~ubuntu-branches/ubuntu/feisty/openssh/feisty-security

SSH(1)                     OpenBSD Reference Manual                     SSH(1)

NAME

     ssh - OpenSSH SSH client (remote login program)

SYNOPSIS

     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]

DESCRIPTION

     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 untrust-

     ed 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 be-

     low).

     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.

     -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

             Use bind_address on the local machine as the source address of

             the connection.  Only useful on systems with more than one ad-

             dress.

     -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

             Selects the cipher specification for encrypting the session.

             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 dif-

             ferent 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 interoperabil-

             ity with legacy protocol 1 implementations that do not support

             the 3des cipher.  Its use is strongly discouraged due to crypto-

             graphic 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, blow-

             fish-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

     -D [bind_address:]port

             Specifies a local ``dynamic'' application-level port forwarding.

             This works by allocating a socket to listen to port on the local

             side, optionally bound to the specified bind_address.  Whenever a

             connection is made to this port, the connection is forwarded over

             the secure channel, and the application protocol is then used to

             determine where to connect to from the remote machine.  Currently

             the SOCKS4 and SOCKS5 protocols are supported, and ssh will act

             as a SOCKS server.  Only root can forward privileged ports.  Dy-

             namic port forwardings can also be specified in the configuration

             file.

             IPv6 addresses can be specified with an alternative syntax:

             [bind_address/]port or by enclosing the address in square brack-

             ets.  Only the superuser can forward privileged ports.  By de-

             fault, 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 `*' indi-

             cates that the port should be available from all interfaces.

     -e escape_char

             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.  Setting the

             character to ``none'' disables any escapes and makes the session

             fully transparent.

     -F configfile

             Specifies an alternative per-user configuration file.  If a con-

             figuration file is given on the command line, the system-wide

             configuration file (/etc/ssh/ssh_config) will be ignored.  The

             default for the per-user configuration file is ~/.ssh/config.

     -f      Requests ssh to go to background just before command execution.

             This is useful if ssh is going to ask for passwords or passphras-

             es, 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.

     -g      Allows remote hosts to connect to local forwarded ports.

     -I smartcard_device

             Specify the device ssh should use to communicate with a smartcard

             used for storing the user's private RSA key.  This option is only

             available if support for smartcard devices is compiled in (de-

             fault is no support).

     -i identity_file

             Selects a file from which the identity (private key) for RSA or

             DSA authentication is read.  The default is ~/.ssh/identity for

             protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro-

             tocol version 2.  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 config-

             uration files).

     -k      Disables forwarding (delegation) of GSSAPI credentials to the

             server.

     -L [bind_address:]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,

             optionally bound to the specified bind_address.  Whenever a con-

             nection is made to this port, the connection is forwarded over

             the secure channel, and a connection is made to host port

             hostport from the remote machine.  Port forwardings can also be

             specified in the configuration file.  IPv6 addresses can be spec-

             ified with an alternative syntax: [bind_address/]port/host/host-

             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.  How-

             ever, an explicit bind_address may be used to bind the connection

             to a specific address.  The bind_address of ``localhost'' indi-

             cates that the listening port be bound for local use only, while

             an empty address or `*' indicates that the port should be avail-

             able from all interfaces.

     -l login_name

             Specifies the user to log in as on the remote machine.  This also

             may be specified on a per-host basis in the configuration file.

     -M      Places the ssh client into ``master'' mode for connection shar-

             ing.  Multiple -M options places ssh into ``master'' mode with

             confirmation required before slave connections are accepted.  Re-

             fer to the description of ControlMaster in ssh_config(5) for de-

             tails.

     -m mac_spec

             Additionally, for protocol version 2 a comma-separated list of

             MAC (message authentication code) algorithms can be specified in

             order of preference.  See the MACs keyword for more information.

     -N      Do not execute a remote command.  This is useful for just for-

             warding 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 ma-

             chine.  For example, ssh -n shadows.cs.hut.fi emacs & will start

             an emacs on shadows.cs.hut.fi, and the X11 connection will be au-

             tomatically 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

             Control an active connection multiplexing master process.  When

             the -O option is specified, the ctl_cmd argument is interpreted

             and passed to the master process.  Valid commands are: ``check''

             (check that the master process is running) and ``exit'' (request

             the master to exit).

     -o option

             Can be used to give options in the format used in the configura-

             tion file.  This is useful for specifying options for which there

             is no separate command-line flag.  For full details of the op-

             tions listed below, and their possible values, see ssh_config(5).

                   AddressFamily

                   BatchMode

                   BindAddress

                   ChallengeResponseAuthentication

                   CheckHostIP

                   Cipher

                   Ciphers

                   ClearAllForwardings

                   Compression

                   CompressionLevel

                   ConnectionAttempts

                   ConnectTimeout

                   ControlMaster

                   ControlPath

                   DynamicForward

                   EscapeChar

                   ForwardAgent

                   ForwardX11

                   ForwardX11Trusted

                   GatewayPorts

                   GlobalKnownHostsFile

                   GSSAPIAuthentication

                   GSSAPIDelegateCredentials

                   HashKnownHosts

                   Host

                   HostbasedAuthentication

                   HostKeyAlgorithms

                   HostKeyAlias

                   HostName

                   IdentityFile

                   IdentitiesOnly

                   KbdInteractiveDevices

                   LocalCommand

                   LocalForward

                   LogLevel

                   MACs

                   NoHostAuthenticationForLocalhost

                   NumberOfPasswordPrompts

                   PasswordAuthentication

                   PermitLocalCommand

                   Port

                   PreferredAuthentications

                   Protocol

                   ProxyCommand

                   PubkeyAuthentication

                   RekeyLimit

                   RemoteForward

                   RhostsRSAAuthentication

                   RSAAuthentication

                   SendEnv

                   ServerAliveInterval

                   ServerAliveCountMax

                   SmartcardDevice

                   StrictHostKeyChecking

                   TCPKeepAlive

                   Tunnel

                   TunnelDevice

                   UsePrivilegedPort

                   User

                   UserKnownHostsFile

                   VerifyHostKeyDNS

                   XAuthLocation

     -p port

             Port to connect to on the remote host.  This can be specified on

             a per-host basis in the configuration file.

     -q      Quiet mode.  Causes all warning and diagnostic messages to be

             suppressed.

     -R [bind_address:]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 connec-

             tion is forwarded over the secure channel, and a connection is

             made to host port hostport from the local machine.

             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 `*', indi-

             cates that the remote socket should listen on all interfaces.

             Specifying a remote bind_address will only succeed if the serv-

             er's GatewayPorts option is enabled (see sshd_config(5)).

     -S ctl_path

             Specifies the location of a control socket for connection shar-

             ing.  Refer to the description of ControlPath and ControlMaster

             in ssh_config(5) for details.

     -s      May be used to request invocation of a subsystem on the remote

             system.  Subsystems are a feature of the SSH2 protocol which fa-

             cilitate the use of SSH as a secure transport for other applica-

             tions (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 arbi-

             trary 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, authentica-

             tion, and configuration problems.  Multiple -v options increase

             the verbosity.  The maximum is 3.

     -w tunnel:tunnel

             Requests a tun(4) device on the client (first tunnel arg) and

             server (second tunnel arg).  The devices may be specified by nu-

             merical ID or the keyword ``any'', which uses the next available

             tunnel device.  See also the Tunnel directive in ssh_config(5).

     -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 ex-

             tension restrictions by default.  Please refer to the ssh -Y op-

             tion 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.

     ssh may additionally obtain configuration data from a per-user configura-

     tion file and a system-wide configuration file.  The file format and con-

     figuration 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

     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 pre-

     ferred 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 pass-

     word authentication.  Authentication methods are tried in the order spec-

     ified 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/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 se-

     curity 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 pub-

     lic-key cryptography, using cryptosystems where encryption and decryption

     are done using separate keys, and it is unfeasible to derive the decryp-

     tion 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 us-

     er should then copy the public key to ~/.ssh/authorized_keys in his/her

     home directory on the remote machine.  The authorized_keys file corre-

     sponds 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 with-

     out 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 al-

     lows 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 identifica-

     tion 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 iden-

     tification ever changes, ssh warns about this and disables password au-

     thentication 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 ei-

     ther 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

     When a pseudo-terminal has been requested, ssh supports a number of func-

     tions 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 charac-

     ter can be changed in configuration files using the EscapeChar configura-

     tion directive or on the command line by the -e option.

     The supported escapes (assuming the default `~') are:

     ~.      Disconnect.

     ~^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 and -R options (see above).  It also al-

             lows the cancellation of existing remote port-forwardings using

             -KR hostport.  !command allows the user to execute a local com-

             mand if the PermitLocalCommand option is enabled in

             ssh_config(5).  Basic help is available, using the -h option.

     ~R      Request rekeying of the connection (only useful for SSH protocol

             version 2 and if the peer supports it).

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 con-

     nections 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 mat-

     ter 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

     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 auto-

     matically forwarded to the remote side in such a way that any X11 pro-

     grams 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 re-

     mote 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