| 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]
[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.
If command is specified, command is executed on the remote host instead of a login shell.
As a second 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 ~/.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 challenge 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 automatically. The user creates his/her RSA key pair by running
ssh-keygen(1). This stores the private key in
~/.ssh/identity and stores the public key in
~/.ssh/identity.pub in the user's home directory.
The user should then copy the identity.pub 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 RSA authentication may be with an authentication agent. See ssh-agent(1) for more information.
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.
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, ~/.ssh/id_dsa or ~/.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 ~/.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 to prove the user's identity.
Additionally, ssh supports hostbased or
challenge response authentication.
Protocol 2 provides additional mechanisms for confidentiality (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour) and integrity (hmac-md5, hmac-sha1, hmac-ripemd160). Note that protocol 1 lacks a strong mechanism for ensuring the integrity of the connection.
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. The exit
status of the remote program is returned as the exit status of
ssh.
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:
~.~^Zssh.~#~&ssh at logout when waiting for
forwarded connection / X11 sessions to terminate.~?~B~C-L and -R
options (see below). It also allows the cancellation of existing remote
port-forwardings using -KR
hostport. Basic help is available, using the
-h option.~RForwardX11 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. 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 described
later) 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 automatically maintains and checks a database
containing identifications 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 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 circumvent the
encryption. The StrictHostKeyChecking option can be
used to prevent 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).
The options are as follows:
-1ssh to try protocol version 1 only.-2ssh to try protocol version 2 only.-4ssh to use IPv4 addresses only.-6ssh to use IPv6 addresses only.-AAgent 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-CCompressionLevel 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
ch | ^ch |
none~’). 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-fssh 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.-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-Mssh client into “master”
mode for connection sharing. Refer to the description of
ControlMaster in ssh_config(5)
for details.-m
mac_specMACs keyword for more
information.-N-nssh 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-vssh 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.-XX11 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-Yssh 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 will normally set the following environment
variables:
DISPLAYDISPLAY 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).HOMELOGNAMEUSER; set for compatibility with
systems that use this variable.MAILPATHPATH, as specified when
compiling ssh.SSH_ASKPASSssh 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_SOCKSSH_CONNECTIONSSH_ORIGINAL_COMMANDSSH_TTYTZUSERAdditionally, ssh reads
~/.ssh/environment, and adds lines of the format
“VARNAME=value” to the environment if the file exists and if
users are allowed to change their environment. For more information, see the
PermitUserEnvironment option in
sshd_config(5).
ssh ignores
a private key file if it is accessible by others. 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.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.
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.RhostsRSAAuthentication and
HostbasedAuthentication 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 partition, 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 sshd(8) allows authentication only in combination with client host key authentication before permitting log in. If the server machine does not have the client's host key in /etc/ssh/ssh_known_hosts, it can be stored in ~/.ssh/known_hosts. The easiest way to do this is to connect back to the client from the server machine using ssh; this will automatically add the host key to ~/.ssh/known_hosts.
RhostsRSAAuthentication and
HostbasedAuthentication authentication without
permitting login with rlogin or
rsh(1).RhostsRSAAuthentication
and HostbasedAuthentication authentication. It
contains canonical hosts names, one per line (the full format is described
in 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 client host key
authentication is required. This file should only be writable by
root.ssh but not using
rsh/rlogin.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.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.ssh exits with the exit status of the remote command or
with 255 if an error occurred.
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 |