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authorDamien Miller <djm@mindrot.org>1999-10-28 14:03:14 +1000
committerDamien Miller <djm@mindrot.org>1999-10-28 14:03:14 +1000
commit062307657e30c9b970514eb53b4b743f9d571231 (patch)
tree04b1d6b094442720b4197eb442ee53c010cdc1d7 /ssh.1
parent7f6ea0264d01aa40b20df8dc75141631479da054 (diff)
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-.\" -*- nroff -*-
-.\"
-.\" ssh.1.in
-.\"
-.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
-.\"
-.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
-.\" All rights reserved
-.\"
-.\" Created: Sat Apr 22 21:55:14 1995 ylo
-.\"
-.\" $Id: ssh.1,v 1.1 1999/10/27 03:42:45 damien Exp $
-.\"
-.Dd September 25, 1999
-.Dt SSH 1
-.Os
-.Sh NAME
-.Nm ssh
-.Nd OpenSSH secure shell client (remote login program)
-.Sh SYNOPSIS
-.Nm ssh
-.Op Fl l Ar login_name
-.Op Ar hostname | user@hostname
-.Op Ar command
-.Pp
-.Nm ssh
-.Op Fl afgknqtvxCPX
-.Op Fl c Ar blowfish | 3des
-.Op Fl e Ar escape_char
-.Op Fl i Ar identity_file
-.Op Fl l Ar login_name
-.Op Fl o Ar option
-.Op Fl p Ar port
-.Oo Fl L Xo
-.Sm off
-.Ar host :
-.Ar port :
-.Ar hostport
-.Sm on
-.Xc
-.Oc
-.Oo Fl R Xo
-.Sm off
-.Ar host :
-.Ar port :
-.Ar hostport
-.Sm on
-.Xc
-.Oc
-.Op Ar hostname | user@hostname
-.Op Ar command
-.Sh DESCRIPTION
-.Nm
-(Secure Shell) is a program for logging into a remote machine and for
-executing commands on a remote machine. It is intended to replace
-rlogin and rsh, and provide secure encrypted communications between
-two untrusted hosts over an insecure network. X11 connections and
-arbitrary TCP/IP ports can also be forwarded over the secure channel.
-.Pp
-.Nm
-connects and logs into the specified
-.Ar hostname .
-The user must prove
-his/her identity to the remote machine using one of several methods.
-.Pp
-First, if the machine the user logs in from is listed in
-.Pa /etc/hosts.equiv
-or
-.Pa /etc/shosts.equiv
-on the remote machine, and the user names are
-the same on both sides, the user is immediately permitted to log in.
-Second, if
-.Pa \&.rhosts
-or
-.Pa \&.shosts
-exists in the user's home directory on the
-remote machine and contains a line containing the name of the client
-machine and the name of the user on that machine, the user is
-permitted to log in. This form of authentication alone is normally not
-allowed by the server because it is not secure.
-.Pp
-The second (and primary) authentication method is the
-.Pa rhosts
-or
-.Pa hosts.equiv
-method combined with RSA-based host authentication. It
-means that if the login would be permitted by
-.Pa \&.rhosts ,
-.Pa \&.shosts ,
-.Pa /etc/hosts.equiv ,
-or
-.Pa /etc/shosts.equiv ,
-and if additionally the server can verify the client's
-host key (see
-.Pa /etc/ssh_known_hosts
-in the
-.Sx FILES
-section), only then login is
-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
-As a third authentication method,
-.Nm
-supports RSA based authentication.
-The scheme is based on public-key cryptography: there are cryptosystems
-where encryption and decryption are done using separate keys, and it
-is not possible to derive the decryption key from the encryption key.
-RSA is one such system. 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.
-The file
-.Pa $HOME/.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 server checks if this key is permitted, and if
-so, sends the user (actually the
-.Nm
-program running on behalf of the user) a challenge, a random number,
-encrypted by the user's public key. The challenge can only be
-decrypted using the proper private key. The user's client then decrypts the
-challenge using the private key, proving that he/she knows the private
-key but without disclosing it to the server.
-.Pp
-.Nm
-implements the RSA authentication protocol automatically. The user
-creates his/her RSA key pair by running
-.Xr ssh-keygen 1 .
-This stores the private key in
-.Pa \&.ssh/identity
-and the public key in
-.Pa \&.ssh/identity.pub
-in the user's home directory. The user should then
-copy the
-.Pa identity.pub
-to
-.Pa \&.ssh/authorized_keys
-in his/her 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. RSA authentication is much
-more secure than rhosts authentication.
-.Pp
-The most convenient way to use RSA authentication may be with an
-authentication agent. See
-.Xr ssh-agent 1
-for more information.
-.Pp
-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
-When the user's identity has been accepted by the server, the server
-either executes the given command, or logs into the machine and gives
-the user a normal shell on the remote machine. All communication with
-the remote command or shell will be automatically encrypted.
-.Pp
-If a pseudo-terminal has been allocated (normal login session), the
-user can disconnect with
-.Ic ~. ,
-and suspend
-.Nm
-with
-.Ic ~^Z .
-All forwarded connections can be listed with
-.Ic ~#
-and if
-the session blocks waiting for forwarded X11 or TCP/IP
-connections to terminate, it can be backgrounded with
-.Ic ~&
-(this should not be used while the user shell is active, as it can cause the
-shell to hang). All available escapes can be listed with
-.Ic ~? .
-.Pp
-A single tilde character can be sent as
-.Ic ~~
-(or by following the tilde by a character other than those described above).
-The escape character must always follow a newline to be interpreted as
-special. The escape character can be changed in configuration files
-or on the command line.
-.Pp
-If no pseudo tty has been allocated, the
-session is transparent and can be used to reliably transfer binary
-data. On most systems, setting the escape character to
-.Dq none
-will also make the session transparent even if a tty is used.
-.Pp
-The session terminates when the command or shell in on the remote
-machine exists and all X11 and TCP/IP connections have been closed.
-The exit status of the remote program is returned as the exit status
-of
-.Nm ssh .
-.Pp
-If 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 user is using an authentication agent, the connection to the agent
-is automatically forwarded to the remote side unless disabled on
-command line or in a configuration file.
-.Pp
-Forwarding of arbitrary TCP/IP connections over the secure channel can
-be specified either on command line or in a configuration file. One
-possible application of TCP/IP forwarding is a secure connection to an
-electronic purse; another is going trough firewalls.
-.Pp
-.Nm
-automatically maintains and checks a database containing RSA-based
-identifications for all hosts it has ever been used with. The
-database is stored in
-.Pa \&.ssh/known_hosts
-in the user's home directory. Additionally, the file
-.Pa /etc/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 a
-trojan horse from getting the user's password. Another purpose of
-this mechanism is to prevent man-in-the-middle attacks which could
-otherwise be used to circumvent the encryption. The
-.Cm StrictHostKeyChecking
-option (see below) can be used to prevent logins to machines whose
-host key is not known or has changed.
-.Sh OPTIONS
-.Bl -tag -width Ds
-.It Fl a
-Disables forwarding of the authentication agent connection. This may
-also be specified on a per-host basis in the configuration file.
-.It Fl c Ar blowfish|3des
-Selects the cipher to use for encrypting the session.
-.Ar 3des
-is used by default. It is believed to be secure.
-.Ar 3des
-(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
-It is presumably more secure than the
-.Ar des
-cipher which is no longer supported in ssh.
-.Ar blowfish
-is a fast block cipher, it appears very secure and is much faster than
-.Ar 3des .
-.It Fl e Ar ch|^ch|none
-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.
-.It Fl f
-Requests
-.Nm
-to go to background after authentication. 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 .
-.It Fl i Ar identity_file
-Selects the file from which the identity (private key) for
-RSA authentication is read. Default is
-.Pa \&.ssh/identity
-in the user's home directory. 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).
-.It Fl g
-Allows remote hosts to connect to local forwarded ports.
-.It Fl k
-Disables forwarding of Kerberos tickets and AFS tokens. This may
-also be specified on a per-host basis in the configuration file.
-.It Fl l Ar login_name
-Specifies the user to log in as on the remote machine. This may also
-be specified on a per-host basis in the configuration file.
-.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 in 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.)
-.It Fl o Ar option
-Can be used to give options in the format used in the config file.
-This is useful for specifying options for which there is no separate
-command-line flag. The option has the same format as a line in the
-configuration file.
-.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.
-.It Fl P
-Use a non-privileged port for outgoing connections.
-This can be used if your firewall does
-not permit connections from privileged ports.
-Note that this option turns of
-.Cm RhostsAuthentication
-and
-.Cm RhostsRSAAuthentication .
-.It Fl q
-Quiet mode. Causes all warning and diagnostic messages to be
-suppressed. Only fatal errors are displayed.
-.It Fl t
-Force pseudo-tty allocation. This can be used to execute arbitary
-screen-based programs on a remote machine, which can be very useful
-e.g. when implementing menu services.
-.It Fl v
-Verbose mode. Causes
-.Nm
-to print debugging messages about its progress. This is helpful in
-debugging connection, authentication, and configuration problems.
-The verbose mode is also used to display
-.Xr skey 1
-challenges, if the user entered "s/key" as password.
-.It Fl x
-Disables X11 forwarding. This can also be specified on a per-host
-basis in a configuration file.
-.It Fl X
-Enables X11 forwarding.
-.It Fl C
-Requests compression of all data (including stdin, stdout, stderr, and
-data for forwarded X11 and TCP/IP connections). The compression
-algorithm is the same used by gzip, and the
-.Dq level
-can be controlled by the
-.Cm CompressionLevel
-option (see below). 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 Compress
-option below.
-.It Fl L Ar port:host:hostport
-Specifies that the given port on the local (client) host is to be
-forwarded to the given host and port on the remote side. This works
-by allocating a socket to listen to
-.Ar port
-on the local side, and whenever a connection is made to this port, the
-connection is forwarded over the secure channel, and a connection is
-made to
-.Ar host:hostport
-from the remote machine. Port forwardings can also be specified in the
-configuration file. Only root can forward privileged ports.
-.It Fl R Ar port:host:hostport
-Specifies that the given port on the remote (server) host is to be
-forwarded to the given host and port on the local side. This works
-by allocating a socket to listen to
-.Ar port
-on the remote side, and whenever a connection is made to this port, the
-connection is forwarded over the secure channel, and a connection is
-made to
-.Ar host:hostport
-from the local machine. Port forwardings can also be specified in the
-configuration file. Privileged ports can be forwarded only when
-logging in as root on the remote machine.
-.El
-.Sh CONFIGURATION FILES
-.Nm
-obtains configuration data from the following sources (in this order):
-command line options, user's configuration file
-.Pq Pa $HOME/.ssh/config ,
-and system-wide configuration file
-.Pq Pa /etc/ssh_config .
-For each parameter, the first obtained value
-will be used. The configuration files contain sections bracketed by
-"Host" specifications, and that section is only applied for hosts that
-match one of the patterns given in the specification. The matched
-host name is the one given on the command line.
-.Pp
-Since the first obtained value for each parameter is used, more
-host-specific declarations should be given near the beginning of the
-file, and general defaults at the end.
-.Pp
-The configuration file has the following format:
-.Pp
-Empty lines and lines starting with
-.Ql #
-are comments.
-.Pp
-Otherwise a line is of the format
-.Dq keyword arguments .
-The possible
-keywords and their meanings are as follows (note that the
-configuration files are case-sensitive):
-.Bl -tag -width Ds
-.It Cm Host
-Restricts the following declarations (up to the next
-.Cm Host
-keyword) to be only for those hosts that match one of the patterns
-given after the keyword.
-.Ql \&*
-and
-.Ql ?
-can be used as wildcards in the
-patterns. A single
-.Ql \&*
-as a pattern can be used to provide global
-defaults for all hosts. The host is the
-.Ar hostname
-argument given on the command line (i.e., the name is not converted to
-a canonicalized host name before matching).
-.It Cm AFSTokenPassing
-Specifies whether to pass AFS tokens to remote host. The argument to
-this keyword must be
-.Dq yes
-or
-.Dq no .
-.It Cm BatchMode
-If set to
-.Dq yes ,
-passphrase/password querying will be disabled. This
-option is useful in scripts and other batch jobs where you have no
-user to supply the password. The argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm Cipher
-Specifies the cipher to use for encrypting the session. Currently,
-.Dq blowfish ,
-and
-.Dq 3des
-are supported. The default is
-.Dq 3des .
-.It Cm Compression
-Specifies whether to use compression. The argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm CompressionLevel
-Specifies the compression level to use if compression is enable. The
-argument must be an integer from 1 (fast) to 9 (slow, best). The
-default level is 6, which is good for most applications. The meaning
-of the values is the same as in GNU GZIP.
-.It Cm ConnectionAttempts
-Specifies the number of tries (one per second) to make before falling
-back to rsh or exiting. The argument must be an integer. This may be
-useful in scripts if the connection sometimes fails.
-.It Cm EscapeChar
-Sets the escape character (default:
-.Ql ~ ) .
-The escape character can also
-be set on the command line. The argument should be a single
-character,
-.Ql ^
-followed by a letter, or
-.Dq none
-to disable the escape
-character entirely (making the connection transparent for binary
-data).
-.It Cm FallBackToRsh
-Specifies that if connecting via
-.Nm
-fails due to a connection refused error (there is no
-.Xr sshd 8
-listening on the remote host),
-.Xr rsh 1
-should automatically be used instead (after a suitable warning about
-the session being unencrypted). The argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm ForwardAgent
-Specifies whether the connection to the authentication agent (if any)
-will be forwarded to the remote machine. The argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm ForwardX11
-Specifies whether X11 connections will be automatically redirected
-over the secure channel and
-.Ev DISPLAY
-set. The argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm GatewayPorts
-Specifies whether remote hosts are allowed to connect to local
-forwarded ports.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq no .
-.It Cm GlobalKnownHostsFile
-Specifies a file to use instead of
-.Pa /etc/ssh_known_hosts .
-.It Cm HostName
-Specifies the real host name to log into. This can be used to specify
-nicnames or abbreviations for hosts. Default is the name given on the
-command line. Numeric IP addresses are also permitted (both on the
-command line and in
-.Cm HostName
-specifications).
-.It Cm IdentityFile
-Specifies the file from which the user's RSA authentication identity
-is read (default
-.Pa .ssh/identity
-in the user's home directory).
-Additionally, any identities represented by the authentication agent
-will be used for authentication. The file name may use the tilde
-syntax to refer to a user's home directory. It is possible to have
-multiple identity files specified in configuration files; all these
-identities will be tried in sequence.
-.It Cm KeepAlive
-Specifies whether the system should send keepalive messages to the
-other side. If they are sent, death of the connection or crash of one
-of the machines will be properly noticed. However, this means that
-connections will die if the route is down temporarily, and some people
-find it annoying.
-.Pp
-The default is
-.Dq yes
-(to send keepalives), and the client will notice
-if the network goes down or the remote host dies. This is important
-in scripts, and many users want it too.
-.Pp
-To disable keepalives, the value should be set to
-.Dq no
-in both the server and the client configuration files.
-.It Cm KerberosAuthentication
-Specifies whether Kerberos authentication will be used. The argument to
-this keyword must be
-.Dq yes
-or
-.Dq no .
-.It Cm KerberosTgtPassing
-Specifies whether a Kerberos TGT will be forwarded to the server. This
-will only work if the Kerberos server is actually an AFS kaserver. The
-argument to this keyword must be
-.Dq yes
-or
-.Dq no .
-.It Cm LocalForward
-Specifies that a TCP/IP port on the local machine be forwarded over
-the secure channel to given host:port from the remote machine. The
-first argument must be a port number, and the second must be
-host:port. Multiple forwardings may be specified, and additional
-forwardings can be given on the command line. Only the root can
-forward privileged ports.
-.It Cm PasswordAuthentication
-Specifies whether to use password authentication. The argument to
-this keyword must be
-.Dq yes
-or
-.Dq no .
-.It Cm NumberOfPasswordPrompts
-Specifies the number of password prompts before giving up. The
-argument to this keyword must be an integer. Default is 3.
-.It Cm Port
-Specifies the port number to connect on the remote host. Default is
-22.
-.It Cm ProxyCommand
-Specifies the command to use to connect to the server. The command
-string extends to the end of the line, and is executed with /bin/sh.
-In the command string, %h will be substituted by the host name to
-connect and %p by the port. The command can be basically anything,
-and should read from its stdin and write to its stdout. It should
-eventually connect an
-.Xr sshd 8
-server running on some machine, or execute
-.Ic sshd -i
-somewhere. Host key management will be done using the
-HostName of the host being connected (defaulting to the name typed by
-the user).
-.Pp
-.It Cm RemoteForward
-Specifies that a TCP/IP port on the remote machine be forwarded over
-the secure channel to given host:port from the local machine. The
-first argument must be a port number, and the second must be
-host:port. Multiple forwardings may be specified, and additional
-forwardings can be given on the command line. Only the root can
-forward privileged ports.
-.It Cm RhostsAuthentication
-Specifies whether to try rhosts based authentication. Note that this
-declaration only affects the client side and has no effect whatsoever
-on security. Disabling rhosts authentication may reduce
-authentication time on slow connections when rhosts authentication is
-not used. Most servers do not permit RhostsAuthentication because it
-is not secure (see RhostsRSAAuthentication). The argument to this
-keyword must be
-.Dq yes
-or
-.Dq no .
-.It Cm RhostsRSAAuthentication
-Specifies whether to try rhosts based authentication with RSA host
-authentication. This is the primary authentication method for most
-sites. The argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm RSAAuthentication
-Specifies whether to try RSA authentication. The argument to this
-keyword must be
-.Dq yes
-or
-.Dq no .
-RSA authentication will only be
-attempted if the identity file exists, or an authentication agent is
-running.
-.It Cm CheckHostIP
-If this flag is set to
-.Dq yes ,
-ssh will additionally check the host ip address in the
-.Pa known_hosts
-file. This allows ssh to detect if a host key changed due to DNS spoofing.
-If the option is set to
-.Dq no ,
-the check will not be executed.
-.It Cm StrictHostKeyChecking
-If this flag is set to
-.Dq yes ,
-.Nm
-ssh will never automatically add host keys to the
-.Pa $HOME/.ssh/known_hosts
-file, and refuses to connect hosts whose host key has changed. This
-provides maximum protection against trojan horse attacks. However, it
-can be somewhat annoying if you don't have good
-.Pa /etc/ssh_known_hosts
-files installed and frequently
-connect new hosts. Basically this option forces the user to manually
-add any new hosts. Normally this option is disabled, and new hosts
-will automatically be added to the known host files. The host keys of
-known hosts will be verified automatically in either case. The
-argument must be
-.Dq yes
-or
-.Dq no .
-.It Cm User
-Specifies the user to log in as. This can be useful if you have a
-different user name in different machines. This saves the trouble of
-having to remember to give the user name on the command line.
-.It Cm UserKnownHostsFile
-Specifies a file to use instead of
-.Pa $HOME/.ssh/known_hosts .
-.It Cm UsePrivilegedPort
-Specifies whether to use a privileged port for outgoing connections.
-The argument must be
-.Dq yes
-or
-.Dq no .
-The default is
-.Dq yes .
-Note that setting this option to
-.Dq no
-turns of
-.Cm RhostsAuthentication
-and
-.Cm RhostsRSAAuthentication .
-.It Cm UseRsh
-Specifies that rlogin/rsh should be used for this host. It is
-possible that the host does not at all support the
-.Nm
-protocol. This causes
-.Nm
-to immediately exec
-.Xr rsh 1 .
-All other options (except
-.Cm HostName )
-are ignored if this has been specified. The argument must be
-.Dq yes
-or
-.Dq no .
-.Sh ENVIRONMENT
-.Nm
-will normally set the following environment variables:
-.Bl -tag -width Ds
-.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 hostname indicates
-the host where the shell runs, and n is an integer >= 1. Ssh uses
-this special value to forward X11 connections over the secure
-channel. The user should normally not set DISPLAY explicitly, as that
-will render the X11 connection insecure (and will require the user to
-manually copy any required authorization cookies).
-.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 point the user's mailbox.
-.It Ev PATH
-Set to the default
-.Ev PATH ,
-as specified when compiling
-.Nm ssh .
-.It Ev SSH_AUTH_SOCK
-indicates the path of a unix-domain socket used to communicate with the
-agent.
-.It Ev SSH_CLIENT
-Identifies the client end of the connection. The variable contains
-three space-separated values: client ip-address, client port number,
-and server port number.
-.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 TZ
-The timezone variable is set to indicate the present timezone if it
-was set when the daemon was started (e.i., 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 $HOME/.ssh/environment ,
-and adds lines of the format
-.Dq VARNAME=value
-to the environment.
-.Sh FILES
-.Bl -tag -width $HOME/.ssh/known_hosts
-.It Pa $HOME/.ssh/known_hosts
-Records host keys for all hosts the user has logged into (that are not
-in
-.Pa /etc/ssh_known_hosts ) .
-See
-.Xr sshd 8 .
-.It Pa $HOME/.ssh/random_seed
-Used for seeding the random number generator. This file contains
-sensitive data and should read/write for the user and not accessible
-for others. This file is created the first time the program is run
-and updated automatically. The user should never need to read or
-modify this file.
-.It Pa $HOME/.ssh/identity
-Contains the RSA authentication identity of the user. This file
-contains sensitive data and should be readable by the user but not
-accessible by others (read/write/execute).
-Note that
-.Nm
-ignores this file if it is accessible by others.
-It is possible to specify a passphrase when
-generating the key; the passphrase will be used to encrypt the
-sensitive part of this file using 3DES.
-.It Pa $HOME/.ssh/identity.pub
-Contains the public key for authentication (public part of the
-identity file in human-readable form). The contents of this file
-should be added to
-.Pa $HOME/.ssh/authorized_keys
-on all machines
-where you wish to log in using RSA authentication. This file is not
-sensitive and can (but need not) be readable by anyone. This file is
-never used automatically and is not necessary; it is only provided for
-the convenience of the user.
-.It Pa $HOME/.ssh/config
-This is the per-user configuration file. The format of this file is
-described above. This file is used by the
-.Nm
-client. This file does not usually contain any sensitive information,
-but the recommended permissions are read/write for the user, and not
-accessible by others.
-.It Pa $HOME/.ssh/authorized_keys
-Lists the RSA keys that can be used for logging in as this user. The
-format of this file is described in the
-.Xr sshd 8
-manual page. In the simplest form the format is the same as the .pub
-identity files (that is, each line contains the number of bits in
-modulus, public exponent, modulus, and comment fields, separated by
-spaces). This file is not highly sensitive, but the recommended
-permissions are read/write for the user, and not accessible by others.
-.It Pa /etc/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. This file should be world-readable. This file contains
-public keys, one per line, in the following format (fields separated
-by spaces): system name, number of bits in modulus, public exponent,
-modulus, and optional comment field. When different names are used
-for the same machine, all such names should be listed, separated by
-commas. The format is described on the
-.Xr sshd 8
-manual page.
-.Pp
-The canonical system name (as returned by name servers) is used by
-.Xr sshd 8
-to verify the client host when logging in; other names are needed because
-.Nm
-does not convert the user-supplied name to a canonical name before
-checking the key, because someone with access to the name servers
-would then be able to fool host authentication.
-.It Pa /etc/ssh_config
-Systemwide configuration file. This file provides defaults for those
-values that are not specified in the user's configuration file, and
-for those users who do not have a configuration file. This file must
-be world-readable.
-.It Pa $HOME/.rhosts
-This file is used in
-.Pa \&.rhosts
-authentication to list the
-host/user pairs that are permitted to log in. (Note that this file is
-also used by rlogin and rsh, which makes using this file insecure.)
-Each line of the file contains a host name (in the canonical form
-returned by name servers), and then a user name on that host,
-separated by a space. One some machines this file may need to be
-world-readable if the user's home directory is on a 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
-Note that by default
-.Xr sshd 8
-will be installed so that it requires successful RSA host
-authentication before permitting \s+2.\s0rhosts authentication. If your
-server machine does not have the client's host key in
-.Pa /etc/ssh_known_hosts ,
-you can store it in
-.Pa $HOME/.ssh/known_hosts .
-The easiest way to do this is to
-connect back to the client from the server machine using ssh; this
-will automatically add the host key inxi
-.Pa $HOME/.ssh/known_hosts .
-.It Pa $HOME/.shosts
-This file is used exactly the same way as
-.Pa \&.rhosts .
-The purpose for
-having this file is to be able to use rhosts authentication with
-.Nm
-without permitting login with
-.Xr rlogin 1
-or
-.Xr rsh 1 .
-.It Pa /etc/hosts.equiv
-This file is used during
-.Pa \&.rhosts authentication. It contains
-canonical hosts names, one per line (the full format is described on
-the
-.Xr sshd 8
-manual page). If the client host is found in this file, login is
-automatically permitted provided client and server user names are the
-same. Additionally, successful RSA host authentication is normally
-required. This file should only be writable by root.
-.It Pa /etc/shosts.equiv
-This file is processed exactly as
-.Pa /etc/hosts.equiv .
-This file may be useful to permit logins using
-.Nm
-but not using rsh/rlogin.
-.It Pa /etc/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.
-.It Pa $HOME/.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.
-.It Pa libcrypto.so.X.1
-A version of this library which includes support for the RSA algorithm
-is required for proper operation.
-.Sh AUTHOR
-Tatu Ylonen <ylo@cs.hut.fi>
-.Pp
-Issues can be found from the SSH WWW home page:
-.Pp
-.Dl http://www.cs.hut.fi/ssh
-.Pp
-OpenSSH
-is a derivative of the original (free) ssh 1.2.12 release, but with bugs
-removed and newer features re-added. Rapidly after the 1.2.12 release,
-newer versions bore successively more restrictive licenses. This version
-of OpenSSH
-.Bl -bullet
-.It
-has all components of a restrictive nature (ie. patents, see
-.Xr ssl 8 )
-directly removed from the source code; any licensed or patented components
-are chosen from
-external libraries.
-.It
-has been updated to support ssh protocol 1.5.
-.It
-contains added support for
-.Xr kerberos 8
-authentication and ticket passing.
-.It
-supports one-time password authentication with
-.Xr skey 1 .
-.El
-.Pp
-The libraries described in
-.Xr ssl 8
-are required for proper operation.
-.Sh SEE ALSO
-.Xr rlogin 1 ,
-.Xr rsh 1 ,
-.Xr scp 1 ,
-.Xr ssh-add 1 ,
-.Xr ssh-agent 1 ,
-.Xr ssh-keygen 1 ,
-.Xr telnet 1 ,
-.Xr sshd 8 ,
-.Xr ssl 8