SSH

NAME

ssh - secure shell client (remote login program)

SYNOPSIS

ssh [-l login_name] hostname [command]

ssh [-a] [-c idea|blowfish|des|3des|arcfour|none] [-e escape_char] [-i identity_file] [-l login_name] [-n] [-k] [-V] [-o option] [-p port] [-q] [-P] [-t] [-v] [-x] [-C] [-g] [-L port:host:hostport] [-R port:host:hostport] hostname [command]

DESCRIPTION

Ssh (Secure Shell) a program for logging into a remote machine and for executing commands in 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.

Ssh connects and logs into the specified hostname. The user must prove his/her identity to the remote machine using one of several methods.

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 . rhosts, . shosts, /etc/hosts.equiv, or /etc/shosts.equiv, and additionally it can verify the client’s host key (see $HOME/ . ssh/known_hosts and /etc/ssh_known_hosts in the FILES section), only then login is 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 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, 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.

Ssh 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 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. 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.

As a fourth authentication method, ssh supports authentication through TIS authentication server. The idea is that ssh asks TIS authsrv(8) to authenticate the user. Sometime, usernames in the TIS database cannot be the same as the local users. This can be the case if the user authenticates itself with a smartcard or a Digipass. In that case, the username in the database is usually known as the serial number of the authentification device. The file /etc/sshd_tis.map contains the mapping between local users and their corresponding name in the TIS database. If the file does not exist or the user is not found, the corresponding name in the TIS database is supposed to be the same.

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 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 "~#", and if the session blocks waiting for forwarded X11 or TCP/IP connections to terminate, it can be backgrounded with "~&" (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 on the remote machine exists 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, 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 user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side unless disabled on command line or in a configuration file.

Forwarding of arbitrary TCP/IP connections over the secure channel can be specified either on 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 trough firewalls.

Ssh automatically maintains and checks a database containing RSA-based identifications for all hosts it has ever been used with. The database is stored in . 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.

OPTIONS

-c idea|des|3des|blowfish|arcfour|none

Selects the cipher to use for encrypting the session. idea is used by default. It is believed to be secure. des is the data encryption standard, but is breakable by governments, large corporations, and major criminal organizations. 3des (triple-des) is encrypt-decrypt-encrypt triple with three different keys. It is presumably more secure than DES. It is used as default if both sites do not support IDEA. blowfish is an encryption algorithm invented by Bruce Schneier. It uses 128 bit keys. arcfour is an algorithm published in the Usenet News in 1995. This algorithm is believed to be equivalent with the RC4 cipher from RSA Data Security (RC4 is a trademark of RSA Data Security). This is the fastest algorithm currently supported. none disables encryption entirely; it is only intended for debugging, and it renders the connection insecure.

-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 ´none´ disables any escapes and makes the session fully transparent.

-i identity_file

Selects the file from which the identity (private key) for RSA authentication is read. Default is . 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).

-l login_name

Specifies the user to log in as on the remote machine. This may also be specified on a per-host basis in the configuration file.

-o ’option’

Can be used to give options in the format used in the config 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.

-p port

Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.

-L 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, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host:hostport from the remote machine. Port forwardings can also be specified in the configuration file. Only root can forward privileged ports.

-R 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: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.

CONFIGURATION FILES

Ssh obtains configuration data from the following sources (in this order): command line options, user’s configuration file ($HOME/ . ssh/config), and system-wide configuration file (/etc/ssh_config). For each parameter, the first obtained value will be used. The configuration files contain sections bracketed by "Host" specifications, and that section is only applied for hosts that match one of the patterns given in the specification. The matched host name is the one given on the command line.

Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end.

The configuration file has the following format:

Empty lines and lines starting with ´#´ are comments.

Otherwise a line is of the format "keyword arguments" or "keyword = arguments". The possible keywords and their meanings are as follows (note that the configuration files are case-sensitive, but keywords are case-insensitive):

BatchMode

If set to "yes", passphrase/password querying will be disabled. This option is useful in scripts and other batch jobs where you have no user to supply the password. The argument must be "yes" or "no".

ClearAllForwardings

Clears all forwardings after reading all config files and parsing command line. This is usefull to disable forwardings in config file when you want to make second connection to host having forwardings in config file. Scp sets this on by default so it will not fail even if you have some forwardings set in config file.

Compression

Specifies whether to use compression. The argument must be "yes" or "no".

CompressionLevel

Specifies the compression level to use if compression is enable. The argument must be an integer from 1 (fast) to 9 (slow, best). The default level is 6, which is good for most applications. The meaning of the values is the same as in GNU GZIP.

ConnectionAttempts

Specifies the number of tries (one per second) to make before falling back to rsh or exiting. The argument must be an integer. This may be useful in scripts if the connection sometimes fails.

EscapeChar

Sets the escape character (default: ~). The escape character can also be set on the command line. The argument should be a single character, ´^´ followed by a letter, or ’’none’’ to disable the escape character entirely (making the connection transparent for binary data).

FallBackToRsh

Specifies that if connecting via ssh fails due to a connection refused error (there is no sshd listening on the remote host), rsh should automatically be used instead (after a suitable warning about the session being unencrypted). The argument must be "yes" or "no".

ForwardAgent

Specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. The argument must be "yes" or "no".

ForwardX11

Specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be "yes" or "no".

GatewayPorts

Specifies that also remote hosts may connect to locally forwarded ports. The argument must be "yes" or "no".

GlobalKnownHostsFile

Specifies a file to use instead of /etc/ssh_known_hosts.

HostName

Specifies the real host name to log into. This can be used to specify nicnames or abbreviations for hosts. Default is the name given on the command line. Numeric IP addresses are also permitted (both on the command line and in HostName specifications).

IdentityFile

Specifies the file from which the user’s RSA authentication identity is read (default . ssh/identity in the user’s home directory). Additionally, any identities represented by the authentication agent will be used for authentication. The file name may use the tilde syntax to refer to a user’s home directory. It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence.

KeepAlive

Specifies whether the system should send keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections will die if the route is down temporarily, and some people find it annoying.

The default is "yes" (to send keepalives), and the client will notice if the network goes down or the remote host dies. This is important in scripts, and many users want it too.

To disable keepalives, the value should be set to "no" in both the server and the client configuration files.

KerberosAuthentication

Specifies whether Kerberos V5 authentication will be used.

KerberosTgtPassing

Specifies whether a Kerberos V5 TGT will be forwarded to the server.

LocalForward

Specifies that a TCP/IP port on the local machine be forwarded over the secure channel to given host:port from the remote machine. The first argument must be a port number, and the second must be host:port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the root can forward privileged ports.

NumberOfPasswordPrompts

Specifies number of password prompts before giving up. The argument to must be integer. Note that server also limits number of attempts (currently 5), so setting this larger doesn’t have any effect. Default value is one.

PasswordAuthentication

Specifies whether to use password authentication. The argument to this keyword must be "yes" or "no".

PasswordPromptHost

Specifies whether to include the remote host name in the password prompt. The argument to this keyword must be "yes" or "no".

PasswordPromptLogin

Specifies whether to include the remote login name in the password prompt. The argument to this keyword must be "yes" or "no".

ProxyCommand

Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed with /bin/sh. In the command string, %h will be substituted by the host name to connect and %p by the port. The command can be basically anything, and should read from its stdin and write to its stdout. It should eventually connect an sshd server running on some machine, or execute "sshd -i" somewhere. Host key management will be done using the HostName of the host being connected (defaulting to the name typed by the user).

Note that ssh can also be configured to support the SOCKS system using the --with-socks4 or --with-socks5 compile-time configuration option.

RemoteForward

Specifies that a TCP/IP port on the remote machine be forwarded over the secure channel to given host:port from the local machine. The first argument must be a port number, and the second must be host:port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the root can forward privileged ports.

RhostsAuthentication

Specifies whether to try rhosts based authentication. Note that this declaration only affects the client side and has no effect whatsoever on security. Disabling rhosts authentication may reduce authentication time on slow connections when rhosts authentication is not used. Most servers do not permit RhostsAuthentication because it is not secure (see RhostsRSAAuthentication). The argument to this keyword must be "yes" or "no".

RhostsRSAAuthentication

Specifies whether to try rhosts based authentication with RSA host authentication. This is the primary authentication method for most sites. The argument must be "yes" or "no".

RSAAuthentication

Specifies whether to try RSA authentication. The argument to this keyword must be "yes" or "no". RSA authentication will only be attempted if the identity file exists, or an authentication agent is running.

StrictHostKeyChecking

If this flag is set to "yes", ssh ssh will never automatically add host keys to the $HOME/.ssh/known_hosts file, and refuses to connect hosts whose host key has changed. This provides maximum protection against trojan horse attacks. However, it can be somewhat annoying if you don’t have good /etc/ssh_known_hosts files installed and frequently connect new hosts. Basically this option forces the user to manually add any new hosts. Normally this option is set to "ask", and new hosts will automatically be added to the known host files after you have confirmed you really want to do that. If this is set to "no" then new host will automatically be added to the known host files. The host keys of known hosts will be verified automatically in either case.

The argument must be "yes", "no" or "ask".

TISAuthentication

Specifies whether to try TIS authentication. The argument to this keyword must be "yes" or "no".

UsePriviledgedPort

Specifies whether to use priviledged port when connecting to other end. The default is yes if rhosts or rsarhosts authentications are enabled.

UserKnownHostsFile

Specifies a file to use instead of $HOME/ . ssh/known_hosts.

XAuthLocation

Specifies the path to xauth program.

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 explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies).

LOGNAME

Synonym for USER; set for compatibility with systems that use this variable.

SSH_AUTH_SOCK

if exists, is used to indicate the path of a unix-domain socket used to communicate with the authentication agent (or its local representative).

SSH_CLIENT

Identifies the client end of the connection. The variable contains three space-separated values: client ip-address, client port number, and server port number.

SSH_ORIGINAL_COMMAND

This will be the original command line of given by protocol if forced command is run. It can be used to fetch arguments etc from the other end.

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.

Additionally, ssh reads /etc/environment and $HOME/.ssh/environment, and adds lines of the format VARNAME=value to the environment. Some systems may have still additional mechanisms for setting up the environment, such as /etc/default/login on Solaris.

FILES

$HOME/ . ssh/known_hosts

Records host keys for all hosts the user has logged into (that are not in /etc/ssh_known_hosts). See sshd manual page.

$HOME/ . ssh/random_seed

Used for seeding the random number generator. This file contains sensitive data and should read/write for the user and not accessible for others. This file is created the first time the program is run and updated automatically. The user should never need to read or modify this file.

$HOME/ . ssh/identity

Contains the RSA authentication identity of the user. This file contains sensitive data and should be readable by the user but not 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 IDEA .

$HOME/ . ssh/identity.pub

Contains the public key for authentication (public part of the identity file in human-readable form). The contents of this file should be added to $HOME/ . ssh/authorized_keys on all machines where you wish to log in using RSA authentication. This file is not sensitive and can (but need not) be readable by anyone. This file is never used automatically and is not necessary; it is only provided for the convenience of the user.

$HOME/ . ssh/config

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.

$HOME/ . ssh/authorized_keys

Lists the RSA keys 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 (that is, each line contains the number of bits in modulus, public exponent, modulus, and comment fields, separated by spaces). This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.

/etc/ssh_known_hosts

Systemwide list of known host keys. This file should be prepared by the system administrator to contain the public host keys of all machines in the organization. This file should be world-readable. This file contains public keys, one per line, in the following format (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. The format is described on 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 ssh 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.

/etc/ssh_config

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.

$HOME/ . rhosts

This file is used in . rhosts 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. This file must be owned by the user, and must not have write permissions for anyone else. The recommended permission is read/write for the user, and not accessible by others.

Note that by default sshd 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 in $HOME/ . ssh/known_hosts.

$HOME/ . shosts

This file is used exactly the same way as . rhosts. The purpose for having this file is to be able to use rhosts authentication with ssh without permitting login with rlogin or rsh.

/etc/hosts.equiv

This file is used during . rhosts authentication. It contains canonical hosts names, one per line (the full format is described on 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 RSA host authentication is normally required. This file should only be writable by root.

/etc/shosts.equiv

This file is processed exactly as /etc/hosts.equiv. This file may be useful to permit logins using ssh but not using rsh/rlogin.

/etc/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 manual page for more information.

$HOME/.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 manual page for more information.

INSTALLATION

Ssh is normally installed as suid root. It needs root privileges only for rhosts authentication (rhosts authentication requires that the connection must come from a privileged port, and allocating such a port requires root privileges). It also needs to be able to read /etc/ssh_host_key to perform RSA host authentication. It is possible to use ssh without root privileges, but rhosts authentication will then be disabled. Ssh drops any extra privileges immediately after the connection to the remote host has been made.

Considerable work has been put into making ssh secure. However, if you find a security problem, please report it immediately to <ssh-bugs@cs.hut.fi>.

AUTHOR

Tatu Ylonen <ylo@ssh.fi>

Information about new releases, mailing lists, and other related issues can be found from the ssh WWW home page at http://www.cs.hut.fi/ssh.

SEE ALSO

sshd(8), ssh-keygen(1), ssh-agent(1), ssh-add(1), scp(1), make-ssh-known-hosts(1), rlogin(1), rsh(1), telnet(1)