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SSH(1)									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\fP
       [bind_address:]	port:host:hostport ] [-l login_name] [-m mac_spec] [-O
       ctl_cmd] [-o option] [-p port] [-R\fP  [bind_address:]  port:host:host‐
       port ] [-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
       untrusted hosts over an insecure network.  X11  connections  and	 arbi‐
       trary 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
       combined	 with  RSA-based host authentication.  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, 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 machine 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 per‐
       mitted.	This authentication method closes security  holes  due	to  IP
       spoofing,  DNS spoofing and routing spoofing.  [Note to the administra‐
       tor: /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  authentica‐
       tion.   The scheme is based on public-key cryptography: there are cryp‐
       tosystems 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
       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(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 corre‐
       sponds 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
       authentication 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 PreferredAuthenti‐
       cations, 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 to prove the user's identity.

       Additionally, ssh supports hostbased or challenge response  authentica‐
       tion.

       Protocol	 2  provides  additional  mechanisms  for confidentiality (the
       traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour) and
       integrity  (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
       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
       status 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
       functions through the use of an escape character.

       A single tilde character can be sent as ~~ or by following the tilde by
       a  character  other  than  those described below.  The escape character
       must always follow a newline to be interpreted as special.  The	escape
       character  can  be  changed in configuration files using the EscapeChar
       configuration 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
	      allows 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
       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 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 for‐
       warding 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  for‐
       warded  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‐
       warded 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 identifi‐
       cations 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 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 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
       resource 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 mate‐
	      rial 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
	      different	 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 interop‐
	      erability with legacy protocol 1	implementations	 that  do  not
	      support the 3des cipher.	Its use is strongly discouraged due to
	      cryptographic 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 con‐
	      nection is forwarded over the secure channel, and	 the  applica‐
	      tion 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/con‐
	      fig.

       -f     Requests	ssh to go to background just before command execution.
	      This is  useful  if  ssh	is  going  to  ask  for	 passwords  or
	      passphrases,  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
	      device 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
	      also 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 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.	 IPv6 addresses can be
	      specified with an alternative syntax:
	       [bind_address/] port/host/ hostport 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.	 However, an explicit bind_address may
	      be used to bind the  connection  to  a  specific	address.   The
	      bind_address  of ``localhost'' indicates that the listening port
	      be bound for local use only, while an empty address or `*' indi‐
	      cates that the port should be available 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
	      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.)

       -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
	      options  listed  below,  and their possible values, see ssh_con‐
	      fig(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 con‐
	      nection 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  specify‐
	      ing  a bind_address.  An empty bind_address, or the address `*',
	      indicates that the remote socket should  listen  on  all	inter‐
	      faces.   Specifying  a  remote bind_address will only succeed if
	      the server'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
	      facilitate the use of SSH as a secure transport for other appli‐
	      cations (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  dis‐
	      play  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
	      extension	 restrictions  by default.  Please refer to the ssh -Y
	      option 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 configu‐
       ration  file and a system-wide configuration file.  The file format and
       configuration 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  re‐
	      direct 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.

       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, respec‐
	      tively.  These files contain sensitive data and should be	 read‐
	      able  by	the user but not accessible by others (read/write/exe‐
	      cute).  Note that ssh ignores a private key file if it is acces‐
	      sible  by	 others.   It is possible to specify a passphrase when
	      generating the key; the passphrase will be used to  encrypt  the
	      sensitive part of this file using 3DES.

       $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 automatically and are not necessary; they are only provided
	      for the convenience 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  permis‐
	      sions: 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 sensi‐
	      tive,  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  pre‐
	      pared  by	 the  system  administrator to contain the public host
	      keys of all machines in the organization.	 This file  should  be
	      world-readable.	This  file contains public keys, one per line,
	      in the following format (fields  separated  by  spaces):	system
	      name,  public  key  and  optional comment field.	When different
	      names are used for the same machine, all such  names  should  be
	      listed,  separated  by  commas.	The format is described in the
	      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 some‐
	      one with access to the name servers would then be able  to  fool
	      host authentication.

       /etc/ssh/ssh_config
	      Systemwide  configuration	 file.	The file format and configura‐
	      tion 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  HostbasedAuthentica‐
	      tion.   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 elim‐
	      inates the requirement that ssh be setuid root when that authen‐
	      tication method is used.	By default ssh is not setuid root.

       $HOME/.rhosts
	      This file is used in  RhostsRSAAuthentication  and  HostbasedAu‐
	      thentication 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 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  RhostsRSAAuthentica‐
	      tion  and HostbasedAuthentication authentication without permit‐
	      ting login with rlogin() or rsh(1).

       /etc/hosts.equiv
	      This file	 is  used  during  RhostsRSAAuthentication  and	 Host‐
	      basedAuthentication 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 authen‐
	      tication is 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
	      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-key‐
       gen(1),	telnet(1),  hosts.equiv(5),   ssh_config(5),   ssh-keysign(8),
       sshd(8)

       S.  Lehtinen, T. Rinne, M. Saarinen, T. Kivinen and T. Ylonen, SSH Pro‐
       tocol 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.

			      September 25, 1999			SSH(1)
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