ssh [-afgknqtvxACNPTX246] [-c cipher_spec] [-e escape_char]
[-i identity_file] [-l login_name] [-o option] [-p port] [-L
port:host:hostport] [-R port:host:hostport]
[hostname | user@hostname] [command]
DESCRIPTION
ssh (Secure Shell) 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 (and primary) authentication method is the rhosts or
hosts.equiv method combined 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_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-
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: At first, the client attempts to authenticate
using the public key method. If this method fails password authentica-
tion is tried.
The public key method is similar to RSA authentication described in the
previous section except that the DSA algorithm is used instead of the
patented RSA algorithm. The client uses his private DSA key
$HOME/.ssh/id_dsa 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
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. This
protocol 2 implementation does not yet support Kerberos or S/Key authen-
tication.
Protocol 2 provides additional mechanisms for confidentiality (the traf-
fic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity
(hmac-sha1, hmac-md5). 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
can disconnect with ~., and suspend ssh with ~^Z. All forwarded connec-
tions can be listed with ~# and if the session blocks waiting for for-
warded X11 or TCP/IP connections to terminate, it can be backgrounded
with ~& (this should not be used while the user shell is active, as it
can cause the shell to hang). All available escapes can be listed with
~?.
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 or on the command line.
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 trough firewalls.
Server authentication
ssh automatically maintains and checks a database containing identifica-
tions for all hosts it has ever been used with. RSA host keys are stored
in $HOME/.ssh/known_hosts and DSA host keys are stored in
$HOME/.ssh/known_hosts2 in the user's home directory. Additionally, the
files /etc/ssh_known_hosts and /etc/ssh_known_hosts2 are 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 disables password authentication to prevent a trojan horse from
getting the user's password. Another purpose of this mechanism is to
prevent man-in-the-middle attacks which could otherwise be used to cir-
cumvent the encryption. The StrictHostKeyChecking option (see below) can
be used to prevent logins to machines whose host key is not known or has
changed.
OPTIONS
-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.
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 the file from which the identity (private key) for RSA
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 config-
uration 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.
-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 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 usefull 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
progress. This is helpful in debugging connection, authentica-
tion, and configuration problems. The verbose mode is also used
to display skey(1) challenges, if the user entered "s/key" as
password. 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-
figuration files; see the Compress option below.
-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.
-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
order): command line options, user's configuration file
($HOME/.ssh/config), and system-wide configuration file
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 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''.
BatchMode
If set to ``yes'', passphrase/password querying will be disabled.
This option is useful in scripts and other batch jobs where you
have no user to supply the password. The argument must be
``yes'' or ``no''.
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.
Cipher Specifies the cipher to use for encrypting the session in proto-
col version 1. Currently, ``blowfish'' and ``3des'' are sup-
ported. 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 ``3des-cbc,blowfish-cbc,cast128-cbc,arcfour''.
Compression
Specifies whether to use compression. The argument must be
``yes'' or ``no''.
CompressionLevel
Specifies the compression level to use if compression is enable.
The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications. The
meaning of the values is the same as in gzip(1).
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.
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''.
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. The argument must be ``yes'' or ``no''. The
default is ``no''.
GlobalKnownHostsFile
Specifies a file to use instead of /etc/ssh_known_hosts.
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 authentication iden-
tity 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 direc-
tory. It is possible to have multiple identity files specified
in configuration files; all these identities will be tried in
sequence.
IdentityFile2
Specifies the file from which the user's DSA authentication iden-
tity is read (default $HOME/.ssh/id_dsa in the user's home direc-
tory). The file name may use the tilde syntax to refer to a
user's home directory. It is possible to have multiple identity
files specified in configuration files; all these identities will
be tried in sequence.
KeepAlive
Specifies whether the system should send keepalive messages to
the other side. If they are sent, death of the connection or
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 given host:port from the remote
machine. The first argument must be a port number, and the sec-
ond 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.
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''. Note that this option
applies to both protocol version 1 and 2.
Port Specifies the port number to connect on the remote host. Default
is 22.
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 ``1,2''. This
means that ssh tries version 1 and falls back to version 2 if
version 1 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.
sAuthentication because it is not secure (see RhostsRSAAuthenti-
cation). The argument to this keyword must be ``yes'' or ``no''.
RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA
host authentication. This is the primary authentication method
for most sites. The argument must be ``yes'' or ``no''.
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. Note that this option applies to protocol
version 1 only.
SkeyAuthentication
Specifies whether to use skey(1) authentication. The argument to
this keyword must be ``yes'' or ``no''. The default is ``no''.
StrictHostKeyChecking
If this flag is set to ``yes'', ssh ssh will never automatically
add host keys to the $HOME/.ssh/known_hosts and
$HOME/.ssh/known_hosts2 files, and refuses to connect hosts whose
host key has changed. This provides maximum protection against
trojan horse attacks. However, it can be somewhat annoying if
you don't have good /etc/ssh_known_hosts and
/etc/ssh_known_hosts2 files installed and frequently connect new
hosts. Basically this option forces the user to manually add any
new hosts. Normally this option is disabled, and new hosts will
automatically be added to the known host files. The host keys of
known hosts will be verified automatically in either case. The
argument must be ``yes'' or ``no''.
UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connec-
tions. The argument must be ``yes'' or ``no''. The default is
``yes''. Note that setting this option to ``no'' turns off
RhostsAuthentication and RhostsRSAAuthentication.
User Specifies the user to log in as. This can be useful if you have
a different user name on different machines. This saves the
trouble of having to remember to give the user name on the com-
mand line.
UserKnownHostsFile
Specifies a file to use 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 specified. The
argument must be ``yes'' or ``no''.
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_TTY
This is set to the name of the tty (path to the device) associ-
ated with the current shell or command. If the current session
has no tty, this variable is not set.
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_known_hosts). See sshd(8).
$HOME/.ssh/identity, $HOME/.ssh/id_dsa
Contains the RSA and the DSA authentication identity of the user.
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
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 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 spa-
ces). 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 DSA keys that can be used for logging in as this user.
This file is not highly sensitive, but the recommended permis-
sions are read/write for the user, and not accessible by others.
/etc/ssh_known_hosts, /etc/ssh_known_hosts2
Systemwide list of known host keys. /etc/ssh_known_hosts con-
tains RSA and /etc/ssh_known_hosts2 contains DSA keys. These
files 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 spa-
ces): system name, number of bits in modulus, public exponent,
modulus, and optional comment field. When different names are
used for the same machine, all such names should be listed, sepa-
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_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,
$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
This file is processed exactly as /etc/hosts.equiv. This file
may be useful to permit logins using ssh but not using
rsh/rlogin.
/etc/sshrc
Commands in this file are executed by ssh when the user logs in
just before the user's shell (or command) is started. 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 started. See the
sshd(8) manual page for more information.
$HOME/.ssh/environment
Contains additional definitions for environment variables, see
section ENVIRONMENT above.
libcrypto.so.X.1
A version of this library which includes support for the RSA
algorithm is required for proper operation.
AUTHOR
OpenSSH is a derivative of the original (free) ssh 1.2.12 release by Tatu
Ylonen, but with bugs removed and newer features re-added. Rapidly after
the 1.2.12 release, newer versions of the original ssh bore successively
more restrictive licenses, and thus demand for a free version was born.
This version of OpenSSH
o has all components of a restrictive nature (i.e., patents, see
crypto(3)) directly removed from the source code; any licensed or
patented components are chosen from external libraries.
o has been updated to support SSH protocol 1.5 and 2, making it compat-
ible with all other SSH clients and servers.
BSD September 25, 1999 BSD
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