(SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP/IP ports can also be forwarded over the secure channel.
connects and logs into the specified hostname (with optional user name). The user must prove his/her identity to the remote machine using one of several methods depending on the protocol version used.
If command is specified, command is executed on the remote host instead of a login shell.
SSH protocol version 1
The first authentication method is the or method combined with RSA-based host authentication. If the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/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 log in. Additionally, if the server can verify the client's host key (see /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts in the section), only then is login 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.]
As a second authentication method, supports RSA based authentication. The scheme is based on public-key cryptography: there are cryptosystems where encryption and decryption are done using separate keys, and it is not possible to derive the decryption key from the encryption key. RSA is one such system. 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.
The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the program tells the server which key pair it would like to use for authentication. The server checks if this key is permitted, and if so, sends the user (actually the program running on behalf of the user) a challenge, a random number, encrypted by the user's public key. The challenge can only be decrypted using the proper private key. The user's client then decrypts the challenge using the private key, proving that he/she knows the private key but without disclosing it to the server.
implements the RSA authentication protocol automatically. The user creates his/her RSA key pair by running ssh-keygen(1) . This stores the private key in ~/.ssh/identity and stores the public key in ~/.ssh/identity.pub in the user's home directory. The user should then copy the identity.pub 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.
The most convenient way to use RSA authentication may be with an authentication agent. See ssh-agent() for more information.
If other authentication methods fail, 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 protocol version 2
When a user connects using protocol version 2, similar authentication methods are available. Using the default values for PreferredAuthentications , the client will try to authenticate first using the hostbased method; if this method fails, public key authentication is attempted, and finally if this method fails, keyboard-interactive and password authentication are tried.
The public key method is similar to RSA authentication described in the previous section and allows the RSA or DSA algorithm to be used: The client uses his private key, ~/.ssh/id_dsa or ~/.ssh/id_rsa , to sign the session identifier and sends the result to the server. The server checks whether the matching public key is listed in ~/.ssh/authorized_keys and grants access if both the key is found and the signature is correct. The session identifier is derived from a shared Diffie-Hellman value and is only known to the client and the server.
If public key authentication fails or is not available, a password can be sent encrypted to the remote host to prove the user's identity.
Additionally, supports hostbased or challenge response authentication.
Protocol 2 provides additional mechanisms for confidentiality (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour) and integrity (hmac-md5, hmac-sha1, hmac-ripemd160). Note that protocol 1 lacks a strong mechanism for ensuring the integrity of the connection.
Login session and remote execution
When the user's identity has been accepted by the server, the server either executes the given command, or logs into the machine and gives the user a normal shell on the remote machine. All communication with the remote command or shell will be automatically encrypted.
If a pseudo-terminal has been allocated (normal login session), the user may use the escape characters noted below.
If no pseudo-tty has been allocated, the session is transparent and can be used to reliably transfer binary data. On most systems, setting the escape character to 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/IP connections have been closed. The exit status of the remote program is returned as the exit status of ssh .
When a pseudo-terminal has been requested, supports a number of functions through the use of an escape character.
A single tilde character can be sent as ~~ or by following the tilde by a character other than those described below. The escape character must always follow a newline to be interpreted as special. The escape character can be changed in configuration files using the EscapeChar configuration directive or on the command line by the -e option.
The supported escapes (assuming the default are:
- Background ssh .
- List forwarded connections.
- Background at logout when waiting for forwarded connection / X11 sessions to terminate.
- Display a list of escape characters.
- Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer supports it).
- Open command line. Currently this allows the addition of port forwardings using the -L and -R options (see below). It also allows the cancellation of existing remote port-forwardings using -KR hostport . Basic help is available, using the -h option.
- Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it).
X11 and TCP forwarding
If the ForwardX11 variable is set to (or see the description of the -X and -x options described later) 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 will point to the server machine, but with a display number greater than zero. This is normal, and happens because creates a X server on the server machine for forwarding the connections over the encrypted channel.
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 (or see the description of the -A and -a options described later) and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.
Forwarding of arbitrary TCP/IP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP/IP forwarding is a secure connection to an electronic purse; another is going through firewalls.
automatically maintains and checks a database containing identifications 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, warns about this and disables password authentication to prevent a trojan horse from getting the user's password. Another purpose of this mechanism is to prevent man-in-the-middle attacks which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can be used to prevent logins to machines whose host key is not known or has changed.
can be configured to verify host identification using fingerprint resource records (SSHFP) published in DNS. The VerifyHostKeyDNS option can be used to control how DNS lookups are performed. SSHFP resource records can be generated using ssh-keygen(1) .
The options are as follows:
- Forces to try protocol version 1 only.
- Forces to try protocol version 2 only.
- Forces to use IPv4 addresses only.
- Forces to use IPv6 addresses only.
- Enables forwarding of the authentication agent connection. 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.
- Disables forwarding of the authentication agent connection.
- -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.
- Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11 and TCP/IP connections). The compression algorithm is the same used by gzip(1) , and the can be controlled by the CompressionLevel option for protocol version 1. Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option.
- -c cipher_spec
- Selects the cipher specification for encrypting the session. Protocol version 1 allows specification of a single cipher. The suported values are and 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. It is believed to be secure. blowfish is a fast block cipher; it appears very secure and is much faster than 3des . des is only supported in the client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged due to cryptographic weaknesses. The default is For protocol version 2 cipher_spec is a comma-separated list of ciphers listed in order of preference. The supported ciphers are and The default is ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, aes192-ctr,aes256-ctr''
- -D port
- Specifies a local application-level port forwarding. This works by allocating a socket to listen to port on the local side, and 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 will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.
- -e ch | ^ch | none
- Sets the escape character for sessions with a pty (default: The escape character is only recognized at the beginning of a line. The escape character followed by a dot closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once. Setting the character to 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 will be ignored. The default for the per-user configuration file is ~/.ssh/config .
- Requests to go to background just before command execution. This is useful if 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 .
- Allows remote hosts to connect to local forwarded ports.
- -I smartcard_device
- Specifies which smartcard device to use. The argument is the device should use to communicate with a smartcard used for storing the user's private RSA key.
- -i identity_file
- Selects a file from which the identity (private key) for RSA or DSA authentication is read. The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for protocol version 2. 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).
- Disables forwarding (delegation) of GSSAPI credentials to the server.
- -L Xo
- port : host : hostport Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side. 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 a connection is made to host port hostport from the remote machine. Port forwardings can also be specified in the configuration file. IPv6 addresses can be specified with an alternative syntax: [bind_address No /] port No / host No / hostport or 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 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.
- Places the client into mode for connection sharing. Refer to the description of ControlMaster in ssh_config() for details.
- -m mac_spec
- Additionally, for protocol version 2 a comma-separated list of MAC (message authentication code) algorithms can be specified in order of preference. See the MACs keyword for more information.
- Do not execute a remote command. This is useful for just forwarding ports (protocol version 2 only).
- Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when 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 program will be put in the background. (This does not work if 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: (check that the master process is running) and (request the master to exit).
- -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
For full details of the options listed below, and their possible values, see
- -p port
- Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.
- Quiet mode. Causes all warning and diagnostic messages to be suppressed. Only fatal errors are displayed. If a second -q is given then even fatal errors are suppressed.
- -R Xo
- port : host : hostport Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side. This works by allocating a socket to listen to port on the remote side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the local machine. Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the address in square braces or using an alternative syntax: [bind_address No /] host No / port No / hostport By default, the listening socket on the server will be bound to the loopback interface only. This may be overriden 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) ) .
- -S ctl_path
- Specifies the location of a control socket for connection sharing. Refer to the description of ControlPath and ControlMaster in ssh_config() for details.
- May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use of SSH as a secure transport for other applications (eg. sftp(1) ) . The subsystem is specified as the remote command.
- Disable pseudo-tty allocation.
- Force pseudo-tty 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 has no local tty.
- Display the version number and exit.
- Verbose mode. Causes 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.
- 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 -Y option and the ForwardX11Trusted directive in ssh_config() for more information.
- Disables X11 forwarding.
- Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.
will normally set the following environment variables:
- The DISPLAY variable indicates the location of the X11 server. It is automatically set by to point to a value of the form where hostname indicates the host where the shell runs, and n is an integer 1. 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).
- Set to the path of the user's home directory.
- Synonym for USER ; set for compatibility with systems that use this variable.
- Set to the path of the user's mailbox.
- Set to the default PATH , as specified when compiling ssh .
- If needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If 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 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.)
- Identifies the path of a unix-domain socket used to communicate with the agent.
- 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.
- The variable contains the original command line if a forced command is executed. It can be used to extract the original arguments.
- 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.
- The timezone variable is set to indicate the present timezone if it was set when the daemon was started (i.e., the daemon passes the value on to new connections).
- Set to the name of the user logging in.
Additionally, reads ~/.ssh/environment , and adds lines of the format to the environment if the file exists and if users are allowed to change their environment. For more information, see the PermitUserEnvironment option in sshd_config(5) .
- Records host keys for all hosts the user has logged into that are not in /etc/ssh/ssh_known_hosts . See sshd(8) .
- ~/.ssh/identity, ~/.ssh/id_dsa, ~/.ssh/id_rsa
- Contains the authentication identity of the user. They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). Note that ignores a private key file if it is accessible by others. It is possible to specify a passphrase when generating the key; the passphrase will be used to encrypt the sensitive part of this file using 3DES.
- ~/.ssh/identity.pub, ~/.ssh/id_dsa.pub, ~/.ssh/id_rsa.pub
- Contains the public key for authentication (public part of the identity file in human-readable form). The contents of the ~/.ssh/identity.pub file should be added to the file ~/.ssh/authorized_keys on all machines where the user wishes to log in using protocol version 1 RSA authentication. The contents of the ~/.ssh/id_dsa.pub and ~/.ssh/id_rsa.pub file should be added to ~/.ssh/authorized_keys on all machines where the user wishes to log in using protocol version 2 DSA/RSA authentication. These files are not sensitive and can (but need not) be readable by anyone. These files are never used automatically and are not necessary; they are only provided for the convenience of the user.
- 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 accessible by others. It may be group-writable provided that the group in question contains only the user.
- Lists the public keys (RSA/DSA) that can be used for logging in as this user. The format of this file is described in the sshd() manual page. In the simplest form the format is the same as the .pub identity files. This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.
- 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. This file should be world-readable. This file contains public keys, one per line, in the following format (fields separated by spaces): system name, public key and optional comment field. When different names are used for the same machine, all such names should be listed, separated by commas. The format is described in the sshd() manual page. The canonical system name (as returned by name servers) is used by sshd() to verify the client host when logging in; other names are needed because does not convert the user-supplied name to a canonical name before checking the key, because someone with access to the name servers would then be able to fool host authentication.
- 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_rsa_key
- These three files contain the private parts of the host keys and are used for RhostsRSAAuthentication and HostbasedAuthentication . If the protocol version 1 RhostsRSAAuthentication method is used, must be setuid root, since the host key is readable only by root. For protocol version 2, uses ssh-keysign() to access the host keys for HostbasedAuthentication . This eliminates the requirement that be setuid root when that authentication method is used. By default is not setuid root.
- This file is used in RhostsRSAAuthentication and HostbasedAuthentication authentication to list the host/user pairs that are permitted to log in. (Note that this file is also used by rlogin and rsh, which makes using this file insecure.) Each line of the file contains a host name (in the canonical form returned by name servers), and then a user name on that host, separated by a space. On some machines this file may need to be world-readable if the user's home directory is on a NFS partition, because sshd() 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. Note that sshd() allows authentication only in combination with client host key authentication before permitting log in. If the server machine does not have the client's host key in /etc/ssh/ssh_known_hosts , it can be stored in ~/.ssh/known_hosts . The easiest way to do this is to connect back to the client from the server machine using ssh; this will automatically add the host key to ~/.ssh/known_hosts .
- This file is used exactly the same way as .rhosts . The purpose for having this file is to be able to use RhostsRSAAuthentication and HostbasedAuthentication authentication without permitting login with () or rsh(1) .
- This file is used during RhostsRSAAuthentication and HostbasedAuthentication authentication. It contains canonical hosts names, one per line (the full format is described in the sshd() manual page). If the client host is found in this file, login is automatically permitted provided client and server user names are the same. Additionally, successful client host key authentication is required. This file should only be writable by root.
- This file is processed exactly as /etc/hosts.equiv . This file may be useful to permit logins using but not using rsh/rlogin.
- Commands in this file are executed by when the user logs in just before the user's shell (or command) is started. See the sshd() manual page for more information.
- Commands in this file are executed by when the user logs in just before the user's shell (or command) is started. See the sshd() manual page for more information.
- Contains additional definitions for environment variables, see section above.
exits with the exit status of the remote command or with 255 if an error occurred.
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.