path: root/ssh.0

SSH(1)                     OpenBSD Reference Manual                     SSH(1)

NAME
     ssh - OpenSSH SSH client (remote login program)

SYNOPSIS
     ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec]
         [-D port] [-e escape_char] [-F configfile] [-i identity_file]
         [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec]
         [-O ctl_cmd] [-o option] [-p port]
         [-R [bind_address:]port:host:hostport] [-S ctl_path] [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 untrust-
     ed 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 (with optional user
     name).  The user must prove his/her identity to the remote machine using
     one of several methods depending on the protocol version used.

     If command is specified, command is executed on the remote host instead
     of a login shell.

   SSH protocol version 1
     The first authentication method is the rhosts or hosts.equiv method com-
     bined with RSA-based host authentication.  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, or if the files
     $HOME/.rhosts or $HOME/.shosts exist in the user's home directory on the
     remote machine and contain a line containing the name of the client ma-
     chine and the name of the user on that machine, the user is considered
     for log in.  Additionally, if 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 is login permitted.  This authentication method clos-
     es security holes due to IP spoofing, 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 second 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 per-
     mitted 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 serv-
     er 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 num-
     ber, encrypted by the user's public key.  The challenge can only be de-
     crypted using the proper private key.  The user's client then decrypts
     the challenge using the private key, proving that he/she knows the pri-
     vate 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 stores 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.

     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 serv-
     er.

     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, ssh supports hostbased or challenge response authentica-
     tion.

     Protocol 2 provides additional mechanisms for confidentiality (the traf-
     fic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour) and in-
     tegrity (hmac-md5, hmac-sha1, hmac-ripemd160).  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 ei-
     ther 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 sta-
     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 func-
     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 charac-
     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.  Currently this allows the addition of port
             forwardings using the -L and -R options (see below).  It also al-
             lows the cancellation of existing remote port-forwardings using
             -KR hostport.  Basic help is available, using the -h option.

     ~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 encrypt-
     ed 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 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 authenti-
     cation 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.

   Server authentication
     ssh automatically maintains and checks a database containing identifica-
     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
     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.

     ssh can be configured to verify host identification using fingerprint re-
     source records (SSHFP) published in DNS.  The VerifyHostKeyDNS option can
     be used to control how DNS lookups are performed.  SSHFP resource records
     can be generated using ssh-keygen(1).

     The options are as follows:

     -1      Forces ssh to try protocol version 1 only.

     -2      Forces ssh to try protocol version 2 only.

     -4      Forces ssh to use IPv4 addresses only.

     -6      Forces ssh to use IPv6 addresses only.

     -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.

     -a      Disables forwarding of the authentication agent connection.

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

     -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.

     -c cipher_spec
             Selects the cipher specification for encrypting the session.

             Protocol version 1 allows specification of a single cipher.  The
             suported values are ``3des'', ``blowfish'' and ``des''.  3des
             (triple-des) is an encrypt-decrypt-encrypt triple with three dif-
             ferent keys.  It is believed to be secure.  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 interoperabil-
             ity with legacy protocol 1 implementations that do not support
             the 3des cipher.  Its use is strongly discouraged due to crypto-
             graphic weaknesses.  The default is ``3des''.

             For protocol version 2 cipher_spec is a comma-separated list of
             ciphers listed in order of preference.  The supported ciphers are
             ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
             ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour'',
             ``blowfish-cbc'', and ``cast128-cbc''.  The default is

               ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
                 aes192-cbc,aes256-cbc''

     -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
             protocol is then used to determine where to connect to from the
             remote machine.  Currently the SOCKS4 and SOCKS5 protocols are
             supported, and ssh will act as a SOCKS server.  Only root can
             forward privileged ports.  Dynamic port forwardings can also be
             specified in the configuration file.

     -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 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.

     -f      Requests ssh to go to background just before command execution.
             This is useful if ssh is going to ask for passwords or passphras-
             es, but the user wants it in the background.  This implies -n.
             The recommended way to start X11 programs at a remote site is
             with something like ssh -f host xterm.

     -g      Allows remote hosts to connect to local forwarded ports.

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

     -i identity_file
             Selects a file from which the identity (private key) for RSA or
             DSA authentication is read.  The default is $HOME/.ssh/identity
             for protocol version 1, and $HOME/.ssh/id_rsa and
             $HOME/.ssh/id_dsa for protocol version 2.  Identity files may al-
             so be specified on a per-host basis in the configuration file.
             It is possible to have multiple -i options (and multiple identi-
             ties specified in configuration files).

     -k      Disables forwarding (delegation) of GSSAPI credentials to the
             server.

     -L [bind_address:]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,
             optionally bound to the specified bind_address.  Whenever a con-
             nection 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.  IPv6 addresses can be spec-
             ified with an alternative syntax: [bind_address/]port/host/host-
             port or by enclosing the address in square brackets.  Only the
             superuser can forward privileged ports.  By default, the local
             port is bound in accordance with the GatewayPorts setting.  How-
             ever, an explicit bind_address may be used to bind the connection
             to a specific address.  The bind_address of ``localhost'' indi-
             cates that the listening port be bound for local use only, while
             an empty address or `*' indicates that the port should be avail-
             able from all interfaces.

     -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      Places the ssh client into ``master'' mode for connection shar-
             ing.  Refer to the description of ControlMaster in ssh_config(5)
             for details.

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

     -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 ma-
             chine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
             an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
             tomatically 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.)

     -O ctl_cmd
             Control an active connection multiplexing master process.  When
             the -O option is specified, the ctl_cmd argument is interpreted
             and passed to the master process.  Valid commands are: ``check''
             (check that the master process is running) and ``exit'' (request
             the master to exit).

     -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.  For full details of the op-
             tions listed below, and their possible values, see ssh_config(5).

                   AddressFamily
                   BatchMode
                   BindAddress
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   DynamicForward
                   EscapeChar
                   ForwardAgent
                   ForwardX11
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentityFile
                   IdentitiesOnly
                   KbdInteractiveDevices
                   LocalForward
                   LogLevel
                   MACs
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   Port
                   PreferredAuthentications
                   Protocol
                   ProxyCommand
                   PubkeyAuthentication
                   RemoteForward
                   RhostsRSAAuthentication
                   RSAAuthentication
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   SmartcardDevice
                   StrictHostKeyChecking
                   TCPKeepAlive
                   UsePrivilegedPort
                   User
                   UserKnownHostsFile
                   VerifyHostKeyDNS
                   XAuthLocation

     -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.

     -R [bind_address:]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 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 by enclosing
             the address in square braces or using an alternative syntax:
             [bind_address/]host/port/hostport.

             By default, the listening socket on the server will be bound to
             the loopback interface only.  This may be overriden by specifying
             a bind_address.  An empty bind_address, or the address `*', indi-
             cates that the remote socket should listen on all interfaces.
             Specifying a remote bind_address will only succeed if the serv-
             er's GatewayPorts option is enabled (see sshd_config(5)).

     -S ctl_path
             Specifies the location of a control socket for connection shar-
             ing.  Refer to the description of ControlPath and ControlMaster
             in ssh_config(5) for details.

     -s      May be used to request invocation of a subsystem on the remote
             system.  Subsystems are a feature of the SSH2 protocol which fa-
             cilitate the use of SSH as a secure transport for other applica-
             tions (eg. sftp(1)).  The subsystem is specified as the remote
             command.

     -T      Disable pseudo-tty allocation.

     -t      Force pseudo-tty allocation.  This can be used to execute arbi-
             trary screen-based programs on a remote machine, which can be
             very useful, e.g., when implementing menu services.  Multiple -t
             options force tty allocation, even if ssh has no local tty.

     -V      Display the version number and exit.

     -v      Verbose mode.  Causes ssh to print debugging messages about its
             progress.  This is helpful in debugging connection, authentica-
             tion, and configuration problems.  Multiple -v options increase
             the verbosity.  The maximum is 3.

     -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.

             For this reason, X11 forwarding is subjected to X11 SECURITY ex-
             tension restrictions by default.  Please refer to the ssh -Y op-
             tion and the ForwardX11Trusted directive in ssh_config(5) for
             more information.

     -x      Disables X11 forwarding.

     -Y      Enables trusted X11 forwarding.  Trusted X11 forwardings are not
             subjected to the X11 SECURITY extension controls.

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 manu-
              ally 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 particu-
              larly 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-ad-
              dress, 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.

     SSH_TTY  This is set to the name of the tty (path to the device) associ-
              ated with the current shell or command.  If the current session
              has no tty, this variable is not set.

     TZ       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).

     USER     Set to the name of the user logging in.

     Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the
     format ``VARNAME=value'' to the environment if the file exists and if
     users are allowed to change their environment.  For more information, see
     the 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 the file
             $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).  Because of
             the potential for abuse, this file must have strict permissions:
             read/write for the user, and not accessible by others.

     $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 in the sshd(8) manual
             page.

             The canonical system name (as returned by name servers) is used
             by sshd(8) 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 au-
             thentication.

     /etc/ssh/ssh_config
             Systemwide configuration file.  The file format and configuration
             options are described in ssh_config(5).

     /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key,
             /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 RhostsRSAAuthentication and
             HostbasedAuthentication 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 sshd(8) allows authentication only in combination with
             client host key authentication before permitting log in.  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 connect
             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 RhostsRSAAuthentication
             and HostbasedAuthentication authentication without permitting lo-
             gin with rlogin or rsh(1).

     /etc/hosts.equiv
             This file is used during RhostsRSAAuthentication and
             HostbasedAuthentication authentication.  It contains canonical
             hosts names, one per line (the full format is described in 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 client host key au-
             thentication is required.  This file should only be writable by
             root.

     /etc/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
             just before the user's shell (or command) is started.  See the
             sshd(8) manual page for more information.

     $HOME/.ssh/rc
             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(8) manual page for more information.

     $HOME/.ssh/environment
             Contains additional definitions for environment variables, see
             section ENVIRONMENT above.

DIAGNOSTICS
     ssh exits with the exit status of the remote command or with 255 if an
     error occurred.

SEE ALSO
     gzip(1), rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1),
     ssh-keygen(1), telnet(1), hosts.equiv(5), 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
     created OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.

OpenBSD 3.6                   September 25, 1999                            11