ssh (Secure Shell) 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 to 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.
ssh connects and logs into the specified hostname. The user must prove his or her identity to the remote machine using one of several methods depending on the protocol version used:
First, if the machine the user logs in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote machine, and the user names are the same on both sides, the user is immediately permitted to log in. Second, if .rhosts or .shosts exists in the user's home directory on the remote machine and contains a line containing the name of the client machine and the name of the user on that machine, the user is permitted to log in. This form of authentication alone is normally not allowed by the server because it is not secure.
The second (and primary) authentication method is the rhosts or hosts.equiv method combined with RSA-based host authentication. It means that if the login would be permitted by $HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or /etc/shosts.equiv, and if additionally the server can verify the client's host key (see /etc/ssh_known_hosts in the FILES 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, $HOME/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.
As a third authentication method, ssh 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 $HOME/.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 server checks if this key is permitted, and if so, sends the user (actually the ssh program running on behalf of the user) a challenge in the form of 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 or she knows the private key but without disclosing it to the server.
ssh implements the RSA authentication protocol automatically. The user creates his or her RSA key pair by running ssh-keygen(1). This stores the private key in $HOME/.ssh/identity and the public key in $HOME/.ssh/identity.pub in the user's home directory. The user should then copy the identity.pub to $HOME/.ssh/authorized_keys in his or her home directory on the remote machine (the authorized_keys file corresponds to the conventional $HOME/.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. RSA authentication is much more secure than rhosts authentication.
The most convenient way to use RSA authentication may be with an authentication agent. See ssh-agent(1) for more information.
If other authentication methods fail, ssh prompts the user for a password. The password is sent to the remote host for checking. However, since all communications are encrypted, the password cannot be seen by someone listening on the network.
When a user connects using the protocol version 2, different authentication methods are available. At first, the client attempts to authenticate using the public key method. If this method fails, password authentication is tried.
The public key method is similar to RSA authentication described in the previous section except that the DSA algorithm is used instead of the patented RSA algorithm. The client uses his private DSA key $HOME/.ssh/id_dsa to sign the session identifier and sends the result to the server. The server checks whether the matching public key is listed in $HOME/.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 known only 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 for proving the user's identity. This protocol 2 implementation does not yet support Kerberos or S/Key authentication.
Protocol 2 provides additional mechanisms for confidentiality (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity (hmac-sha1, hmac-md5). Notice that protocol 1 lacks a strong mechanism for ensuring the integrity of the connection.
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 can disconnect with ~., and suspend ssh with ~^Z. All forwarded connections can be listed with ~#. If the session blocks waiting for forwarded X11 or TCP/IP connections to terminate, ssh can be backgrounded with ~&, although this should not be used while the user shell is active, as it can cause the shell to hang. All available escapes can be listed with ~?.
A single tilde character can be sent as ~~ (or by following the tilde by a character other than those described above). The escape character must always follow a newline to be interpreted as special. The escape character can be changed in configuration files or on the command line.
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 “none” will also make the session transparent even if a tty is used.
The session terminates when the command or shell in 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.
If the user is using X11 (the DISPLAY environment variable is set), the connection to the X11 display is automatically forwarded to the remote side in such a way that any X11 programs started from the shell (or command) will go through the encrypted channel, and the connection to the real X server will be made from the local machine. The user should not manually set DISPLAY. Forwarding of X11 connections can be configured on the command line or in configuration files.
The DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero. This is normal behavior, 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 user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side unless disabled on the command line or in a configuration file.
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 possible application is going through firewalls.
ssh automatically maintains and checks a database containing identifications for all hosts it has ever been used with. RSA host keys are stored in $HOME/.ssh/known_hosts in the user's home directory. Additionally, the file /etc/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 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 (see below) can be used to prevent logins to machines whose host key is not known or has changed.
The following options are supported:
Forces ssh to try protocol version 2 only.
Forces ssh to use IPv4 addresses only.
Forces ssh to use IPv6 addresses only.
Disables forwarding of the authentication agent connection.
Enables forwarding of the authentication agent connection. This can also be specified on a per-host basis in a configuration file.
Selects the cipher to use for encrypting the session. 3des is used by default. It is believed to be secure. 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. It is presumably more secure than the des cipher, which is no longer fully supported in ssh. blowfish is a fast block cipher, it appears very secure and is much faster than 3des.
Additionally, for protocol version 2 a comma-separated list of ciphers can be specified in order of preference. Protocol version 2 supports 3DES, Blowfish, and AES 128 in CBC mode.
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). (The gzip man page is available in the SUNWsfman package.) The “level” can be controlled by the CompressionLevel option (see below). 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 Compress option below.
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. If followed by control-Z, the escape character suspends the connection. If followed by itself, the escape character sends itself once. Setting the character to “none” disables any escapes and makes the session fully transparent.
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 the -n option. 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.
Selects the file from which the identity (private key) for RSA authentication is read. Default is $HOME/.ssh/identity in the user's home directory. 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).
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.
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 the port on the local side. Then, 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. Only root can forward privileged ports. IPv6 addresses can be specified with an alternative syntax: port/host/hostport.
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.)
Does not execute a remote command. This is useful if you just want to forward ports (protocol version 2 only).
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. The option has the same format as a line in the configuration file.
Specifies the port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.
Uses a non-privileged port for outgoing connections. This can be used if your firewall does not permit connections from privileged ports. Notice that this option turns off RhostsAuthentication and RhostsRSAAuthentication.
Quiet mode. Causes all warning and diagnostic messages to be suppressed. Only fatal errors are displayed.
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 the port on the remote side. Then, 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.
Forces pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, for example, when implementing menu services.
Disables pseudo-tty allocation (protocol version 2 only).
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. Maximum is 3.
Disables X11 forwarding.
Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
ssh will normally set the following environment variables:
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 greater than or equal to 1. ssh uses this special value to forward X11 connections over the secure channel. The user should normally not set DISPLAY explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies).
Set to the path of the user's home directory.
Synonym for USER. Set for compatibility with systems that use this variable.
Set to point to the user's mailbox.
Set to the default PATH, as specified when compiling ssh.
Indicates the path of a unix-domain socket used to communicate with the agent.
Identifies the client end of the connection. The variable contains three space-separated values: client ip-address, client port number, and server port number.
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, that is, the daemon passes the value on to new connections.
Set to the name of the user logging in.
Additionally, ssh reads $HOME/.ssh/environment and adds lines of the format VARNAME=value to the environment
Records host keys for all hosts the user has logged into that are not in /etc/ssh_known_hosts. See sshd(1M).
Contains the RSA and the DSA authentication identity of the user. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). Notice that ssh 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.
Contains the public key for authentication, that is, the public part of the identity file in human-readable form. The contents of the $HOME/.ssh/identity.pub file should be added to $HOME/.ssh/authorized_keys on all machines where you wish to log in using RSA authentication. The contents of the $HOME/.ssh/id_dsa.pub file should be added to $HOME/.ssh/authorized_keys on all machines where you wish to log in using DSA 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 provided only for the convenience of the user.
This is the per-user configuration file. The format of this file is described above. This file is used by the ssh client. This file does not usually contain any sensitive information, but the recommended permissions are read/write for the user and not accessible by others.
Lists the DSA keys that can be used for logging in as this user. 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. /etc/ssh_known_hosts contains RSA keys. This file should be prepared by the system administrator to contain the public host keys of all machines in the organization and should be world-readable. The file contains public keys, one per line, in the following format, with fields separated by spaces: system name, number of bits in modulus, public exponent, modulus, and optional comment field. When different names are used for the same machine, all such names should be listed, separated by commas. See sshd(1M).
The canonical system name (as returned by name servers) is used by sshd(1M) to verify the client host when logging in. Other names are needed because ssh does not convert the user-supplied name to a canonical name before checking the key, to prevent someone with access to the name servers from being able able to fool host authentication.
Systemwide configuration file. This file provides defaults for those values that are not specified in the user's configuration file, and for those users who do not have a configuration file.
This file must be world-readable.
This file is used in .rhosts authentication to list the host/user pairs that are permitted to log in. (Notice 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 an NFS partition, because sshd(1M) 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.
Notice that, by default, sshd(1M) will be installed so that it requires successful RSA host authentication before permitting .rhosts authentication. If your server machine does not have the client's host key in /etc/ssh_known_hosts, you can store it in $HOME/.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 $HOME/.ssh/known_hosts.
This file is used during .rhosts authentication. It contains canonical hosts names, one per line. (See sshd(1M) for the full format description.). If the client host is found in this file, login is automatically permitted, provided that client and server user names are the same. In addition, successful RSA host authentication is normally 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 ssh but not using rsh or rlogin.
Commands in this file are executed by ssh when the user logs in just before the user's shell or command is started. See sshd(1M) for more information.
Commands in this file are executed by ssh when the user logs in just before the user's shell or command is started. See sshd(1M) for more information.
Contains additional definitions for environment variables. See ENVIRONMENT VARIABLES.
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|
To view license terms, attribution, and copyright for OpenSSH, the default path is /var/sadm/pkg/SUNWsshdr/install/copyright. If the Solaris operating environment has been installed anywhere other than the default, modify the given path to access the file at the installed location.
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, added newer features and created Open SSH. Markus Friedl contributed the support for SSH protocol versions 1.4 and 2.0.