast home-grown algorithm based on MD5. none disables encryption entirely; it is only intended for debugging, and it renders the connection insecure.
Disables the VT220 emulation which by default is enabled. The VT220 emulator is writtem by Robert Muchsel. It is fast enough (although it uses the 16 Bit VIO calls) and passes the vttest (OS/2 telnet does not, check it!) and other vt100 test suites. The code was also enhanced to support all extended Linux escape codes and key codes. This enables you to log into a Linux box, set term=console and everything will work. The function keys return Linux key codes, however SHIFT-F1 to SHIFT-F4 return the standard vt100 key codes.
-e ch|^ch|none
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 a number sign (#) lists forwared connections, followed by the letter 'r' resets the terminal emulation, followed by a question mark (?) lists supported escape sequences, and followed by itself sends the escape character once. Setting the character to `none' disables any escapes and makes the session fully transparent.
-i identity_file
Selects the file from which the identity (private key) for RSA authentication is read. Default is .ssh/identity in the user's home directory. Identity files may also be specified on a per-host basis in the configuration file. It is possible to have multiple -i options (and multiple identities specified in configuration files).
-l login_name
Specifies the user to log in as on the remote machine. This may also be specified on a per-host basis in the configuration file.
-o option
Can be used to give options in the format used in the config file. This is useful for specifying options for which there is no separate command-line flag. The option has the same format as a line in the configuration file.
-p port
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.
Force pseudo-tty allocation. This can be used to execute arbitary screen-based programs on a remote machine, which can be very useful e.g. when implementing menu services.
Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems.
Disables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
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 gzip, and the "level" can be controlled by the CompressionLevel option (see below). 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 Compress option below.
-L port:host:hostport
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 port 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 host:hostport from the remote machine. Port forwardings can also be specified in the configuration file. Only root can forward privileged ports.
-R port:host:hostport
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 port 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 host:hostport 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.

Configuration Files

Ssh obtains configuration data from the following sources (in this order): command line options, user's configuration file (%HOME%/.ssh/config), and system-wide configuration file (%ETC%/ssh_config). For each parameter, the first obtained value will be used. The configuration files contain sections bracketed by "Host" specifications, and that section is only applied for hosts that match one of the patterns given in the specification. The matched host name is the one given on the command line.

Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end.

The configuration file has the following format: Empty lines and lines starting with `#' are comments. Otherwise a line is of the format "keyword arguments".

The possible keywords and their meanings are as follows (note that the configuration files are case-sensitive):

Restricts the following declarations (up to the next Host keyword) to be only for those hosts that match one of the patterns given after the keyword. `*' and `?' can be as wildcards in the patterns. A single `*' as a pattern can be used to provide global defaults for all hosts. The host is the hostname argument given on the command line (i.e., the name is not converted to a canonicalized host name before matching).
If set to "yes", passphrase/password querying will be disabled. This option is useful in scripts and other batch jobs where you have no user to supply the password. The argument must be "yes" or "no".
Specifies the cipher to use for encrypting the session. Currently, idea, des, 3des, blowfish, arcfour, tss, and none are supported. The default is "idea" (or "3des" if "idea" is not supported by both hosts). Using "none" (no encryption) is intended only for debugging, and will render the connection insecure.
Specifies whether to use compression. The argument must be "yes" or "no".
Specifies the compression level to use if compression is enable. The argument must be an integer from 1 (fast) to 9 (slow, best). The default level is 6, which is good for most applications. The meaning of the values is the same as in GNU GZIP.
Specifies the number of tries (one per second) to make before falling back to rsh or exiting. The argument must be an integer. This may be useful in scripts if the connection sometimes fails.
Sets the escape character (default: ~). The escape character can also be set on the command line. The argument should be a single character, `^' followed by a letter, or ``none'' to disable the escape character entirely (making the connection transparent for binary data).
Specifies that if connecting via ssh fails due to a connection refused error (there is no sshd listening on the remote host), rsh should automatically be used instead (after a suitable warning about the session being unencrypted). The argument must be "yes" or "no".
Specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. The argument must be "yes" or "no".
Specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be "yes" or "no".
Specifies a file to use instead of %ETC%/ssh_known_hosts.
Specifies the real host name to log into. This can be used to specify nicnames or abbreviations for hosts. Default is the name given on the command line. Numeric IP addresses are also permitted (both on the command line and in HostName specifications).
Specifies the file from which the user's RSA authentication identity is read (default .ssh/identity in the user's home directory). Additionally, any identities represented by the authentication agent will be used for authentication. The file name may use the tilde syntax to refer to a user's home directory. It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence.
Specifies whether the system should send keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections will die if the route is down temporarily, and some people find it annoying.

The default is "yes" (to send keepalives), and the client will notice if the network goes down or the remote host dies. This is important in scripts, and many users want it too.

To disable keepalives, the value should be set to "no" in both the server and the client configuration files.

Specifies that a TCP/IP port on the local machine be forwarded over the secure channel to given host:port from the remote machine. The first argument must be a port number, and the second must be host:port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the root can forward privileged ports.
Specifies whether to use password authentication. The argument to this keyword must be "yes" or "no".
Specifies the port number to connect on the remote host. Default is 22.
Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed with /bin/sh. In the command string, %h will be substituted by the host name to connect and %p by the port. The command can be basically anything, and should read from its stdin and write to its stdout. It should eventually connect an sshd server running on some machine, or execute "sshd -i" somewhere. Host key management will be done using the HostName of the host being connected (defaulting to the name typed by the user).

Note that ssh can also be configured to support the SOCKS system using the --with-socks4 or --with socks5 compile-time configuration option.

Specifies that a TCP/IP port on the remote machine be forwarded over the secure channel to given host:port from the local machine. The first argument must be a port number, and the second must be host:port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the root can forward privileged ports.
Specifies whether to try rhosts based authentication. Note that this declaration only affects the client side and has no effect whatsoever on security. Disabling rhosts authentication may reduce authentication time on slow connections when rhosts authentication is not used. Most servers do not permit RhostsAuthentication because it is not secure (see RhostsRSAAuthentication). The argument to this keyword must be "yes" or "no".
Specifies whether to try rhosts based authentication with RSA host authentication. This is the primary authentication method for most sites. The argument must be "yes" or "no".
Specifies whether to try RSA authentication. The argument to this keyword must be "yes" or "no". RSA authentication will only be attempted if the identity file exists, or an authentication agent is running.
If this flag is set to "yes", ssh ssh will never automatically add host keys to the %HOME%/.ssh/known_hosts file, and refuses to connect hosts whose host key has changed. This provides maximum protection against trojan horse attacks. However, it can be somewhat annoying if you don't have good %ETC%/ssh_known_hosts files installed and frequently connect new hosts. Basically this option forces the user to manually add any new hosts. Normally this option is disabled, and new hosts will automatically be added to the known host files. The host keys of known hosts will be verified automatically in either case. The argument must be "yes" or "no".
Specifies the user to log in as. This can be useful if you have a different user name in different machines. This saves the trouble of having to remember to give the user name on the command line.
Specifies a file to use instead of %HOME%/.ssh/known_hosts.
Specifies that rlogin/rsh should be used for this host. It is possible that the host does not at all support the ssh protocol. This causes ssh to immediately exec rsh. All other options (except HostName) are ignored if this has been specified. The argument must be "yes" or "no".


Ssh expects the following environment variables to be set when invoked on the OS/2 machine. They are best defined in config.sys. Environment variables marked with * usually are already defined there, the other ones need to be added.
Full path name of OS/2 command interpreter.
Defined if an X server is installed on the local machine, e.g. IBM's PMX.
Path of the directory storing TCP/IP administrative and configuration data.
Path of the user's home directory.
Hostname of the local machine.
Name of the directory where temporary files are stored.
Type of the terminal. Should be aither vt220 or console for remote Linux machines.
Account name of the user.
(OS/2 specific) If this variable is defined its value determines the userid ssh and all other programs run under. By default, all programs run with userid 0 i.e. root. This effects the way ports are selected. For example, ssh to connect to the remote sshd will use a privileged port < 1024 by default and a non-privileged port > 1024 if SSH_UID is set to a value different from 0. Set this variable if you want to connect to a remote machine which refuses the usage of privileged ports.

Ssh will normally set the following environment variables in the remote shell (or command):

The DISPLAY variable indicates the location of the X11 server. It is automatically set by ssh to point to a value of the form "hostname:n" where hostname indicates the host where the shell runs, and n is an integer >= 1. Ssh uses this special value to forward X11 connections over the secure channel. The user should normally not set DISPLAY 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 USER; set for compatibility with systems that use this variable.
Set to point the user's mailbox.
Set to the default PATH, as specified when compiling ssh or, on some systems, %ETC%/environment or %ETC%/default/login.
This is set to an integer value if you are using the authentication agent and a connection to it has been forwarded. The value indicates a file descriptor number used for communicating with the agent.
Identifies the client end of the connection. The variable contains three space-separated values: client ip-address, client port number, and server port number.
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 (e.i., the daemon passes the value on to new connections).
Set to the name of the user logging in.
Additionally, ssh reads %ETC%/environment and %HOME%/.ssh/environment, and adds lines of the format VARNAME=value to the environment. Some systems may have still additional mechanisms for setting up the environment, such as %ETC%/default/login on Solaris.


Note that ssh under OS/2 uses the Unix line end convention which terminates lines in textual files with a Line Feed character only whereas OS/2 usually terminates lines with a two character combination consisting of Carriage Return and Line Feed. Note that you must save all files following this convention when you edit a file (this can be done with EPM if you use the save /u command). Especially having an additional carriage return character separate the lines in the key files (e.g. .ssh/identity) will lead to strange error messages.

Records host keys for all hosts the user has logged into (that are not in %ETC%/ssh_known_hosts). See sshd manual page.
Used for seeding the random number generator. This file contains sensitive data and should read/write for the user and not accessible for others. This file is created the first time the program is run and updated automatically. The user should never need to read or modify this file.
Contains the RSA authentication identity of the user. This file contains sensitive data and should be readable by the user but not 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 IDEA.
Contains the public key for authentication (public part of the identity file in human-readable form). The contents of this file should be added to %HOME%/.ssh/authorized_keys on all machines where you wish to log in using RSA authentication. This file is not sensitive and can (but need not) be readable by anyone. This file is never used automatically and is not necessary; it is only provided for the convenience of the user.
This is the per-user configuration file. The format of this file is described above. This file is used by the ssh client. This file does not usually contain any sensitive information, but the recommended permissions are read/write for the user, and not accessible by others.
Lists the RSA keys that can be used for logging in as this user. The format of this file is described in the sshd manual page. In the simplest form the format is the same as the .pub identity files (that is, each line contains the number of bits in modulus, public exponent, modulus, and comment fields, separated by spaces). 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, number of bits in modulus, public exponent, modulus, 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 on the sshd manual page.

The canonical system name (as returned by name servers) is used by sshd to verify the client host when logging in; other names are needed because ssh 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. This file provides defaults for those values that are not specified in the user's configuration file, and for those users who do not have a configuration file. This file must be world-readable.
This file is used in .rhosts 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. This file must be owned by the user, and must not have write permissions for anyone else. The recommended permission is read/write for the user, and not accessible by others.

Note that by default sshd will be installed so that it requires successful RSA host authentication before permitting .rhosts authentication. If your server machine does not have the client's host key in %ETC%/ssh_known_hosts, you can store it in %HOME%/.ssh/known_hosts. 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 in %HOME%/.ssh/known_hosts.

This file is used exactly the same way as .rhosts. The purpose for having this file is to be able to use rhosts authentication with ssh without permitting login with rlogin or rsh.
This file is used during .rhosts authentication. It contains canonical hosts names, one per line (the full format is described on the sshd 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 %ETC%/hosts.equiv. This file may be useful to permit logins using ssh but not using rsh/rlogin.
Commands in this file are executed by ssh when the user logs in just before the user's shell (or command) is started. See the sshd manual page for more information.
Commands in this file are executed by ssh when the user logs in just before the user's shell (or command) is started. See the sshd manual page for more information.


Tatu Ylönen <ylo@cs.hut.fi>;
VT220 emulation by Robert Muchsel <muchsel@acm.org>;
OS/2 adaptation by Hans-Michael Stahl <hmstahl@berlin.snafu.de>

Information about new releases, mailing lists, and other related issues can be found from the ssh WWW home page at http://www.cs.hut.fi/ssh/.

See also

sshd, ssh-keygen, ssh-agent, ssh-add, scp, rsh, telnet
Last update: 13 February 1997 by hmstahl@berlin.snafu.de