ssh - secure shell client (remote login program)
ssh [-l login_name] hostname | user@hostname [ command]
ssh [-afgknqstvxACNTX1246] [-b bind_address] [-m mac_spec] [-c cipher_spec] [-e escape_char] [-i identity_file] [-l login_name] [-F configfile] [-o option] [-p port] [-L [bind_address:]port:host:hostport] [-R [bind_address:]port:host:hostport] [-D [bind_address:]port] hostname | user@hostname [command]
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 can 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.
The SSH version 2 protocol supports multiple user authentication methods, some of which are similar to those available with the SSH protocol version 1. These authentication mechanisms are negotiated by the client and server, with the client trying methods in the order specified in the PreferredAuthentications client configuration option. The server decides when enough authentication methods have passed successfully so as to complete the authentication phase of the protocol.
When a user connects by using protocol version 2, similar authentication methods are available. Using the default values for PreferredAuthentications, the client tries to authenticate first by using the hostbased method. If this method fails, public key authentication is attempted. 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 or her private key, $HOME/.ssh/id_dsa or $HOME/.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 $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 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 for proving the user's identity, or an extended prompt/reply protocol can be engaged.
Additionally, ssh supports hostbased or challenge response authentication.
Protocol 2 provides additional mechanisms for confidentiality (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity (hmac-sha1, hmac-md5). 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 specified 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 is automatically encrypted.
If a pseudo-terminal has been allocated (normal login session), the user can use the escape characters noted below. 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 with 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" also makes 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, ssh supports a number of functions through the use of an escape character.
A single tilde character can be sent as ~~ or by following the tilde with 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:
If the ForwardX11 variable is set to ``yes'' (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) goes through the encrypted channel, and the connection to the real X server is 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 points to the server machine, but with a display number greater than zero. This is normal behavior, because ssh creates a "proxy" X11 server on the server machine for forwarding the connections over the encrypted channel.
ssh also automatically sets up Xauthority data on the server machine. For this purpose, it generates a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain).
If the ForwardAgent variable is set to "yes" (or, see the description of the -A and -a options 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 possible application is firewall traversal.
ssh automatically maintains and checks a database containing identifications for all hosts it has ever been used with. 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. The behavior of ssh with respect to unknown host keys is controlled by the StrictHostKeyChecking parameter. 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 attacks by intermediaries 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.
However, when using key exchange protected by GSS-API, the server can advertise a host key. The client automatically adds this host key to its known hosts file, $HOME/.ssh/known_hosts, regardless of the setting of the StrictHostKeyChecking option, unless the advertised host key collides with an existing known hosts entry.
When the user's GSS-API credentials expire, the client continues to be able to rekey the session using the server's public host key to protect the key exchanges.
ssh uses the user's GSS-API credentials to authenticate the client to the server wherever possible, if GssKeyEx and/or GssAuthentication are set.
With GssKeyEx, one can have an SSHv2 server that has no host public keys, so that only GssKeyEx can be used. With such servers, rekeying fails if the client's credentials are expired.
GSS-API user authentication has the disadvantage that it does not obviate the need for SSH host keys, but its failure does not impact rekeying. ssh can try other authentication methods (such as public key, password, and so on) if GSS-API authentication fails.
Delegation of GSS-API credentials can be quite useful, but is not without danger. As with passwords, users should not delegate GSS credentials to untrusted servers, since a compromised server can use a user's delegated GSS credentials to impersonate the user.
GSS-API user authorization is covered in gss_auth_rules(5).
Rekeying can be used to redelegate credentials when GssKeyEx is "yes". (See ~R under Escape Characters above.)
The following options are supported:
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, the attacker can perform operations on the keys that enable the attacker to authenticate using the identities loaded into the agent.
-c blowfish | 3des | des
IPv6 addresses can be specified with an alternative syntax: [bind_address/]port or by enclosing the address in square brackets. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address can be used to bind the connection to a specific address. The bind_address of localhost indicates that the listening port be bound for local use only, while an empty address or * indicates that the port should be available from all interfaces.
-e ch | ^ch | none
By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address can be used to bind the connection to a specific address. The bind_address of localhost indicates that the listening port be bound for local use only, while an empty address or * indicates that the port should be available from all interfaces.
ssh -n shadows.cs.hut.fi emacs &
starts an emacs on shadows.cs.hut.fi, and the X11 connection is automatically forwarded over an encrypted channel. The ssh program is put in the background. This does not work if ssh needs to ask for a password or passphrase. See also the -f option.
IPv6 addresses can be specified by enclosing the address in square braces or using an alternative syntax: [bind_address/]host/port/hostport.
By default, the listening socket on the server is bound to the loopback interface only. This can be overridden by specifying a bind_address. An empty bind_address, or the address *, indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address only succeeds if the server's GatewayPorts option is enabled. See sshd_config(4).
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 can then be able to perform activities such as keystroke monitoring.
For this reason, X11 forwarding might be subjected to X11 SECURITY extension restrictions. Refer to the ForwardX11Trusted directive in ssh_config(4) for more information.
If X11 forwarding is enabled, remote X11 clients is trusted by default. This means that they have full access to the original X11 display.
ssh normally sets the following environment variables:
LANG, LC_ALL, LC_CoLLATE, LC_CTYPE,
LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME
See the ENVIRONMENT VARIABLES section in the sshd(1M) man page for more information on how locale setting can be further changed depending on server side configuration.
The status of the remote program is returned as the exit status of ssh. 255 is returned if an error occurred at anytime during the ssh connection, including the initial key exchange.
See attributes(5) for descriptions of the following attributes:
The command line syntax is Committed. The remote locale selection through passing LC_* environment variables is Uncommitted.
rlogin(1), rsh(1), scp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-http-proxy-connect(1), ssh-socks5-proxy-connect(1), telnet(1), sshd(1M), ssh_config(4), sshd_config(4), attributes(5), gss_auth_rules(5), kerberos(5), privileges(5)
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Created 1996-2022 by Maxim Chirkov
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