SSH(1) | General Commands Manual | SSH(1) |
ssh
—
ssh |
[-1246AaCfgkMNnqsTtVvXxY ]
[-b bind_address]
[-c cipher_spec]
[-D
[bind_address:]port]
[-e escape_char]
[-F configfile]
[-i identity_file]
[-L
[bind_address:]port:host:hostport]
[-l login_name]
[-m mac_spec]
[-O ctl_cmd]
[-o option]
[-p port]
[-R
[bind_address:]port:host:hostport]
[-S ctl_path]
[-w tunnel:tunnel]
[user@]hostname
[command] |
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 (with optional user
name). The user must prove his/her identity to the remote machine using one
of several methods depending on the protocol version used (see below).
If command is specified, it is executed on the remote host instead of a login shell.
The options are as follows:
-1
ssh
to try protocol version 1 only.-2
ssh
to try protocol version 2 only.-4
ssh
to use IPv4 addresses only.-6
ssh
to use IPv6 addresses only.-A
Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.
-a
-b
bind_address-C
CompressionLevel
option for protocol version 1.
Compression is desirable on modem lines and other slow connections, but
will only slow down things on fast networks. The default value can be set
on a host-by-host basis in the configuration files; see the
Compression
option.-c
cipher_specProtocol version 1 allows specification of a single cipher.
The supported values are “3des”, “blowfish”,
and “des”. 3des (triple-des) is an
encrypt-decrypt-encrypt triple with three different keys. It is believed
to be secure. 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. The default is “3des”.
For protocol version 2, cipher_spec is a comma-separated list of ciphers listed in order of preference. The supported ciphers are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blowfish-cbc, and cast128-cbc. The default is:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, aes192-ctr,aes256-ctr
-D
[bind_address:]portssh
will act as a SOCKS server.
Only root can forward privileged ports. Dynamic port forwardings can also
be specified in the configuration file.
IPv6 addresses can be specified with an alternative syntax:
[bind_address/]port
or by enclosing the address in square brackets. Only the superuser can
forward privileged ports. By default, the local port is bound in
accordance with the GatewayPorts
setting.
However, an explicit bind_address may be used to
bind the connection to a specific address. The
bind_address of “localhost”
indicates that the listening port be bound for local use only, while an
empty address or ‘*’ indicates that the port should be
available from all interfaces.
-e
escape_char~
’). 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
configfile-f
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
-I
smartcard_devicessh
should use to communicate
with a smartcard used for storing the user's private RSA key. This option
is only available if support for smartcard devices is compiled in (default
is no support).-i
identity_file-i
options (and
multiple identities specified in configuration files).-k
-L
[bind_address:]port:host:hostportGatewayPorts
setting. However,
an explicit bind_address may be used to bind the
connection to a specific address. The bind_address
of “localhost” indicates that the listening port be bound
for local use only, while an empty address or ‘*’ indicates
that the port should be available from all interfaces.-l
login_name-M
ssh
client into “master”
mode for connection sharing. Multiple -M
options
places ssh
into “master” mode with
confirmation required before slave connections are accepted. Refer to the
description of ControlMaster
in
ssh_config(5) for details.-m
mac_specMACs
keyword for more
information.-N
-n
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.)-O
ctl_cmd-O
option is specified, the
ctl_cmd argument is interpreted and passed to the
master process. Valid commands are: “check” (check that the
master process is running) and “exit” (request the master to
exit).-o
option-p
port-q
-R
[bind_address:]port:host:hostportPort forwardings 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 by enclosing the address in square braces or using an alternative syntax: [bind_address/]host/port/hostport.
By default, the listening socket on the server will be bound
to the loopback interface only. This may be overriden by specifying a
bind_address. An empty
bind_address, or the address
‘*
’, indicates that the remote
socket should listen on all interfaces. Specifying a remote
bind_address will only succeed if the server's
GatewayPorts
option is enabled (see
sshd_config(5)).
-S
ctl_pathControlPath
and
ControlMaster
in ssh_config(5)
for details.-s
-T
-t
-t
options force tty allocation, even if ssh
has no
local tty.-V
-v
ssh
to print debugging
messages about its progress. This is helpful in debugging connection,
authentication, and configuration problems. Multiple
-v
options increase the verbosity. The maximum is
3.-w
tunnel:tunnelTunnel
directive in ssh_config(5).-X
X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring.
For this reason, X11 forwarding is subjected to X11 SECURITY
extension restrictions by default. Please refer to the
ssh
-Y
option and the
ForwardX11Trusted
directive in
ssh_config(5) for more information.
-x
-Y
ssh
may additionally obtain configuration
data from a per-user configuration file and a system-wide configuration
file. The file format and configuration options are described in
ssh_config(5).
ssh
exits with the exit status of the
remote command or with 255 if an error occurred.
ssh
falling back to protocol 1 if it
detects protocol 2 is unsupported. These settings may be altered using the
Protocol
option in ssh_config(5), or
enforced using the -1
and -2
options (see above). Both protocols support similar authentication methods,
but protocol 2 is preferred since it provides additional mechanisms for
confidentiality (the traffic is encrypted using AES, 3DES, Blowfish, CAST128,
or Arcfour) and integrity (hmac-md5, hmac-sha1, hmac-ripemd160). Protocol 1
lacks a strong mechanism for ensuring the integrity of the connection.
The methods available for authentication are: host-based
authentication, public key authentication, challenge-response
authentication, and password authentication. Authentication methods are
tried in the order specified above, though protocol 2 has a configuration
option to change the default order:
PreferredAuthentications
.
Host-based authentication works as follows: 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, or if the files ~/.rhosts or ~/.shosts exist in the user's home directory on the remote machine and contain a line containing the name of the client machine and the name of the user on that machine, the user is considered for login. Additionally, the server must be able to verify the client's host key (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing, and routing spoofing. [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.]
Public key authentication works as follows: The scheme is based on
public-key cryptography, using cryptosystems where encryption and decryption
are done using separate keys, and it is unfeasible to derive the decryption
key from the encryption key. 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.
ssh
implements public key authentication protocol
automatically, using either the RSA or DSA algorithms. Protocol 1 is
restricted to using only RSA keys, but protocol 2 may use either. The
HISTORY section of
ssl(8) contains a brief discussion of the two
algorithms.
The file ~/.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 client proves that it has access to the
private key and the server checks that the corresponding public key is
authorized to accept the account.
The user creates his/her key pair by running ssh-keygen(1). This stores the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The user should then copy the public key to ~/.ssh/authorized_keys in his/her home directory on the remote machine. The authorized_keys file corresponds to the conventional ~/.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.
The most convenient way to use public key authentication may be with an authentication agent. See ssh-agent(1) for more information.
Challenge-response authentication works as follows: The server sends an arbitrary “challenge” text, and prompts for a response. Protocol 2 allows multiple challenges and responses; protocol 1 is restricted to just one challenge/response. Examples of challenge-response authentication include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD systems).
Finally, if other authentication methods fail,
ssh
prompts the user for a password. The password is
sent to the remote host for checking; however, since all communications are
encrypted, the password cannot be seen by someone listening on the
network.
ssh
automatically maintains and checks a
database containing identification for all hosts it has ever been used with.
Host keys are stored in ~/.ssh/known_hosts in the
user's home directory. Additionally, 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 disables password authentication to prevent server spoofing
or man-in-the-middle attacks, which could otherwise be used to circumvent
the encryption. The StrictHostKeyChecking
option can
be used to control logins to machines whose host key is not known or has
changed.
ssh
can be configured to verify host
identification using fingerprint resource records (SSHFP) published in DNS.
The VerifyHostKeyDNS
option can be used to control
how DNS lookups are performed. SSHFP resource records can be generated using
ssh-keygen(1).
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 exits and all X11 and TCP/IP connections have been closed.
ssh
supports
a number of functions 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 character can be changed in configuration files using
the EscapeChar
configuration directive or on the
command line by the -e
option.
The supported escapes (assuming the default
‘~
’) are:
~.
~^Z
ssh
.~#
~&
ssh
at logout when waiting for
forwarded connection / X11 sessions to terminate.~?
~B
~C
-L
and -R
options (see above). It also allows the cancellation of existing remote
port-forwardings using -KR
hostport.
!
command allows the user to
execute a local command if the PermitLocalCommand
option is enabled in ssh_config(5). Basic help is
available, using the -h
option.~R
ForwardX11
variable is set to “yes”
(or see the description of the -X
,
-x
, and -Y
options above) 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
. 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 ForwardAgent
variable is set to
“yes” (or see the description of the
-A
and -a
options above) and
the user is using an authentication agent, the connection to the agent is
automatically forwarded to the remote side.
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.
ssh
will normally set the following environment
variables:
DISPLAY
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
LOGNAME
USER
; set for compatibility with
systems that use this variable.MAIL
PATH
PATH
, as specified when
compiling ssh
.SSH_ASKPASS
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
SSH_CONNECTION
SSH_ORIGINAL_COMMAND
SSH_TTY
TZ
USER
Additionally, ssh
reads
~/.ssh/environment, and adds lines of the format
“VARNAME=value” to the environment if the file exists and
users are allowed to change their environment. For more information, see the
PermitUserEnvironment
option in
sshd_config(5).
ssh
will simply ignore a
private key file if it is accessible by others. It is possible to specify
a passphrase when generating the key which will be used to encrypt the
sensitive part of this file using 3DES.
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.
RhostsRSAAuthentication
and
HostbasedAuthentication
. If the protocol version 1
RhostsRSAAuthentication
method is used,
ssh
must be setuid root, since the host key is
readable only by root. For protocol version 2, ssh
uses ssh-keysign(8) to access the host keys for
HostbasedAuthentication
. This eliminates the
requirement that ssh
be setuid root when that
authentication method is used. By default ssh
is
not setuid root.
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.
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.T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January 2002, work in progress material.
September 25, 1999 | BSD |