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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 log_file] [-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]  [-Q
       cipher  |  cipher-auth | mac | kex | key] [-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 log_file
	      Append debug logs to log_file instead of standard error.

       -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,
	      ~/.ssh/id_ed25519 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

       CanonicalDomains

       CanonicalizeFallbackLocal

       CanonicalizeHostname

       CanonicalizeMaxDots

       CanonicalizePermittedCNAMEs

       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

       Match

       NoHostAuthenticationForLocalhost

       NumberOfPasswordPrompts

       PasswordAuthentication

       PermitLocalCommand

       PKCS11Provider

       Port

       PreferredAuthentications

       Protocol

       ProxyCommand

       ProxyUseFdpass

       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 cipher | cipher-auth | mac | kex | key
	      Queries  ssh for the algorithms supported for the specified ver‐
	      sion 2.  The available features are: cipher (supported symmetric
	      ciphers),	 cipher-auth (supported symmetric ciphers that support
	      authenticated  encryption),  mac	(supported  message  integrity
	      codes), kex (key exchange algorithms), key (key types).

       -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 brackets.

	      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.  Works with Proto‐
	      col 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, umac-128, hmac-ripemd160).  Protocol 1 lacks a	strong	mecha‐
       nism 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,  ED25519  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), ~/.ssh/id_ed25519
       (protocol 2 ED25519), or ~/.ssh/id_rsa (protocol 2 RSA) and stores  the
       public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (pro‐
       tocol	2    DSA),    ~/.ssh/id_ecdsa.pub    (protocol	  2    ECDSA),
       ~/.ssh/id_ed25519.pub  (protocol 2 ED25519), or ~/.ssh/id_rsa.pub (pro‐
       tocol 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  con‐
       ventional  ~/.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 Bx 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).

       ~V     Decrease the verbosity (LogLevel) when errors are being  written
	      to stderr.

       ~v     Increase	the verbosity (LogLevel) when errors are being written
	      to stderr.

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, ED25519,  RSA)	 that  can  be
	      used  for	 logging  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 writable by others.

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

       ~/.ssh/identity

       ~/.ssh/id_dsa

       ~/.ssh/id_ecdsa

       ~/.ssh/id_ed25519

       ~/.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_ed25519.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_ed25519_key

       /etc/ssh/ssh_host_rsa_key
	      These 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)

STANDARDS
       C.  Lonvick  and	 S. Lehtinen, The Secure Shell (SSH) Protocol Assigned
       Numbers, RFC 4250, January 2006.

       C. Lonvick and T. Ylonen, The Secure Shell (SSH) Protocol Architecture,
       RFC 4251, January 2006.

       C.  Lonvick and T. Ylonen, The Secure Shell (SSH) Authentication Proto‐
       col, RFC 4252, January 2006.

       C. Lonvick and T. Ylonen, The Secure Shell (SSH) Transport Layer Proto‐
       col, RFC 4253, January 2006.

       C.  Lonvick  and T. Ylonen, The Secure Shell (SSH) Connection Protocol,
       RFC 4254, January 2006.

       W. Griffin and J. Schlyter, Using DNS to Securely Publish Secure	 Shell
       (SSH) Key Fingerprints, RFC 4255, January 2006.

       M.  Forssen  and F. Cusack, Generic Message Exchange Authentication for
       the Secure Shell Protocol (SSH), RFC 4256, January 2006.

       P. Remaker and J. Galbraith, The Secure	Shell  (SSH)  Session  Channel
       Break Extension, RFC 4335, January 2006.

       C.  Namprempre,	T. Kohno and M. Bellare, The Secure Shell (SSH) Trans‐
       port Layer Encryption Modes, RFC 4344, January 2006.

       B. Harris, Improved Arcfour Modes for the Secure Shell (SSH)  Transport
       Layer Protocol, RFC 4345, January 2006.

       W.  Simpson, N. Provos and M. Friedl, Diffie-Hellman Group Exchange for
       the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.

       R. Thayer and J. Galbraith, The Secure Shell (SSH) Public Key File For‐
       mat, RFC 4716, November 2006.

       J.  Green  and  D. Stebila, Elliptic Curve Algorithm Integration in the
       Secure Shell Transport Layer, RFC 5656, December 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.

			       December 7 2013				SSH(1)
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