| SSH(1) | General Commands Manual | SSH(1) |
ssh —
ssh |
[-1246AaCfgKkMNnqsTtVvXxY]
[-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 local_tun[:remote_tun]]
[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 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:
-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
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-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. 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-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. 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-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.-w
local_tun[:remote_tun]The devices may be specified by numerical ID or the keyword
“any”, which uses the next available tunnel device. If
remote_tun is not specified, it defaults to
“any”. See also the Tunnel and
TunnelDevice directives in
ssh_config(5). If the Tunnel
directive is unset, it is set to the default tunnel mode, which is
“point-to-point”.
-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 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, umac-64, hmac-ripemd160).
Protocol 1 lacks a strong mechanism for ensuring the integrity of the
connection.
The methods available for authentication are: GSSAPI-based
authentication, 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.
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 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:
~.~^Zssh.~#~&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[bind_address:]port.
!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.~RIn the example below, we look at encrypting communication between
an IRC client and server, even though the IRC server does not directly
support encrypted communications. This works as follows: the user connects
to the remote host using ssh, specifying a port to
be used to forward connections to the remote server. After that it is
possible to start the service which is to be encrypted on the client
machine, connecting to the same local port, and ssh
will encrypt and forward the connection.
The following example tunnels an IRC session from client machine “127.0.0.1” (localhost) to remote server “server.example.com”:
$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 $ irc -c '#users' -p 1234 pinky 127.0.0.1
This tunnels a connection to IRC server “server.example.com”, joining channel “#users”, nickname “pinky”, using port 1234. It doesn't matter which port is used, as long as it's greater than 1023 (remember, only root can open sockets on privileged ports) and doesn't conflict with any ports already in use. The connection is forwarded to port 6667 on the remote server, since that's the standard port for IRC services.
The -f option backgrounds
ssh and the remote command “sleep 10”
is specified to allow an amount of time (10 seconds, in the example) to
start the service which is to be tunnelled. If no connections are made
within the time specified, ssh will exit.
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.
StrictHostKeyChecking has been disabled). Fingerprints
can be determined using ssh-keygen(1):
$ ssh-keygen -l -f
/etc/ssh/ssh_host_rsa_keyIf the fingerprint is already known, it can be matched and the key
can be accepted or rejected. Because of the difficulty of comparing host
keys just by looking at hex strings, there is also support to compare host
keys visually, using random art. By setting the
VisualHostKey option to “yes”, a small
ASCII graphic gets displayed on every login to a server, no matter if the
session itself is interactive or not. By learning the pattern a known server
produces, a user can easily find out that the host key has changed when a
completely different pattern is displayed. Because these patterns are not
unambiguous however, a pattern that looks similar to the pattern remembered
only gives a good probability that the host key is the same, not guaranteed
proof.
To get a listing of the fingerprints along with their random art for all known hosts, the following command line can be used:
$ ssh-keygen -lv -f
~/.ssh/known_hostsIf the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. An additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to match the fingerprint with that of the key presented.
In this example, we are connecting a client to a server, “host.example.com”. The SSHFP resource records should first be added to the zonefile for host.example.com:
$ ssh-keygen -r host.example.com.
The output lines will have to be added to the zonefile. To check that the zone is answering fingerprint queries:
$ dig -t SSHFP
host.example.comFinally the client connects:
$ ssh -o "VerifyHostKeyDNS ask" host.example.com [...] Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?
See the VerifyHostKeyDNS option in
ssh_config(5) for more information.
ssh contains support for Virtual Private Network (VPN)
tunnelling using the tun(4) network pseudo-device, allowing
two networks to be joined securely. The sshd_config(5)
configuration option PermitTunnel controls whether the
server supports this, and at what level (layer 2 or 3 traffic).
The following example would connect client network 10.0.50.0/24 with remote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway to the remote network, at 192.168.1.15, allows it.
On the client:
# ssh -f -w 0:1 192.168.1.15 true # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 # route add 10.0.99.0/24 10.1.1.2
On the server:
# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 # route add 10.0.50.0/24 10.1.1.1
Client access may be more finely tuned via the
/root/.ssh/authorized_keys file (see below) and the
PermitRootLogin server option. The following entry
would permit connections on tun(4) device 1 from user
“jane” and on tun device 2 from user “john”, if
PermitRootLogin is set to
“forced-commands-only”:
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
Since an SSH-based setup entails a fair amount of overhead, it may be more suited to temporary setups, such as for wireless VPNs. More permanent VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8).
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
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.
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, eliminating
the requirement that ssh be setuid root when
host-based authentication is used. By default ssh
is not setuid root.
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.The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, 2006.
Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, 2006.
The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, 2006.
Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, 2006.
The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-World Security, 1999, International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99).
| June 26, 2008 | BSD |