|SSH(1)||General Commands Manual||SSH(1)|
ssh — OpenSSH SSH
client (remote login program)
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
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:
sshto try protocol version 1 only.
sshto try protocol version 2 only.
sshto use IPv4 addresses only.
sshto use IPv6 addresses only.
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.
CompressionLeveloption 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
Protocol 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. See the
Ciphers keyword in
sshwill 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 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.
~’). 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.
sshto go to background just before command execution. This is useful if
sshis 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.
configuration option is set to “yes”, then a client
-f will wait for all remote port
forwards to be successfully established before placing itself in the
sshshould use to communicate with a PKCS#11 token providing the user's private RSA key.
-ioptions (and multiple identities specified in configuration files).
sshwill also try to load certificate information from the filename obtained by appending -cert.pub to identity filenames.
GatewayPortssetting. 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.
sshclient into “master” mode for connection sharing. Multiple
sshinto “master” mode with confirmation required before slave connections are accepted. Refer to the description of
ControlMasterin ssh_config(5) for details.
MACskeyword for more information.
sshis 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
sshprogram will be put in the background. (This does not work if
sshneeds to ask for a password or passphrase; see also the
-Ooption 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), “forward” (request forwardings without command execution), “cancel” (cancel forwardings), “exit” (request the master to exit), and “stop” (request the master to stop accepting further multiplexing requests).
Port 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.
By default, the listening socket on the server will be bound
to the loopback interface only. This may be overridden 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
If the port argument is
0’, the listen port will be
dynamically allocated on the server and reported to the client at run
time. When used together with
-O forward the
allocated port will be printed to the standard output.
ControlMasterin ssh_config(5) for details.
-toptions force tty allocation, even if
sshhas no local tty.
sshto print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple
-voptions increase the verbosity. The maximum is 3.
ClearAllForwardingsand works with Protocol version 2 only.
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
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”.
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
-Y option and the
ForwardX11Trusted directive in
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
The OpenSSH SSH client supports SSH protocols 1 and 2. The default
is to use protocol 2 only, though this can be changed via the
Protocol option in
ssh_config(5) or the
-2 options (see
above). Both protocols support similar authentication methods, but protocol
2 is the default 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-sha2-256, hmac-sha2-512, 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:
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 one of the DSA, ECDSA or RSA algorithms. Protocol 1 is
restricted to using only RSA keys, but protocol 2 may use any. The
HISTORY section of
ssl(8) contains a brief
discussion of the DSA and RSA 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), ~/.ssh/id_ecdsa (protocol 2 ECDSA), 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), ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), 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.
A variation on public key authentication is available in the form of certificate authentication: instead of a set of public/private keys, signed certificates are used. This has the advantage that a single trusted certification authority can be used in place of many public/private keys. See the CERTIFICATES section of ssh-keygen(1) for more information.
The most convenient way to use public key or certificate 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
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,
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
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.
When a pseudo-terminal has been requested,
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
EscapeChar configuration directive or on the
command line by the
The supported escapes (assuming the default
sshat logout when waiting for forwarded connection / X11 sessions to terminate.
-Doptions (see above). It also allows the cancellation of existing port-forwardings with
-KL[bind_address:]port for local,
-KR[bind_address:]port for remote and
-KD[bind_address:]port for dynamic port-forwardings.
!command allows the user to execute a local command if the
PermitLocalCommandoption is enabled in ssh_config(5). Basic help is available, using the
Forwarding of arbitrary TCP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP forwarding is a secure connection to a mail server; another is going through firewalls.
In 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
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.
-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
-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.
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
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
ForwardAgent variable is set to
“yes” (or see the description of the
-a options above) and
the user is using an authentication agent, the connection to the agent is
automatically forwarded to the remote side.
When connecting to a server for the first time, a fingerprint of
the server's public key is presented to the user (unless the option
StrictHostKeyChecking has been disabled).
Fingerprints can be determined using
$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
If 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
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_hosts
If 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.com
Finally 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)?
VerifyHostKeyDNS option in
ssh_config(5) for more
ssh contains support for Virtual Private
Network (VPN) tunnelling using the
tun(4) network pseudo-device,
allowing two networks to be joined securely. The
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
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
DISPLAYvariable indicates the location of the X11 server. It is automatically set by
sshto point to a value of the form “hostname:n”, where “hostname” indicates the host where the shell runs, and ‘n’ is an integer ≥ 1.
sshuses this special value to forward X11 connections over the secure channel. The user should normally not set
DISPLAYexplicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies).
USER; set for compatibility with systems that use this variable.
PATH, as specified when compiling
sshneeds a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If
sshdoes not have a terminal associated with it but
SSH_ASKPASSare set, it will execute the program specified by
SSH_ASKPASSand open an X11 window to read the passphrase. This is particularly useful when calling
sshfrom 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/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
sshwill 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.
sshwhen the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.
sshmust be setuid root, since the host key is readable only by root. For protocol version 2,
sshuses ssh-keysign(8) to access the host keys, eliminating the requirement that
sshbe setuid root when host-based authentication is used. By default
sshis not setuid root.
sshwhen 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.
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.
Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, 2009.
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).
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 features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
|September 11, 2011||OpenBSD-5.1|