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Diffstat (limited to 'ssh.1')
| -rw-r--r-- | ssh.1 | 1783 |
1 files changed, 1783 insertions, 0 deletions
@@ -0,0 +1,1783 @@ +.\" +.\" 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 1.433 2022/11/28 01:37:36 djm Exp $ +.Dd $Mdocdate: November 28 2022 $ +.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 port +.Op Fl Q Ar query_option +.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 ... +.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 a +.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 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-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. |