ssh - OpenSSH SSH client (remote login program)
ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-e escape_char] [-F configfile] [-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 tunnel:tunnel] [user@]hostname [command]
ssh (SSH client) 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 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.
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.
The options are as follows:
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 they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.
Protocol version 1 allows specification of a single cipher. The supported values are “3des", “blowfish", and “des". 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. It is believed to be secure. blowfish is a fast block cipher; it appears very secure and is much faster than 3des. des is only supported in the ssh client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged due to cryptographic weaknesses. The default is “3des".
For protocol version 2, cipher_spec is a comma-separated list of ciphers listed in order of preference. The supported ciphers are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blowfish-cbc, and cast128-cbc. The default is:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, aes192-ctr,aes256-ctr
IPv6 addresses can be specified with an alternative syntax: [bind_address/]port or by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may 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.
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. 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 will be bound to the loopback interface only. This may be overriden 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 will only succeed if the server’s GatewayPorts option is enabled (see sshd_config(5) ).
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 may then be able to perform activities such as keystroke monitoring.
For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default. Please refer to the ssh -Y option and the ForwardX11Trusted directive in ssh_config(5) for more information.
ssh may additionally obtain configuration data from a per-user configuration file and a system-wide configuration file. The file format and configuration options are described in ssh_config(5) .
ssh exits with the exit status of the remote command or with 255 if an error occurred.
The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the default, with ssh falling back to protocol 1 if it detects protocol 2 is unsupported. These settings may be altered using the Protocol option in ssh_config(5) , or enforced using the -1 and -2 options (see above). Both protocols support similar authentication methods, but protocol 2 is preferred 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-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the integrity of the connection.
The methods available for authentication are: host-based authentication, public key authentication, challenge-response authentication, and password authentication. 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 either the RSA or DSA algorithms. Protocol 1 is restricted to using only RSA keys, but protocol 2 may use either. The HISTORY section of ssl(8) contains a brief discussion of the two algorithms.
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 for authentication. The client proves that it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.
The user creates his/her key pair by running ssh-keygen(1) . This stores the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 2 DSA), 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), 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.
The most convenient way to use public key authentication may be with an authentication agent. See ssh-agent(1) 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 just one challenge/response. Examples of challenge-response authentication include BSD Authentication (see login.conf(5) ) and PAM (some nonOpenBSD 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 be used to control logins to machines whose host key is not known or has changed.
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 may use the escape characters noted below.
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 on the remote machine exits and all X11 and TCP connections have been closed.
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 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:
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 which 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 which is to be tunnelled. If no connections are made within the time specified, ssh will exit.
If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options above) 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) 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 ForwardAgent variable is set to “yes” (or see the description of the -A and -a options above) and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.
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 has been disabled). Fingerprints can be determined using ssh-keygen(1) :
$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
If the fingerprint is already known, it can be matched and verified, and the key can be accepted. If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. An additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to 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 -f /etc/ssh/ssh_host_rsa_key.pub -r host.example.com. $ ssh-keygen -f /etc/ssh/ssh_host_dsa_key.pub -r host.example.com.
The output lines will 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
Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?
See the VerifyHostKeyDNS option in ssh_config(5) for more information.
ssh contains support for Virtual Private Network (VPN) tunnelling using the tun(4) network pseudo-device, allowing two networks to be joined securely. The sshd_config(5) 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, provided that the SSH server running on the gateway to the remote network, at 192.168.1.15, allows it:
# ssh -f -w 0:1 192.168.1.15 true # ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
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 the first tun(4) device from user “jane” and on the second device 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 tun1” ssh-rsa ... john
Since a 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(8) and isakmpd(8) .
ssh will normally set the following environment variables:
SSH_ORIGINAL_COMMAND This variable contains the original command line if a forced command is executed. It can be used to extract the original arguments.
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(5) .
This file is used for host-based authentication (see above). On some machines this file may need to be world-readable if the user’s home directory is on an NFS partition, because sshd(8) 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.
This file is used in exactly the same way as .rhosts, but allows host-based authentication without permitting login with rlogin/rsh.
Lists the public keys (RSA/DSA) that can be used for logging in as this user. The format of this file is described in the sshd(8) manual page. This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.
This is the per-user configuration file. The file format and configuration options are described in ssh_config(5) . Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not accessible by others.
Contains additional definitions for environment variables; see ENVIRONMENT, above.
Contains the private key for authentication. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). ssh will simply ignore a private key file if it is accessible by others. It is possible to specify a passphrase when generating the key which will be used to encrypt the sensitive part of this file using 3DES.
Contains the public key for authentication. These files are not sensitive and can (but need not) be readable by anyone.
Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys. See sshd(8) for further details of the format of this file.
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(8) manual page for more information.
This file is for host-based authentication (see above). It should only be writable by root.
This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh.
Systemwide configuration file. The file format and configuration options are described in ssh_config(5) .
These three files contain the private parts of the host keys and are used for host-based authentication. If protocol version 1 is used, ssh must be setuid root, since the host key is readable only by root. For protocol version 2, ssh uses ssh-keysign(8) to access the host keys, eliminating the requirement that ssh be setuid root when host-based authentication is used. By default ssh is not setuid root.
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. It should be world-readable. See sshd(8) for further details of the format of this file.
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(8) manual page for more information.
scp(1) , sftp(1) , ssh-add(1) , ssh-agent(1) , ssh-keygen(1) , ssh-keyscan(1) , tun(4) , hosts.equiv(5) , ssh_config(5) , ssh-keysign(8) , sshd(8)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January 2002, work in progress material.
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, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
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