ssh [-l login_name] [hostname | user@hostname] [command]
ssh [-afgknqstvxACNPTX1246] [-c cipher_spec] [-e escape_char] [-i
identity_file] [-l login_name] [-m mac_spec] [-o option] [-p port]
[-L port:host:hostport] [-R port:host:hostport] [hostname |
user@hostname] [command]
DESCRIPTION
ssh (SSH client) 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 untrust
ed hosts over an insecure network. X11 connections and arbitrary TCP/IP
ports can also be forwarded over the secure channel.
ssh connects and logs into the specified hostname. The user must prove
his/her identity to the remote machine using one of several methods de
pending on the protocol version used:
SSH protocol version 1
First, if the machine the user logs in from is listed in /etc/hosts.equiv
or /etc/ssh/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 .rhosts or .shosts exists in the user's home directory on the
remote machine and contains a line containing the name of the client ma
chine 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.
The second authentication method is the rhosts or hosts.equiv method com
bined with RSA-based host authentication. It means that if the login
would be permitted by $HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or
/etc/ssh/shosts.equiv, and if additionally the server can verify the
client's host key (see /etc/ssh/ssh_known_hosts and
$HOME/.ssh/known_hosts in the FILES section), only then login is permit
ted. This authentication method closes security holes due to IP spoof
ing, DNS spoofing and routing spoofing. [Note to the administrator:
/etc/hosts.equiv, $HOME/.rhosts, and the rlogin/rsh protocol in general,
are inherently insecure and should be disabled if security is desired.]
As a third authentication method, ssh 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
$HOME/.ssh/authorized_keys lists the public keys that are permitted for
logging in. When the user logs in, the ssh 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
ssh 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 chal
lenge using the private key, proving that he/she knows the private key
but without disclosing it to the server.
the password. RSA authentication is much more secure than rhosts authen
tication.
The most convenient way to use RSA authentication may be with an authen
tication agent. See ssh-agent(1) for more information.
If other authentication methods fail, ssh prompts the user for a pass
word. 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.
SSH protocol version 2
When a user connects using the protocol version 2 different authentica
tion methods are available. Using the default values for
PreferredAuthentications, the client will try to authenticate first using
the public key method; if this method fails password authentication is
attempted, and finally if this method fails keyboard-interactive authen
tication is attempted. If this method fails password authentication is
tried.
The public key method is similar to RSA authentication described in the
previous section and allows the RSA or DSA algorithm to be used: The
client uses his private key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to
sign the session identifier and sends the result to the server. The
server checks whether the matching public key is listed in
$HOME/.ssh/authorized_keys2 and grants access if both the key is found
and the signature is correct. The session identifier is derived from a
shared Diffie-Hellman value and is only known to the client and the serv
er.
If public key authentication fails or is not available a password can be
sent encrypted to the remote host for proving the user's identity.
Additionally, ssh supports hostbased or challenge response authentica
tion.
Protocol 2 provides additional mechanisms for confidentiality (the traf
fic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity
(hmac-md5, hmac-sha1). Note that protocol 1 lacks a strong mechanism for
ensuring the integrity of the connection.
Login session and remote execution
When the user's identity has been accepted by the server, the server ei
ther 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.
If a pseudo-terminal has been allocated (normal login session), the user
may use the escape characters noted below.
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 ``none'' will also make the session transparent even
if a tty is used.
A single tilde character can be sent as ~~ (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 charac
ter can be changed in configuration files using the EscapeChar configura
tion directive or on the command line by the -e option.
The supported escapes (assuming the default `~') are:
~. Disconnect
~^Z Background ssh
~# List forwarded connections
~& Background ssh at logout when waiting for forwarded connection /
X11 sessions to terminate (protocol version 1 only)
~? Display a list of escape characters
~R Request rekeying of the connection (only useful for SSH protocol
version 2 and if the peer supports it)
X11 and TCP forwarding
If the user is using X11 (the 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 com
mand) 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 DISPLAY. Forwarding of X11 connections can be configured on
the command line or in configuration files.
The DISPLAY value set by ssh will point to the server machine, but with a
display number greater than zero. This is normal, and happens because
ssh creates a ``proxy'' X server on the server machine for forwarding the
connections over the encrypted channel.
ssh 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).
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.
Forwarding of arbitrary TCP/IP connections over the secure channel can be
specified either on command line or in a configuration file. One possi
ble application of TCP/IP forwarding is a secure connection to an elec
tronic purse; another is going through firewalls.
Server authentication
which could otherwise be used to circumvent the encryption. The
StrictHostKeyChecking option (see below) can be used to prevent logins to
machines whose host key is not known or has changed.
The options are as follows:
-a Disables forwarding of the authentication agent connection.
-A Enables forwarding of the authentication agent connection. This
can also be specified on a per-host basis in a configuration
file.
-c blowfish|3des
Selects the cipher to use for encrypting the session. 3des is
used by default. It is believed to be secure. 3des (triple-des)
is an encrypt-decrypt-encrypt triple with three different keys.
It is presumably more secure than the des cipher which is no
longer fully supported in ssh. blowfish is a fast block cipher,
it appears very secure and is much faster than 3des.
-c cipher_spec
Additionally, for protocol version 2 a comma-separated list of
ciphers can be specified in order of preference. See Ciphers for
more information.
-e ch|^ch|none
Sets the escape character for sessions with a pty (default: `~').
The escape character is only recognized at the beginning of a
line. The escape character followed by a dot (`.') closes the
connection, followed by control-Z suspends the connection, and
followed by itself sends the escape character once. Setting the
character to ``none'' disables any escapes and makes the session
fully transparent.
-f Requests ssh to go to background just before command execution.
This is useful if ssh is going to ask for passwords or passphras
es, but the user wants it in the background. This implies -n.
The recommended way to start X11 programs at a remote site is
with something like ssh -f host xterm.
-g Allows remote hosts to connect to local forwarded ports.
-i identity_file
Selects the file from which the identity (private key) for RSA or
DSA authentication is read. Default is $HOME/.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 -i options (and multiple identities specified in
configuration files).
-k Disables forwarding of Kerberos tickets and AFS tokens. This may
also be specified on a per-host basis in the configuration file.
-l login_name
Specifies the user to log in as on the remote machine. This also
may be specified on a per-host basis in the configuration file.
an emacs on shadows.cs.hut.fi, and the X11 connection will be au
tomatically forwarded over an encrypted channel. The ssh program
will be put in the background. (This does not work if ssh needs
to ask for a password or passphrase; see also the -f option.)
-N Do not execute a remote command. This is useful if you just want
to forward ports (protocol version 2 only).
-o 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.
-p port
Port to connect to on the remote host. This can be specified on
a per-host basis in the configuration file.
-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 RhostsAuthentication and
RhostsRSAAuthentication for older servers.
-q Quiet mode. Causes all warning and diagnostic messages to be
suppressed. Only fatal errors are displayed.
-s May be used to request invocation of a subsystem on the remote
system. Subsystems are a feature of the SSH2 protocol which fa
cilitate the use of SSH as a secure transport for other applica
tion (eg. sftp). The subsystem is specified as the remote com
mand.
-t Force pseudo-tty allocation. This can be used to execute arbi
trary screen-based programs on a remote machine, which can be
very useful, e.g., when implementing menu services. Multiple -t
options force tty allocation, even if ssh has no local tty.
-T Disable pseudo-tty allocation.
-v Verbose mode. Causes ssh to print debugging messages about its
progress. This is helpful in debugging connection, authentica
tion, and configuration problems. Multiple -v options increases
the verbosity. Maximum is 3.
-x Disables X11 forwarding.
-X Enables X11 forwarding. This can also be specified on a per-host
basis in a configuration file.
-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(1), and the
``level'' can be controlled by the 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 con
host port hostport from the remote machine. Port forwardings can
also be specified in the configuration file. Only root can for
ward privileged ports. IPv6 addresses can be specified with an
alternative syntax: port/host/hostport
-R 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 port on the remote
side, and whenever a connection is made to this port, the connec
tion is forwarded over the secure channel, and a connection is
made to host port hostport from the local machine. Port forward
ings can also be specified in the configuration file. Privileged
ports can be forwarded only when logging in as root on the remote
machine. IPv6 addresses can be specified with an alternative
syntax: port/host/hostport
-1 Forces ssh to try protocol version 1 only.
-2 Forces ssh to try protocol version 2 only.
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh to use IPv6 addresses only.
CONFIGURATION FILES
ssh obtains configuration data from the following sources (in this or
der): command line options, user's configuration file
($HOME/.ssh/config), and system-wide configuration file
(/etc/ssh/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.
Since the first obtained value for each parameter is used, more host-spe
cific declarations should be given near the beginning of the file, and
general defaults at the end.
The configuration file has the following format:
Empty lines and lines starting with `#' are comments.
Otherwise a line is of the format ``keyword arguments''. The possible
keywords and their meanings are as follows (note that the configuration
files are case-sensitive):
Host Restricts the following declarations (up to the next Host key
word) to be only for those hosts that match one of the patterns
given after the keyword. `*' and `?' can be used as wildcards in
the patterns. A single `*' as a pattern can be used to provide
global defaults for all hosts. The host is the hostname argument
given on the command line (i.e., the name is not converted to a
canonicalized host name before matching).
AFSTokenPassing
CheckHostIP
If this flag is set to ``yes'', ssh will additionally check the
host IP address in the known_hosts file. This allows ssh to de
tect if a host key changed due to DNS spoofing. If the option is
set to ``no'', the check will not be executed. The default is
``yes''.
Cipher Specifies the cipher to use for encrypting the session in proto
col version 1. Currently, ``blowfish'' and ``3des'' are support
ed. The default is ``3des''.
Ciphers
Specifies the ciphers allowed for protocol version 2 in order of
preference. Multiple ciphers must be comma-separated. The de
fault is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc''
Compression
Specifies whether to use compression. The argument must be
``yes'' or ``no''. The default is ``no''.
CompressionLevel
Specifies the compression level to use if compression is enabled.
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 gzip(1). Note that this
option applies to protocol version 1 only.
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.
The default is 4.
EscapeChar
Sets the escape character (default: `~'). The escape character
can also be set on the command line. The argument should be a
single character, `^' followed by a letter, or ``none'' to dis
able the escape character entirely (making the connection trans
parent for binary data).
FallBackToRsh
Specifies that if connecting via ssh fails due to a connection
refused error (there is no sshd(8) listening on the remote host),
rsh(1) should automatically be used instead (after a suitable
warning about the session being unencrypted). The argument must
be ``yes'' or ``no''. The default is ``no''.
ForwardAgent
Specifies whether the connection to the authentication agent (if
any) will be forwarded to the remote machine. The argument must
be ``yes'' or ``no''. The default is ``no''.
GlobalKnownHostsFile
Specifies a file to use for the protocol version 1 global host
key database instead of /etc/ssh/ssh_known_hosts.
GlobalKnownHostsFile2
Specifies a file to use for the protocol version 2 global host
key database instead of /etc/ssh/ssh_known_hosts2.
HostbasedAuthentication
Specifies whether to try rhosts based authentication with public
key authentication. The argument must be ``yes'' or ``no''. The
default is ``yes''. This option applies to protocol version 2 on
ly and is similar to RhostsRSAAuthentication.
HostKeyAlgorithms
Specfies the protocol version 2 host key algorithms that the
client wants to use in order of preference. The default for this
option is: ``ssh-rsa,ssh-dss''
HostKeyAlias
Specifies an alias that should be used instead of the real host
name when looking up or saving the host key in the host key
database files. This option is useful for tunneling ssh connec
tions or if you have multiple servers running on a single host.
HostName
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 HostName specifica
tions).
IdentityFile
Specifies the file from which the user's RSA or DSA authentica
tion identity is read (default $HOME/.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 directo
ry. It is possible to have multiple identity files specified in
configuration files; all these identities will be tried in se
quence.
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 tem
porarily, and some people find it annoying.
The default is ``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.
To disable keepalives, the value should be set to ``no'' in both
the server and the client configuration files.
LocalForward
Specifies that a TCP/IP port on the local machine be forwarded
over the secure channel to given host:port from the remote ma
chine. 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.
LogLevel
Gives the verbosity level that is used when logging messages from
ssh. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE
and DEBUG. The default is INFO.
MACs Specifies the MAC (message authentication code) algorithms in or
der of preference. The MAC algorithm is used in protocol version
2 for data integrity protection. Multiple algorithms must be
comma-separated. The default is
``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
hmac-sha1-96,hmac-md5-96''
NumberOfPasswordPrompts
Specifies the number of password prompts before giving up. The
argument to this keyword must be an integer. Default is 3.
PasswordAuthentication
Specifies whether to use password authentication. The argument
to this keyword must be ``yes'' or ``no''. The default is
``yes''.
Port Specifies the port number to connect on the remote host. Default
is 22.
PreferredAuthentications
Specifies the order in which the client should try protocol 2 au
thentication methods. This allows a client to prefer one method
(e.g. keyboard-interactive) over another method (e.g. password)
The default for this option is: ``publickey, password, keyboard-
interactive''
Protocol
Specifies the protocol versions ssh should support in order of
preference. The possible values are ``1'' and ``2''. Multiple
versions must be comma-separated. The default is ``2,1''. This
means that ssh tries version 2 and falls back to version 1 if
version 2 is not available.
ProxyCommand
Specifies the command to use to connect to the server. The com
mand 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 standard input and
write to its standard output. It should eventually connect an
sshd(8) server running on some machine, or execute sshd -i some
RemoteForward
Specifies that a TCP/IP port on the remote machine be forwarded
over the secure channel to given host:port from the local ma
chine. 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.
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 re
duce authentication time on slow connections when rhosts authen
tication is not used. Most servers do not permit RhostsAuthenti
cation because it is not secure (see RhostsRSAAuthentication ).
The argument to this keyword must be ``yes'' or ``no''. The de
fault is ``yes''. This option applies to protocol version 1 only.
RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA
host authentication. The argument must be ``yes'' or ``no''. The
default is ``yes''. This option applies to protocol version 1 on
ly.
RSAAuthentication
Specifies whether to try RSA authentication. The argument to
this keyword must be ``yes'' or ``no''. RSA authentication will
only be attempted if the identity file exists, or an authentica
tion agent is running. The default is ``yes''. Note that this
option applies to protocol version 1 only.
ChallengeResponseAuthentication
Specifies whether to use challenge response authentication. Cur
rently there is only support for skey(1) authentication. The ar
gument to this keyword must be ``yes'' or ``no''. The default is
``no''.
StrictHostKeyChecking
If this flag is set to ``yes'', ssh will never automatically add
host keys to the $HOME/.ssh/known_hosts and
$HOME/.ssh/known_hosts2 files, and refuses to connect to hosts
whose host key has changed. This provides maximum protection
against trojan horse attacks. However, it can be somewhat annoy
ing if you don't have good /etc/ssh/ssh_known_hosts and
/etc/ssh/ssh_known_hosts2 files installed and frequently connect
to new hosts. This option forces the user to manually add all
new hosts. If this flag is set to ``no'', ssh will automatically
add new host keys to the user known hosts files. If this flag is
set to ``ask'', new host keys will be added to the user known
host files only after the user has confirmed that is what they
really want to do, and ssh will refuse to connect to hosts whose
host key has changed. The host keys of known hosts will be veri
fied automatically in all cases. The argument must be ``yes'',
``no'' or ``ask''. The default is ``ask''.
UsePrivilegedPort
UserKnownHostsFile
Specifies a file to use for the protocol version 1 user host key
database instead of $HOME/.ssh/known_hosts.
UserKnownHostsFile2
Specifies a file to use for the protocol version 2 user host key
database instead of $HOME/.ssh/known_hosts2.
UseRsh Specifies that rlogin/rsh should be used for this host. It is
possible that the host does not at all support the ssh protocol.
This causes ssh to immediately execute rsh(1). All other options
(except HostName) are ignored if this has been specified. The
argument must be ``yes'' or ``no''.
XAuthLocation
Specifies the location of the xauth(1) program. The default is
/usr/X11R6/bin/xauth.
ENVIRONMENT
ssh will normally set the following environment variables:
DISPLAY
The DISPLAY variable indicates the location of the X11 server.
It is automatically set by ssh to point to a value of the form
``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).
HOME Set to the path of the user's home directory.
LOGNAME
Synonym for USER; set for compatibility with systems that use
this variable.
MAIL Set to point the user's mailbox.
PATH Set to the default PATH, as specified when compiling ssh.
SSH_AUTH_SOCK
indicates the path of a unix-domain socket used to communicate
with the agent.
SSH_CLIENT
Identifies the client end of the connection. The variable con
tains three space-separated values: client ip-address, client
port number, and server port number.
SSH_ORIGINAL_COMMAND
The variable contains the original command line if a forced com
mand is executed. It can be used to extract the original argu
ments.
Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the
format ``VARNAME=value'' to the environment.
FILES
$HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
Records host keys for all hosts the user has logged into (that
are not in /etc/ssh/ssh_known_hosts for protocol version 1 or
/etc/ssh/ssh_known_hosts2 for protocol version 2). See sshd(8).
$HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
Contains the authentication identity of the user. They are for
protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
These files contain sensitive data and should be readable by the
user but not accessible by others (read/write/execute). Note
that ssh ignores a private key file if it is accessible by oth
ers. 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.
$HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
Contains the public key for authentication (public part of the
identity file in human-readable form). The contents of the
$HOME/.ssh/identity.pub file should be added to
$HOME/.ssh/authorized_keys on all machines where you wish to log
in using protocol version 1 RSA authentication. The contents of
the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file should
be added to $HOME/.ssh/authorized_keys2 on all machines where you
wish to log in using protocol version 2 DSA/RSA authentication.
These files are not sensitive and can (but need not) be readable
by anyone. These files are never used automatically and are not
necessary; they are only provided for the convenience of the us
er.
$HOME/.ssh/config
This is the per-user configuration file. The format of this file
is described above. This file is used by the ssh client. This
file does not usually contain any sensitive information, but the
recommended permissions are read/write for the user, and not ac
cessible by others.
$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 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.
$HOME/.ssh/authorized_keys2
Lists the public keys (RSA/DSA) that can be used for logging in
as this user. This file is not highly sensitive, but the recom
mended permissions are read/write for the user, and not accessi
ble by others.
ment 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 sshd(8) manual page.
The canonical system name (as returned by name servers) is used
by sshd(8) to verify the client host when logging in; other names
are needed because ssh 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 au
thentication.
/etc/ssh/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.
$HOME/.rhosts
This file is used in .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. On some machines this file may need to be
world-readable if the user's home directory is on a NFS parti
tion, because 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.
Note that by default sshd(8) will be installed so that it re
quires successful RSA host authentication before permitting
.rhosts authentication. If your server machine does not have the
client's host key in /etc/ssh/ssh_known_hosts, you can store it
in $HOME/.ssh/known_hosts. The easiest way to do this is to con
nect back to the client from the server machine using ssh; this
will automatically add the host key to $HOME/.ssh/known_hosts.
$HOME/.shosts
This file is used exactly the same way as .rhosts. The purpose
for having this file is to be able to use rhosts authentication
with ssh without permitting login with rlogin(1) or rsh(1).
/etc/hosts.equiv
This file is used during .rhosts authentication. It contains
canonical hosts names, one per line (the full format is described
on the 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 au
thentication is normally required. This file should only be
writable by root.
/etc/ssh/shosts.equiv
This file is processed exactly as /etc/hosts.equiv. This file may
be useful to permit logins using ssh but not using rsh/rlogin.
$HOME/.ssh/environment
Contains additional definitions for environment variables, see
section ENVIRONMENT above.
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
de Raadt and Dug Song removed many bugs, re-added newer features and cre
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
SEE ALSO
rlogin(1), rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-
keygen(1), telnet(1), sshd(8)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-07.txt, January
2001, work in progress material.
BSD Experimental September 25, 1999 14
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