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SSH(1)									SSH(1)

NAME
       ssh - OpenSSH SSH client (remote login program)

SYNOPSIS
       ssh  [-1246AaCfgKkMNnqsTtVvXxYy]	 [-b  bind_address]  [-c  cipher_spec]
       [-D[bind_address :]port] [-e escape_char] [-F configfile]  [-I  pkcs11]
       [-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] [-W host :port]
       [-w local_tun[:remote_tun] [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 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
       (see below).

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

       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
	      Use bind_address on the local machine as the source  address  of
	      the  connection.	 Only  useful  on  systems  with more than one
	      address.

       -C     Requests compression  of	all  data  (including  stdin,  stdout,
	      stderr,  and  data  for forwarded X11 and TCP 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
	      supported 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.  See the Ciphers  keyword
	      in ssh_config(5) for more information.

       -D [bind_address:] port
	      Specifies a local ``dynamic'' application-level port forwarding.
	      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 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.

	      IPv6 addresses can be specified  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.

       -e escape_char
	      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 ~/.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
	      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.

	      If the  ExitOnForwardFailure  configuration  option  is  set  to
	      ``yes'',	then a client started with -f will wait for all remote
	      port forwards to	be  successfully  established  before  placing
	      itself in the background.

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

       -I pkcs11
	      Specify the PKCS#11 shared library ssh should use to communicate
	      with a PKCS#11 token providing the user's private RSA key.

       -i identity_file
	      Selects a file from which the identity (private key) for	public
	      key  authentication is read.  The default is ~/.ssh/identity for
	      protocol	version	 1,  and  ~/.ssh/id_dsa,  ~/.ssh/id_ecdsa  and
	      ~/.ssh/id_rsa  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 identities
	      specified in configuration files).  ssh will also	 try  to  load
	      certificate  information from the filename obtained by appending
	      -cert.pub to identity filenames.

       -K     Enables GSSAPI-based authentication and forwarding  (delegation)
	      of GSSAPI credentials to the server.

       -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 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 connec‐
	      tion 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 `*' indicates 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.  Multiple -M options places ssh into ``master''  mode  with
	      confirmation  required  before  slave  connections are accepted.
	      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), ``forward'' (request
	      forwardings without command execution), ``cancel'' (cancel  for‐
	      wardings),  ``exit''  (request the master to exit), and ``stop''
	      (request the  master  to	stop  accepting	 further  multiplexing
	      requests).

       -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

       ControlPersist

       DynamicForward

       EscapeChar

       ExitOnForwardFailure

       ForwardAgent

       ForwardX11

       ForwardX11Timeout

       ForwardX11Trusted

       GatewayPorts

       GlobalKnownHostsFile

       GSSAPIAuthentication

       GSSAPIDelegateCredentials

       HashKnownHosts

       Host

       HostbasedAuthentication

       HostKeyAlgorithms

       HostKeyAlias

       HostName

       IdentityFile

       IdentitiesOnly

       IPQoS

       KbdInteractiveAuthentication

       KbdInteractiveDevices

       KexAlgorithms

       LocalCommand

       LocalForward

       LogLevel

       MACs

       NoHostAuthenticationForLocalhost

       NumberOfPasswordPrompts

       PasswordAuthentication

       PermitLocalCommand

       PKCS11Provider

       Port

       PreferredAuthentications

       Protocol

       ProxyCommand

       PubkeyAuthentication

       RekeyLimit

       RemoteForward

       RequestTTY

       RhostsRSAAuthentication

       RSAAuthentication

       SendEnv

       ServerAliveInterval

       ServerAliveCountMax

       StrictHostKeyChecking

       TCPKeepAlive

       Tunnel

       TunnelDevice

       UsePrivilegedPort

       User

       UserKnownHostsFile

       VerifyHostKeyDNS

       VisualHostKey

       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 most 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.

	      By  default, the listening socket on the server will be bound to
	      the loopback interface only.  This may be overridden 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))
	      .

	      If the port argument is `0', the listen port will be dynamically
	      allocated on the server and reported to the client at run	 time.
	      When  used  together  with -O forward the allocated port will be
	      printed to the standard output.

       -S ctl_path
	      Specifies the location of a control socket for connection	 shar‐
	      ing,  or	the  string  ``none''  to  disable connection sharing.
	      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.

       -W host :port
	      Requests	that  standard	input and output on the client be for‐
	      warded to host on port over the secure channel.  Implies -N, -T,
	      ExitOnForwardFailure and ClearAllForwardings and works with Pro‐
	      tocol version 2 only.

       -w local_tun[:remote_tun]
	      Requests tunnel device  forwarding  with	the  specified	tun(4)
	      devices	between	  the	client	 (local_tun)  and  the	server
	      (remote_tun.)

	      The devices may be specified by  numerical  ID  or  the  keyword
	      ``any'',	which  uses  the  next	available  tunnel  device.  If
	      remote_tun is not specified, it defaults to ``any''.   See  also
	      the Tunnel and TunnelDevice directives in ssh_config(5).	If the
	      Tunnel directive is unset, it is set to the default tunnel mode,
	      which is ``point-to-point''.

       -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.

       -y     Send log information using  the  syslog(3)  system  module.   By
	      default this information is sent to stderr.

	      ssh  may	additionally obtain configuration data from a per-user
	      configuration file and a system-wide  configuration  file.   The
	      file  format and configuration options are described in ssh_con‐
	      fig(5).

AUTHENTICATION
       The OpenSSH SSH client supports SSH protocols 1 and 2.  The default  is
       to  use	protocol  2  only, though this can be changed via the Protocol
       option in ssh_config(5) or the -1 and -2	 options  (see	above).	  Both
       protocols support similar authentication methods, but protocol 2 is the
       default since it provides  additional  mechanisms  for  confidentiality
       (the  traffic  is encrypted using AES, 3DES, Blowfish, CAST128, or Arc‐
       four) and integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512,
       umac-64,	 hmac-ripemd160).   Protocol  1	 lacks	a strong mechanism for
       ensuring the integrity of the connection.

       The methods available for authentication are: GSSAPI-based  authentica‐
       tion,  host-based authentication, public key authentication, challenge-
       response authentication, and password  authentication.	Authentication
       methods are tried in the order specified above, though protocol 2 has a
       configuration option to change the default order:  PreferredAuthentica‐
       tions.

       Host-based  authentication  works  as  follows: 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 ~/.rhosts or ~/.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 login.  Additionally, the server must be able to verify
       the client's host key (see the description of  /etc/ssh/ssh_known_hosts
       and ~/.ssh/known_hosts, below) for login to be permitted.  This authen‐
       tication method closes security holes due to IP spoofing, DNS spoofing,
       and  routing  spoofing.	 [Note to the administrator: /etc/hosts.equiv,
       ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
       cure and should be disabled if security is desired.]

       Public key authentication works as follows: The scheme is based on pub‐
       lic-key cryptography, using cryptosystems where encryption and  decryp‐
       tion  are  done using separate keys, and it is unfeasible to derive the
       decryption key from the encryption key.	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.
       ssh  implements public key authentication protocol automatically, using
       one of the DSA, ECDSA or RSA algorithms.	 Protocol 1 is	restricted  to
       using  only  RSA keys, but protocol 2 may use any.  The HISTORY section
       of ssl(8) contains a brief discussion of the DSA and RSA algorithms.

       The file ~/.ssh/authorized_keys lists the public keys that are  permit‐
       ted  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
       client  proves  that  it	 has  access to the private key and the server
       checks that the corresponding public key is authorized  to  accept  the
       account.

       The  user  creates  his/her  key	 pair  by running ssh-keygen(1).  This
       stores the private key in ~/.ssh/identity (protocol  1),	 ~/.ssh/id_dsa
       (protocol  2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa
       (protocol 2 RSA) and stores the public key in ~/.ssh/identity.pub (pro‐
       tocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (pro‐
       tocol 2 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2	 RSA)  in  the	user's
       home   directory.   The	user  should  then  copy  the  public  key  to
       ~/.ssh/authorized_keys in his/her home directory on the remote machine.
       The  authorized_keys  file  corresponds	to  the conventional ~/.rhosts
       file, and has one key per line, though the  lines  can  be  very	 long.
       After this, the user can log in without giving the password.

       A  variation  on	 public key authentication is available in the form of
       certificate authentication: instead of a set  of	 public/private	 keys,
       signed  certificates  are  used.	  This has the advantage that a single
       trusted certification authority can be  used  in	 place	of  many  pub‐
       lic/private  keys.   See	 the CERTIFICATES section of ssh-keygen(1) for
       more information.

       The most convenient way to use public key or certificate authentication
       may  be with an authentication agent.  See ssh-agent(1) for more infor‐
       mation.

       Challenge-response authentication works as follows: The server sends an
       arbitrary  Qq  challenge	 text, and prompts for a response.  Protocol 2
       allows multiple challenges and responses; protocol 1 is	restricted  to
       just  one challenge/response.  Examples of challenge-response authenti‐
       cation include BSD Authentication (see  login.conf(5))  and  PAM	 (some
       non-OpenBSD systems).

       Finally, if other authentication methods fail, ssh prompts the user for
       a password.  The password is sent to the remote host for checking; how‐
       ever,  since  all  communications are encrypted, the password cannot be
       seen by someone listening on the network.

       ssh automatically maintains and checks a database containing  identifi‐
       cation  for all hosts it has ever been used with.  Host keys are stored
       in ~/.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 server spoofing or man-in-the-middle attacks,
       which could otherwise be used to circumvent the encryption.  The Stric‐
       tHostKeyChecking option can be used to control logins to machines whose
       host key is not known or has changed.

       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 connections have been closed.

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 .

       ~#     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,  -R and -D options (see above).  It
	      also allows the cancellation of existing	port-forwardings  with
	      -KL[bind_address:]port  for  local,  -KR[bind_address:]port  for
	      remote and -KD[bind_address:]port for dynamic  port-forwardings.
	      !	 Ns  command allows the user to execute a local command if the
	      PermitLocalCommand option is enabled  in	ssh_config(5).	 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).

TCP FORWARDING
       Forwarding of arbitrary TCP connections over the secure channel can  be
       specified  either  on the command line or in a configuration file.  One
       possible application of TCP forwarding is a secure connection to a mail
       server; another is going through firewalls.

       In  the	example	 below, we look at encrypting communication between an
       IRC client and server, even though the IRC  server  does	 not  directly
       support encrypted communications.  This works as follows: the user con‐
       nects to the remote host using , specifying a port to be used  to  for‐
       ward  connections  to  the remote server.  After that it is possible to
       start the service which is to be encrypted on the client machine,  con‐
       necting	to  the	 same local port, and ssh will encrypt and forward the
       connection.

       The following example  tunnels  an  IRC	session	 from  client  machine
       ``127.0.0.1'' (localhost) to remote server ``server.example.com :''

       $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
       $ irc -c '#users' -p 1234 pinky 127.0.0.1

       This tunnels a connection to IRC server ``server.example.com'', joining
       channel ``#users'', nickname ``pinky'', using port  1234.   It  doesn't
       matter which port is used, as long as it's greater than 1023 (remember,
       only root can open sockets on privileged ports)	and  doesn't  conflict
       with  any  ports	 already  in use.  The connection is forwarded to port
       6667 on the remote server, since that's the standard port for IRC  ser‐
       vices.

       The  -f	option	backgrounds ssh and the remote command ``sleep 10'' is
       specified to allow an amount of time (10 seconds, in  the  example)  to
       start the service which is to be tunnelled.  If no connections are made
       within the time specified, ssh will exit.

X11 FORWARDING
       If the ForwardX11 variable is set to ``yes'' (or see the description of
       the  -X,	 -x, and -Y options above) and the user is using X11 (the DIS‐
       PLAY 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 above) and the user is using an authentication
       agent, the connection to the agent is automatically  forwarded  to  the
       remote side.

VERIFYING HOST KEYS
       When  connecting	 to  a server for the first time, a fingerprint of the
       server's public key is presented to the user (unless the option	Stric‐
       tHostKeyChecking	 has  been  disabled).	Fingerprints can be determined
       using ssh-keygen(1):

       Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

       If the fingerprint is already known, it can be matched and the key  can
       be  accepted  or rejected.  Because of the difficulty of comparing host
       keys just by looking at hex strings, there is also support  to  compare
       host  keys  visually,  using  randomart.	  By setting the VisualHostKey
       option to ``yes'', a small ASCII graphic gets displayed on every	 login
       to a server, no matter if the session itself is interactive or not.  By
       learning the pattern a known server produces, a user  can  easily  find
       out  that  the host key has changed when a completely different pattern
       is displayed.  Because these patterns are not  unambiguous  however,  a
       pattern	that looks similar to the pattern remembered only gives a good
       probability that the host key is the same, not guaranteed proof.

       To get a listing of the fingerprints along with their  random  art  for
       all known hosts, the following command line can be used:

       Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts

       If the fingerprint is unknown, an alternative method of verification is
       available: SSH fingerprints verified by DNS.   An  additional  resource
       record (RR), SSHFP, is added to a zonefile and the connecting client is
       able to match the fingerprint with that of the key presented.

       In this example, we are connecting a client to a	 server,  ``host.exam‐
       ple.com''.   The	 SSHFP	resource  records should first be added to the
       zonefile for host.example.com:

       $ ssh-keygen -r host.example.com.

       The output lines will have to be added to the zonefile.	To check  that
       the zone is answering fingerprint queries:

       Dl $ dig -t SSHFP host.example.com

       Finally the client connects:

       $ ssh -o "VerifyHostKeyDNS ask" host.example.com
       [...]
       Matching host key fingerprint found in DNS.
       Are you sure you want to continue connecting (yes/no)?

       See the VerifyHostKeyDNS option in ssh_config(5) for more information.

SSH-BASED VIRTUAL PRIVATE NETWORKS
       ssh contains support for Virtual Private Network (VPN) tunnelling using
       the tun(4) network pseudo-device, allowing two networks	to  be	joined
       securely.   The	sshd_config(5)	configuration option PermitTunnel con‐
       trols whether the server supports this, and at what level (layer 2 or 3
       traffic).

       The  following  example	would connect client network 10.0.50.0/24 with
       remote network 10.0.99.0/24  using  a  point-to-point  connection  from
       10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gate‐
       way to the remote network, at 192.168.1.15, allows it.

       On the client:

       # ssh -f -w 0:1 192.168.1.15 true
       # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
       # route add 10.0.99.0/24 10.1.1.2

       On the server:

       # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
       # route add 10.0.50.0/24 10.1.1.1

       Client access may  be  more  finely  tuned  via	the  /root/.ssh/autho‐
       rized_keys file (see below) and the PermitRootLogin server option.  The
       following entry would permit connections on tun(4) device 1  from  user
       ``jane''	 and on tun device 2 from user ``john'', if PermitRootLogin is
       set to ``forced-commands-only :''

       tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
       tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john

       Since an SSH-based setup entails a fair amount of overhead, it  may  be
       more  suited to temporary setups, such as for wireless VPNs.  More per‐
       manent VPNs are better  provided	 by  tools  such  as  ipsecctl(8)  and
       isakmpd(8).

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_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
	      This variable contains the original command  line	 if  a	forced
	      command  is  executed.   It  can be used to extract the original
	      arguments.

       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     This variable is set to indicate the present time zone 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 ~/.ssh/environment, and  adds  lines  of
	      the  format  ``VARNAME=value''  to  the  environment if the file
	      exists and users are allowed to change their  environment.   For
	      more   information,  see	the  PermitUserEnvironment  option  in
	      sshd_config(5).

FILES
       ~/.rhosts
	      This file is used for host-based authentication (see above).  On
	      some  machines  this  file  may need to be world-readable if the
	      user's home directory is on an NFS  partition,  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.

       ~/.shosts
	      This file is used in exactly the same way as .rhosts, but allows
	      host-based   authentication   without   permitting   login  with
	      rlogin/rsh.

       ~/.ssh/
	      This directory is the default  location  for  all	 user-specific
	      configuration  and authentication information.  There is no gen‐
	      eral requirement to keep the entire contents of  this  directory
	      secret,  but  the recommended permissions are read/write/execute
	      for the user, and not accessible by others.

       ~/.ssh/authorized_keys
	      Lists the public keys (DSA/ECDSA/RSA) that can be used for  log‐
	      ging  in	as this user.  The format of this file is described in
	      the sshd(8) manual page.	This file is not highly sensitive, but
	      the recommended permissions are read/write for the user, and not
	      accessible by others.

       ~/.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.

       ~/.ssh/environment
	      Contains	additional  definitions for environment variables; see
	      ENVIRONMENT , above.

       ~/.ssh/identity

       ~/.ssh/id_dsa

       ~/.ssh/id_ecdsa

       ~/.ssh/id_rsa
	      Contains the private key for authentication.  These  files  con‐
	      tain  sensitive  data and should be readable by the user but not
	      accessible by  others  (read/write/execute).   ssh  will	simply
	      ignore  a private key file if it is accessible by others.	 It is
	      possible to specify a passphrase when generating the  key	 which
	      will  be	used  to encrypt the sensitive part of this file using
	      3DES.

       ~/.ssh/identity.pub

       ~/.ssh/id_dsa.pub

       ~/.ssh/id_ecdsa.pub

       ~/.ssh/id_rsa.pub
	      Contains the public key for authentication.  These files are not
	      sensitive and can (but need not) be readable by anyone.

       ~/.ssh/known_hosts
	      Contains	a  list of host keys for all hosts the user has logged
	      into that are not already in the systemwide list of  known  host
	      keys.   See  sshd(8)  for	 further details of the format of this
	      file.

       ~/.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.

       /etc/hosts.equiv
	      This file is for	host-based  authentication  (see  above).   It
	      should only be writable by root.

       /etc/ssh/shosts.equiv
	      This  file  is  used in exactly the same way as hosts.equiv, but
	      allows host-based authentication without permitting  login  with
	      rlogin/rsh.

       /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_ecdsa_key

       /etc/ssh/ssh_host_rsa_key
	      These three files contain the private parts of the host keys and
	      are  used	 for host-based authentication.	 If protocol version 1
	      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, eliminating the requirement that ssh be
	      setuid  root when host-based authentication is used.  By default
	      ssh is not setuid root.

       /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.	 It should  be	world-
	      readable.	 See sshd(8) for further details of the format of this
	      file.

       /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.

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

SEE ALSO
       scp(1),	 sftp(1),   ssh-add(1),	  ssh-agent(1),	  ssh-keygen(1),  ssh-
       keyscan(1),  tun(4),  hosts.equiv(5),  ssh_config(5),   ssh-keysign(8),
       sshd(8)

       The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.

       The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.

       The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.

       The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.

       The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.

       Using  DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
       4255, 2006.

       Generic Message Exchange Authentication for the Secure  Shell  Protocol
       (SSH), RFC 4256, 2006.

       The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.

       The  Secure  Shell  (SSH)  Transport  Layer Encryption Modes, RFC 4344,
       2006.

       Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer  Pro‐
       tocol, RFC 4345, 2006.

       Diffie-Hellman  Group  Exchange	for  the  Secure Shell (SSH) Transport
       Layer Protocol, RFC 4419, 2006.

       The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

       Elliptic Curve Algorithm Integration  in	 the  Secure  Shell  Transport
       Layer, RFC 5656, 2009.

       D.  Song	 and A. Perrig, Hash Visualization: a New Technique to improve
       Real-World Security, 1999,  "International  Workshop  on	 Cryptographic
       Techniques and E-Commerce (CrypTEC '99)".

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 11 2011				SSH(1)
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