Manual Pages  — SSH

ssh connects and logs into the specified destination, which may be specified as either [user@ ]hostnameor 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 Ns -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 Ns -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 : ]portSpecifies 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 (‘amp;.’) 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_socketSpecifies 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).
AddKeysToAgent AddressFamily BatchMode BindAddress CanonicalDomains CanonicalizeFallbackLocal CanonicalizeHostname CanonicalizeMaxDots CanonicalizePermittedCNAMEs CASignatureAlgorithms CertificateFile CheckHostIP Ciphers ClearAllForwardings Compression ConnectionAttempts ConnectTimeout ControlMaster ControlPath ControlPersist DynamicForward EscapeChar ExitOnForwardFailure FingerprintHash ForkAfterAuthentication ForwardAgent ForwardX11 ForwardX11Timeout ForwardX11Trusted GatewayPorts GlobalKnownHostsFile GSSAPIAuthentication GSSAPIDelegateCredentials HashKnownHosts Host HostbasedAcceptedAlgorithms HostbasedAuthentication HostKeyAlgorithms HostKeyAlias Hostname IdentitiesOnly IdentityAgent IdentityFile IPQoS KbdInteractiveAuthentication KbdInteractiveDevices KexAlgorithms KnownHostsCommand LocalCommand LocalForward LogLevel MACs Match NoHostAuthenticationForLocalhost NumberOfPasswordPrompts PasswordAuthentication PermitLocalCommand PermitRemoteOpen PKCS11Provider Port PreferredAuthentications ProxyCommand ProxyJump ProxyUseFdpass PubkeyAcceptedAlgorithms PubkeyAuthentication RekeyLimit RemoteCommand RemoteForward RequestTTY RequiredRSASize SendEnv ServerAliveInterval ServerAliveCountMax SessionType SetEnv StdinNull StreamLocalBindMask StreamLocalBindUnlink StrictHostKeyChecking TCPKeepAlive Tunnel TunnelDevice UpdateHostKeys User UserKnownHostsFile VerifyHostKeyDNS VersionAddendum 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 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), 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 : ]portSpecifies 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.amp; 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.
-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.

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/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) 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).