path: root/usr.bin/ssh/ssh.1

.\"  -*- 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 1.3 1999/09/29 00:10:16 deraadt Exp $
.\"
.Dd September 25, 1999
.Dt SSH 1
.Os
.Sh NAME
.Nm ssh
.Nd secure shell client (remote login program)
.Sh SYNOPSIS
.Nm ssh
.Op Fl l Ar login_name
.Ar hostname
.Op Ar command
.Pp
.Nm ssh
.Op Fl gknqtvxXC
.Op Fl c Ar blowfish|idea|des|3des|none
.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
.Op Fl L Ar port:host:hostport
.Op Fl R Ar port:host:hostport
.Ar hostname
.Op Ar command
.Sh DESCRIPTION 
.Nm
(Secure Shell) a program for logging into a remote machine and for
executing commands in 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 additionally it can verify the client's
host key (see 
.Pa $HOME/.ssh/known_hosts
and
.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 c Ar blowfish|idea|des|3des|none
Selects the cipher to use for encrypting the session. 
.Ar 3des
is used by default.  It is believed to be secure. 
.Ar des
is the data encryption standard, but is breakable by 
governments, large corporations, and major criminal organizations.
.Ar 3des
(triple-des) is encrypt-decrypt-encrypt triple with three different keys.
It is presumably more secure than DES.
.Ar none
disables encryption entirely; it is only intended for debugging, and
it renders the connection insecure.
.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 / 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 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.
.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 ,
.Dq idea ,
.Dq des ,
.Dq 3des ,
and
.Dq none
are supported.  The default is
.Dq 3des .
Using
.Dq none
(no encryption) is intended only for debugging, and will render the connection
insecure.
.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. 
.It Cm KerberosTgtPassing
Specifies whether a Kerberos TGT will be forwarded to the server.
Note that TGT forwarding is normally not enabled in the server.
.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 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
Note that
.Nm
can also be configured to support the SOCKS system using the
--with-socks compile-time configuration option.
.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 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 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
or, on some systems, 
.Pa /etc/environment 
or 
.Pa /etc/default/login .
.It Ev SSH_AUTHENTICATION_FD
This is set to an integer value if you are using the authentication
agent and a connection to it has been forwarded.  The value indicates
a file descriptor number used for communicating with the agent.  On
some systems, 
.Ev SSH_AUTHENTICATION_SOCKET
may be used instead to
indicate the path of a unix-domain socket used to communicate with the
agent (this method is less secure, and is only used on systems that
don't support the first method).
.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 /etc/environment 
and 
.Pa $HOME/.ssh/environment , 
and adds lines of the format
.Dq VARNAME=value
to the environment.  Some systems may have
still additional mechanisms for setting up the environment, such as
.Pa /etc/default/login
on Solaris.
.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.  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 IDEA.
.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.
.Sh INSTALLATION
.Nm
is normally installed as suid root.  It needs root privileges only for
rhosts authentication (rhosts authentication requires that the
connection must come from a privileged port, and allocating such a
port requires root privileges).  It also needs to be able to read
.Pa /etc/ssh_host_key
to perform RSA
host authentication.  It is possible to use
.Nm
without root privileges, but rhosts authentication will then be
disabled.  
.Nm
drops any extra privileges immediately after the connection to the
remote host has been made.
.Pp
Considerable work has been put into making
.Xr sshd 8
secure.  However, if you find a security problem, please report it
immediately to <ssh-bugs@cs.hut.fi>.
.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
.Sh SEE ALSO
.Xr make-ssh-known-hosts 1 ,
.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