# man > ssh(1) > **TLDR:** Secure Shell is a protocol used to securely log onto remote systems. > - Connect to a remote server: `ssh {{username}}@{{remote_host}}` - Connect to a remote server with a specific [i]dentity (private key): `ssh {{username}}@{{remote_host}} -i {{path/to/key_file}}` - Connect to a remote server with IP `10.0.0.1` and using a specific [p]ort (Note: `10.0.0.1` can be shortened to `10.1`): `ssh {{username}}@10.0.0.1 -p {{2222}}` - Run a command on a remote server with a [t]ty allocation allowing interaction with the remote command: `ssh {{username}}@{{remote_host}} -t {{command}} {{command_arguments}}` - SSH tunneling: [D]ynamic port forwarding (SOCKS proxy on `localhost:1080`): `ssh {{username}}@{{remote_host}} -D {{1080}}` - SSH tunneling: Forward a specific port (`localhost:9999` to `example.org:80`) along with disabling pseudo-[T]ty allocation and executio[N] of remote commands: `ssh {{username}}@{{remote_host}} -L {{9999}}:{{example.org}}:{{80}} -N -T` - SSH [J]umping: Connect through a jumphost to a remote server (Multiple jump hops may be specified separated by comma characters): `ssh {{username}}@{{remote_host}} -J {{username}}@{{jump_host}}` - Close a hanged session: `<~><.>` *Source: tldr-pages* --- [_SSH_(1)](https://www.chedong.com/phpMan.php/man/SSH/1/markdown) General Commands Manual [_SSH_(1)](https://www.chedong.com/phpMan.php/man/SSH/1/markdown) ## NAME ssh — 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 _**tag_] [**-p _**port_] [**-R _**address_] [**-S _**ctl_path_] [**-W _**host_:_port_] [**-w _**local_tun_[:_remote_tun_]] _destination_ [_command_ [_argument_ _..._]] **ssh **[**-Q _**query_option_] ## 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 un‐ trusted 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 ### -6 ### -A 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 ma‐ terial 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 ### -B _ 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 _ Use _bind_address_ on the local machine as the source address of the connection. Only useful on systems with more than one address. ### -C warded X11, TCP and Unix-domain connections). The compression algorithm is the same used by [_gzip_(1)](https://www.chedong.com/phpMan.php/man/gzip/1/markdown). 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). ### -c _ 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information. ### -D Specifies a local “dynamic” application-level port forwarding. This works by allo‐ cating a socket to listen to _port_ on the local side, optionally bound to the speci‐ fied _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 de‐ termine 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 configura‐ tion 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 _ Append debug logs to _log_file_ instead of standard error. ### -e _ Sets the escape character for sessions with a pty (default: ‘~’). The escape charac‐ ter 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 _ 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 **ssh **is going to ask for passwords or passphrases, but the user wants it in the back‐ ground. 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for details. ### -G exit. ### -g connection, then this option must be specified on the master process. ### -I _ Specify the PKCS#11 shared library **ssh **should use to communicate with a PKCS#11 token providing keys for user authentication. ### -i _ 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)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown) 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 _ Connect to the target host by first making an **ssh **connection to the jump host de‐ scribed by _destination_ and then establishing a TCP forwarding to the ultimate desti‐ nation from there. Multiple jump hops may be specified separated by comma charac‐ ters. This is a shortcut to specify a **ProxyJump **configuration directive. Note that configuration directives supplied on the command-line generally apply to the destina‐ tion host and not any specified jump hosts. Use _~/.ssh/config_ to specify configura‐ tion for jump hosts. ### -K to the server. ### -k ### -L ### -L ### -L _ ### -L _ 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 ad‐ dress 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 _ 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 -M places **ssh **into “master” mode but with confirmation required using [_ssh-askpass_(1)](https://www.chedong.com/phpMan.php/man/ssh-askpass/1/markdown) be‐ fore each operation that changes the multiplexing state (e.g. opening a new session). Refer to the description of **ControlMaster **in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for details. ### -m _ A comma-separated list of MAC (message authentication code) algorithms, specified in order of preference. See the **MACs **keyword in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information. ### -N the description of **SessionType **in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for details. ### -n 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for details. ### -O _ 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 _ 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). AddKeysToAgent AddressFamily BatchMode BindAddress CanonicalDomains CanonicalizeFallbackLocal CanonicalizeHostname CanonicalizeMaxDots CanonicalizePermittedCNAMEs CASignatureAlgorithms CertificateFile CheckHostIP Ciphers ClearAllForwardings Compression ConnectionAttempts ConnectTimeout ControlMaster ControlPath ControlPersist DynamicForward EnableEscapeCommandline EscapeChar ExitOnForwardFailure FingerprintHash ForkAfterAuthentication ForwardAgent ForwardX11 ForwardX11Timeout ForwardX11Trusted GatewayPorts GlobalKnownHostsFile GSSAPIAuthentication GSSAPIKeyExchange GSSAPIClientIdentity GSSAPIDelegateCredentials GSSAPIKexAlgorithms GSSAPIRenewalForcesRekey GSSAPIServerIdentity GSSAPITrustDns 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 VisualHostKey XAuthLocation ### -P _ to the **Tag **and **Match **keywords in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information. ### -p _ Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file. ### -Q _ Queries for the algorithms supported by one of the following features: _cipher_ (sup‐ ported symmetric ciphers), _cipher-auth_ (supported symmetric ciphers that support au‐ thenticated 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-ca-sign_ (valid CA signature algorithms for certificates), _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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) or [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown) that takes an algorithm list may be used as an alias for the corresponding query_option. ### -q ### -R ### -R ### -R _ ### -R _ ### -R 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 inter‐ face 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)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown)). 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 _ 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for details. ### -s facilitate the use of SSH as a secure transport for other applications (e.g. [_sftp_(1)](https://www.chedong.com/phpMan.php/man/sftp/1/markdown)). The subsystem is specified as the remote command. Refer to the descrip‐ tion of **SessionType **in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for details. ### -T ### -t 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 ### -v helpful in debugging connection, authentication, and configuration problems. Multi‐ ple **-v **options increase the verbosity. The maximum is 3. ### -W _ 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 us‐ ing **-o **command line options. ### -w _ Requests tunnel device forwarding with the specified [_tun_(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). 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 ration 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information. (Debian-specific: X11 forwarding is not subjected to X11 SECURITY extension restric‐ tions 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 ### -Y 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 is sent to stderr. **ssh **may additionally obtain configuration data from a per-user configuration file and a sys‐ tem-wide configuration file. The file format and configuration options are described in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). ## AUTHENTICATION The OpenSSH SSH client supports SSH protocol 2. The methods available for authentication are: GSSAPI-based authentication, host-based authen‐ tication, public key authentication, keyboard-interactive authentication, and password au‐ thentication. 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 au‐ thentication 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 proto‐ col 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 pro‐ tocol automatically, using one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of [_ssl_(8)](https://www.chedong.com/phpMan.php/man/ssl/8/markdown) (on non-OpenBSD systems, see ) contains a brief discus‐ sion 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 suc‐ ceeding after authentication completes using a different method. These may be viewed by in‐ creasing the **LogLevel **to **DEBUG **or higher (e.g. by using the **-v **flag). The user creates their key pair by running [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown). 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 authentica‐ tion: 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 pub‐ lic/private keys. See the CERTIFICATES section of [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown) for more information. The most convenient way to use public key or certificate authentication may be with an au‐ thentication agent. See [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown) and (optionally) the **AddKeysToAgent **directive in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) 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)](https://www.chedong.com/phpMan.php/man/login.conf/5/markdown)) 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 en‐ crypted, 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 di‐ rectory. 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 en‐ cryption. 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 reli‐ ably 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. ## 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 inter‐ preted 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 ter‐ minate. **~? **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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). 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 se‐ cure 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. ## X11 FORWARDING If the **ForwardX11 **variable is set to “yes” (or see the description of the **-X**, **-x**, and **-Y **op‐ tions above) and the user is using X11 (the DISPLAY environment variable is set), the connec‐ tion 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 automat‐ ically 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). Finger‐ prints can be determined using [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown): **$ 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 re‐ jected. If only legacy (MD5) fingerprints for the server are available, the [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown) **-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 com‐ pletely 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 fin‐ gerprints 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 re‐ source 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information. ## SSH-BASED VIRTUAL PRIVATE NETWORKS **ssh **contains support for Virtual Private Network (VPN) tunnelling using the [_tun_(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) network pseudo-device, allowing two networks to be joined securely. The [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown) 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)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) 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 tempo‐ rary setups, such as for wireless VPNs. More permanent VPNs are better provided by tools such as [_ipsecctl_(8)](https://www.chedong.com/phpMan.php/man/ipsecctl/8/markdown) and [_isakmpd_(8)](https://www.chedong.com/phpMan.php/man/isakmpd/8/markdown). ## 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 “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 ex‐ plicitly, as that will render the X11 connection insecure (and will re‐ quire 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 termi‐ nal associated with it but DISPLAY and SSH_ASKPASS are set, it will ex‐ ecute 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)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) to contain the interface names assigned if tunnel forwarding was requested by the client. SSH_USER_AUTH Optionally set by [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown), 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)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown). ## 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 parti‐ tion, because [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) reads it as root. Additionally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permis‐ sion 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 authenti‐ cation without permitting login with rlogin/rsh. _~/.ssh/_ This directory is the default location for all user-specific configuration and au‐ thentication information. There is no general requirement to keep the entire con‐ tents of this directory secret, but the recommended permissions are read/write/exe‐ cute 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)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) 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)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). 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 pos‐ sible 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 al‐ ready in the systemwide list of known host keys. See [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) 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)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) manual page for more informa‐ tion. _/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 au‐ thentication without permitting login with rlogin/rsh. _/etc/ssh/ssh_config_ Systemwide configuration file. The file format and configuration options are de‐ scribed in [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). _/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 ad‐ ministrator to contain the public host keys of all machines in the organization. It should be world-readable. See [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) 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)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) manual page for more informa‐ tion. ## EXIT STATUS **ssh **exits with the exit status of the remote command or with 255 if an error occurred. ## SEE ALSO [_scp_(1)](https://www.chedong.com/phpMan.php/man/scp/1/markdown), [_sftp_(1)](https://www.chedong.com/phpMan.php/man/sftp/1/markdown), [_ssh-add_(1)](https://www.chedong.com/phpMan.php/man/ssh-add/1/markdown), [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown), [_ssh-argv0_(1)](https://www.chedong.com/phpMan.php/man/ssh-argv0/1/markdown), [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown), [_ssh-keyscan_(1)](https://www.chedong.com/phpMan.php/man/ssh-keyscan/1/markdown), [_tun_(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown), [_ssh_config_(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown), [_ssh-keysign_(8)](https://www.chedong.com/phpMan.php/man/ssh-keysign/8/markdown), [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) ## 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. Debian October 11, 2023 [_SSH_(1)](https://www.chedong.com/phpMan.php/man/SSH/1/markdown)