ssh.0

OpenSSL Source code for SFTP, SSH, and many others

SSH(1)                  System General Commands Manual                  SSH(1)

NAME
     ssh - OpenSSH SSH client (remote login program)

SYNOPSIS
     ssh [-l login_name] hostname | user@hostname [command]

     ssh [-afgknqstvxACNPTX1246] [-b bind_address] [-c cipher_spec]
         [-e escape_char] [-i identity_file] [-l login_name] [-m mac_spec]
         [-o option] [-p port] [-F configfile] [-L port:host:hostport] [-R
         port:host:hostport] [-D port] hostname | user@hostname [command]

DESCRIPTION
     ssh (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.

     ssh connects and logs into the specified hostname.  The user must prove
     his/her identity to the remote machine using one of several methods
     depending on the protocol version used:

   SSH protocol version 1

     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 authentication method is the rhosts or hosts.equiv method comM--
     bined with RSA-based host authentication.  It means that if the login
     would be permitted by $HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or
     /etc/shosts.equiv, and if additionally the server can verify the client's
     host key (see /etc/ssh/ssh_known_hosts and $HOME/.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 spoofM--
     ing.  [Note to the administrator: /etc/hosts.equiv, $HOME/.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: 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
     $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 chalM--
     lenge 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(1).  This stores the
     private key in $HOME/.ssh/identity and the public key in
     $HOME/.ssh/identity.pub in the user's home directory.  The user should
     then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home
     directory on the remote machine (the authorized_keys file corresponds to
     the conventional $HOME/.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 authenM--
     tication.

     The most convenient way to use RSA authentication may be with an authenM--
     tication agent.  See ssh-agent(1) for more information.

     If other authentication methods fail, ssh prompts the user for a passM--
     word.  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.

   SSH protocol version 2

     When a user connects using protocol version 2 similar authentication
     methods are available.  Using the default values for
     PreferredAuthentications, 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, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to
     sign the session identifier and sends the result to the server.  The
     server checks whether the matching public key is listed in
     $HOME/.ssh/authorized_keys 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 for proving the user's identity.

     Additionally, ssh supports hostbased or challenge response authenticaM--
     tion.

     Protocol 2 provides additional mechanisms for confidentiality (the trafM--
     fic 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.

   Login session and remote execution

     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 ``none'' 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 staM--
     tus of the remote program is returned as the exit status of ssh.

   Escape Characters

     When a pseudo terminal has been requested, ssh supports a number of funcM--
     tions 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 characM--
     ter can be changed in configuration files using the EscapeChar configuraM--
     tion directive or on the command line by the -e option.

     The supported escapes (assuming the default `~') are:

     ~.      Disconnect

     ~^Z     Background ssh

     ~#      List forwarded connections

     ~&      Background ssh at logout when waiting for forwarded connection /
             X11 sessions to terminate

     ~?      Display a list of escape characters

     ~C      Open command line (only useful for adding port forwardings using
             the -L and -R options)

     ~R      Request rekeying of the connection (only useful for SSH protocol
             version 2 and if the peer supports it)

   X11 and TCP forwarding

     If the ForwardX11 variable is set to ``yes'' (or, see the description of
     the -X and -x options described later) and the user is using X11 (the
     DISPLAY 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 DISPLAY.  ForM--
     warding of X11 connections can be configured on the command line or in
     configuration files.

     The DISPLAY value set by ssh 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 the comM--
     mand line or in a configuration file.

     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.

   Server authentication

     ssh automatically maintains and checks a database containing identificaM--
     tions for all hosts it has ever been used with.  Host keys are stored in
     $HOME/.ssh/known_hosts in the user's home directory.  Additionally, the
     file /etc/ssh/ssh_known_hosts is automatically checked for known hosts.
     Any new hosts are automatically added to the user's file.  If a host's