ssh(1) General Commands Manual ssh(1)

NAME

 ssh - OpenSSH remote login client

SYNOPSIS

 ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
 [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
 [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
 [-J destination] [-L address] [-l login_name] [-m mac_spec]
 [-O ctl_cmd] [-o option] [-P tag] [-p port] [-R address]
 [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination
 [command [argument ...]]
 ssh [-Q query_option]

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 provide secure
 encrypted communications between two untrusted hosts over an insecure
 network. X11 connections, arbitrary TCP ports and UNIX-domain sockets
 can also be forwarded over the secure channel.
 ssh connects and logs into the specified destination, which may be
 specified as either [user@]hostname or a URI of the form
 ssh://[user@]hostname[:port]. The user must prove their identity to the
 remote machine using one of several methods (see below).
 If a command is specified, it will be executed on the remote host instead
 of a login shell. A complete command line may be specified as command,
 or it may have additional arguments. If supplied, the arguments will be
 appended to the command, separated by spaces, before it is sent to the
 server to be executed.
 The options are as follows:
 -4 Forces ssh to use IPv4 addresses only.
 -6 Forces ssh to use IPv6 addresses only.
 -A Enables forwarding of connections from an authentication agent
 such as ssh-agent(1) . 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 safer alternative may be to use a jump host (see
 -J).
 -a Disables forwarding of the authentication agent connection.
 -B bind_interface
 Bind to the address of bind_interface before attempting to
 connect to the destination host. This is only useful on systems
 with more than one address.
 -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, TCP and UNIX-domain
 connections). The compression algorithm is the same used by
 gzip(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 in ssh_config(5) .
 -c cipher_spec
 Selects the cipher specification for encrypting the session.
 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 `*' indicates
 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
 configuration file is given on the command line, the system-wide
 configuration file (/opt/local/etc/ssh/ssh_config) will be
 ignored. The default for the per-user configuration file is
 ~/.ssh/config. If set to "none", no configuration files will be
 read.
 -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. Refer to the description of
 ForkAfterAuthentication in ssh_config(5)  for details.
 -G Causes ssh to print its configuration after evaluating Host and
 Match blocks and exit.
 -g Allows remote hosts to connect to local forwarded ports. If used
 on a multiplexed connection, then this option must be specified
 on the master process.
 -I pkcs11
 Specify the PKCS#11 shared library ssh should use to communicate
 with a PKCS#11 token providing keys for user authentication.
 -i identity_file
 Selects a file from which the identity (private key) for public
 key authentication is read. You can also specify a public key
 file to use the corresponding private key that is loaded in
 ssh-agent(1)  when the private key file is not present locally.
 The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa,
 ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519 and ~/.ssh/id_ed25519_sk.
 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). If
 no certificates have been explicitly specified by the
 CertificateFile directive, ssh will also try to load certificate
 information from the filename obtained by appending -cert.pub to
 identity filenames.
 -J destination
 Connect to the target host by first making an ssh connection to
 the jump host described by destination and then establishing a
 TCP forwarding to the ultimate destination from there. Multiple
 jump hops may be specified separated by comma characters. IPv6
 addresses can be specified by enclosing the address in square
 brackets. This is a shortcut to specify a ProxyJump
 configuration directive. Note that configuration directives
 supplied on the command-line generally apply to the destination
 host and not any specified jump hosts. Use ~/.ssh/config to
 specify configuration for jump hosts.
 -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
 -L [bind_address:]port:remote_socket
 -L local_socket:host:hostport
 -L local_socket:remote_socket
 Specifies that connections to the given TCP port or Unix socket
 on the local (client) host are to be forwarded to the given host
 and port, or Unix socket, on the remote side. This works by
 allocating a socket to listen to either a TCP port on the local
 side, optionally bound to the specified bind_address, or to a
 Unix socket. Whenever a connection is made to the local port or
 socket, the connection is forwarded over the secure channel, and
 a connection is made to either host port hostport, or the Unix
 socket remote_socket, from the remote machine.
 Port forwardings can also be specified in the configuration file.
 Only the superuser can forward privileged ports. IPv6 addresses
 can be specified by enclosing the address in square brackets.
 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.
 -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 but with
 confirmation required using ssh-askpass(1) before each operation
 that changes the multiplexing state (e.g. opening a new session).
 Refer to the description of ControlMaster in ssh_config(5)  for
 details.
 -m mac_spec
 A comma-separated list of MAC (message authentication code)
 algorithms, specified in order of preference. See the MACs
 keyword in ssh_config(5)  for more information.
 -N Do not execute a remote command. This is useful for just
 forwarding ports. Refer to the description of SessionType in
 ssh_config(5)  for details.
 -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.) Refer to the description of StdinNull in ssh_config(5) 
 for details.
 -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
 forwardings), "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
 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) .
 AddKeysToAgent
 AddressFamily
 BatchMode
 BindAddress
 CanonicalDomains
 CanonicalizeFallbackLocal
 CanonicalizeHostname
 CanonicalizeMaxDots
 CanonicalizePermittedCNAMEs
 CASignatureAlgorithms
 CertificateFile
 CheckHostIP
 Ciphers
 ClearAllForwardings
 Compression
 ConnectionAttempts
 ConnectTimeout
 ControlMaster
 ControlPath
 ControlPersist
 DynamicForward
 EnableEscapeCommandline
 EscapeChar
 ExitOnForwardFailure
 FingerprintHash
 ForkAfterAuthentication
 ForwardAgent
 ForwardX11
 ForwardX11Timeout
 ForwardX11Trusted
 GatewayPorts
 GlobalKnownHostsFile
 GSSAPIAuthentication
 GSSAPIDelegateCredentials
 HashKnownHosts
 Host
 HostbasedAcceptedAlgorithms
 HostbasedAuthentication
 HostKeyAlgorithms
 HostKeyAlias
 Hostname
 IdentitiesOnly
 IdentityAgent
 IdentityFile
 IPQoS
 KbdInteractiveAuthentication
 KbdInteractiveDevices
 KexAlgorithms
 KnownHostsCommand
 LocalCommand
 LocalForward
 LogLevel
 MACs
 Match
 NoHostAuthenticationForLocalhost
 NumberOfPasswordPrompts
 PasswordAuthentication
 PermitLocalCommand
 PermitRemoteOpen
 PKCS11Provider
 Port
 PreferredAuthentications
 ProxyCommand
 ProxyJump
 ProxyUseFdpass
 PubkeyAcceptedAlgorithms
 PubkeyAuthentication
 RekeyLimit
 RemoteCommand
 RemoteForward
 RequestTTY
 RequiredRSASize
 SendEnv
 ServerAliveInterval
 ServerAliveCountMax
 SessionType
 SetEnv
 StdinNull
 StreamLocalBindMask
 StreamLocalBindUnlink
 StrictHostKeyChecking
 TCPKeepAlive
 Tunnel
 TunnelDevice
 UpdateHostKeys
 User
 UserKnownHostsFile
 VerifyHostKeyDNS
 VisualHostKey
 XAuthLocation
 -P tag Specify a tag name that may be used to select configuration in
 ssh_config(5) . Refer to the Tag and Match keywords in
 ssh_config(5)  for more information.
 -p port
 Port to connect to on the remote host. This can be specified on
 a per-host basis in the configuration file.
 -Q query_option
 Queries for the algorithms supported by one of the following
 features: cipher (supported symmetric ciphers), cipher-auth
 (supported symmetric ciphers that support authenticated
 encryption), help (supported query terms for use with the -Q
 flag), mac (supported message integrity codes), kex (key exchange
 algorithms), key (key types), key-ca-sign (valid CA signature
 algorithms for certificates), key-cert (certificate key types),
 key-plain (non-certificate key types), key-sig (all key types and
 signature algorithms), protocol-version (supported SSH protocol
 versions), and sig (supported signature algorithms).
 Alternatively, any keyword from ssh_config(5)  or sshd_config(5) 
 that takes an algorithm list may be used as an alias for the
 corresponding query_option.
 -q Quiet mode. Causes most warning and diagnostic messages to be
 suppressed.
 -R [bind_address:]port:host:hostport
 -R [bind_address:]port:local_socket
 -R remote_socket:host:hostport
 -R remote_socket:local_socket
 -R [bind_address:]port
 Specifies that connections to the given TCP port or Unix socket
 on the remote (server) host are to be forwarded to the local
 side.
 This works by allocating a socket to listen to either a TCP port
 or to a Unix socket on the remote side. Whenever a connection is
 made to this port or Unix socket, the connection is forwarded
 over the secure channel, and a connection is made from the local
 machine to either an explicit destination specified by host port
 hostport, or local_socket, or, if no explicit destination was
 specified, ssh will act as a SOCKS 4/5 proxy and forward
 connections to the destinations requested by the remote SOCKS
 client.
 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, TCP listening sockets 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.
 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
 sharing, 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 facilitate the use of SSH as a secure
 transport for other applications (e.g. sftp(1) ). The subsystem
 is specified as the remote command. Refer to the description of
 SessionType in ssh_config(5)  for details.
 -T Disable pseudo-terminal allocation.
 -t Force pseudo-terminal 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, though these
 can be overridden in the configuration file or using -o command
 line options.
 -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 will be set to the default
 tunnel mode, which is "point-to-point". If a different Tunnel
 forwarding mode it desired, then it should be specified before
 -w.
 -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 monitoring.
 For this reason, X11 forwarding is subjected to X11 SECURITY
 extension restrictions by default. 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) .

AUTHENTICATION

 The OpenSSH SSH client supports SSH protocol 2.
 The methods available for authentication are: GSSAPI-based
 authentication, host-based authentication, public key authentication,
 keyboard-interactive authentication, and password authentication.
 Authentication methods are tried in the order specified above, though
 PreferredAuthentications can be used to change the default order.
 Host-based authentication works as follows: If the machine the user logs
 in from is listed in /etc/hosts.equiv or /opt/local/etc/ssh/shosts.equiv
 on the remote machine, the user is non-root 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
 /opt/local/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 cryptography, 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 implements public key authentication protocol automatically, using
 one of the ECDSA, Ed25519 or RSA 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 server may inform the client of errors that prevented public key
 authentication from succeeding after authentication completes using a
 different method. These may be viewed by increasing the LogLevel to
 DEBUG or higher (e.g. by using the -v flag).
 The user creates their key pair by running ssh-keygen(1) . This stores
 the private key in ~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ecdsa_sk
 (authenticator-hosted ECDSA), ~/.ssh/id_ed25519 (Ed25519),
 ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or ~/.ssh/id_rsa
 (RSA) and stores the public key in ~/.ssh/id_ecdsa.pub (ECDSA),
 ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted ECDSA),
 ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (authenticator-
 hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's home directory.
 The user should then copy the public key to ~/.ssh/authorized_keys in
 their 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)  and (optionally)
 the AddKeysToAgent directive in ssh_config(5)  for more information.
 Keyboard-interactive authentication works as follows: The server sends an
 arbitrary "challenge" text and prompts for a response, possibly multiple
 times. Examples of keyboard-interactive 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 communications 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 /opt/local/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 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 in a non-interactive session or, if no
 command has been specified, logs into the machine and gives the user a
 normal shell as an interactive session. All communication with the
 remote command or shell will be automatically encrypted.
 If an interactive session is requested, ssh by default will only request
 a pseudo-terminal (pty) for interactive sessions when the client has one.
 The flags -T and -t can be used to override this behaviour.
 If a pseudo-terminal has been allocated, the user may use the escape
 characters noted below.
 If no pseudo-terminal 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 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.
 !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 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 a 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 for an IRC
 client, even though the IRC server it connects to does not directly
 support encrypted communication. This works as follows: the user
 connects to the remote host using ssh, specifying the ports to be used to
 forward the connection. After that it is possible to start the program
 locally, and ssh will encrypt and forward the connection to the remote
 server.
 The following example tunnels an IRC session from the client to an IRC
 server at "server.example.com", joining channel "#users", nickname
 "pinky", using the standard IRC port, 6667:
 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
 $ irc -c '#users' pinky IRC/127.0.0.1
 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 program which is going to use the tunnel. 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 disabled). Fingerprints can be determined
 using ssh-keygen(1) :
 $ ssh-keygen -l -f /opt/local/etc/ssh/ssh_host_rsa_key
 If the fingerprint is already known, it can be matched and the key can be
 accepted or rejected. If only legacy (MD5) fingerprints for the server
 are available, the ssh-keygen(1)  -E option may be used to downgrade the
 fingerprint algorithm to match.
 Because of the difficulty of comparing host keys just by looking at
 fingerprint 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 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:
 $ 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 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
 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 necessary to
 redirect the input from /dev/null to make this
 work.)
 SSH_ASKPASS_REQUIRE Allows further control over the use of an askpass
 program. If this variable is set to "never" then
 ssh will never attempt to use one. If it is set to
 "prefer", then ssh will prefer to use the askpass
 program instead of the TTY when requesting
 passwords. Finally, if the variable is set to
 "force", then the askpass program will be used for
 all passphrase input regardless of whether DISPLAY
 is set.
 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) associated with the current shell or
 command. If the current session has no tty, this
 variable is not set.
 SSH_TUNNEL Optionally set by sshd(8)  to contain the interface
 names assigned if tunnel forwarding was requested
 by the client.
 SSH_USER_AUTH Optionally set by sshd(8) , this variable may
 contain a pathname to a file that lists the
 authentication methods successfully used when the
 session was established, including any public keys
 that were used.
 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
 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 (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 permissions:
 read/write for the user, and not writable by others.
 ~/.ssh/environment
 Contains additional definitions for environment variables; see
 ENVIRONMENT, above.
 ~/.ssh/id_ecdsa
 ~/.ssh/id_ecdsa_sk
 ~/.ssh/id_ed25519
 ~/.ssh/id_ed25519_sk
 ~/.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
 AES-128.
 ~/.ssh/id_ecdsa.pub
 ~/.ssh/id_ecdsa_sk.pub
 ~/.ssh/id_ed25519.pub
 ~/.ssh/id_ed25519_sk.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.
 /opt/local/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.
 /opt/local/etc/ssh/ssh_config
 Systemwide configuration file. The file format and configuration
 options are described in ssh_config(5) .
 /opt/local/etc/ssh/ssh_host_ecdsa_key
 /opt/local/etc/ssh/ssh_host_ed25519_key
 /opt/local/etc/ssh/ssh_host_rsa_key
 These files contain the private parts of the host keys and are
 used for host-based authentication.
 /opt/local/etc/ssh/ssh_known_hosts
 Systemwide list of known host keys. This file should be prepared
 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.
 /opt/local/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) , ssh_config(5) , ssh-keysign(8) , sshd(8) 

STANDARDS

 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
 Numbers, RFC 4250, January 2006.
 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
 RFC 4251, January 2006.
 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
 RFC 4252, January 2006.
 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
 Protocol, RFC 4253, January 2006.
 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
 4254, January 2006.
 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
 (SSH) Key Fingerprints, RFC 4255, January 2006.
 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
 Secure Shell Protocol (SSH), RFC 4256, January 2006.
 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
 Extension, RFC 4335, January 2006.
 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
 Layer Encryption Modes, RFC 4344, January 2006.
 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
 Layer Protocol, RFC 4345, January 2006.
 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
 Format, RFC 4716, November 2006.
 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
 Secure Shell Transport Layer, RFC 5656, December 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 contributed the support for SSH protocol
 versions 1.5 and 2.0.
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