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ssh



     ssh [-afgknqstvxACNTVX1246] [-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/ssh/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 com­
     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/ssh/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 permit­
     ted.  This authentication method closes security holes due to IP spoof­
     ing, DNS spoofing and routing spoofing.  [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

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

     If other authentication methods fail, ssh prompts the user for a pass­
     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 authentica­
     tion.

     Protocol 2 provides additional mechanisms for confidentiality (the traf­
     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
     ter can be changed in configuration files using the EscapeChar configura­
     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

     ~B      Send a BREAK to the remote system (only useful for SSH protocol
             version 2 and if the peer supports it)

     ~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.  For­
     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 ForwardAgent variable is set to ``yes'' (or, see the description
     of the -A and -a options described later) and the user is using an
     authentication agent, the connection to the agent is automatically for­
     identification ever changes, ssh warns about this and disables password
     authentication to prevent a trojan horse from getting the user's pass­
     word.  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 can be used to prevent logins to machines
     whose host key is not known or has changed.

     The options are as follows:

     -a      Disables forwarding of the authentication agent connection.

     -A      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.

     -b bind_address
             Specify the interface to transmit from on machines with multiple
             interfaces or aliased addresses.

     -c blowfish|3des|des
             Selects the cipher to use for encrypting the session.  3des is
             used by default.  It is believed to be secure.  3des (triple-des)
             is an encrypt-decrypt-encrypt triple with three different keys.
             blowfish is a fast block cipher, it appears very secure and is
             much faster than 3des.  des is only supported in the ssh client
             for interoperability with legacy protocol 1 implementations that
             do not support the 3des cipher.  Its use is strongly discouraged
             due to cryptographic weaknesses.

     -c cipher_spec
             Additionally, for protocol version 2 a comma-separated list of
             ciphers can be specified in order of preference.  See Ciphers for
             more information.

     -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 control-Z suspends the connection, and
             followed by itself sends the escape character once.  Setting the
             character to ``none'' disables any escapes and makes the session
             fully transparent.

     -f      Requests ssh to go to background just before command execution.
             ties specified in configuration files).

     -I smartcard_device
             Specifies which smartcard device to use.  The argument is the
             device ssh should use to communicate with a smartcard used for
             storing the user's private RSA key.

     -k      Disables forwarding of Kerberos tickets.  This may also be speci­
             fied on a per-host basis in the configuration file.

     -l login_name
             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.

     -m mac_spec
             Additionally, for protocol version 2 a comma-separated list of
             MAC (message authentication code) algorithms can be specified in
             order of preference.  See the MACs keyword for more information.

     -n      Redirects stdin from /dev/null (actually, prevents reading from
             stdin).  This must be used when ssh is run in the background.  A
             common trick is to use this to run X11 programs on a remote
             machine.  For example, ssh -n shadows.cs.hut.fi emacs & will
             start an emacs on shadows.cs.hut.fi, and the X11 connection will
             be automatically forwarded over an encrypted channel.  The ssh
             program will be put in the background.  (This does not work if
             ssh needs to ask for a password or passphrase; see also the -f
             option.)

     -N      Do not execute a remote command.  This is useful for just for­
             warding ports (protocol version 2 only).

     -o option
             Can be used to give options in the format used in the configura­
             tion file.  This is useful for specifying options for which there
             is no separate command-line flag.

     -p port
             Port to connect to on the remote host.  This can be specified on
             a per-host basis in the configuration file.

     -q      Quiet mode.  Causes all warning and diagnostic messages to be
             suppressed.

     -s      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 appli­
             cations (eg. sftp).  The subsystem is specified as the remote
             command.

     -t      Force pseudo-tty allocation.  This can be used to execute arbi­
             trary screen-based programs on a remote machine, which can be

     -X      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.

     -C      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(1), and the
             ``level'' can be controlled by the CompressionLevel 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 Compression option.

     -F configfile
             Specifies an alternative per-user configuration file.  If a con­
             figuration file is given on the command line, the system-wide
             configuration file (/etc/ssh/ssh_config) will be ignored.  The
             default for the per-user configuration file is $HOME/.ssh/config.

     -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 port hostport from the remote machine.  Port forwardings can
             also be specified in the configuration file.  Only root can for­
             ward privileged ports.  IPv6 addresses can be specified with an
             alternative syntax: port/host/hostport

     -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 connec­
             tion is forwarded over the secure channel, and a connection is
             made to host port hostport from the local machine.  Port forward­
             ings 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: port/host/hostport

     -D port
             Specifies a local ``dynamic'' application-level port forwarding.
             This works by allocating a socket to listen to port on the local
             side, and whenever a connection is made to this port, the connec­
             tion is forwarded over the secure channel, and the application


CONFIGURATION FILES

     ssh may additionally obtain configuration data from a per-user configura­
     tion file and a system-wide configuration file.  The file format and con­
     figuration options are described in ssh_config(5).


ENVIRONMENT

     ssh will normally set the following environment variables:

     DISPLAY
             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).

     HOME    Set to the path of the user's home directory.

     LOGNAME
             Synonym for USER; set for compatibility with systems that use
             this variable.

     MAIL    Set to the path of the user's mailbox.

     PATH    Set to the default PATH, as specified when compiling ssh.

     SSH_ASKPASS
             If ssh needs a passphrase, it will read the passphrase from the
             current terminal if it was run from a terminal.  If ssh does not
             have a terminal associated with it but DISPLAY and SSH_ASKPASS
             are set, it will execute the program specified by SSH_ASKPASS and
             open an X11 window to read the passphrase.  This is particularly
             useful when calling ssh from a .Xsession or related script.
             (Note that on some machines it may be necessary to redirect the
             input from /dev/null to make this work.)

     SSH_AUTH_SOCK
             Identifies the path of a unix-domain socket used to communicate
             with the agent.

     SSH_CONNECTION
             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.

     SSH_ORIGINAL_COMMAND
             The variable contains the original command line if a forced com­
             mand is executed.  It can be used to extract the original argu­
             ments.

     PermitUserEnvironment option in sshd_config(5).


FILES

     $HOME/.ssh/known_hosts
             Records host keys for all hosts the user has logged into that are
             not in /etc/ssh/ssh_known_hosts.  See sshd(8).

     $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
             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 ssh ignores a private key file if it is accessible by oth­
             ers.  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.

     $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
             Contains the public key for authentication (public part of the
             identity file in human-readable form).  The contents of the
             $HOME/.ssh/identity.pub file should be added to
             $HOME/.ssh/authorized_keys on all machines where the user wishes
             to log in using protocol version 1 RSA authentication.  The con­
             tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file
             should be added to $HOME/.ssh/authorized_keys 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 automati­
             cally and are not necessary; they are only provided for the con­
             venience of the user.

     $HOME/.ssh/config
             This is the per-user configuration file.  The file format and
             configuration options are described in ssh_config(5).

     $HOME/.ssh/authorized_keys
             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
             sshd(8) manual page.  In the simplest form the format is the same
             as the .pub identity files.  This file is not highly sensitive,
             but the recommended permissions are read/write for the user, and
             not accessible by others.

     /etc/ssh/ssh_known_hosts
             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, pub­
             lic key and optional comment field.  When different names are
             used for the same machine, all such names should be listed, sepa­
             rated by commas.  The format is described on the sshd(8) manual
             /etc/ssh/ssh_host_rsa_key
             These three files contain the private parts of the host keys and
             are used for RhostsRSAAuthentication and HostbasedAuthentication.
             If the protocol version 1 RhostsRSAAuthentication method is used,
             ssh must be setuid root, since the host key is readable only by
             root.  For protocol version 2, ssh uses ssh-keysign(8) to access
             the host keys for HostbasedAuthentication.  This eliminates the
             requirement that ssh be setuid root when that authentication
             method is used.  By default ssh is not setuid root.

     $HOME/.rhosts
             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.  On some machines this file may need to be
             world-readable if the user's home directory is on a NFS parti­
             tion, because sshd(8) 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 sshd(8) will be installed so that it
             requires successful RSA host authentication before permitting
             .rhosts authentication.  If the server machine does not have the
             client's host key in /etc/ssh/ssh_known_hosts, it can be stored
             in $HOME/.ssh/known_hosts.  The easiest way to do this is to con­
             nect back to the client from the server machine using ssh; this
             will automatically add the host key to $HOME/.ssh/known_hosts.

     $HOME/.shosts
             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(1).

     /etc/hosts.equiv
             This file is used during .rhosts authentication.  It contains
             canonical hosts names, one per line (the full format is described
             on the sshd(8) 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.

     /etc/ssh/shosts.equiv
             This file is processed exactly as /etc/hosts.equiv.  This file
             may be useful to permit logins using ssh but not using
             rsh/rlogin.

     /etc/ssh/sshrc
             Commands in this file are executed by ssh when the user logs in

     error occurred.


SEE ALSO

     rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
     telnet(1), ssh_config(5), ssh-keysign(8), sshd(8)

     T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
     Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
     2002, work in progress material.


AUTHORS

     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 cre­
     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.

BSD                           September 25, 1999                           BSD
  




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