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SSH(1)                    BSD General Commands Manual                   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 arbitrary TCP ports can also be forwarded over the secure chan-
     nel.

     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 material from the agent, however they can perform
             operations on the keys that enable them to authenticate using the identities
             loaded into the agent.

     -a      Disables forwarding of the authentication agent connection.

     -b bind_address
             Use bind_address on the local machine as the source address of the connec-
             tion.  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 interoperabil-
             ity 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 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.  Cur-
             rently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a
             SOCKS server.  Only root can forward privileged ports.  Dynamic port forward-
             ings 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 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 '*' 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 charac-
             ter followed by a dot ('.') closes the connection; followed by control-Z sus-
             pends 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 configuration
             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 configu-
             ration file is ~/.ssh/config.

     -f      Requests ssh to go to background just before command execution.  This is use-
             ful 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 success-
             fully established before placing itself in the background.

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

     -I pkcs11
             Specify the PKCS#11 shared libarary ssh should use to communicate with a
             PKCS#11 token used for storing the user's private RSA key.  This option is
             only available if support for PKCS#11 is compiled in (default is no support).

     -i identity_file
             Selects a file from which the identity (private key) for public key authenti-
             cation 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 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 listen-
             ing 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 sharing.  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 forwarding 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 run-
             ning) and "exit" (request the master to exit).

     -o option
             Can be used to give options in the format used in the configuration 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_config(5).

                   AddressFamily
                   BatchMode
                   BindAddress
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   ForwardAgent
                   ForwardX11
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIKeyExchange
                   GSSAPIClientIdentity
                   GSSAPIDelegateCredentials
                   GSSAPIRenewalForcesRekey
                   GSSAPITrustDns
                   GSSAPIKexAlgorithms
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentityFile
                   IdentitiesOnly
                   KbdInteractiveAuthentication
                   KbdInteractiveDevices
                   KexAlgorithms
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   Protocol
                   ProxyCommand
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteForward
                   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 connection 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 overridden by specifying a bind_address.  An
             empty bind_address, or the address '*', indicates that the remote socket
             should listen on all interfaces.  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.

     -S ctl_path
             Specifies the location of a control socket for 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.  Sub-
             systems are a feature of the SSH2 protocol which facilitate the use of SSH as
             a secure transport for other applications (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 arbitrary 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, authentication, 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 forwarded to host on
             port over the secure channel.  Implies -N, -T, ExitOnForwardFailure and
             ClearAllForwardings and works with Protocol 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 display through the forwarded connection.
             An attacker may then be able to perform activities such as keystroke monitor-
             ing.

             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_config(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 con-
     fidentiality (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 authentication, 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:
     PreferredAuthentications.

     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 con-
     sidered 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 authentication 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
     insecure and should be disabled if security is desired.]

     Public key authentication works as follows: The scheme is based on public-key cryp-
     tography, using cryptosystems where encryption and decryption 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 imple-
     ments 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 two
     algorithms.

     The file ~/.ssh/authorized_keys lists the public keys that are permitted 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 (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA),
     ~/.ssh/id_ecdsa.pub (protocol 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 public/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 information.

     Challenge-response authentication works as follows: The server sends an arbitrary
     "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 authentication 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; however, since all communica-
     tions are encrypted, the password cannot be seen by someone listening on the network.

     ssh automatically maintains and checks a database containing identification 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 automati-
     cally 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
     StrictHostKeyChecking 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 automati-
     cally 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 ssh.

     ~#      List forwarded connections.

     ~&      Background ssh at logout when waiting for forwarded connection / X11 sessions
             to terminate.

     ~?      Display a list of escape characters.

     ~B      Send a BREAK to the remote system (only useful for SSH protocol version 2 and
             if the peer supports it).

     ~C      Open command line.  Currently this allows the addition of port forwardings
             using the -L, -R and -D options (see above).  It also allows the cancellation
             of existing remote port-forwardings using -KR[bind_address:]port.  !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 communica-
     tions.  This works as follows: the user connects to the remote host using ssh, speci-
     fying a port to be used to forward 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" (local-
     host) 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 for-
     warded 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 DISPLAY environment variable is
     set), the connection to the X11 display is automatically forwarded to the remote side
     in such a way that any X11 programs started from the shell (or command) will go
     through the encrypted channel, and the connection to the real X server will be made
     from the local machine.  The user should not manually set DISPLAY.  Forwarding of X11
     connections can be configured on the command line or in configuration files.

     The DISPLAY value set by ssh will point to the server machine, but with a display
     number greater than zero.  This is normal, and happens because ssh creates a "proxy"
     X server on the server machine for forwarding the connections over the encrypted
     channel.

     ssh will also automatically set up Xauthority data on the server machine.  For this
     purpose, it will generate a random authorization cookie, store it in Xauthority on
     the server, and verify that any forwarded connections carry this cookie and replace
     it by the real cookie when the connection is opened.  The real authentication cookie
     is never sent to the server machine (and no cookies are sent in the plain).

     If the ForwardAgent variable is set to "yes" (or see the description of the -A and -a
     options 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 StrictHostKeyChecking has been dis-
     abled).  Fingerprints can be determined using ssh-keygen(1):

           $ 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 random art.  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 guaran-
     teed proof.

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

           $ 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.example.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:

           $ 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 controls 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 gateway 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/authorized_keys file (see
     below) and the PermitRootLogin server option.  The following entry would permit con-
     nections 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 permanent 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.

     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 particularly useful when calling ssh from a .xsession
                           or related script.  (Note that on some machines it may be nec-
                           essary to redirect the input from /dev/null to make this work.)

     SSH_AUTH_SOCK         Identifies the path of a UNIX-domain socket used to communicate
                           with the agent.

     SSH_CONNECTION        Identifies the client and server ends of the connection.  The
                           variable contains four space-separated values: client IP
                           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).

ENVIRONMENT
     SSH_USE_STRONG_RNG
             The reseeding of the OpenSSL random generator is usually done from
             /dev/urandom.  If the SSH_USE_STRONG_RNG environment variable is set to value
             other than 0 the OpenSSL random generator is reseeded from /dev/random.  The
             number of bytes read is defined by the SSH_USE_STRONG_RNG value.  Minimum is
             14 bytes.  This setting is not recommended on the computers without the hard-
             ware random generator because insufficient entropy causes the connection to
             be blocked until enough entropy is available.

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 any-
             one 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 general 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 (RSA/ECDSA/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 permissions: 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 contain 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 fur-
             ther 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 configuration 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 prepared by the sys-
             tem 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.

IPV6
     IPv6 address can be used everywhere where IPv4 address. In all entries must be the
     IPv6 address enclosed in square brackets. Note: The square brackets are metacharac-
     ters for the shell and must be escaped in shell.

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 Protocol, 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.

     A. Perrig and D. Song, 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 con-
     tributed the support for SSH protocol versions 1.5 and 2.0.

BSD                            December 12, 2017                           BSD

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