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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\fP
	[bind_address:] port ] [-e  escape_char]  [-F  configfile]  [-i	 iden-
       tity_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:hostport	  ]    [-S    ctl_path]	   [-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.   The  supported  ciphers
	      are:  3des-cbc,  aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr,
	      aes192-ctr, aes256-ctr, arcfour128, arcfour256,  arcfour,	 blow-
	      fish-cbc, and cast128-cbc.  The default is:

	      aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
	      arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
	      aes192-ctr,aes256-ctr

       -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 with an alternative syntax:
	       [bind_address/] port or by  enclosing  the  address  in	square
	      brackets.	  Only the superuser can forward privileged ports.  By
	      default, the local port is bound in accordance with the Gateway-
	      Ports 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 `*' indicates 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.

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

       -I smartcard_device
	      Specify the device ssh should use to communicate with  a	smart-
	      card  used  for storing the user's private RSA key.  This option
	      is only available if support for smartcard devices  is  compiled
	      in (default is no support).

       -i identity_file
	      Selects  a file from which the identity (private key) for RSA or
	      DSA authentication is read.  The default is ~/.ssh/identity  for
	      protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro-
	      tocol 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  con-
	      figuration 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.  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) 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

       ExitOnForwardFailure

       ForwardAgent

       ForwardX11

       ForwardX11Trusted

       GatewayPorts

       GlobalKnownHostsFile

       GSSAPIAuthentication

       GSSAPIDelegateCredentials

       HashKnownHosts

       Host

       HostbasedAuthentication

       HostKeyAlgorithms

       HostKeyAlias

       HostName

       IdentityFile

       IdentitiesOnly

       KbdInteractiveDevices

       LocalCommand

       LocalForward

       LogLevel

       MACs

       NoHostAuthenticationForLocalhost

       NumberOfPasswordPrompts

       PasswordAuthentication

       PermitLocalCommand

       Port

       PreferredAuthentications

       Protocol

       ProxyCommand

       PubkeyAuthentication

       RekeyLimit

       RemoteForward

       RhostsRSAAuthentication

       RSAAuthentication

       SendEnv

       ServerAliveInterval

       ServerAliveCountMax

       SmartcardDevice

       StrictHostKeyChecking

       TCPKeepAlive

       Tunnel

       TunnelDevice

       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.

       -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.

	      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).

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

AUTHENTICATION
       The OpenSSH SSH client supports SSH protocols 1 and 2.  Protocol	 2  is
       the default, with ssh falling back to protocol 1 if it detects protocol
       2 is unsupported.  These settings may be	 altered  using	 the  Protocol
       option  in  ssh_config(5), or enforced using the -1 and -2 options (see
       above).	Both protocols support	similar	 authentication	 methods,  but
       protocol	 2  is	preferred  since it provides additional mechanisms for
       confidentiality (the traffic is encrypted using	AES,  3DES,  Blowfish,
       CAST128,	  or   Arcfour)	 and  integrity	 (hmac-md5,  hmac-sha1,	 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
       either  the  RSA	 or DSA algorithms.  Protocol 1 is restricted to using
       only RSA keys, but protocol 2 may use either.  The HISTORY  section  of
       ssl(8) contains a brief discussion of the two 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), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the pub-
       lic  key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (proto-
       col 2 DSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in  the  user's  home
       directory.   The	 user should then copy the public key to ~/.ssh/autho-
       rized_keys in his/her home directory on the remote machine.  The autho-
       rized_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.

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

       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 and -R options (see above).  It also
	      allows the  cancellation	of  existing  remote  port-forwardings
	      using  -KR[bind_address:]port.   ! 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 verified,
       and the key can be accepted.  If the fingerprint is unknown, an	alter-
       native  method  of verification is available: SSH fingerprints verified
       by DNS.	An additional resource record (RR), SSHFP, is added to a zone-
       file  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 Ux-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/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.  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_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_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_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.

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, 2006.

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

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

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

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

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

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

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

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

       ,  Improved  Arcfour  Modes  for the Secure Shell (SSH) Transport Layer
       Protocol, 2006.

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

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

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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