992 lines
49 KiB
Plaintext
992 lines
49 KiB
Plaintext
SSH(1) General Commands Manual SSH(1)
|
|
|
|
NAME
|
|
ssh M-bM-^@M-^S OpenSSH remote login client
|
|
|
|
SYNOPSIS
|
|
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]
|
|
|
|
DESCRIPTION
|
|
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 his/her identity to
|
|
the remote machine using one of several methods (see below).
|
|
|
|
If a command is specified, it is executed on the remote host instead of a
|
|
login shell.
|
|
|
|
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.
|
|
|
|
-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 M-bM-^@M-^\dynamicM-bM-^@M-^] 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 M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
|
|
bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y 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: M-bM-^@M-^X~M-bM-^@M-^Y).
|
|
The escape character is only recognized at the beginning of a
|
|
line. The escape character followed by a dot (M-bM-^@M-^X.M-bM-^@M-^Y) closes the
|
|
connection; followed by control-Z suspends the connection; and
|
|
followed by itself sends the escape character once. Setting the
|
|
character to M-bM-^@M-^\noneM-bM-^@M-^] 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.
|
|
|
|
-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 M-bM-^@M-^\yesM-bM-^@M-^],
|
|
then a client started with -f will wait for all remote port
|
|
forwards to be successfully established before placing itself in
|
|
the background.
|
|
|
|
-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. The default is ~/.ssh/id_dsa,
|
|
~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
|
|
~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa. 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 M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
|
|
bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y 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 M-bM-^@M-^\masterM-bM-^@M-^] mode for connection sharing.
|
|
Multiple -M options places ssh into M-bM-^@M-^\masterM-bM-^@M-^] 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 for more information.
|
|
|
|
-N Do not execute a remote command. This is useful for just
|
|
forwarding ports.
|
|
|
|
-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.)
|
|
|
|
-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: M-bM-^@M-^\checkM-bM-^@M-^]
|
|
(check that the master process is running), M-bM-^@M-^\forwardM-bM-^@M-^] (request
|
|
forwardings without command execution), M-bM-^@M-^\cancelM-bM-^@M-^] (cancel
|
|
forwardings), M-bM-^@M-^\exitM-bM-^@M-^] (request the master to exit), and M-bM-^@M-^\stopM-bM-^@M-^]
|
|
(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).
|
|
|
|
AddKeysToAgent
|
|
AddressFamily
|
|
BatchMode
|
|
BindAddress
|
|
CanonicalDomains
|
|
CanonicalizeFallbackLocal
|
|
CanonicalizeHostname
|
|
CanonicalizeMaxDots
|
|
CanonicalizePermittedCNAMEs
|
|
CASignatureAlgorithms
|
|
CertificateFile
|
|
ChallengeResponseAuthentication
|
|
CheckHostIP
|
|
Ciphers
|
|
ClearAllForwardings
|
|
Compression
|
|
ConnectionAttempts
|
|
ConnectTimeout
|
|
ControlMaster
|
|
ControlPath
|
|
ControlPersist
|
|
DynamicForward
|
|
EscapeChar
|
|
ExitOnForwardFailure
|
|
FingerprintHash
|
|
ForwardAgent
|
|
ForwardX11
|
|
ForwardX11Timeout
|
|
ForwardX11Trusted
|
|
GatewayPorts
|
|
GlobalKnownHostsFile
|
|
GSSAPIAuthentication
|
|
GSSAPIDelegateCredentials
|
|
HashKnownHosts
|
|
Host
|
|
HostbasedAuthentication
|
|
HostbasedKeyTypes
|
|
HostKeyAlgorithms
|
|
HostKeyAlias
|
|
Hostname
|
|
IdentitiesOnly
|
|
IdentityAgent
|
|
IdentityFile
|
|
IPQoS
|
|
KbdInteractiveAuthentication
|
|
KbdInteractiveDevices
|
|
KexAlgorithms
|
|
LocalCommand
|
|
LocalForward
|
|
LogLevel
|
|
MACs
|
|
Match
|
|
NoHostAuthenticationForLocalhost
|
|
NumberOfPasswordPrompts
|
|
PasswordAuthentication
|
|
PermitLocalCommand
|
|
PKCS11Provider
|
|
Port
|
|
PreferredAuthentications
|
|
ProxyCommand
|
|
ProxyJump
|
|
ProxyUseFdpass
|
|
PubkeyAcceptedKeyTypes
|
|
PubkeyAuthentication
|
|
RekeyLimit
|
|
RemoteCommand
|
|
RemoteForward
|
|
RequestTTY
|
|
SendEnv
|
|
ServerAliveInterval
|
|
ServerAliveCountMax
|
|
SetEnv
|
|
StreamLocalBindMask
|
|
StreamLocalBindUnlink
|
|
StrictHostKeyChecking
|
|
TCPKeepAlive
|
|
Tunnel
|
|
TunnelDevice
|
|
UpdateHostKeys
|
|
User
|
|
UserKnownHostsFile
|
|
VerifyHostKeyDNS
|
|
VisualHostKey
|
|
XAuthLocation
|
|
|
|
-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 ssh for the algorithms supported for the specified
|
|
version 2. The available features are: 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), 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
|
|
M-bM-^@M-^X*M-bM-^@M-^Y, 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 M-bM-^@M-^X0M-bM-^@M-^Y, 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 M-bM-^@M-^\noneM-bM-^@M-^] 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.
|
|
|
|
-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
|
|
M-bM-^@M-^\anyM-bM-^@M-^], which uses the next available tunnel device. If
|
|
remote_tun is not specified, it defaults to M-bM-^@M-^\anyM-bM-^@M-^]. 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 M-bM-^@M-^\point-to-pointM-bM-^@M-^]. 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. Please refer to the ssh -Y
|
|
option and the ForwardX11Trusted directive in ssh_config(5) for
|
|
more information.
|
|
|
|
-x Disables X11 forwarding.
|
|
|
|
-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
|
|
subjected to the X11 SECURITY extension controls.
|
|
|
|
-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).
|
|
|
|
AUTHENTICATION
|
|
The OpenSSH SSH client supports SSH protocol 2.
|
|
|
|
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
|
|
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/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, Ed25519 or RSA algorithms. 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 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 his/her 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 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) and (optionally)
|
|
the AddKeysToAgent directive in ssh_config(5) for more information.
|
|
|
|
Challenge-response authentication works as follows: The server sends an
|
|
arbitrary "challenge" text, and prompts for a 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 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 M-bM-^@M-^\noneM-bM-^@M-^] 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.
|
|
|
|
ESCAPE CHARACTERS
|
|
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 M-bM-^@M-^X~M-bM-^@M-^Y) 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.
|
|
|
|
TCP FORWARDING
|
|
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 M-bM-^@M-^\server.example.comM-bM-^@M-^], joining channel M-bM-^@M-^\#usersM-bM-^@M-^], nickname
|
|
M-bM-^@M-^\pinkyM-bM-^@M-^], 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 M-bM-^@M-^\sleep 10M-bM-^@M-^] 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.
|
|
|
|
X11 FORWARDING
|
|
If the ForwardX11 variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (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 M-bM-^@M-^\proxyM-bM-^@M-^] 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 M-bM-^@M-^\yesM-bM-^@M-^] (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.
|
|
|
|
VERIFYING HOST KEYS
|
|
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 M-bM-^@M-^\yesM-bM-^@M-^], 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,
|
|
M-bM-^@M-^\host.example.comM-bM-^@M-^]. 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-BASED VIRTUAL PRIVATE NETWORKS
|
|
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 M-bM-^@M-^\janeM-bM-^@M-^] and on
|
|
tun device 2 from user M-bM-^@M-^\johnM-bM-^@M-^], if PermitRootLogin is set to
|
|
M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^]:
|
|
|
|
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).
|
|
|
|
ENVIRONMENT
|
|
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 M-bM-^@M-^\hostname:nM-bM-^@M-^], where
|
|
M-bM-^@M-^\hostnameM-bM-^@M-^] indicates the host where the shell runs,
|
|
and M-bM-^@M-^XnM-bM-^@M-^Y is an integer M-bM-^IM-% 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_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
|
|
M-bM-^@M-^\VARNAME=valueM-bM-^@M-^] 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).
|
|
|
|
FILES
|
|
~/.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.
|
|
|
|
~/.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/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.
|
|
|
|
EXIT STATUS
|
|
ssh exits with the exit status of the remote command or with 255 if an
|
|
error occurred.
|
|
|
|
SEE ALSO
|
|
scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
|
|
tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
|
|
|
|
STANDARDS
|
|
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).
|
|
|
|
AUTHORS
|
|
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.
|
|
|
|
OpenBSD 6.6 February 7, 2020 OpenBSD 6.6
|