SSH(1)

NAME
ssh - secure shell client (remote login program)

SYNOPSIS
ssh [-l login_name] hostname [command] ssh [-a] [-c idea|des|3des|rc4|tss|none] [-e escape_char] [-i identity_file] [-l login_name] [-n] [-p port] [-t] [-v] [-x] [-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 from is listed in /etc/hosts.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 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 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 or /etc/hosts.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 idea 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 pairs for authentication purposes. The server knows the public key, and only the user knows the private key. The server has a list of 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 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. 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". 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. 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. 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. 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.

OPTIONS
-a Disables forwarding of the authentication agent connection. This may also be specified on a per-host basis in the configuration file. -c idea | des | 3des | rc4 | tss | 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 may be breakable by major governments, large corporations, and major criminal organizations. 3DES (triple-des) is encrypt-decrypt-encrypt triple with two different keys. It is presumably more secure than DES. RC4 is the (alleged) RC4 algorithm posted in the Usenet News in 1995 (RC4 was originally a trade secret algorithm of RSA Data Security and has not been published; however, this implementation is widely believed to be equivalent). This is the fastest algorithm currently supported. TSS is a fast home-grown algorithm based on MD5. The algorithm may also be selected on a per-host basis 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. -f Requests ssh to go to background after authentication. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n. -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. -n Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when ssh is run in the background. A common trick is to use this to run X11 programs in a remote machine. For example, "ssh -n shadows.cs.hut.fi emacs &" will start an emacs in 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. -p port Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file. -t Force pseudo-tty allocation. This can be used to execute arbitary screen-based programs on a remote machine, which can be very useful e.g. when implementing menu services. -v Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. -x Disables X11 forwarding. This can also be specified on a per-host basis in a 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. Only root can forward privileged ports.

CONFIGURATION FILES
Ssh obtains configuration data from the following sources (in this order): command line options, user's configuration file (.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. 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". The possible keywords and their meanings are as follows (note that the configuration files are case-sensitive): Host Restricts the following declarations (up to the next Host keyword) to be only for those hosts that match one of the patterns given after the keyword. '*' and '?' can be as wildcards in the patterns. A single '*' as a pattern can be used to provide global defaults for all hosts. The host is the hostname argument given on the command line. UseRsh Specifies that rlogin/rsh should be used for this host. It is possible that the host does not at all support the ssh protocol. This causes ssh to exec rsh. All other options (except HostName ) are ignored if this has been specified. 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). 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). Port Specifies the port number to connect on the remote host. Default is 22. User Specifies the user to log in as. This can be useful if you have a different user name in different machines. This saves the trouble of having to remember to give the user name on the command line. 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''. Cipher Specifies the cipher to use for encrypting the session. Currently, idea, des, 3des, tss, and none are supported. IDEA is the default. 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 support 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. The argument must be "yes" or "no". PasswordAuthentication Specifies whether to use password authentication. The argument to this keyword must be "yes" or "no". RSAAuthentication Specifies whether to use 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. 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. 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". 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. 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.

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.

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. This file is usually not very sensitive and can be readable by anyone (it may need to be if the user's home directory is on a NFS partition and sshd reads it as root).

$HOME/.ssh/random_seed Used for seeding the random number generator. This file contains sensitive data and should not be readable by anyone but the user. 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 not be readable by anyone but the user. 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.

$HOME/.ssh/authorized_keys Lists the RSA keys that can be used for logging in as this user. This file may beed to be world-readable if the user's home directory is on an NFS module (the file must be readable by root). 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).

/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 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. Note that by default sshd will be compiled 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. 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 must be readable by root.

AUTHOR
Tatu Ylonen <ylo@cs.hut.fi>

SEE ALSO
sshd(8), ssh-keygen(1), ssh-agent(1), ssh-add(1), scp(1), rlogin(1), rsh(1), telnet(1)