SSH(1) BSD General Commands Manual SSH(1)
NAME
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ssh − OpenSSH SSH client (remote login program) |
SYNOPSIS
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ssh [−1246AaCfgKkMNnqsTtVvXxYy] [−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 local_tun[:remote_tun]] [ user@]hostname [command]
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DESCRIPTION |
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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: |
−1’ Forces ssh to try protocol version 1 only.
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−2’ Forces ssh to try protocol version 2 only. −4’ Forces ssh to use IPv4 addresses only. −6’ Forces ssh to use IPv6 addresses only. −A’ Enables forwarding of the authentication agent connection. This can also be specified on a per-host basis in a configuration file. 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. −a’ Disables forwarding of the authentication agent connection. −b bind_address −C’ Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11 and TCP connections). The compression algorithm is the same used by gzip(1), and the ‘‘level’’ can be controlled by the CompressionLevel option for protocol version 1. Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option. −c cipher_spec 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. See the Ciphers keyword for more information. −D IPv6 addresses can be specified with an alternative
syntax: −e escape_char −F configfile −f’ Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies −n. The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm. If the ExitOnForwardFailure configuration option is set to ‘‘yes’’, then a client started with −f will wait for all remote port forwards to be successfully established before placing itself in the background. −g’ Allows remote hosts to connect to local forwarded ports. −I smartcard_device −i identity_file −K’ Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server. −k’ Disables forwarding (delegation) of GSSAPI credentials to the server. −L −l login_name −M’ Places the ssh client into ‘‘master’’ mode for connection sharing. Multiple −M options places ssh into ‘‘master’’ mode with confirmation required before slave connections are accepted. Refer to the description of ControlMaster in ssh_config(5) for details. −m mac_spec −N’ Do not execute a remote command. This is useful for just forwarding ports (protocol version 2 only). −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 on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on 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. (This does not work if ssh needs to ask for a password or passphrase; see also the −f option.) −O ctl_cmd −o option AddressFamily −p port −q’ Quiet mode. Causes most warning and diagnostic messages to be suppressed. −R 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: By default, the listening socket on the server will be bound to the loopback interface only. This may 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 will only succeed if the server’s GatewayPorts option is enabled (see sshd_config(5)). If the port argument is ’0’, the listen port will be dynamically allocated on the server and reported to the client at run time. −S ctl_path −s’ May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use of SSH as a secure transport for other applications (eg. sftp(1)). The subsystem is specified as the remote command. −T’ Disable pseudo-tty allocation. −t’ Force pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple −t options force tty allocation, even if ssh has no local tty. −V’ Display the version number and exit. −v’ Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple −v options increase the verbosity. The maximum is 3. −w The devices may be specified by numerical ID or the keyword ‘‘any’’, which uses the next available tunnel device. If remote_tun is not specified, it defaults to ‘‘any’’. See also the Tunnel and TunnelDevice directives in ssh_config(5). If the Tunnel directive is unset, it is set to the default tunnel mode, which is ‘‘point-to-point’’. −X’ Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file. 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. −x’ Disables X11 forwarding. −Y’ Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls. −y’ Send log information using the syslog(3) system module. By default this information is sent to stderr. 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. AUTHENTICATION |
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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, 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 authentication, 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 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 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. |
ESCAPE CHARACTERS
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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: |
~.’ Disconnect.
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~^Z’ Background ssh. ~#’ List forwarded connections. ~&’ Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate. ~?’ Display a list of escape characters. ~B’ Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer supports it). ~C’ Open command line. Currently this allows
the addition of port forwardings using the −L,
−R and −D options (see above). It
also allows the cancellation of existing remote
port-forwardings using −KR[ ~R’ Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it). TCP FORWARDING |
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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. |
X11 FORWARDING
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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. |
VERIFYING HOST KEYS
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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): |
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$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key |
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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 just 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 every login to a server, no matter if the session itself 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 along with their random art for all known hosts, the following command line can be used: |
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$ ssh-keygen -lv -f ~/.ssh/known_hosts |
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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 -r host.example.com. The output lines will have to be added to the zonefile. To check that the zone is answering fingerprint queries: |
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$ dig -t SSHFP host.example.com |
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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-BASED VIRTUAL PRIVATE NETWORKS
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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 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(4) 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(8) and isakmpd(8). |
ENVIRONMENT
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ssh will normally set the following environment variables: |
DISPLAY’ The DISPLAY variable indicates thelocation of the X11 server. It is automaticallyset by ssh to point to a value of the form‘‘hostname:n’’, where ‘‘hostname’’ indicates thehost where the shell runs, and ’n’ is an integer ≥1. ssh uses this special value to forward X11connections over the secure channel. The usershould normally not set DISPLAY explicitly, as thatwill render the X11 connection insecure (and willrequire the user to manually copy any requiredauthorization cookies).
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HOME’ Set to the path of the user’s home directory. LOGNAME’ Synonym for USER; set for compatibility with systems that use this variable. MAIL’ Set to the path of the user’s mailbox. PATH’ Set to the default PATH, as specified when compiling ssh. SSH_ASKPASS’ If ssh needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If ssh does not have a terminal associated with it but DISPLAY and SSH_ASKPASS are set, it will execute the program specified by SSH_ASKPASS and open an X11 window to read the passphrase. This is particularly useful when calling ssh from a .xsession or related script. (Note that on some machines it may be necessary to redirect the input from /dev/null to make this work.) SSH_AUTH_SOCK’ Identifies the path of a UNIX-domain socket used to communicate with the agent. SSH_CONNECTION’ Identifies the client and server ends of the connection. The variable contains four space-separated values: client IP address, client port number, server IP address, and server port number. SSH_ORIGINAL_COMMAND 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. TZ’ This variable is set to indicate the present time zone if it was set when the daemon was started (i.e. the daemon passes the value on to new connections). USER’ Set to the name of the user logging in. 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). ENVIRONMENT |
SSH_USE_STRONG_RNG
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The reseeding of the OpenSSL random generator is usually done from /dev/urandom. If the SSH_USE_STRONG_RNG environment variable is set to value other than 0 the OpenSSL random generator is reseeded from /dev/random. The number of bytes read is defined by the SSH_USE_STRONG_RNG value. Minimum is 6 bytes. This setting is not recommended on the computers without the hardware random generator because insufficient entropy causes the connection to be blocked until enough entropy is available. FILES |
~/.rhosts
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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. ~/.shosts ~/.ssh/ ~/.ssh/authorized_keys ~/.ssh/config ~/.ssh/environment ~/.ssh/identity ~/.ssh/identity.pub ~/.ssh/known_hosts ~/.ssh/rc /etc/hosts.equiv /etc/ssh/shosts.equiv /etc/ssh/ssh_config /etc/ssh/ssh_host_key /etc/ssh/ssh_known_hosts /etc/ssh/sshrc IPV6 |
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IPv6 address can be used everywhere where IPv4 address. In all entries must be the IPv6 address enclosed in square brackets. Note: The square brackets are metacharacters for the shell and must be escaped in shell. |
SEE ALSO
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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) |
The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, 2006.
Generic Message Exchange Authentication for the Secure Shell Protocol(SSH), RFC 4256, 2006.
The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
Improved Arcfour Modes for the Secure Shell (SSH) Transport LayerProtocol, RFC 4345, 2006.
Diffie-Hellman Group Exchange for the Secure Shell (SSH) TransportLayer Protocol, RFC 4419, 2006.
The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
A. Perrig and D. Song, Hash Visualization: a New Technique toimprove Real-World Security, 1999, International Workshop onCryptographic Techniques and E-Commerce (CrypTEC ’99).
AUTHORS
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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. BSD September 27, 2013 BSD |