freebsd-nq/crypto/openssh/ssh.1

1106 lines
32 KiB
Groff
Raw Normal View History

2000-02-24 14:29:47 +00:00
.\" -*- 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
.\"
2000-03-26 07:37:48 +00:00
.\" $Id: ssh.1,v 1.43 2000/03/24 03:04:46 brad Exp $
.\" $FreeBSD$
2000-02-24 14:29:47 +00:00
.\"
.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 afgknqtvxCPX46
.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 port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Oo Fl R Xo
.Sm off
.Ar port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Op Ar hostname | user@hostname
.Op Ar command
.Sh DESCRIPTION
2000-02-24 14:29:47 +00:00
.Nm
(Secure Shell) is a program for logging into a remote machine and for
2000-03-26 07:37:48 +00:00
executing commands on a remote machine.
It is intended to replace
2000-02-24 14:29:47 +00:00
rlogin and rsh, and provide secure encrypted communications between
2000-03-26 07:37:48 +00:00
two untrusted hosts over an insecure network.
X11 connections and
2000-02-24 14:29:47 +00:00
arbitrary TCP/IP ports can also be forwarded over the secure channel.
.Pp
.Nm
connects and logs into the specified
2000-02-24 14:29:47 +00:00
.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/ssh/shosts.equiv
2000-02-24 14:29:47 +00:00
on the remote machine, and the user names are
the same on both sides, the user is immediately permitted to log in.
Second, if
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
permitted to log in.
This form of authentication alone is normally not
2000-02-24 14:29:47 +00:00
allowed by the server because it is not secure.
.Pp
The second (and primary) authentication method is the
.Pa rhosts
or
.Pa hosts.equiv
2000-03-26 07:37:48 +00:00
method combined with RSA-based host authentication.
It means that if the login would be permitted by
2000-02-24 14:29:47 +00:00
.Pa \&.rhosts ,
.Pa \&.shosts ,
.Pa /etc/hosts.equiv ,
or
.Pa /etc/ssh/shosts.equiv ,
2000-02-24 14:29:47 +00:00
and if additionally the server can verify the client's
host key (see
.Pa /etc/ssh/ssh_known_hosts
2000-02-24 14:29:47 +00:00
and
.Pa $HOME/.ssh/known_hosts
in the
.Sx FILES
2000-03-26 07:37:48 +00:00
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:
2000-02-24 14:29:47 +00:00
.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,
2000-02-24 14:29:47 +00:00
.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.
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
.Pa $HOME/.ssh/authorized_keys
lists the public keys that are permitted for logging
2000-03-26 07:37:48 +00:00
in.
When the user logs in, the
2000-02-24 14:29:47 +00:00
.Nm
program tells the server which key pair it would like to use for
2000-03-26 07:37:48 +00:00
authentication.
The server checks if this key is permitted, and if
2000-02-24 14:29:47 +00:00
so, sends the user (actually the
.Nm
program running on behalf of the user) a challenge, a random number,
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
challenge using the private key, proving that he/she knows the private
key but without disclosing it to the server.
.Pp
.Nm
2000-03-26 07:37:48 +00:00
implements the RSA authentication protocol automatically.
The user creates his/her RSA key pair by running
2000-02-24 14:29:47 +00:00
.Xr ssh-keygen 1 .
This stores the private key in
2000-02-24 14:29:47 +00:00
.Pa \&.ssh/identity
and the public key in
.Pa \&.ssh/identity.pub
2000-03-26 07:37:48 +00:00
in the user's home directory.
The user should then copy the
2000-02-24 14:29:47 +00:00
.Pa identity.pub
to
2000-02-24 14:29:47 +00:00
.Pa \&.ssh/authorized_keys
in his/her home directory on the remote machine (the
2000-02-24 14:29:47 +00:00
.Pa authorized_keys
file corresponds to the conventional
2000-02-24 14:29:47 +00:00
.Pa \&.rhosts
file, and has one key
2000-03-26 07:37:48 +00:00
per line, though the lines can be very long).
After this, the user can log in without giving the password.
RSA authentication is much
2000-02-24 14:29:47 +00:00
more secure than rhosts authentication.
.Pp
The most convenient way to use RSA authentication may be with an
2000-03-26 07:37:48 +00:00
authentication agent.
See
2000-02-24 14:29:47 +00:00
.Xr ssh-agent 1
for more information.
.Pp
If other authentication methods fail,
2000-02-24 14:29:47 +00:00
.Nm
2000-03-26 07:37:48 +00:00
prompts the user for a password.
The password is sent to the remote
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
the user a normal shell on the remote machine.
All communication with
2000-02-24 14:29:47 +00:00
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 ~#
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
shell to hang).
All available escapes can be listed with
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
special.
The escape character can be changed in configuration files
or on the command line.
2000-02-24 14:29:47 +00:00
.Pp
If no pseudo tty has been allocated, the
session is transparent and can be used to reliably transfer binary
2000-03-26 07:37:48 +00:00
data.
On most systems, setting the escape character to
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
from the local machine.
The user should not manually set
2000-02-24 14:29:47 +00:00
.Ev DISPLAY .
Forwarding of X11 connections can be
configured on the command line or in configuration files.
.Pp
The
.Ev DISPLAY
2000-02-24 14:29:47 +00:00
value set by
.Nm
will point to the server machine, but with a display number greater
2000-03-26 07:37:48 +00:00
than zero.
This is normal, and happens because
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
the connection is opened.
The real authentication cookie is never
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
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.
2000-02-24 14:29:47 +00:00
.Pp
.Nm
automatically maintains and checks a database containing RSA-based
2000-03-26 07:37:48 +00:00
identifications for all hosts it has ever been used with.
The database is stored in
2000-02-24 14:29:47 +00:00
.Pa \&.ssh/known_hosts
2000-03-26 07:37:48 +00:00
in the user's home directory.
Additionally, the file
.Pa /etc/ssh/ssh_known_hosts
2000-03-26 07:37:48 +00:00
is automatically checked for known hosts.
Any new hosts are automatically added to the user's file.
If a host's identification
2000-02-24 14:29:47 +00:00
ever changes,
.Nm
warns about this and disables password authentication to prevent a
2000-03-26 07:37:48 +00:00
trojan horse from getting the user's password.
Another purpose of
2000-02-24 14:29:47 +00:00
this mechanism is to prevent man-in-the-middle attacks which could
2000-03-26 07:37:48 +00:00
otherwise be used to circumvent the encryption.
The
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
Disables forwarding of the authentication agent connection.
This may also be specified on a per-host basis in the configuration file.
2000-02-24 14:29:47 +00:00
.It Fl c Ar blowfish|3des
Selects the cipher to use for encrypting the session.
2000-02-24 14:29:47 +00:00
.Ar 3des
2000-03-26 07:37:48 +00:00
is used by default.
It is believed to be secure.
2000-02-24 14:29:47 +00:00
.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 .
2000-02-24 14:29:47 +00:00
.It Fl e Ar ch|^ch|none
Sets the escape character for sessions with a pty (default:
.Ql ~ ) .
2000-03-26 07:37:48 +00:00
The escape character is only recognized at the beginning of a line.
The escape character followed by a dot
2000-02-24 14:29:47 +00:00
.Pq Ql \&.
closes the connection, followed
by control-Z suspends the connection, and followed by itself sends the
2000-03-26 07:37:48 +00:00
escape character once.
Setting the character to
2000-02-24 14:29:47 +00:00
.Dq none
disables any escapes and makes the session fully transparent.
.It Fl f
Requests
.Nm
2000-03-26 07:37:48 +00:00
to go to background just before command execution.
This is useful if
2000-02-24 14:29:47 +00:00
.Nm
is going to ask for passwords or passphrases, but the user
2000-03-26 07:37:48 +00:00
wants it in the background.
This implies
2000-02-24 14:29:47 +00:00
.Fl n .
The recommended way to start X11 programs at a remote site is with
something like
.Ic ssh -f host xterm .
.It Fl g
Allows remote hosts to connect to local forwarded ports.
.It Fl i Ar identity_file
2000-03-26 07:37:48 +00:00
Selects the file from which the identity (private key) for
RSA authentication is read.
Default is
2000-02-24 14:29:47 +00:00
.Pa \&.ssh/identity
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
.Fl i
options (and multiple identities specified in
configuration files).
.It Fl k
2000-03-26 07:37:48 +00:00
Disables forwarding of Kerberos tickets and AFS tokens.
This may also be specified on a per-host basis in the configuration file.
2000-02-24 14:29:47 +00:00
.It Fl l Ar login_name
2000-03-26 07:37:48 +00:00
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.
2000-02-24 14:29:47 +00:00
.It Fl n
Redirects stdin from
.Pa /dev/null
(actually, prevents reading from stdin).
This must be used when
.Nm
2000-03-26 07:37:48 +00:00
is run in the background.
A common trick is to use this to run X11 programs on a remote machine.
For example,
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
command-line flag.
The option has the same format as a line in the configuration file.
2000-02-24 14:29:47 +00:00
.It Fl p Ar port
2000-03-26 07:37:48 +00:00
Port to connect to on the remote host.
This can be specified on a
2000-02-24 14:29:47 +00:00
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 off
.Cm RhostsAuthentication
and
.Cm RhostsRSAAuthentication .
.It Fl q
2000-03-26 07:37:48 +00:00
Quiet mode.
Causes all warning and diagnostic messages to be suppressed.
Only fatal errors are displayed.
2000-02-24 14:29:47 +00:00
.It Fl t
2000-03-26 07:37:48 +00:00
Force pseudo-tty allocation.
This can be used to execute arbitrary
screen-based programs on a remote machine, which can be very useful,
e.g., when implementing menu services.
2000-02-24 14:29:47 +00:00
.It Fl v
2000-03-26 07:37:48 +00:00
Verbose mode.
Causes
2000-02-24 14:29:47 +00:00
.Nm
2000-03-26 07:37:48 +00:00
to print debugging messages about its progress.
This is helpful in
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
Disables X11 forwarding.
This can also be specified on a per-host basis in a configuration file.
2000-02-24 14:29:47 +00:00
.It Fl X
Enables X11 forwarding.
.It Fl C
Requests compression of all data (including stdin, stdout, stderr, and
2000-03-26 07:37:48 +00:00
data for forwarded X11 and TCP/IP connections).
The compression algorithm is the same used by
2000-02-24 14:29:47 +00:00
.Xr gzip 1 ,
and the
.Dq level
can be controlled by the
.Cm CompressionLevel
2000-03-26 07:37:48 +00:00
option (see below).
Compression is desirable on modem lines and other
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
forwarded to the given host and port on the remote side.
This works by allocating a socket to listen to
2000-02-24 14:29:47 +00:00
.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
port
.Ar hostport
2000-03-26 07:37:48 +00:00
from the remote machine.
Port forwardings can also be specified in the configuration file.
Only root can forward privileged ports.
2000-02-24 14:29:47 +00:00
IPv6 addresses can be specified with an alternative syntax:
.Ar port/host/hostport
.It Fl R Ar port:host:hostport
Specifies that the given port on the remote (server) host is to be
2000-03-26 07:37:48 +00:00
forwarded to the given host and port on the local side.
This works by allocating a socket to listen to
2000-02-24 14:29:47 +00:00
.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
port
.Ar hostport
2000-03-26 07:37:48 +00:00
from the local machine.
Port forwardings can also be specified in the configuration file.
Privileged ports can be forwarded only when
2000-02-24 14:29:47 +00:00
logging in as root on the remote machine.
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.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/ssh_config .
2000-02-24 14:29:47 +00:00
For each parameter, the first obtained value
2000-03-26 07:37:48 +00:00
will be used.
The configuration files contain sections bracketed by
.Dq 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.
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
patterns.
A single
2000-02-24 14:29:47 +00:00
.Ql \&*
as a pattern can be used to provide global
2000-03-26 07:37:48 +00:00
defaults for all hosts.
The host is the
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
Specifies whether to pass AFS tokens to remote host.
The argument to this keyword must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm BatchMode
If set to
.Dq yes ,
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm CheckHostIP
If this flag is set to
.Dq yes ,
ssh will additionally check the host ip address in the
.Pa known_hosts
2000-03-26 07:37:48 +00:00
file.
This allows ssh to detect if a host key changed due to DNS spoofing.
2000-02-24 14:29:47 +00:00
If the option is set to
.Dq no ,
the check will not be executed.
.It Cm Cipher
2000-03-26 07:37:48 +00:00
Specifies the cipher to use for encrypting the session.
Currently,
2000-02-24 14:29:47 +00:00
.Dq blowfish ,
and
.Dq 3des
2000-03-26 07:37:48 +00:00
are supported.
The default is
2000-02-24 14:29:47 +00:00
.Dq 3des .
.It Cm Compression
2000-03-26 07:37:48 +00:00
Specifies whether to use compression.
The argument must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm CompressionLevel
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
.Xr gzip 1 .
.It Cm ConnectionAttempts
Specifies the number of tries (one per second) to make before falling
2000-03-26 07:37:48 +00:00
back to rsh or exiting.
The argument must be an integer.
This may be useful in scripts if the connection sometimes fails.
2000-02-24 14:29:47 +00:00
.It Cm EscapeChar
Sets the escape character (default:
.Ql ~ ) .
The escape character can also
2000-03-26 07:37:48 +00:00
be set on the command line.
The argument should be a single character,
2000-02-24 14:29:47 +00:00
.Ql ^
followed by a letter, or
.Dq none
to disable the escape
character entirely (making the connection transparent for binary
data).
.It Cm FallBackToRsh
2000-02-24 14:29:47 +00:00
Specifies that if connecting via
.Nm
fails due to a connection refused error (there is no
.Xr sshd 8
listening on the remote host),
2000-02-24 14:29:47 +00:00
.Xr rsh 1
should automatically be used instead (after a suitable warning about
2000-03-26 07:37:48 +00:00
the session being unencrypted).
The argument must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm ForwardAgent
Specifies whether the connection to the authentication agent (if any)
2000-03-26 07:37:48 +00:00
will be forwarded to the remote machine.
The argument must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm ForwardX11
Specifies whether X11 connections will be automatically redirected
over the secure channel and
2000-02-24 14:29:47 +00:00
.Ev DISPLAY
2000-03-26 07:37:48 +00:00
set.
The argument must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
The default is
.Dq no .
2000-02-24 14:29:47 +00:00
.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/ssh_known_hosts .
2000-02-24 14:29:47 +00:00
.It Cm HostName
2000-03-26 07:37:48 +00:00
Specifies the real host name to log into.
This can be used to specify nicknames 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
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
connections will die if the route is down temporarily, and some people
find it annoying.
2000-02-24 14:29:47 +00:00
.Pp
The default is
.Dq yes
(to send keepalives), and the client will notice
2000-03-26 07:37:48 +00:00
if the network goes down or the remote host dies.
This is important in scripts, and many users want it too.
2000-02-24 14:29:47 +00:00
.Pp
To disable keepalives, the value should be set to
.Dq no
in both the server and the client configuration files.
.It Cm KerberosAuthentication
2000-03-26 07:37:48 +00:00
Specifies whether Kerberos authentication will be used.
The argument to this keyword must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm KerberosTgtPassing
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm LocalForward
Specifies that a TCP/IP port on the local machine be forwarded over
2000-03-26 07:37:48 +00:00
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 superuser can forward privileged ports.
2000-02-24 14:29:47 +00:00
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Nm ssh .
The possible values are:
2000-03-26 07:37:48 +00:00
QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
2000-02-24 14:29:47 +00:00
The default is INFO.
.It Cm NumberOfPasswordPrompts
2000-03-26 07:37:48 +00:00
Specifies the number of password prompts before giving up.
The argument to this keyword must be an integer.
Default is 3.
2000-02-24 14:29:47 +00:00
.It Cm PasswordAuthentication
2000-03-26 07:37:48 +00:00
Specifies whether to use password authentication.
The argument to this keyword must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm Port
2000-03-26 07:37:48 +00:00
Specifies the port number to connect on the remote host.
Default is 22.
2000-02-24 14:29:47 +00:00
.It Cm ProxyCommand
2000-03-26 07:37:48 +00:00
Specifies the command to use to connect to the server.
The command
string extends to the end of the line, and is executed with
.Pa /bin/sh .
In the command string,
2000-03-26 07:37:48 +00:00
.Ql %h
will be substituted by the host name to
connect and
2000-03-26 07:37:48 +00:00
.Ql %p
by the port.
The command can be basically anything,
and should read from its standard input and write to its standard output.
It should eventually connect an
2000-02-24 14:29:47 +00:00
.Xr sshd 8
server running on some machine, or execute
.Ic sshd -i
2000-03-26 07:37:48 +00:00
somewhere.
Host key management will be done using the
2000-02-24 14:29:47 +00:00
HostName of the host being connected (defaulting to the name typed by
the user).
Note that
.Cm CheckHostIP
is not available for connects with a proxy command.
.Pp
.It Cm RemoteForward
Specifies that a TCP/IP port on the remote machine be forwarded over
2000-03-26 07:37:48 +00:00
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 superuser can forward privileged ports.
2000-02-24 14:29:47 +00:00
.It Cm RhostsAuthentication
2000-03-26 07:37:48 +00:00
Specifies whether to try rhosts based authentication.
Note that this
2000-02-24 14:29:47 +00:00
declaration only affects the client side and has no effect whatsoever
2000-03-26 07:37:48 +00:00
on security.
Disabling rhosts authentication may reduce
2000-02-24 14:29:47 +00:00
authentication time on slow connections when rhosts authentication is
2000-03-26 07:37:48 +00:00
not used.
Most servers do not permit RhostsAuthentication because it
is not secure (see RhostsRSAAuthentication).
The argument to this keyword must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA host
2000-03-26 07:37:48 +00:00
authentication.
This is the primary authentication method for most sites.
The argument must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.It Cm RSAAuthentication
2000-03-26 07:37:48 +00:00
Specifies whether to try RSA authentication.
The argument to this keyword must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
RSA authentication will only be
attempted if the identity file exists, or an authentication agent is
running.
.It Cm SkeyAuthentication
Specifies whether to use
.Xr skey 1
2000-03-26 07:37:48 +00:00
authentication.
The argument to this keyword must be
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm StrictHostKeyChecking
If this flag is set to
.Dq yes ,
2000-02-24 14:29:47 +00:00
.Nm
ssh will never automatically add host keys to the
.Pa $HOME/.ssh/known_hosts
2000-03-26 07:37:48 +00:00
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/ssh_known_hosts
2000-02-24 14:29:47 +00:00
files installed and frequently
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
.Dq yes
or
.Dq no .
.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 off
.Cm RhostsAuthentication
and
.Cm RhostsRSAAuthentication .
.It Cm User
2000-03-26 07:37:48 +00:00
Specifies the user to log in as.
This can be useful if you have a different user name on different machines.
This saves the trouble of
2000-02-24 14:29:47 +00:00
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 UseRsh
2000-03-26 07:37:48 +00:00
Specifies that rlogin/rsh should be used for this host.
It is possible that the host does not at all support the
2000-02-24 14:29:47 +00:00
.Nm
2000-03-26 07:37:48 +00:00
protocol.
This causes
2000-02-24 14:29:47 +00:00
.Nm
2000-03-26 07:37:48 +00:00
to immediately execute
2000-02-24 14:29:47 +00:00
.Xr rsh 1 .
All other options (except
.Cm HostName )
2000-03-26 07:37:48 +00:00
are ignored if this has been specified.
The argument must be
2000-02-24 14:29:47 +00:00
.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
2000-03-26 07:37:48 +00:00
variable indicates the location of the X11 server.
It is automatically set by
2000-02-24 14:29:47 +00:00
.Nm
to point to a value of the form
.Dq hostname:n
where hostname indicates
2000-03-26 07:37:48 +00:00
the host where the shell runs, and n is an integer \*(>= 1.
.Nm
uses this special value to forward X11 connections over the secure
channel.
The user should normally not set DISPLAY explicitly, as that
2000-02-24 14:29:47 +00:00
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.
2000-03-26 07:37:48 +00:00
.It Ev PATH
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
Identifies the client end of the connection.
The variable contains
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
with the current shell or command.
If the current session has no tty,
2000-02-24 14:29:47 +00:00
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,
2000-02-24 14:29:47 +00:00
.Nm
reads
.Pa $HOME/.ssh/environment ,
2000-02-24 14:29:47 +00:00
and adds lines of the format
.Dq VARNAME=value
to the environment.
.Sh FILES
.Bl -tag -width Ds
.It Pa $HOME/.ssh/known_hosts
Records host keys for all hosts the user has logged into (that are not
in
.Pa /etc/ssh/ssh_known_hosts ) .
2000-02-24 14:29:47 +00:00
See
.Xr sshd 8 .
.It Pa $HOME/.ssh/identity
2000-03-26 07:37:48 +00:00
Contains the RSA authentication identity of the user.
This file
2000-02-24 14:29:47 +00:00
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
2000-02-24 14:29:47 +00:00
Contains the public key for authentication (public part of the
2000-03-26 07:37:48 +00:00
identity file in human-readable form).
The contents of this file should be added to
2000-02-24 14:29:47 +00:00
.Pa $HOME/.ssh/authorized_keys
on all machines
2000-03-26 07:37:48 +00:00
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
2000-02-24 14:29:47 +00:00
never used automatically and is not necessary; it is only provided for
the convenience of the user.
.It Pa $HOME/.ssh/config
2000-03-26 07:37:48 +00:00
This is the per-user configuration file.
The format of this file is described above.
This file is used by the
2000-02-24 14:29:47 +00:00
.Nm
2000-03-26 07:37:48 +00:00
client.
This file does not usually contain any sensitive information,
2000-02-24 14:29:47 +00:00
but the recommended permissions are read/write for the user, and not
accessible by others.
.It Pa $HOME/.ssh/authorized_keys
2000-03-26 07:37:48 +00:00
Lists the RSA keys that can be used for logging in as this user.
The format of this file is described in the
2000-02-24 14:29:47 +00:00
.Xr sshd 8
2000-03-26 07:37:48 +00:00
manual page.
In the simplest form the format is the same as the .pub
2000-02-24 14:29:47 +00:00
identity files (that is, each line contains the number of bits in
modulus, public exponent, modulus, and comment fields, separated by
2000-03-26 07:37:48 +00:00
spaces).
This file is not highly sensitive, but the recommended
2000-02-24 14:29:47 +00:00
permissions are read/write for the user, and not accessible by others.
.It Pa /etc/ssh/ssh_known_hosts
2000-03-26 07:37:48 +00:00
Systemwide list of known host keys.
This file should be prepared by the
2000-02-24 14:29:47 +00:00
system administrator to contain the public host keys of all machines in the
2000-03-26 07:37:48 +00:00
organization.
This file should be world-readable.
This file contains
2000-02-24 14:29:47 +00:00
public keys, one per line, in the following format (fields separated
by spaces): system name, number of bits in modulus, public exponent,
2000-03-26 07:37:48 +00:00
modulus, and optional comment field.
When different names are used
2000-02-24 14:29:47 +00:00
for the same machine, all such names should be listed, separated by
2000-03-26 07:37:48 +00:00
commas.
The format is described on the
2000-02-24 14:29:47 +00:00
.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/ssh_config
2000-03-26 07:37:48 +00:00
Systemwide configuration file.
This file provides defaults for those
2000-02-24 14:29:47 +00:00
values that are not specified in the user's configuration file, and
2000-03-26 07:37:48 +00:00
for those users who do not have a configuration file.
This file must be world-readable.
2000-02-24 14:29:47 +00:00
.It Pa $HOME/.rhosts
This file is used in
.Pa \&.rhosts
authentication to list the
2000-03-26 07:37:48 +00:00
host/user pairs that are permitted to log in.
(Note that this file is
2000-02-24 14:29:47 +00:00
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,
2000-03-26 07:37:48 +00:00
separated by a space.
One some machines this file may need to be
2000-02-24 14:29:47 +00:00
world-readable if the user's home directory is on a NFS partition,
because
.Xr sshd 8
2000-03-26 07:37:48 +00:00
reads it as root.
Additionally, this file must be owned by the user,
and must not have write permissions for anyone else.
The recommended
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
authentication before permitting \s+2.\s0rhosts authentication.
If your server machine does not have the client's host key in
.Pa /etc/ssh/ssh_known_hosts ,
2000-02-24 14:29:47 +00:00
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
2000-03-26 07:37:48 +00:00
authentication.
It contains
2000-02-24 14:29:47 +00:00
canonical hosts names, one per line (the full format is described on
the
.Xr sshd 8
2000-03-26 07:37:48 +00:00
manual page).
If the client host is found in this file, login is
2000-02-24 14:29:47 +00:00
automatically permitted provided client and server user names are the
2000-03-26 07:37:48 +00:00
same.
Additionally, successful RSA host authentication is normally
required.
This file should only be writable by root.
.It Pa /etc/ssh/shosts.equiv
2000-03-26 07:37:48 +00:00
This file is processed exactly as
2000-02-24 14:29:47 +00:00
.Pa /etc/hosts.equiv .
This file may be useful to permit logins using
.Nm
but not using rsh/rlogin.
.It Pa /etc/ssh/sshrc
2000-02-24 14:29:47 +00:00
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
2000-02-24 14:29:47 +00:00
.Xr sshd 8
manual page for more information.
.It Pa $HOME/.ssh/environment
Contains additional definitions for environment variables, see section
.Sx ENVIRONMENT
above.
.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
OpenSSH
is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
2000-03-26 07:37:48 +00:00
but with bugs removed and newer features re-added.
Rapidly after the
2000-02-24 14:29:47 +00:00
1.2.12 release, newer versions of the original ssh bore successively
more restrictive licenses, and thus demand for a free version was born.
This version of OpenSSH
.Bl -bullet
.It
has all components of a restrictive nature (i.e., 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, making it compatible with
all other ssh protocol 1 clients and servers.
.It
contains added support for
2000-02-24 14:29:47 +00:00
.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.
.Pp
OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
Niels Provos, Theo de Raadt, and Dug Song.
.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