On Unix-like operating systems, the slogin command is an alias for the ssh client, which is used to connect securely to a remote shell.

Description

ssh (the SSH client) is a program for logging into a remote machine and executing commands. 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 ports can also be forwarded over the secure channel.

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ssh 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 (see below).

If command is specified, it is executed on the remote host instead of a login shell.

This particular documentation refers heavily to the OpenSSH implementation of SSH. Other implementations are available; if you have one of those versions installed, consult your manual for unique options and function details.

Syntax

ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file] [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] [user@]hostname [command]

Options

Authentication

The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to use protocol 2 only, though this can be changed via the Protocol option in ssh_config or the -1 and -2 options (see above). Both protocols support similar authentication methods, but protocol 2 is the default since it provides additional mechanisms for confidentiality (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, hmac-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the integrity of the connection.

The methods available for authentication are: GSSAPI-based, host-based, public key, challenge-response, and password. Authentication methods are tried in the order specified above, though protocol 2 has a configuration option to change the default order: PreferredAuthentications.

Host-based authentication works as follows: 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 login. Additionally, the server must be able to verify the client’s host key (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be 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.

Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosystems where encryption and decryption are done using separate keys, and it is unfeasible to derive the decryption key from the encryption key. 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. ssh implements public key authentication protocol automatically, using one of the DSA, ECDSA or RSA algorithms. Protocol 1 is restricted to using only RSA keys, but protocol 2 may use any.

The file ~/.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 checks that the corresponding public key is authorized to accept the account.

The user creates his/her key pair by running ssh-keygen. This stores the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user’s home directory. The user should then copy the public key 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.

A variation on public key authentication is available in the form of certificate authentication: instead of a set of public/private keys, signed certificates are used. This has the advantage that a single trusted certification authority can be used in place of many public/private keys. See the CERTIFICATES section of ssh-keygen for more information.

The most convenient way to use public key or certificate authentication may be with an authentication agent. See ssh-agent for more information.

Challenge-response authentication works as follows: The server sends an arbitrary “challenge” text, and prompts for a response. Protocol 2 allows multiple challenges and responses; protocol 1 is restricted to only one challenge/response. Examples of challenge-response authentication include BSD Authentication (see login.conf) and PAM (some non-OpenBSD systems).

Finally, 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.

ssh automatically maintains and checks a database containing identification 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, ssh warns about this and disables password authentication to prevent server spoofing or man-in-the-middle attacks, which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can control logins to machines whose host key is not known or has changed.

When the user’s identity is 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 was allocated (normal login session), the user may use the escape characters noted below.

If no pseudo-tty was allocated, the session is transparent and can 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 connections are closed.

Escape Characters

When a pseudo-terminal is requested, ssh supports some 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:

TCP Forwarding

Forwarding of arbitrary TCP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP forwarding is a secure connection to a mail server; another is going through firewalls.

In the example below, we look at encrypting communication between an IRC client and server, even though the IRC server does not directly support encrypted communications. This works as follows: the user connects to the remote host using ssh, specifying a port to be used to forward connections to the remote server. After that it is possible to start the service that is to be encrypted on the client machine, connecting to the same local port, and ssh will encrypt and forward the connection.

The following example tunnels an IRC session from client machine “127.0.0.1” (localhost) to remote server “server.example.com”:

ssh -f -L 1234:localhost:6667 server.example.com sleep 10

irc -c ‘#users’ -p 1234 pinky 127.0.0.1

This tunnels a connection to IRC server “server.example.com”, joining channel “#users”, nickname “pinky”, using port 1234. It doesn’t matter which port is used, as long as it’s greater than 1023 (remember, only root can open sockets on privileged ports) and doesn’t conflict with any ports already in use. The connection is forwarded to port 6667 on the remote server since that’s the standard port for IRC services.

The -f option backgrounds ssh and the remote command “sleep 10” is specified to allow an amount of time (10 seconds, in the example) to start the service that is to be tunnelled. If no connections are made in the time specified, ssh will exit.

X11 Forwarding

If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options above) and the user uses 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 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 points 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 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 above) and the user uses an authentication agent, the connection to the agent is automatically forwarded to the remote side.

Verifying Host Keys

When connecting to a server for the first time, a fingerprint of the server’s public key is presented to the user (unless the option StrictHostKeyChecking is disabled). Fingerprints can be determined using ssh-keygen:

ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

If the fingerprint is already known, it can be matched and the key can be accepted or rejected. Because of the difficulty of comparing host keys only by looking at hex strings, there is also support to compare host keys visually, using random art. By setting the VisualHostKey option to “yes”, a small ASCII graphic gets displayed on eresource recordvery login to a server, no matter if the session is interactive or not. By learning the pattern a known server produces, a user can easily find out that the host key has changed when a completely different pattern is displayed. Because these patterns are not unambiguous however, a pattern that looks similar to the pattern remembered only gives a good probability that the host key is the same, not guaranteed proof.

To get a listing of the fingerprints with their random art for all known hosts, the following command line can be used:

ssh-keygen -lv -f ~/.ssh/known_hosts

If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. An additional RR (resource record), SSHFP, is added to a zone file and the connecting client can match the fingerprint with that of the key presented.

In this example, we are connecting a client to a server, “host.example.com”. The SSHFP resource records should first be added to the zonefile for host.example.com:

ssh-keygen -r host.example.com.

The output lines have to be added to the zonefile. To check that the zone is answering fingerprint queries:

dig -t SSHFP host.example.com

Finally the client connects:

ssh -o “VerifyHostKeyDNS ask” host.example.com

and outputs:

[…] Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?

See the VerifyHostKeyDNS option in ssh_config for more information.

SSH-based Virtual Private Networks

ssh contains support for Virtual Private Network (VPN) tunnelling using the tun network pseudo-device, allowing two networks to be joined securely. The sshd_config configuration option PermitTunnel controls whether the server supports this, and at what level (layer 2 or 3 traffic).

The following example would connect client network 10.0.50.0/24 with remote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway to the remote network, at 192.168.1.15, allows it.

On the client:

ssh -f -w 0:1 192.168.1.15 true

ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252

route add 10.0.99.0/24 10.1.1.2

On the server:

ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252

route add 10.0.50.0/24 10.1.1.1

Client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the PermitRootLogin server option. The following entry would permit connections on tun device 1 from user “jane” and on tun device 2 from user “john”, if PermitRootLogin is set to “forced-commands-only”:

tunnel=“1”,command=“sh /etc/netstart tun1” ssh-rsa … jane

tunnel=“2”,command=“sh /etc/netstart tun2” ssh-rsa … john

Since an SSH-based setup entails a fair amount of overhead, it may be more suited to temporary setups, such as for wireless VPNs. More permanent VPNs are better provided by tools such as ipsecctl and isakmpd.

Environment

ssh will normally set the following environment variables:

Additionally, ssh reads ~/.ssh/environment, and adds lines of the format “VARNAME=value” to the environment if the file exists and users are allowed to change their environment. For more information, see the PermitUserEnvironment option in sshd_config.

Files

Examples

slogin shell.computerhope.com

The above example would initiate a secure connection to shell.computerhope.com. Below is an example of what would you might see during the login:

The authenticity of host ‘shell.computerhope.com (204.228.150.3)’ can’t be established. DSA key fingerprint is 58:1f:6d:32:8d:1e:2d:5c:8f:00:f7:14:02:f0:c5:cb. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added ‘shell.computerhope.com,204.228.150.3’ (DSA) to the list of known hosts. Username: hopeuser Password: Linux computerhope 2.4.30-grsec #4 SMP Mon Jun 13 19:38:13 MDT 2005 i686 GNU/Linux Welcome to the computerhope shell server. …

The server provides you with a DSA fingerprint key, verifies that you want to connect, and if verified, adds the address to its list of known hosts. It then prompts for username and password.

scp — Copy files securely over a network connection.sftp — Conduct an interactive FTP session over a secure network connection.