ssh [-l login_name] hostname | user@hostname [command]
ssh [-afgknqstvxACNPTX1246] [-b bind_address] [-c ci-
pher_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 ma-
chine 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 comM--
bined with RSA-based host authentication. It means that if
the login
would be permitted by $HOME/.rhosts, $HOME/.shosts,
/etc/hosts.equiv, or
As a third authentication method, ssh supports RSA based au-
thentication.
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 encryp-
tion 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 de-
crypts the chalM--
lenge using the private key, proving that he/she knows the
private key
but without disclosing it to the server.
ssh implements the RSA authentication protocol automatical-
ly. The user
creates his/her RSA key pair by running ssh-keygen(1). This
stores the
private key in $HOME/.ssh/identity and the public key in
$HOME/.ssh/identity.pub in the user's home directory. The
user should
then copy the identity.pub to $HOME/.ssh/authorized_keys in
his/her home
directory on the remote machine (the authorized_keys file
corresponds to
the conventional $HOME/.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 authenM--
tication.
The most convenient way to use RSA authentication may be
with an authenM--
methods are available. Using the default values for
PreferredAuthentications, the client will try to authenti-
cate first using
the hostbased method; if this method fails public key au-
thentication is
attempted, and finally if this method fails keyboard-inter-
active and
password authentication are tried.
The public key method is similar to RSA authentication de-
scribed 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
authenticaM--
tion.
Protocol 2 provides additional mechanisms for confidentiali-
ty (the trafM--
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 communica-
tion with the
remote command or shell will be automatically encrypted.
mote machine
exits and all X11 and TCP/IP connections have been closed.
The exit staM--
tus of the remote program is returned as the exit status of
ssh.
Escape Characters
When a pseudo terminal has been requested, ssh supports a
number of funcM--
tions through the use of an escape character.
A single tilde character can be sent as ~~ or by following
the tilde by a
character other than those described below. The escape
character must
always follow a newline to be interpreted as special. The
escape characM--
ter can be changed in configuration files using the Es-
capeChar configuraM--
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
~? 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
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 forward-
ed 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 dis-
abled on the comM--
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 con-
nection to an
electronic purse; another is going through firewalls.
Server authentication
ssh automatically maintains and checks a database containing
identificaM--
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. Addi-
tionally, the
file /etc/ssh/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, ssh warns about this and dis-
ables password
authentication to prevent a trojan horse from getting the
user's passM--
word. Another purpose of this mechanism is to prevent man-
in-the-middle
attacks which could otherwise be used to circumvent the en-
cryption. The
StrictHostKeyChecking option (see below) can be used to pre-
vent logins to
machines whose host key is not known or has changed.
interfaces or aliased addresses.
-c blowfish|3des|des
Selects the cipher to use for encrypting the ses-
sion. 3des is
used by default. It is believed to be secure. 3des
(triple-des)
is an encrypt-decrypt-encrypt triple with three dif-
ferent keys.
blowfish is a fast block cipher, it appears very se-
cure and is
much faster than 3des. des is only supported in the
ssh client
for interoperability with legacy protocol 1 imple-
mentations 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-sepa-
rated 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 be-
ginning of a
line. The escape character followed by a dot (`.')
closes the
connection, followed by control-Z suspends the con-
nection, 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 back-
ground. This
implies -n. The recommended way to start X11 pro-
grams at a
remote site is with something like ssh -f host
xterm.
tiple identiM--
ties specified in configuration files).
-I smartcard_device
Specifies which smartcard device to use. The argu-
ment is the
device ssh should use to communicate with a smart-
card used for
storing the user's private RSA key.
-k Disables forwarding of Kerberos tickets and AFS to-
kens. This may
also be specified on a per-host basis in the config-
uration file.
-l login_name
Specifies the user to log in as on the remote ma-
chine. This also
may be specified on a per-host basis in the configu-
ration file.
-m mac_spec
Additionally, for protocol version 2 a comma-sepa-
rated list of
MAC (message authentication code) algorithms can be
specified in
order of preference. See the MACs keyword for more
information.
-n Redirects stdin from /dev/null (actually, prevents
reading from
stdin). This must be used when ssh is run in the
background. A
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 chan-
nel. 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 forM--
warding ports (protocol version 2 only).
-o option
privileged
ports. Note that this option turns off Rhost-
sAuthentication and
RhostsRSAAuthentication for older servers.
-q Quiet mode. Causes all warning and diagnostic mes-
sages to be
suppressed.
-s May be used to request invocation of a subsystem on
the remote
system. Subsystems are a feature of the SSH2 proto-
col which
facilitate the use of SSH as a secure transport for
other appliM--
cations (eg. sftp). The subsystem is specified as
the remote comM--
mand.
-t Force pseudo-tty allocation. This can be used to
execute arbiM--
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 lo-
cal tty.
-T Disable pseudo-tty allocation.
-v Verbose mode. Causes ssh to print debugging mes-
sages about its
progress. This is helpful in debugging connection,
authenticaM--
tion, and configuration problems. Multiple -v op-
tions 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 con-
nections). The
compression algorithm is the same used by gzip(1),
and the
``level'' can be controlled by the CompressionLevel
option (see
nored. The
default for the per-user configuration file is
$HOME/.ssh/config.
-L port:host:hostport
Specifies that the given port on the local (client)
host is to be
forwarded to the given host and port on the remote
side. This
works by allocating a socket to listen to port on
the local side,
and whenever a connection is made to this port, the
connection is
forwarded over the secure channel, and a connection
is made to
host port hostport from the remote machine. Port
forwardings can
also be specified in the configuration file. Only
root can forM--
ward privileged ports. IPv6 addresses can be speci-
fied with an
alternative syntax: port/host/hostport
-R port:host:hostport
Specifies that the given port on the remote (server)
host is to
be forwarded to the given host and port on the local
side. This
works by allocating a socket to listen to port on
the remote
side, and whenever a connection is made to this
port, the connecM--
tion is forwarded over the secure channel, and a
connection is
made to host port hostport from the local machine.
Port forwardM--
ings can also be specified in the configuration
file. Privileged
ports can be forwarded only when logging in as root
on the remote
machine. IPv6 addresses can be specified with an
alternative
syntax: port/host/hostport
-D port
Specifies a local ``dynamic'' application-level port
forwarding.
This works by allocating a socket to listen to port
on the local
side, and whenever a connection is made to this
port, the connecM--
tion is forwarded over the secure channel, and the
-4 Forces ssh to use IPv4 addresses only.
-6 Forces ssh to use IPv6 addresses only.
CONFIGURATION FILES
ssh obtains configuration data from the following sources in
the followM--
ing order: command line options, user's configuration file
($HOME/.ssh/config), and system-wide configuration file
(/etc/ssh/ssh_config). For each parameter, the first ob-
tained 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-speM--
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''.
Configuration
options may be separated by whitespace or optional white-
space and exactly
one `='; the latter format is useful to avoid the need to
quote whitesM--
pace when specifying configuration options using the ssh,
scp and sftp -o
option.
The possible keywords and their meanings are as follows
(note that keyM--
words are case-insensitive and arguments are case-sensi-
tive):
Host Restricts the following declarations (up to the next
Host keyM--
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
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 argu-
ment 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 op-
tion does not
work if UsePrivilegedPort is set to ``yes''.
CheckHostIP
If this flag is set to ``yes'', ssh will additional-
ly check the
host IP address in the known_hosts file. This al-
lows 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 ses-
sion in protoM--
col version 1. Currently, ``blowfish'', ``3des'',
and ``des''
are supported. des is only supported in the ssh
client for
interoperability with legacy protocol 1 implementa-
tions 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-separat-
ed. The
default is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,ar-
cfour,
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 compres-
sion 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 ap-
plications. 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 1.
DynamicForward
Specifies that a TCP/IP port on the local machine be
forwarded
over the secure channel, and the application proto-
col is then
used to determine where to connect to from the re-
mote machine.
The argument must be a port number. Currently the
SOCKS4 protoM--
col is supported, and ssh will act as a SOCKS4 serv-
er. Multiple
forwardings may be specified, and additional for-
wardings can be
given on the command line. Only the superuser can
forward priviM--
leged ports.
EscapeChar
Sets the escape character (default: `~'). The es-
cape character
can also be set on the command line. The argument
should be a
single character, `^' followed by a letter, or
``none'' to disM--
able the escape character entirely (making the con-
nection transM--
parent for binary data).
tion 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 automati-
cally rediM--
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 con-
nect 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 specM--
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''.
GlobalKnownHostsFile
Specifies a file to use for the global host key
database instead
of /etc/ssh/ssh_known_hosts.
HostbasedAuthentication
Specifies whether to try rhosts based authentication
with public
key authentication. The argument must be ``yes'' or
``no''. The
default is ``no''. This option applies to protocol
version 2
only and is similar to RhostsRSAAuthentication.
HostKeyAlgorithms
Specifies the protocol version 2 host key algorithms
that the
client wants to use in order of preference. The de-
fault for this
option is: ``ssh-rsa,ssh-dss''.
HostKeyAlias
Specifies an alias that should be used instead of
es are also
permitted (both on the command line and in HostName
specificaM--
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 protoM--
col version 2. Additionally, any identities repre-
sented by the
authentication agent will be used for authentica-
tion. The file
name may use the tilde syntax to refer to a user's
home direcM--
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 no-
ticed. However,
this means that connections will die if the route is
down temM--
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''.
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.
specified
with an alternative syntax: host/port. Multiple
forwardings may
be specified, and additional forwardings can be giv-
en on the comM--
mand line. Only the superuser can forward privi-
leged ports.
LogLevel
Gives the verbosity level that is used when logging
messages from
ssh. The possible values are: QUIET, FATAL, ERROR,
INFO, VERM--
BOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. The default
is INFO.
DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3
each specify
higher levels of verbose output.
MACs Specifies the MAC (message authentication code) al-
gorithms in
order of preference. The MAC algorithm is used in
protocol verM--
sion 2 for data integrity protection. Multiple al-
gorithms 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 warnM--
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 giv-
ing up. The
argument to this keyword must be an integer. De-
fault is 3.
PasswordAuthentication
Specifies whether to use password authentication.
(e.g. keyboard-interactive) over another method
(e.g. password)
The default for this option is:
``hostbased,publickey,keyboard-interactive,pass-
word''.
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 serv-
er. The comM--
mand string extends to the end of the line, and is
executed with
/bin/sh. In the command string, `%h' will be sub-
stituted by the
host name to connect and `%p' by the port. The com-
mand can be
basically anything, and should read from its stan-
dard input and
write to its standard output. It should eventually
connect an
sshd(8) server running on some machine, or execute
sshd -i someM--
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.
PubkeyAuthentication
Specifies whether to try public key authentication.
The argument
to this keyword must be ``yes'' or ``no''. The de-
fault is
``yes''. This option applies to protocol version 2
only.
RemoteForward
Specifies that a TCP/IP port on the remote machine
be forwarded
over the secure channel to the specified host and
tion. Note that
this declaration only affects the client side and
has no effect
whatsoever on security. Disabling rhosts authenti-
cation may
reduce authentication time on slow connections when
rhosts
authentication is not used. Most servers do not
permit RhostM--
sAuthentication because it is not secure (see
RhostsRSAAuthentication). The argument to this key-
word 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 pro-
tocol version
1 only.
RSAAuthentication
Specifies whether to try RSA authentication. The
argument to
this keyword must be ``yes'' or ``no''. RSA authen-
tication will
only be attempted if the identity file exists, or an
authenticaM--
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 authen-
tication. The
argument to this keyword must be ``yes'' or ``no''.
The default
is ``yes''.
SmartcardDevice
Specifies which smartcard device to use. The argu-
ment to this
keyword is the device ssh should use to communicate
with a smartM--
card used for storing the user's private RSA key. By
default, no
device is specified and smartcard support is not ac-
tivated.
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 af-
ter 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''.
UsePrivilegedPort
Specifies whether to use a privileged port for out-
going connecM--
tions. The argument must be ``yes'' or ``no''. The
default is
``no''. Note that this option must be set to
``yes'' if
RhostsAuthentication and RhostsRSAAuthentication au-
thentications
are needed with older servers.
User Specifies the user to log in as. This can be useful
when a difM--
ferent user name is used on different machines.
This saves the
trouble of having to remember to give the user name
on the comM--
mand line.
UserKnownHostsFile
Specifies a file to use for the user host key data-
base instead of
$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 spec-
ified. The
argument must be ``yes'' or ``no''.
where the shell
runs, and n is an integer >= 1. ssh uses this spe-
cial 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 conM--
tains three space-separated values: client ip-ad-
dress, client
port number, and server port number.
TZ The timezone variable is set to indicate the present
timezone if
it was set when the daemon was started (i.e., the
daemon passes
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/exe-
cute). Note
that ssh ignores a private key file if it is acces-
sible by othM--
ers. It is possible to specify a passphrase when
generating the
key; the passphrase will be used to encrypt the sen-
sitive 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 authentica-
tion. The conM--
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 ver-
sion 2 DSA/RSA
tion, 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 for-
mat 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
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): sys-
tem name, pubM--
lic key and optional comment field. When different
names are
used for the same machine, all such names should be
listed, sepaM--
rated 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
authentication.
/etc/ssh/ssh_config
Systemwide configuration file. This file provides
defaults for
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 partiM--
tion, because sshd(8) reads it as root. Additional-
ly, 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 oth-
ers.
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 conM--
nect back to the client from the server machine us-
ing 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 au-
thentication
with ssh without permitting login with rlogin(1) or
rsh(1).
/etc/hosts.equiv
This file is used during .rhosts authentication. It
contains
This file
may be useful to permit logins using ssh but not us-
ing
rsh/rlogin.
/etc/ssh/sshrc
Commands in this file are executed by ssh when the
user logs in
just before the user's shell (or command) is start-
ed. See the
sshd(8) manual page for more information.
$HOME/.ssh/rc
Commands in this file are executed by ssh when the
user logs in
just before the user's shell (or command) is start-
ed. See the
sshd(8) manual page for more information.
$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 fea-
tures and creM--
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. Lehti-
nen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-09.txt,
July 2001,
work in progress material.
BSD September 25, 1999
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