ssh —
OpenSSH
remote login client
ssh |
[-46AaCfGgKkMNnqsTtVvXxYy]
[-B
bind_interface]
[-b
bind_address]
[-c
cipher_spec]
[-D
[bind_address:]port]
[-E
log_file]
[-e
escape_char]
[-F
configfile]
[-I
pkcs11]
[-i
identity_file]
[-J
destination]
[-L
address]
[-l
login_name]
[-m
mac_spec]
[-O
ctl_cmd]
[-o
option]
[-p
port]
[-Q
query_option]
[-R
address]
[-S
ctl_path]
[-W
host:port]
[-w
local_tun[:remote_tun]]
destination
[command
[argument
...]] |
ssh (SSH client) is a program for logging into a
remote machine and for executing commands on a remote machine. It is intended
to provide secure encrypted communications between two untrusted hosts over an
insecure network. X11 connections, arbitrary TCP ports and
UNIX-domain sockets can also be forwarded over the
secure channel.
ssh connects and logs into the specified
destination, which may be specified as either
[
user@]hostname or a URI of the form
ssh://
[
user@]hostname[
:port]. The
user must prove their identity to the remote machine using one of several
methods (see below).
If a
command is specified, it will be executed
on the remote host instead of a login shell. A complete command line may be
specified as
command, or it may have
additional arguments. If supplied, the arguments will be appended to the
command, separated by spaces, before it is sent to the server to be executed.
The options are as follows:
- -4
- Forces ssh to use IPv4
addresses only.
- -6
- Forces ssh to use IPv6
addresses only.
- -A
- Enables forwarding of connections from an authentication
agent such as ssh-agent(1). This can also be
specified on a per-host basis in a configuration file.
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
safer alternative may be to use a jump host (see
-J).
- -a
- Disables forwarding of the authentication agent connection.
-
-B
bind_interface
- Bind to the address of
bind_interface before attempting to
connect to the destination host. This is only useful on systems with more
than one address.
-
-b
bind_address
- Use bind_address on the
local machine as the source address of the connection. Only useful on
systems with more than one address.
- -C
- Requests compression of all data (including stdin, stdout,
stderr, and data for forwarded X11, TCP and
UNIX-domain connections). The compression
algorithm is the same used by gzip(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 in
ssh_config(5).
-
-c
cipher_spec
- Selects the cipher specification for encrypting the
session. cipher_spec is a comma-separated
list of ciphers listed in order of preference. See the
Ciphers keyword in
ssh_config(5) for more information.
-
-D
[bind_address:]port
- Specifies a local “dynamic” application-level
port forwarding. This works by allocating a socket to listen to
port on the local side, optionally bound
to the specified bind_address. Whenever a
connection is made to this port, the connection is forwarded over the
secure channel, and the application protocol is then used to determine
where to connect to from the remote machine. Currently the SOCKS4 and
SOCKS5 protocols are supported, and ssh 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 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
log_file
- Append debug logs to
log_file instead of standard error.
-
-e
escape_char
- Sets the escape character for sessions with a pty (default:
‘
~
’). 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
- Specifies an alternative per-user configuration file. If a
configuration file is given on the command line, the system-wide
configuration file (/etc/ssh/ssh_config) will
be ignored. The default for the per-user configuration file is
~/.ssh/config. If set to
“none”, no configuration files will be read.
- -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.
If the ExitOnForwardFailure configuration
option is set to “yes”, then a client started with
-f will wait for all remote port forwards to
be successfully established before placing itself in the background. Refer
to the description of ForkAfterAuthentication
in ssh_config(5) for details.
- -G
- Causes ssh to print its
configuration after evaluating Host and
Match blocks and exit.
- -g
- Allows remote hosts to connect to local forwarded ports. If
used on a multiplexed connection, then this option must be specified on
the master process.
-
-I
pkcs11
- Specify the PKCS#11 shared library
ssh should use to communicate with a PKCS#11
token providing keys for user authentication.
-
-i
identity_file
- Selects a file from which the identity (private key) for
public key authentication is read. You can also specify a public key file
to use the corresponding private key that is loaded in
ssh-agent(1) when the private key file is not
present locally. The default is
~/.ssh/id_rsa,
~/.ssh/id_ecdsa,
~/.ssh/id_ecdsa_sk,
~/.ssh/id_ed25519,
~/.ssh/id_ed25519_sk and
~/.ssh/id_dsa. 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 configuration files). If no certificates have been
explicitly specified by the CertificateFile
directive, ssh will also try to load
certificate information from the filename obtained by appending
-cert.pub to identity filenames.
-
-J
destination
- Connect to the target host by first making a
ssh connection to the jump host described by
destination and then establishing a TCP
forwarding to the ultimate destination from there. Multiple jump hops may
be specified separated by comma characters. This is a shortcut to specify
a ProxyJump configuration directive. Note
that configuration directives supplied on the command-line generally apply
to the destination host and not any specified jump hosts. Use
~/.ssh/config to specify configuration for
jump hosts.
- -K
- Enables GSSAPI-based authentication and forwarding
(delegation) of GSSAPI credentials to the server.
- -k
- Disables forwarding (delegation) of GSSAPI credentials to
the server.
-
-L
[bind_address:]port:host:hostport
-
-
-L
[bind_address:]port:remote_socket
-
-
-L
local_socket:host:hostport
-
-
-L
local_socket:remote_socket
- Specifies that connections to the given TCP port or Unix
socket on the local (client) host are to be forwarded to the given host
and port, or Unix socket, on the remote side. This works by allocating a
socket to listen to either a TCP port on
the local side, optionally bound to the specified
bind_address, or to a Unix socket.
Whenever a connection is made to the local port or socket, the connection
is forwarded over the secure channel, and a connection is made to either
host port
hostport, or the Unix socket
remote_socket, from the remote machine.
Port forwardings can also be specified in the configuration file. Only the
superuser can forward privileged ports. IPv6 addresses can be specified by
enclosing the address in square brackets.
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.
-
-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.
- -M
- Places the ssh client into
“master” mode for connection sharing. Multiple
-M options places
ssh into “master” mode but with
confirmation required using ssh-askpass(1)
before each operation that changes the multiplexing state (e.g. opening a
new session). Refer to the description of
ControlMaster in
ssh_config(5) for details.
-
-m
mac_spec
- A comma-separated list of MAC (message authentication code)
algorithms, specified in order of preference. See the
MACs keyword in
ssh_config(5) for more information.
- -N
- Do not execute a remote command. This is useful for just
forwarding ports. Refer to the description of
SessionType in
ssh_config(5) for details.
- -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.) Refer to the description of
StdinNull in
ssh_config(5) for details.
-
-O
ctl_cmd
- Control an active connection multiplexing master process.
When the -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), “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).
-
-o
option
- Can be used to give options in the format used in the
configuration file. This is useful for specifying options for which there
is no separate command-line flag. For full details of the options listed
below, and their possible values, see
ssh_config(5).
-
-p
port
- Port to connect to on the remote host. This can be
specified on a per-host basis in the configuration file.
-
-Q
query_option
- Queries for the algorithms supported by one of the
following features: cipher (supported
symmetric ciphers), cipher-auth
(supported symmetric ciphers that support authenticated encryption),
help (supported query terms for use with
the -Q flag),
mac (supported message integrity codes),
kex (key exchange algorithms),
kex-gss (GSSAPI key exchange algorithms),
key (key types),
key-cert (certificate key types),
key-plain (non-certificate key types),
key-sig (all key types and signature
algorithms), protocol-version (supported
SSH protocol versions), and sig
(supported signature algorithms). Alternatively, any keyword from
ssh_config(5) or
sshd_config(5) that takes an algorithm list
may be used as an alias for the corresponding query_option.
- -q
- Quiet mode. Causes most warning and diagnostic messages to
be suppressed.
-
-R
[bind_address:]port:host:hostport
-
-
-R
[bind_address:]port:local_socket
-
-
-R
remote_socket:host:hostport
-
-
-R
remote_socket:local_socket
-
-
-R
[bind_address:]port
- Specifies that connections to the given TCP port or Unix
socket on the remote (server) host are to be forwarded to the local side.
This works by allocating a socket to listen to either a TCP
port or to a Unix socket on the remote
side. Whenever a connection is made to this port or Unix socket, the
connection is forwarded over the secure channel, and a connection is made
from the local machine to either an explicit destination specified by
host port
hostport, or
local_socket, or, if no explicit
destination was specified, ssh will act as a
SOCKS 4/5 proxy and forward connections to the destinations requested by
the remote SOCKS client.
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
brackets.
By default, TCP listening sockets 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
sshd_config(5)).
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.
-
-S
ctl_path
- Specifies the location of a control socket for connection
sharing, or the string “none” to disable connection sharing.
Refer to the description of ControlPath and
ControlMaster in
ssh_config(5) for details.
- -s
- May be used to request invocation of a subsystem on the
remote system. Subsystems facilitate the use of SSH as a secure transport
for other applications (e.g. sftp(1)). The
subsystem is specified as the remote command. Refer to the description of
SessionType in
ssh_config(5) for details.
- -T
- Disable pseudo-terminal allocation.
- -t
- Force pseudo-terminal allocation. This can be used to
execute arbitrary screen-based programs on a remote machine, which can be
very useful, e.g. when implementing menu services. Multiple
-t options force tty allocation, even if
ssh has no local tty.
- -V
- Display the version number and exit.
- -v
- Verbose mode. Causes 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
host:port
- Requests that standard input and output on the client be
forwarded to host on
port over the secure channel. Implies
-N, -T,
ExitOnForwardFailure and
ClearAllForwardings, though these can be
overridden in the configuration file or using
-o command line options.
-
-w
local_tun[:remote_tun]
- Requests tunnel device forwarding with the specified
tun(4) devices between the client
(local_tun) and the server
(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 will be
set to the default tunnel mode, which is “point-to-point”.
If a different Tunnel forwarding mode it
desired, then it should be specified before
-w.
- -X
- Enables X11 forwarding. This can also be specified on a
per-host basis in a configuration file.
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. Refer to the ssh
-Y option and the
ForwardX11Trusted directive in
ssh_config(5) for more information.
(Debian-specific: X11 forwarding is not subjected to X11 SECURITY extension
restrictions by default, because too many programs currently crash in this
mode. Set the ForwardX11Trusted option to
“no” to restore the upstream behaviour. This may change in
future depending on client-side improvements.)
- -x
- Disables X11 forwarding.
- -Y
- Enables trusted X11 forwarding. Trusted X11 forwardings are
not subjected to the X11 SECURITY extension controls.
(Debian-specific: In the default configuration, this option is equivalent to
-X, since
ForwardX11Trusted defaults to
“yes” as described above. Set the
ForwardX11Trusted option to
“no” to restore the upstream behaviour. This may change in
future depending on client-side improvements.)
- -y
- Send log information using the
syslog(3) system module. By default this
information is sent to stderr.
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).
The OpenSSH SSH client supports SSH protocol 2.
The methods available for authentication are: GSSAPI-based authentication,
host-based authentication, public key authentication, keyboard-interactive
authentication, and password authentication. Authentication methods are tried
in the order specified above, though
PreferredAuthentications can be used 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/ssh/shosts.equiv on the remote machine, the
user is non-root 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,
Ed25519 or RSA algorithms. The HISTORY section of
ssl(8) (on non-OpenBSD systems, see
http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY)
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 server may inform the client of errors that prevented public key
authentication from succeeding after authentication completes using a
different method. These may be viewed by increasing the
LogLevel to
DEBUG or
higher (e.g. by using the
-v flag).
The user creates their key pair by running
ssh-keygen(1). This stores the private key in
~/.ssh/id_dsa (DSA),
~/.ssh/id_ecdsa (ECDSA),
~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA),
~/.ssh/id_ed25519 (Ed25519),
~/.ssh/id_ed25519_sk (authenticator-hosted
Ed25519), or
~/.ssh/id_rsa (RSA) and stores the
public key in
~/.ssh/id_dsa.pub (DSA),
~/.ssh/id_ecdsa.pub (ECDSA),
~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
ECDSA),
~/.ssh/id_ed25519.pub (Ed25519),
~/.ssh/id_ed25519_sk.pub (authenticator-hosted
Ed25519), or
~/.ssh/id_rsa.pub (RSA) in the
user's home directory. The user should then copy the public key to
~/.ssh/authorized_keys in their 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)
and (optionally) the
AddKeysToAgent directive in
ssh_config(5) for more information.
Keyboard-interactive authentication works as follows: The server sends an
arbitrary “challenge” text and prompts for a response, possibly
multiple times. Examples of keyboard-interactive 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 in a non-interactive session or, if no command has
been specified, logs into the machine and gives the user a normal shell as an
interactive session. All communication with the remote command or shell will
be automatically encrypted.
If an interactive session is requested,
ssh by
default will only request a pseudo-terminal (pty) for interactive sessions
when the client has one. The flags
-T and
-t can be used to override this behaviour.
If a pseudo-terminal has been allocated, the user may use the escape characters
noted below.
If no pseudo-terminal 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 the
EscapeChar configuration directive or on the
command line by the
-e option.
The supported escapes (assuming the default
‘
~
’) are:
- ~.
- Disconnect.
- ~^Z
- Background ssh.
- ~#
- List forwarded connections.
- ~&
- Background ssh at logout when
waiting for forwarded connection / X11 sessions to terminate.
- ~?
- Display a list of escape characters.
- ~B
- Send a BREAK to the remote system (only useful if the peer
supports it).
- ~C
- Open command line. Currently this allows the addition of
port forwardings using the -L,
-R and -D
options (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
PermitLocalCommand option is enabled in
ssh_config(5). Basic help is available, using
the -h option.
- ~R
- Request rekeying of the connection (only useful if the peer
supports it).
- ~V
- Decrease the verbosity
(LogLevel) when errors are being written to
stderr.
- ~v
- Increase the verbosity
(LogLevel) when errors are being written to
stderr.
Forwarding of arbitrary TCP connections over a 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 for an IRC client,
even though the IRC server it connects to does not directly support encrypted
communication. This works as follows: the user connects to the remote host
using
ssh, specifying the ports to be used to
forward the connection. After that it is possible to start the program
locally, and
ssh will encrypt and forward the
connection to the remote server.
The following example tunnels an IRC session from the client to an IRC server at
“server.example.com”, joining channel “#users”,
nickname “pinky”, using the standard IRC port, 6667:
$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
$ irc -c '#users' pinky IRC/127.0.0.1
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 program which is going to use the tunnel. If no
connections are made within the time specified,
ssh will exit.
If the
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.
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(1):
$ 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. If only legacy (MD5) fingerprints for the server are
available, the
ssh-keygen(1)
-E option may be used to downgrade the
fingerprint algorithm to match.
Because of the difficulty of comparing host keys just by looking at fingerprint
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_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)?
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:
DISPLAY
- The
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
- 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 the path of the user's mailbox.
PATH
- Set to the default
PATH
,
as specified when compiling ssh.
SSH_ASKPASS
- If 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_ASKPASS_REQUIRE
- Allows further control over the use of an askpass program.
If this variable is set to “never” then
ssh will never attempt to use one. If it is
set to “prefer”, then ssh will
prefer to use the askpass program instead of the TTY when requesting
passwords. Finally, if the variable is set to “force”, then
the askpass program will be used for all passphrase input regardless of
whether
DISPLAY
is set.
SSH_AUTH_SOCK
- Identifies the path of a
UNIX-domain socket used to communicate with the
agent.
SSH_CONNECTION
- Identifies the client and server ends of the connection.
The variable contains four space-separated values: client IP address,
client port number, server IP address, and server port number.
SSH_ORIGINAL_COMMAND
- This variable contains the original command line if a
forced command is executed. It can be used to extract the original
arguments.
SSH_TTY
- This is set to the name of the tty (path to the device)
associated with the current shell or command. If the current session has
no tty, this variable is not set.
SSH_TUNNEL
- Optionally set by sshd(8) to
contain the interface names assigned if tunnel forwarding was requested by
the client.
SSH_USER_AUTH
- Optionally set by sshd(8),
this variable may contain a pathname to a file that lists the
authentication methods successfully used when the session was established,
including any public keys that were used.
TZ
- This variable is set to indicate the present time zone 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
~/.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).
- ~/.rhosts
- This file is used for host-based authentication (see
above). On some machines this file may need to be world-readable if the
user's home directory is on an 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.
- ~/.shosts
- This file is used in exactly the same way as
.rhosts, but allows host-based authentication
without permitting login with rlogin/rsh.
- ~/.ssh/
- This directory is the default location for all
user-specific configuration and authentication information. There is no
general requirement to keep the entire contents of this directory secret,
but the recommended permissions are read/write/execute for the user, and
not accessible by others.
- ~/.ssh/authorized_keys
- Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can
be used for logging in as this user. The format of this file is described
in the sshd(8) manual page. This file is not
highly sensitive, but the recommended permissions are read/write for the
user, and not accessible by others.
- ~/.ssh/config
- This is the per-user configuration file. The file format
and configuration options are described in
ssh_config(5). Because of the potential for
abuse, this file must have strict permissions: read/write for the user,
and not writable by others. It may be group-writable provided that the
group in question contains only the user.
- ~/.ssh/environment
- Contains additional definitions for environment variables;
see ENVIRONMENT, above.
- ~/.ssh/id_dsa
-
- ~/.ssh/id_ecdsa
-
- ~/.ssh/id_ecdsa_sk
-
- ~/.ssh/id_ed25519
-
- ~/.ssh/id_ed25519_sk
-
- ~/.ssh/id_rsa
- Contains the private key for authentication. These files
contain sensitive data and should be readable by the user but not
accessible by others (read/write/execute).
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 AES-128.
- ~/.ssh/id_dsa.pub
-
- ~/.ssh/id_ecdsa.pub
-
- ~/.ssh/id_ecdsa_sk.pub
-
- ~/.ssh/id_ed25519.pub
-
- ~/.ssh/id_ed25519_sk.pub
-
- ~/.ssh/id_rsa.pub
- Contains the public key for authentication. These files are
not sensitive and can (but need not) be readable by anyone.
- ~/.ssh/known_hosts
- Contains a list of host keys for all hosts the user has
logged into that are not already in the systemwide list of known host
keys. See sshd(8) for further details of the
format of this file.
- ~/.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.
- /etc/hosts.equiv
- This file is for host-based authentication (see above). It
should only be writable by root.
- /etc/ssh/shosts.equiv
- This file is used in exactly the same way as
hosts.equiv, but allows host-based
authentication without permitting login with rlogin/rsh.
- /etc/ssh/ssh_config
- Systemwide configuration file. The file format and
configuration options are described in
ssh_config(5).
- /etc/ssh/ssh_host_key
-
- /etc/ssh/ssh_host_dsa_key
-
- /etc/ssh/ssh_host_ecdsa_key
-
- /etc/ssh/ssh_host_ed25519_key
-
- /etc/ssh/ssh_host_rsa_key
- These files contain the private parts of the host keys and
are used for host-based authentication.
- /etc/ssh/ssh_known_hosts
- Systemwide list of known host keys. This file should be
prepared by the system administrator to contain the public host keys of
all machines in the organization. It should be world-readable. See
sshd(8) for further details of the format of
this file.
- /etc/ssh/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.
ssh exits with the exit status of the remote
command or with 255 if an error occurred.
scp(1),
sftp(1),
ssh-add(1),
ssh-agent(1),
ssh-argv0(1),
ssh-keygen(1),
ssh-keyscan(1),
tun(4),
ssh_config(5),
ssh-keysign(8),
sshd(8)
S. Lehtinen and
C. Lonvick, The Secure Shell (SSH)
Protocol Assigned Numbers, RFC 4250,
January 2006.
T. Ylonen and
C. Lonvick, The Secure Shell (SSH)
Protocol Architecture, RFC 4251,
January 2006.
T. Ylonen and
C. Lonvick, The Secure Shell (SSH)
Authentication Protocol, RFC 4252,
January 2006.
T. Ylonen and
C. Lonvick, The Secure Shell (SSH)
Transport Layer Protocol, RFC 4253,
January 2006.
T. Ylonen and
C. Lonvick, The Secure Shell (SSH)
Connection Protocol, RFC 4254,
January 2006.
J. Schlyter and
W. Griffin, Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints, RFC
4255, January 2006.
F. Cusack and
M. Forssen, Generic Message
Exchange Authentication for the Secure Shell Protocol (SSH),
RFC 4256, January
2006.
J. Galbraith and
P. Remaker, The Secure Shell (SSH)
Session Channel Break Extension, RFC 4335,
January 2006.
M. Bellare,
T. Kohno, and C. Namprempre,
The Secure Shell (SSH) Transport Layer Encryption
Modes, RFC 4344, January
2006.
B. Harris,
Improved Arcfour Modes for the Secure Shell (SSH) Transport
Layer Protocol, RFC 4345,
January 2006.
M. Friedl,
N. Provos, and W. Simpson,
Diffie-Hellman Group Exchange for the Secure Shell (SSH)
Transport Layer Protocol, RFC 4419,
March 2006.
J. Galbraith and
R. Thayer, The Secure Shell (SSH)
Public Key File Format, RFC 4716,
November 2006.
D. Stebila and
J. Green, Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer, RFC
5656, December 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.