ssh [-afgknqstvxACNPTX1246] [-b bind_address] [-c cipher_spec]
[-e escape_char] [-i identity_file] [-l login_name] [-m mac_spec]
[-o option] [-p port] [-F configfile] [-L port:host:hostport] [-R
port:host:hostport] [-D port] 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
untrusted 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
depending 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/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 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.
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/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 permitted. This authentication method
closes security holes due to IP spoofing, DNS spoofing and routing spoof-
ing. [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-
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 protocol version 2 similar authentication
methods are available. Using the default values for
PreferredAuthentications, the client will try to authenticate first using
the hostbased method; if this method fails public key authentication is
attempted, and finally if this method fails keyboard-interactive and
password authentication are 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_keys 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
server.
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
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.
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.
The session terminates when the command or shell on the remote machine
~. Disconnect
~^Z Background ssh
~# List forwarded connections
~& Background ssh at logout when waiting for forwarded connection /
X11 sessions to terminate
~? 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 ForwardX11 variable is set to ``yes'' (or, see the description of
the -X and -x options described later) and 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 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 DISPLAY. For-
warding 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 the com-
mand line or in a configuration file.
Forwarding of arbitrary TCP/IP connections over the secure channel can be
specified either on the 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 through firewalls.
Server authentication
ssh automatically maintains and checks a database containing identifica-
tions for all hosts it has ever been used with. Host keys are stored in
$HOME/.ssh/known_hosts in the user's home directory. Additionally, the
file /etc/ssh/ssh_known_hosts is automatically checked for known hosts.
file.
-b bind_address
Specify the interface to transmit from on machines with multiple
interfaces or aliased addresses.
-c blowfish|3des|des
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.
blowfish is a fast block cipher, it appears very secure and is
much faster than 3des. des is only supported in the ssh client
for interoperability with legacy protocol 1 implementations that
do not support the 3des cipher. Its use is strongly discouraged
due to cryptographic weaknesses.
-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
passphrases, 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 a file from which the identity (private key) for RSA or
DSA authentication is read. The default is $HOME/.ssh/identity
for protocol version 1, and $HOME/.ssh/id_rsa and
$HOME/.ssh/id_dsa for protocol version 2. 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 identi-
ties specified in configuration files).
-I smartcard_device
Specifies which smartcard device to use. The argument is the
device ssh should use to communicate with a smartcard used for
storing the user's private RSA key.
common trick is to use this to run X11 programs on a remote
machine. For example, 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 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 for just for-
warding ports (protocol version 2 only).
-o option
Can be used to give options in the format used in the configura-
tion file. This is useful for specifying options for which there
is no separate command-line flag.
-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 a 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.
-s May be used to request invocation of a subsystem on the remote
system. Subsystems are a feature of the SSH2 protocol which
facilitate the use of SSH as a secure transport for other appli-
cations (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,
-L 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 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
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
-D port
Specifies a local ``dynamic'' application-level port forwarding.
This works by allocating a socket to listen to port on the local
side, and whenever a connection is made to this port, the connec-
tion is forwarded over the secure channel, and the application
protocol is then used to determine where to connect to from the
remote machine. Currently the SOCKS4 protocol is supported, and
ssh will act as a SOCKS4 server. Only root can forward privi-
leged ports. Dynamic port forwardings can also be specified in
the configuration file.
-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 the follow-
ing order: 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.
The possible keywords and their meanings are as follows (note that key-
words are case-insensitive and arguments 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 pro-
vide global defaults for all hosts. The host is the hostname
argument given on the command line (i.e., the name is not con-
verted to a canonicalized host name before matching).
AFSTokenPassing
Specifies whether to pass AFS tokens to remote host. The argu-
ment to this keyword must be ``yes'' or ``no''. This option
applies to protocol version 1 only.
BatchMode
If set to ``yes'', passphrase/password querying will be disabled.
This option is useful in scripts and other batch jobs where no
user is present to supply the password. The argument must be
``yes'' or ``no''. The default is ``no''.
BindAddress
Specify the interface to transmit from on machines with multiple
interfaces or aliased addresses. Note that this option does not
work if UsePrivilegedPort is set to ``yes''.
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
detect 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'', ``3des'', and ``des''
are supported. des is only supported in the ssh client for
interoperability with legacy protocol 1 implementations that do
not support the 3des cipher. Its use is strongly discouraged due
to cryptographic weaknesses. The default is ``3des''.
Ciphers
Specifies the ciphers allowed for protocol version 2 in order of
preference. Multiple ciphers must be comma-separated. The
default is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc''
ClearAllForwardings
Specifies that all local, remote and dynamic port forwardings
specified in the configuration files or on the command line be
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 1.
DynamicForward
Specifies that a TCP/IP port on the local machine be forwarded
over the secure channel, and the application protocol is then
used to determine where to connect to from the remote machine.
The argument must be a port number. Currently the SOCKS4 proto-
col is supported, and ssh will act as a SOCKS4 server. Multiple
forwardings may be specified, and additional forwardings can be
given on the command line. Only the superuser can forward privi-
leged ports.
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''.
ForwardX11
Specifies whether X11 connections will be automatically redi-
rected over the secure channel and DISPLAY set. The argument
must be ``yes'' or ``no''. The default is ``no''.
GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports. By default, ssh binds local port forwardings to
the loopback addresss. This prevents other remote hosts from
connecting to forwarded ports. GatewayPorts can be used to spec-
ify that ssh should bind local port forwardings to the wildcard
address, thus allowing remote hosts to connect to forwarded
ports. The argument must be ``yes'' or ``no''. The default is
``no''.
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 data-
base files. This option is useful for tunneling ssh connections
or for 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 a file from which the user's RSA or DSA authentication
identity is read. The default is $HOME/.ssh/identity for protocol
version 1, and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for proto-
col version 2. 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 direc-
tory. It is possible to have multiple identity files specified
in configuration files; all these identities will be tried in
sequence.
KeepAlive
Specifies whether the system should send TCP 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 tempo-
rarily, 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''.
KerberosAuthentication
Specifies whether Kerberos authentication will be used. The
argument to this keyword must be ``yes'' or ``no''.
KerberosTgtPassing
Specifies whether a Kerberos TGT will be forwarded to the server.
This will only work if the Kerberos server is actually an AFS
kaserver. The argument to this keyword must be ``yes'' or
``no''.
LocalForward
Specifies that a TCP/IP port on the local machine be forwarded
over the secure channel to the specified host and port from the
remote machine. The first argument must be a port number, and
sion 2 for data integrity protection. Multiple algorithms must
be comma-separated. The default is
``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.
NoHostAuthenticationForLocalhost
This option can be used if the home directory is shared across
machines. In this case localhost will refer to a different
machine on each of the machines and the user will get many warn-
ings about changed host keys. However, this option disables host
authentication for localhost. The argument to this keyword must
be ``yes'' or ``no''. The default is to check the host key for
localhost.
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
authentication 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:
``hostbased,publickey,keyboard-interactive,password''.
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-
where. Host key management will be done using the HostName of
the host being connected (defaulting to the name typed by the
user). Note that CheckHostIP is not available for connects with
a proxy command.
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 Rhost-
sAuthentication because it is not secure (see
RhostsRSAAuthentication). The argument to this keyword must be
``yes'' or ``no''. The default 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 only.
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. The
argument to this keyword must be ``yes'' or ``no''. The default
is ``yes''.
SmartcardDevice
Specifies which smartcard device to use. The argument to this
keyword is the device ssh should use to communicate with a smart-
card used for storing the user's private RSA key. By default, no
device is specified and smartcard support is not activated.
StrictHostKeyChecking
If this flag is set to ``yes'', ssh will never automatically add
host keys to the $HOME/.ssh/known_hosts file, and refuses to con-
nect to hosts whose host key has changed. This provides maximum
protection against trojan horse attacks, however, can be annoying
when the /etc/ssh/ssh_known_hosts file is poorly maintained, or
connections to new hosts are frequently made. 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 verified automatically in all cases.
The argument must be ``yes'', ``no'' or ``ask''. The default is
``ask''.
$HOME/.ssh/known_hosts.
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 the path of the user's mailbox.
PATH Set to the default PATH, as specified when compiling ssh.
SSH_ASKPASS
If ssh needs a passphrase, it will read the passphrase from the
current terminal if it was run from a terminal. If ssh does not
have a terminal associated with it but DISPLAY and SSH_ASKPASS
are set, it will execute the program specified by SSH_ASKPASS and
open an X11 window to read the passphrase. This is particularly
useful when calling ssh from a .Xsession or related script.
(Note that on some machines it may be necessary to redirect the
input from /dev/null to make this work.)
SSH_AUTH_SOCK
Identifies 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
the value on to new connections).
USER Set to the name of the user logging in.
Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the
format ``VARNAME=value'' to the environment.
FILES
$HOME/.ssh/known_hosts
Records host keys for all hosts the user has logged into that are
not in /etc/ssh/ssh_known_hosts. 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 the user wishes
to log in using protocol version 1 RSA authentication. The con-
tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file
should be added to $HOME/.ssh/authorized_keys on all machines
where the user wishes 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 automati-
cally and are not necessary; they are only provided for the con-
venience of the user.
$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
accessible by others.
$HOME/.ssh/authorized_keys
Lists the public keys (RSA/DSA) 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. This file is not highly sensitive,
but the recommended permissions are read/write for the user, and
not accessible by others.
/etc/ssh/ssh_known_hosts
access to the name servers would then be able to fool host
authentication.
/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.
/etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key,
/etc/ssh/ssh_host_rsa_key
These three files contain the private parts of the host keys and
are used for RhostsRSAAuthentication and HostbasedAuthentication.
Since they are readable only by root ssh must be setuid root if
these authentication methods are desired.
$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
requires successful RSA host authentication before permitting
.rhosts authentication. If the server machine does not have the
client's host key in /etc/ssh/ssh_known_hosts, it can be stored
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
authentication is normally required. This file should only be
writable by root.
/etc/shosts.equiv
$HOME/.ssh/environment
Contains additional definitions for environment variables, see
section ENVIRONMENT above.
DIAGNOSTICS
ssh exits with the exit status of the remote command or with 255 if an
error occurred.
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-09.txt, July 2001,
work in progress material.
BSD September 25, 1999 BSD
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