Adding upstream version 1:9.6p1.upstream/1%9.6p1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1797 insertions, 0 deletions

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+.\"
+.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
+.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
+.\"                    All rights reserved
+.\"
+.\" As far as I am concerned, the code I have written for this software
+.\" can be used freely for any purpose.  Any derived versions of this
+.\" software must be clearly marked as such, and if the derived work is
+.\" incompatible with the protocol description in the RFC file, it must be
+.\" called by a name other than "ssh" or "Secure Shell".
+.\"
+.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
+.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
+.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\" $OpenBSD: ssh.1,v 1.438 2023/10/11 23:14:33 djm Exp $
+.Dd $Mdocdate: October 11 2023 $
+.Dt SSH 1
+.Os
+.Sh NAME
+.Nm ssh
+.Nd OpenSSH remote login client
+.Sh SYNOPSIS
+.Nm ssh
+.Op Fl 46AaCfGgKkMNnqsTtVvXxYy
+.Op Fl B Ar bind_interface
+.Op Fl b Ar bind_address
+.Op Fl c Ar cipher_spec
+.Op Fl D Oo Ar bind_address : Oc Ns Ar port
+.Op Fl E Ar log_file
+.Op Fl e Ar escape_char
+.Op Fl F Ar configfile
+.Op Fl I Ar pkcs11
+.Op Fl i Ar identity_file
+.Op Fl J Ar destination
+.Op Fl L Ar address
+.Op Fl l Ar login_name
+.Op Fl m Ar mac_spec
+.Op Fl O Ar ctl_cmd
+.Op Fl o Ar option
+.Op Fl P Ar tag
+.Op Fl p Ar port
+.Op Fl R Ar address
+.Op Fl S Ar ctl_path
+.Op Fl W Ar host : Ns Ar port
+.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
+.Ar destination
+.Op Ar command Op Ar argument ...
+.Nm
+.Op Fl Q Ar query_option
+.Sh DESCRIPTION
+.Nm
+(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
+.Ux Ns -domain
+sockets can also be forwarded over the secure channel.
+.Pp
+.Nm
+connects and logs into the specified
+.Ar destination ,
+which may be specified as either
+.Sm off
+.Oo user @ Oc hostname
+.Sm on
+or a URI of the form
+.Sm off
+.No ssh:// Oo user @ Oc hostname Op : port .
+.Sm on
+The user must prove
+their identity to the remote machine using one of several methods
+(see below).
+.Pp
+If a
+.Ar command
+is specified,
+it will be executed on the remote host instead of a login shell.
+A complete command line may be specified as
+.Ar 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.
+.Pp
+The options are as follows:
+.Pp
+.Bl -tag -width Ds -compact
+.It Fl 4
+Forces
+.Nm
+to use IPv4 addresses only.
+.Pp
+.It Fl 6
+Forces
+.Nm
+to use IPv6 addresses only.
+.Pp
+.It Fl A
+Enables forwarding of connections from an authentication agent such as
+.Xr ssh-agent 1 .
+This can also be specified on a per-host basis in a configuration file.
+.Pp
+Agent forwarding should be enabled with caution.
+Users with the ability to bypass file permissions on the remote host
+(for the agent's
+.Ux Ns -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
+.Fl J ) .
+.Pp
+.It Fl a
+Disables forwarding of the authentication agent connection.
+.Pp
+.It Fl B Ar bind_interface
+Bind to the address of
+.Ar bind_interface
+before attempting to connect to the destination host.
+This is only useful on systems with more than one address.
+.Pp
+.It Fl b Ar bind_address
+Use
+.Ar bind_address
+on the local machine as the source address
+of the connection.
+Only useful on systems with more than one address.
+.Pp
+.It Fl C
+Requests compression of all data (including stdin, stdout, stderr, and
+data for forwarded X11, TCP and
+.Ux Ns -domain
+connections).
+The compression algorithm is the same used by
+.Xr 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
+.Cm Compression
+option in
+.Xr ssh_config 5 .
+.Pp
+.It Fl c Ar cipher_spec
+Selects the cipher specification for encrypting the session.
+.Ar cipher_spec
+is a comma-separated list of ciphers
+listed in order of preference.
+See the
+.Cm Ciphers
+keyword in
+.Xr ssh_config 5
+for more information.
+.Pp
+.It Fl D Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port
+.Sm on
+.Xc
+Specifies a local
+.Dq dynamic
+application-level port forwarding.
+This works by allocating a socket to listen to
+.Ar port
+on the local side, optionally bound to the specified
+.Ar 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
+.Nm
+will act as a SOCKS server.
+Only root can forward privileged ports.
+Dynamic port forwardings can also be specified in the configuration file.
+.Pp
+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
+.Cm GatewayPorts
+setting.
+However, an explicit
+.Ar bind_address
+may be used to bind the connection to a specific address.
+The
+.Ar bind_address
+of
+.Dq localhost
+indicates that the listening port be bound for local use only, while an
+empty address or
+.Sq *
+indicates that the port should be available from all interfaces.
+.Pp
+.It Fl E Ar log_file
+Append debug logs to
+.Ar log_file
+instead of standard error.
+.Pp
+.It Fl e Ar escape_char
+Sets the escape character for sessions with a pty (default:
+.Ql ~ ) .
+The escape character is only recognized at the beginning of a line.
+The escape character followed by a dot
+.Pq Ql \&.
+closes the connection;
+followed by control-Z suspends the connection;
+and followed by itself sends the escape character once.
+Setting the character to
+.Dq none
+disables any escapes and makes the session fully transparent.
+.Pp
+.It Fl F Ar configfile
+Specifies an alternative per-user configuration file.
+If a configuration file is given on the command line,
+the system-wide configuration file
+.Pq Pa /etc/ssh/ssh_config
+will be ignored.
+The default for the per-user configuration file is
+.Pa ~/.ssh/config .
+If set to
+.Dq none ,
+no configuration files will be read.
+.Pp
+.It Fl f
+Requests
+.Nm
+to go to background just before command execution.
+This is useful if
+.Nm
+is going to ask for passwords or passphrases, but the user
+wants it in the background.
+This implies
+.Fl n .
+The recommended way to start X11 programs at a remote site is with
+something like
+.Ic ssh -f host xterm .
+.Pp
+If the
+.Cm ExitOnForwardFailure
+configuration option is set to
+.Dq yes ,
+then a client started with
+.Fl f
+will wait for all remote port forwards to be successfully established
+before placing itself in the background.
+Refer to the description of
+.Cm ForkAfterAuthentication
+in
+.Xr ssh_config 5
+for details.
+.Pp
+.It Fl G
+Causes
+.Nm
+to print its configuration after evaluating
+.Cm Host
+and
+.Cm Match
+blocks and exit.
+.Pp
+.It Fl 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.
+.Pp
+.It Fl I Ar pkcs11
+Specify the PKCS#11 shared library
+.Nm
+should use to communicate with a PKCS#11 token providing keys for user
+authentication.
+.Pp
+.It Fl i Ar 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
+.Xr ssh-agent 1
+when the private key file is not present locally.
+The default is
+.Pa ~/.ssh/id_rsa ,
+.Pa ~/.ssh/id_ecdsa ,
+.Pa ~/.ssh/id_ecdsa_sk ,
+.Pa ~/.ssh/id_ed25519 ,
+.Pa ~/.ssh/id_ed25519_sk
+and
+.Pa ~/.ssh/id_dsa .
+Identity files may also be specified on
+a per-host basis in the configuration file.
+It is possible to have multiple
+.Fl i
+options (and multiple identities specified in
+configuration files).
+If no certificates have been explicitly specified by the
+.Cm CertificateFile
+directive,
+.Nm
+will also try to load certificate information from the filename obtained
+by appending
+.Pa -cert.pub
+to identity filenames.
+.Pp
+.It Fl J Ar destination
+Connect to the target host by first making an
+.Nm
+connection to the jump host described by
+.Ar destination
+and then establishing a TCP forwarding to the ultimate destination from
+there.
+Multiple jump hops may be specified separated by comma characters.
+This is a shortcut to specify a
+.Cm 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
+.Pa ~/.ssh/config
+to specify configuration for jump hosts.
+.Pp
+.It Fl K
+Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
+credentials to the server.
+.Pp
+.It Fl k
+Disables forwarding (delegation) of GSSAPI credentials to the server.
+.Pp
+.It Fl L Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port : host : hostport
+.Sm on
+.Xc
+.It Fl L Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port : remote_socket
+.Sm on
+.Xc
+.It Fl L Xo
+.Sm off
+.Ar local_socket : host : hostport
+.Sm on
+.Xc
+.It Fl L Xo
+.Sm off
+.Ar local_socket : remote_socket
+.Sm on
+.Xc
+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
+.Ar port
+on the local side, optionally bound to the specified
+.Ar 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
+.Ar host
+port
+.Ar hostport ,
+or the Unix socket
+.Ar remote_socket ,
+from the remote machine.
+.Pp
+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.
+.Pp
+By default, the local port is bound in accordance with the
+.Cm GatewayPorts
+setting.
+However, an explicit
+.Ar bind_address
+may be used to bind the connection to a specific address.
+The
+.Ar bind_address
+of
+.Dq localhost
+indicates that the listening port be bound for local use only, while an
+empty address or
+.Sq *
+indicates that the port should be available from all interfaces.
+.Pp
+.It Fl l Ar 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.
+.Pp
+.It Fl M
+Places the
+.Nm
+client into
+.Dq master
+mode for connection sharing.
+Multiple
+.Fl M
+options places
+.Nm
+into
+.Dq master
+mode but with confirmation required using
+.Xr ssh-askpass 1
+before each operation that changes the multiplexing state
+(e.g. opening a new session).
+Refer to the description of
+.Cm ControlMaster
+in
+.Xr ssh_config 5
+for details.
+.Pp
+.It Fl m Ar mac_spec
+A comma-separated list of MAC (message authentication code) algorithms,
+specified in order of preference.
+See the
+.Cm MACs
+keyword in
+.Xr ssh_config 5
+for more information.
+.Pp
+.It Fl N
+Do not execute a remote command.
+This is useful for just forwarding ports.
+Refer to the description of
+.Cm SessionType
+in
+.Xr ssh_config 5
+for details.
+.Pp
+.It Fl n
+Redirects stdin from
+.Pa /dev/null
+(actually, prevents reading from stdin).
+This must be used when
+.Nm
+is run in the background.
+A common trick is to use this to run X11 programs on a remote machine.
+For example,
+.Ic 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
+.Nm
+program will be put in the background.
+(This does not work if
+.Nm
+needs to ask for a password or passphrase; see also the
+.Fl f
+option.)
+Refer to the description of
+.Cm StdinNull
+in
+.Xr ssh_config 5
+for details.
+.Pp
+.It Fl O Ar ctl_cmd
+Control an active connection multiplexing master process.
+When the
+.Fl O
+option is specified, the
+.Ar ctl_cmd
+argument is interpreted and passed to the master process.
+Valid commands are:
+.Dq check
+(check that the master process is running),
+.Dq forward
+(request forwardings without command execution),
+.Dq cancel
+(cancel forwardings),
+.Dq exit
+(request the master to exit), and
+.Dq stop
+(request the master to stop accepting further multiplexing requests).
+.Pp
+.It Fl o Ar 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
+.Xr ssh_config 5 .
+.Pp
+.Bl -tag -width Ds -offset indent -compact
+.It AddKeysToAgent
+.It AddressFamily
+.It BatchMode
+.It BindAddress
+.It CanonicalDomains
+.It CanonicalizeFallbackLocal
+.It CanonicalizeHostname
+.It CanonicalizeMaxDots
+.It CanonicalizePermittedCNAMEs
+.It CASignatureAlgorithms
+.It CertificateFile
+.It CheckHostIP
+.It Ciphers
+.It ClearAllForwardings
+.It Compression
+.It ConnectionAttempts
+.It ConnectTimeout
+.It ControlMaster
+.It ControlPath
+.It ControlPersist
+.It DynamicForward
+.It EnableEscapeCommandline
+.It EscapeChar
+.It ExitOnForwardFailure
+.It FingerprintHash
+.It ForkAfterAuthentication
+.It ForwardAgent
+.It ForwardX11
+.It ForwardX11Timeout
+.It ForwardX11Trusted
+.It GatewayPorts
+.It GlobalKnownHostsFile
+.It GSSAPIAuthentication
+.It GSSAPIDelegateCredentials
+.It HashKnownHosts
+.It Host
+.It HostbasedAcceptedAlgorithms
+.It HostbasedAuthentication
+.It HostKeyAlgorithms
+.It HostKeyAlias
+.It Hostname
+.It IdentitiesOnly
+.It IdentityAgent
+.It IdentityFile
+.It IPQoS
+.It KbdInteractiveAuthentication
+.It KbdInteractiveDevices
+.It KexAlgorithms
+.It KnownHostsCommand
+.It LocalCommand
+.It LocalForward
+.It LogLevel
+.It MACs
+.It Match
+.It NoHostAuthenticationForLocalhost
+.It NumberOfPasswordPrompts
+.It PasswordAuthentication
+.It PermitLocalCommand
+.It PermitRemoteOpen
+.It PKCS11Provider
+.It Port
+.It PreferredAuthentications
+.It ProxyCommand
+.It ProxyJump
+.It ProxyUseFdpass
+.It PubkeyAcceptedAlgorithms
+.It PubkeyAuthentication
+.It RekeyLimit
+.It RemoteCommand
+.It RemoteForward
+.It RequestTTY
+.It RequiredRSASize
+.It SendEnv
+.It ServerAliveInterval
+.It ServerAliveCountMax
+.It SessionType
+.It SetEnv
+.It StdinNull
+.It StreamLocalBindMask
+.It StreamLocalBindUnlink
+.It StrictHostKeyChecking
+.It TCPKeepAlive
+.It Tunnel
+.It TunnelDevice
+.It UpdateHostKeys
+.It User
+.It UserKnownHostsFile
+.It VerifyHostKeyDNS
+.It VisualHostKey
+.It XAuthLocation
+.El
+.Pp
+.It Fl P Ar tag
+Specify a tag name that may be used to select configuration in
+.Xr ssh_config 5 .
+Refer to the
+.Cm Tag
+and
+.Cm Match
+keywords in
+.Xr ssh_config 5
+for more information.
+.It Fl p Ar port
+Port to connect to on the remote host.
+This can be specified on a
+per-host basis in the configuration file.
+.Pp
+.It Fl Q Ar query_option
+Queries for the algorithms supported by one of the following features:
+.Ar cipher
+(supported symmetric ciphers),
+.Ar cipher-auth
+(supported symmetric ciphers that support authenticated encryption),
+.Ar help
+(supported query terms for use with the
+.Fl Q
+flag),
+.Ar mac
+(supported message integrity codes),
+.Ar kex
+(key exchange algorithms),
+.Ar key
+(key types),
+.Ar key-ca-sign
+(valid CA signature algorithms for certificates),
+.Ar key-cert
+(certificate key types),
+.Ar key-plain
+(non-certificate key types),
+.Ar key-sig
+(all key types and signature algorithms),
+.Ar protocol-version
+(supported SSH protocol versions), and
+.Ar sig
+(supported signature algorithms).
+Alternatively, any keyword from
+.Xr ssh_config 5
+or
+.Xr sshd_config 5
+that takes an algorithm list may be used as an alias for the corresponding
+query_option.
+.Pp
+.It Fl q
+Quiet mode.
+Causes most warning and diagnostic messages to be suppressed.
+.Pp
+.It Fl R Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port : host : hostport
+.Sm on
+.Xc
+.It Fl R Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port : local_socket
+.Sm on
+.Xc
+.It Fl R Xo
+.Sm off
+.Ar remote_socket : host : hostport
+.Sm on
+.Xc
+.It Fl R Xo
+.Sm off
+.Ar remote_socket : local_socket
+.Sm on
+.Xc
+.It Fl R Xo
+.Sm off
+.Oo Ar bind_address : Oc
+.Ar port
+.Sm on
+.Xc
+Specifies that connections to the given TCP port or Unix socket on the remote
+(server) host are to be forwarded to the local side.
+.Pp
+This works by allocating a socket to listen to either a TCP
+.Ar 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
+.Ar host
+port
+.Ar hostport ,
+or
+.Ar local_socket ,
+or, if no explicit destination was specified,
+.Nm
+will act as a SOCKS 4/5 proxy and forward connections to the destinations
+requested by the remote SOCKS client.
+.Pp
+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.
+.Pp
+By default, TCP listening sockets on the server will be bound to the loopback
+interface only.
+This may be overridden by specifying a
+.Ar bind_address .
+An empty
+.Ar bind_address ,
+or the address
+.Ql * ,
+indicates that the remote socket should listen on all interfaces.
+Specifying a remote
+.Ar bind_address
+will only succeed if the server's
+.Cm GatewayPorts
+option is enabled (see
+.Xr sshd_config 5 ) .
+.Pp
+If the
+.Ar port
+argument is
+.Ql 0 ,
+the listen port will be dynamically allocated on the server and reported
+to the client at run time.
+When used together with
+.Ic -O forward ,
+the allocated port will be printed to the standard output.
+.Pp
+.It Fl S Ar ctl_path
+Specifies the location of a control socket for connection sharing,
+or the string
+.Dq none
+to disable connection sharing.
+Refer to the description of
+.Cm ControlPath
+and
+.Cm ControlMaster
+in
+.Xr ssh_config 5
+for details.
+.Pp
+.It Fl 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.\&
+.Xr sftp 1 ) .
+The subsystem is specified as the remote command.
+Refer to the description of
+.Cm SessionType
+in
+.Xr ssh_config 5
+for details.
+.Pp
+.It Fl T
+Disable pseudo-terminal allocation.
+.Pp
+.It Fl 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
+.Fl t
+options force tty allocation, even if
+.Nm
+has no local tty.
+.Pp
+.It Fl V
+Display the version number and exit.
+.Pp
+.It Fl v
+Verbose mode.
+Causes
+.Nm
+to print debugging messages about its progress.
+This is helpful in
+debugging connection, authentication, and configuration problems.
+Multiple
+.Fl v
+options increase the verbosity.
+The maximum is 3.
+.Pp
+.It Fl W Ar host : Ns Ar port
+Requests that standard input and output on the client be forwarded to
+.Ar host
+on
+.Ar port
+over the secure channel.
+Implies
+.Fl N ,
+.Fl T ,
+.Cm ExitOnForwardFailure
+and
+.Cm ClearAllForwardings ,
+though these can be overridden in the configuration file or using
+.Fl o
+command line options.
+.Pp
+.It Fl w Xo
+.Ar local_tun Ns Op : Ns Ar remote_tun
+.Xc
+Requests
+tunnel
+device forwarding with the specified
+.Xr tun 4
+devices between the client
+.Pq Ar local_tun
+and the server
+.Pq Ar remote_tun .
+.Pp
+The devices may be specified by numerical ID or the keyword
+.Dq any ,
+which uses the next available tunnel device.
+If
+.Ar remote_tun
+is not specified, it defaults to
+.Dq any .
+See also the
+.Cm Tunnel
+and
+.Cm TunnelDevice
+directives in
+.Xr ssh_config 5 .
+.Pp
+If the
+.Cm Tunnel
+directive is unset, it will be set to the default tunnel mode, which is
+.Dq point-to-point .
+If a different
+.Cm Tunnel
+forwarding mode it desired, then it should be specified before
+.Fl w .
+.Pp
+.It Fl X
+Enables X11 forwarding.
+This can also be specified on a per-host basis in a configuration file.
+.Pp
+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.
+.Pp
+For this reason, X11 forwarding is subjected to X11 SECURITY extension
+restrictions by default.
+Refer to the
+.Nm
+.Fl Y
+option and the
+.Cm ForwardX11Trusted
+directive in
+.Xr ssh_config 5
+for more information.
+.Pp
+.It Fl x
+Disables X11 forwarding.
+.Pp
+.It Fl Y
+Enables trusted X11 forwarding.
+Trusted X11 forwardings are not subjected to the X11 SECURITY extension
+controls.
+.Pp
+.It Fl y
+Send log information using the
+.Xr syslog 3
+system module.
+By default this information is sent to stderr.
+.El
+.Pp
+.Nm
+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
+.Xr ssh_config 5 .
+.Sh AUTHENTICATION
+The OpenSSH SSH client supports SSH protocol 2.
+.Pp
+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
+.Cm PreferredAuthentications
+can be used to change the default order.
+.Pp
+Host-based authentication works as follows:
+If the machine the user logs in from is listed in
+.Pa /etc/hosts.equiv
+or
+.Pa /etc/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
+.Pa ~/.rhosts
+or
+.Pa ~/.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
+.Em must
+be able to verify the client's
+host key (see the description of
+.Pa /etc/ssh/ssh_known_hosts
+and
+.Pa ~/.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:
+.Pa /etc/hosts.equiv ,
+.Pa ~/.rhosts ,
+and the rlogin/rsh protocol in general, are inherently insecure and should be
+disabled if security is desired.]
+.Pp
+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.
+.Nm
+implements public key authentication protocol automatically,
+using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
+The HISTORY section of
+.Xr ssl 8
+contains a brief discussion of the DSA and RSA algorithms.
+.Pp
+The file
+.Pa ~/.ssh/authorized_keys
+lists the public keys that are permitted for logging in.
+When the user logs in, the
+.Nm
+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.
+.Pp
+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
+.Cm LogLevel
+to
+.Cm DEBUG
+or higher (e.g. by using the
+.Fl v
+flag).
+.Pp
+The user creates their key pair by running
+.Xr ssh-keygen 1 .
+This stores the private key in
+.Pa ~/.ssh/id_dsa
+(DSA),
+.Pa ~/.ssh/id_ecdsa
+(ECDSA),
+.Pa ~/.ssh/id_ecdsa_sk
+(authenticator-hosted ECDSA),
+.Pa ~/.ssh/id_ed25519
+(Ed25519),
+.Pa ~/.ssh/id_ed25519_sk
+(authenticator-hosted Ed25519),
+or
+.Pa ~/.ssh/id_rsa
+(RSA)
+and stores the public key in
+.Pa ~/.ssh/id_dsa.pub
+(DSA),
+.Pa ~/.ssh/id_ecdsa.pub
+(ECDSA),
+.Pa ~/.ssh/id_ecdsa_sk.pub
+(authenticator-hosted ECDSA),
+.Pa ~/.ssh/id_ed25519.pub
+(Ed25519),
+.Pa ~/.ssh/id_ed25519_sk.pub
+(authenticator-hosted Ed25519),
+or
+.Pa ~/.ssh/id_rsa.pub
+(RSA)
+in the user's home directory.
+The user should then copy the public key
+to
+.Pa ~/.ssh/authorized_keys
+in their home directory on the remote machine.
+The
+.Pa authorized_keys
+file corresponds to the conventional
+.Pa ~/.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.
+.Pp
+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
+.Xr ssh-keygen 1
+for more information.
+.Pp
+The most convenient way to use public key or certificate authentication
+may be with an authentication agent.
+See
+.Xr ssh-agent 1
+and (optionally) the
+.Cm AddKeysToAgent
+directive in
+.Xr ssh_config 5
+for more information.
+.Pp
+Keyboard-interactive authentication works as follows:
+The server sends an arbitrary
+.Qq challenge
+text and prompts for a response, possibly multiple times.
+Examples of keyboard-interactive authentication include
+.Bx
+Authentication (see
+.Xr login.conf 5 )
+and PAM (some
+.Pf non- Ox
+systems).
+.Pp
+Finally, if other authentication methods fail,
+.Nm
+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.
+.Pp
+.Nm
+automatically maintains and checks a database containing
+identification for all hosts it has ever been used with.
+Host keys are stored in
+.Pa ~/.ssh/known_hosts
+in the user's home directory.
+Additionally, the file
+.Pa /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,
+.Nm
+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
+.Cm StrictHostKeyChecking
+option can be used to control logins to machines whose
+host key is not known or has changed.
+.Pp
+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.
+.Pp
+If an interactive session is requested,
+.Nm
+by default will only request a pseudo-terminal (pty) for interactive
+sessions when the client has one.
+The flags
+.Fl T
+and
+.Fl t
+can be used to override this behaviour.
+.Pp
+If a pseudo-terminal has been allocated, the
+user may use the escape characters noted below.
+.Pp
+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
+.Dq none
+will also make the session transparent even if a tty is used.
+.Pp
+The session terminates when the command or shell on the remote
+machine exits and all X11 and TCP connections have been closed.
+.Sh ESCAPE CHARACTERS
+When a pseudo-terminal has been requested,
+.Nm
+supports a number of functions through the use of an escape character.
+.Pp
+A single tilde character can be sent as
+.Ic ~~
+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
+.Cm EscapeChar
+configuration directive or on the command line by the
+.Fl e
+option.
+.Pp
+The supported escapes (assuming the default
+.Ql ~ )
+are:
+.Bl -tag -width Ds
+.It Cm ~.
+Disconnect.
+.It Cm ~^Z
+Background
+.Nm .
+.It Cm ~#
+List forwarded connections.
+.It Cm ~&
+Background
+.Nm
+at logout when waiting for forwarded connection / X11 sessions to terminate.
+.It Cm ~?
+Display a list of escape characters.
+.It Cm ~B
+Send a BREAK to the remote system
+(only useful if the peer supports it).
+.It Cm ~C
+Open command line.
+Currently this allows the addition of port forwardings using the
+.Fl L ,
+.Fl R
+and
+.Fl D
+options (see above).
+It also allows the cancellation of existing port-forwardings
+with
+.Sm off
+.Fl KL Oo Ar bind_address : Oc Ar port
+.Sm on
+for local,
+.Sm off
+.Fl KR Oo Ar bind_address : Oc Ar port
+.Sm on
+for remote and
+.Sm off
+.Fl KD Oo Ar bind_address : Oc Ar port
+.Sm on
+for dynamic port-forwardings.
+.Ic !\& Ns Ar command
+allows the user to execute a local command if the
+.Ic PermitLocalCommand
+option is enabled in
+.Xr ssh_config 5 .
+Basic help is available, using the
+.Fl h
+option.
+.It Cm ~R
+Request rekeying of the connection
+(only useful if the peer supports it).
+.It Cm ~V
+Decrease the verbosity
+.Pq Ic LogLevel
+when errors are being written to stderr.
+.It Cm ~v
+Increase the verbosity
+.Pq Ic LogLevel
+when errors are being written to stderr.
+.El
+.Sh 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.
+.Pp
+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
+.Nm ,
+specifying the ports to be used to forward the connection.
+After that it is possible to start the program locally,
+and
+.Nm
+will encrypt and forward the connection to the remote server.
+.Pp
+The following example tunnels an IRC session from the client
+to an IRC server at
+.Dq server.example.com ,
+joining channel
+.Dq #users ,
+nickname
+.Dq pinky ,
+using the standard IRC port, 6667:
+.Bd -literal -offset 4n
+$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
+$ irc -c '#users' pinky IRC/127.0.0.1
+.Ed
+.Pp
+The
+.Fl f
+option backgrounds
+.Nm
+and the remote command
+.Dq 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,
+.Nm
+will exit.
+.Sh X11 FORWARDING
+If the
+.Cm ForwardX11
+variable is set to
+.Dq yes
+(or see the description of the
+.Fl X ,
+.Fl x ,
+and
+.Fl Y
+options above)
+and the user is using X11 (the
+.Ev 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
+.Ev DISPLAY .
+Forwarding of X11 connections can be
+configured on the command line or in configuration files.
+.Pp
+The
+.Ev DISPLAY
+value set by
+.Nm
+will point to the server machine, but with a display number greater than zero.
+This is normal, and happens because
+.Nm
+creates a
+.Dq proxy
+X server on the server machine for forwarding the
+connections over the encrypted channel.
+.Pp
+.Nm
+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).
+.Pp
+If the
+.Cm ForwardAgent
+variable is set to
+.Dq yes
+(or see the description of the
+.Fl A
+and
+.Fl a
+options above) and
+the user is using an authentication agent, the connection to the agent
+is automatically forwarded to the remote side.
+.Sh 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
+.Cm StrictHostKeyChecking
+has been disabled).
+Fingerprints can be determined using
+.Xr ssh-keygen 1 :
+.Pp
+.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
+.Pp
+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
+.Xr ssh-keygen 1
+.Fl E
+option may be used to downgrade the fingerprint algorithm to match.
+.Pp
+Because of the difficulty of comparing host keys
+just by looking at fingerprint strings,
+there is also support to compare host keys visually,
+using
+.Em random art .
+By setting the
+.Cm VisualHostKey
+option to
+.Dq 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.
+.Pp
+To get a listing of the fingerprints along with their random art for
+all known hosts, the following command line can be used:
+.Pp
+.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
+.Pp
+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.
+.Pp
+In this example, we are connecting a client to a server,
+.Dq host.example.com .
+The SSHFP resource records should first be added to the zonefile for
+host.example.com:
+.Bd -literal -offset indent
+$ ssh-keygen -r host.example.com.
+.Ed
+.Pp
+The output lines will have to be added to the zonefile.
+To check that the zone is answering fingerprint queries:
+.Pp
+.Dl $ dig -t SSHFP host.example.com
+.Pp
+Finally the client connects:
+.Bd -literal -offset indent
+$ ssh -o "VerifyHostKeyDNS ask" host.example.com
+[...]
+Matching host key fingerprint found in DNS.
+Are you sure you want to continue connecting (yes/no)?
+.Ed
+.Pp
+See the
+.Cm VerifyHostKeyDNS
+option in
+.Xr ssh_config 5
+for more information.
+.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
+.Nm
+contains support for Virtual Private Network (VPN) tunnelling
+using the
+.Xr tun 4
+network pseudo-device,
+allowing two networks to be joined securely.
+The
+.Xr sshd_config 5
+configuration option
+.Cm PermitTunnel
+controls whether the server supports this,
+and at what level (layer 2 or 3 traffic).
+.Pp
+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.
+.Pp
+On the client:
+.Bd -literal -offset indent
+# 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
+.Ed
+.Pp
+On the server:
+.Bd -literal -offset indent
+# 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
+.Ed
+.Pp
+Client access may be more finely tuned via the
+.Pa /root/.ssh/authorized_keys
+file (see below) and the
+.Cm PermitRootLogin
+server option.
+The following entry would permit connections on
+.Xr tun 4
+device 1 from user
+.Dq jane
+and on tun device 2 from user
+.Dq john ,
+if
+.Cm PermitRootLogin
+is set to
+.Dq forced-commands-only :
+.Bd -literal -offset 2n
+tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
+tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
+.Ed
+.Pp
+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
+.Xr ipsecctl 8
+and
+.Xr isakmpd 8 .
+.Sh ENVIRONMENT
+.Nm
+will normally set the following environment variables:
+.Bl -tag -width "SSH_ORIGINAL_COMMAND"
+.It Ev DISPLAY
+The
+.Ev DISPLAY
+variable indicates the location of the X11 server.
+It is automatically set by
+.Nm
+to point to a value of the form
+.Dq hostname:n ,
+where
+.Dq hostname
+indicates the host where the shell runs, and
+.Sq n
+is an integer \*(Ge 1.
+.Nm
+uses this special value to forward X11 connections over the secure
+channel.
+The user should normally not set
+.Ev DISPLAY
+explicitly, as that
+will render the X11 connection insecure (and will require the user to
+manually copy any required authorization cookies).
+.It Ev HOME
+Set to the path of the user's home directory.
+.It Ev LOGNAME
+Synonym for
+.Ev USER ;
+set for compatibility with systems that use this variable.
+.It Ev MAIL
+Set to the path of the user's mailbox.
+.It Ev PATH
+Set to the default
+.Ev PATH ,
+as specified when compiling
+.Nm .
+.It Ev SSH_ASKPASS
+If
+.Nm
+needs a passphrase, it will read the passphrase from the current
+terminal if it was run from a terminal.
+If
+.Nm
+does not have a terminal associated with it but
+.Ev DISPLAY
+and
+.Ev SSH_ASKPASS
+are set, it will execute the program specified by
+.Ev SSH_ASKPASS
+and open an X11 window to read the passphrase.
+This is particularly useful when calling
+.Nm
+from a
+.Pa .xsession
+or related script.
+(Note that on some machines it
+may be necessary to redirect the input from
+.Pa /dev/null
+to make this work.)
+.It Ev SSH_ASKPASS_REQUIRE
+Allows further control over the use of an askpass program.
+If this variable is set to
+.Dq never
+then
+.Nm
+will never attempt to use one.
+If it is set to
+.Dq prefer ,
+then
+.Nm
+will prefer to use the askpass program instead of the TTY when requesting
+passwords.
+Finally, if the variable is set to
+.Dq force ,
+then the askpass program will be used for all passphrase input regardless
+of whether
+.Ev DISPLAY
+is set.
+.It Ev SSH_AUTH_SOCK
+Identifies the path of a
+.Ux Ns -domain
+socket used to communicate with the agent.
+.It Ev 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.
+.It Ev 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.
+.It Ev 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.
+.It Ev SSH_TUNNEL
+Optionally set by
+.Xr sshd 8
+to contain the interface names assigned if tunnel forwarding was
+requested by the client.
+.It Ev SSH_USER_AUTH
+Optionally set by
+.Xr 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.
+.It Ev 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).
+.It Ev USER
+Set to the name of the user logging in.
+.El
+.Pp
+Additionally,
+.Nm
+reads
+.Pa ~/.ssh/environment ,
+and adds lines of the format
+.Dq VARNAME=value
+to the environment if the file exists and users are allowed to
+change their environment.
+For more information, see the
+.Cm PermitUserEnvironment
+option in
+.Xr sshd_config 5 .
+.Sh FILES
+.Bl -tag -width Ds -compact
+.It Pa ~/.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
+.Xr 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.
+.Pp
+.It Pa ~/.shosts
+This file is used in exactly the same way as
+.Pa .rhosts ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
+.Pp
+.It Pa ~/.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.
+.Pp
+.It Pa ~/.ssh/authorized_keys
+Lists the public keys (DSA, ECDSA, Ed25519, RSA)
+that can be used for logging in as this user.
+The format of this file is described in the
+.Xr 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.
+.Pp
+.It Pa ~/.ssh/config
+This is the per-user configuration file.
+The file format and configuration options are described in
+.Xr 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.
+.Pp
+.It Pa ~/.ssh/environment
+Contains additional definitions for environment variables; see
+.Sx ENVIRONMENT ,
+above.
+.Pp
+.It Pa ~/.ssh/id_dsa
+.It Pa ~/.ssh/id_ecdsa
+.It Pa ~/.ssh/id_ecdsa_sk
+.It Pa ~/.ssh/id_ed25519
+.It Pa ~/.ssh/id_ed25519_sk
+.It Pa ~/.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).
+.Nm
+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.
+.Pp
+.It Pa ~/.ssh/id_dsa.pub
+.It Pa ~/.ssh/id_ecdsa.pub
+.It Pa ~/.ssh/id_ecdsa_sk.pub
+.It Pa ~/.ssh/id_ed25519.pub
+.It Pa ~/.ssh/id_ed25519_sk.pub
+.It Pa ~/.ssh/id_rsa.pub
+Contains the public key for authentication.
+These files are not
+sensitive and can (but need not) be readable by anyone.
+.Pp
+.It Pa ~/.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
+.Xr sshd 8
+for further details of the format of this file.
+.Pp
+.It Pa ~/.ssh/rc
+Commands in this file are executed by
+.Nm
+when the user logs in, just before the user's shell (or command) is
+started.
+See the
+.Xr sshd 8
+manual page for more information.
+.Pp
+.It Pa /etc/hosts.equiv
+This file is for host-based authentication (see above).
+It should only be writable by root.
+.Pp
+.It Pa /etc/shosts.equiv
+This file is used in exactly the same way as
+.Pa hosts.equiv ,
+but allows host-based authentication without permitting login with
+rlogin/rsh.
+.Pp
+.It Pa /etc/ssh/ssh_config
+Systemwide configuration file.
+The file format and configuration options are described in
+.Xr ssh_config 5 .
+.Pp
+.It Pa /etc/ssh/ssh_host_key
+.It Pa /etc/ssh/ssh_host_dsa_key
+.It Pa /etc/ssh/ssh_host_ecdsa_key
+.It Pa /etc/ssh/ssh_host_ed25519_key
+.It Pa /etc/ssh/ssh_host_rsa_key
+These files contain the private parts of the host keys
+and are used for host-based authentication.
+.Pp
+.It Pa /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
+.Xr sshd 8
+for further details of the format of this file.
+.Pp
+.It Pa /etc/ssh/sshrc
+Commands in this file are executed by
+.Nm
+when the user logs in, just before the user's shell (or command) is started.
+See the
+.Xr sshd 8
+manual page for more information.
+.El
+.Sh EXIT STATUS
+.Nm
+exits with the exit status of the remote command or with 255
+if an error occurred.
+.Sh SEE ALSO
+.Xr scp 1 ,
+.Xr sftp 1 ,
+.Xr ssh-add 1 ,
+.Xr ssh-agent 1 ,
+.Xr ssh-keygen 1 ,
+.Xr ssh-keyscan 1 ,
+.Xr tun 4 ,
+.Xr ssh_config 5 ,
+.Xr ssh-keysign 8 ,
+.Xr sshd 8
+.Sh STANDARDS
+.Rs
+.%A S. Lehtinen
+.%A C. Lonvick
+.%D January 2006
+.%R RFC 4250
+.%T The Secure Shell (SSH) Protocol Assigned Numbers
+.Re
+.Pp
+.Rs
+.%A T. Ylonen
+.%A C. Lonvick
+.%D January 2006
+.%R RFC 4251
+.%T The Secure Shell (SSH) Protocol Architecture
+.Re
+.Pp
+.Rs
+.%A T. Ylonen
+.%A C. Lonvick
+.%D January 2006
+.%R RFC 4252
+.%T The Secure Shell (SSH) Authentication Protocol
+.Re
+.Pp
+.Rs
+.%A T. Ylonen
+.%A C. Lonvick
+.%D January 2006
+.%R RFC 4253
+.%T The Secure Shell (SSH) Transport Layer Protocol
+.Re
+.Pp
+.Rs
+.%A T. Ylonen
+.%A C. Lonvick
+.%D January 2006
+.%R RFC 4254
+.%T The Secure Shell (SSH) Connection Protocol
+.Re
+.Pp
+.Rs
+.%A J. Schlyter
+.%A W. Griffin
+.%D January 2006
+.%R RFC 4255
+.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
+.Re
+.Pp
+.Rs
+.%A F. Cusack
+.%A M. Forssen
+.%D January 2006
+.%R RFC 4256
+.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
+.Re
+.Pp
+.Rs
+.%A J. Galbraith
+.%A P. Remaker
+.%D January 2006
+.%R RFC 4335
+.%T The Secure Shell (SSH) Session Channel Break Extension
+.Re
+.Pp
+.Rs
+.%A M. Bellare
+.%A T. Kohno
+.%A C. Namprempre
+.%D January 2006
+.%R RFC 4344
+.%T The Secure Shell (SSH) Transport Layer Encryption Modes
+.Re
+.Pp
+.Rs
+.%A B. Harris
+.%D January 2006
+.%R RFC 4345
+.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
+.Re
+.Pp
+.Rs
+.%A M. Friedl
+.%A N. Provos
+.%A W. Simpson
+.%D March 2006
+.%R RFC 4419
+.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
+.Re
+.Pp
+.Rs
+.%A J. Galbraith
+.%A R. Thayer
+.%D November 2006
+.%R RFC 4716
+.%T The Secure Shell (SSH) Public Key File Format
+.Re
+.Pp
+.Rs
+.%A D. Stebila
+.%A J. Green
+.%D December 2009
+.%R RFC 5656
+.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
+.Re
+.Pp
+.Rs
+.%A A. Perrig
+.%A D. Song
+.%D 1999
+.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
+.%T Hash Visualization: a New Technique to improve Real-World Security
+.Re
+.Sh 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.