ssh.0

OpenSSH 是 SSH （Secure SHell） 协议的免费开源实现。它用安全、加密的网络连接工具代替了 telnet、ftp、 rlogin、rsh 和 rcp 工具。OpenSSH 支持

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 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] [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/IP
     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.

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

   SSH protocol version 1
     The first authentication method is the rhosts or hosts.equiv method com-
     bined with RSA-based host authentication.  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
     $HOME/.rhosts or $HOME/.shosts exist in the user's home directory on the
     remote machine and contain a line containing the name of the client ma-
     chine and the name of the user on that machine, the user is considered
     for log in.  Additionally, if the server can verify the client's host key
     (see /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts in the FILES
     section), only then is login permitted.  This authentication method clos-
     es security holes due to IP spoofing, DNS spoofing and routing spoofing.
     [Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts, and the
     rlogin/rsh protocol in general, are inherently insecure and should be
     disabled if security is desired.]

     As a second authentication method, ssh supports RSA based authentication.
     The scheme is based on public-key cryptography: there are cryptosystems
     where encryption and decryption are done using separate keys, and it is
     not possible to derive the decryption key from the encryption key.  RSA
     is one such system.  The idea is that each user creates a public/private
     key pair for authentication purposes.  The server knows the public key,
     and only the user knows the private key.

     The file $HOME/.ssh/authorized_keys lists the public keys that are per-
     mitted 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 serv-
     er checks if this key is permitted, and if so, sends the user (actually
     the ssh program running on behalf of the user) a challenge, a random num-
     ber, encrypted by the user's public key.  The challenge can only be de-
     crypted using the proper private key.  The user's client then decrypts
     the challenge using the private key, proving that he/she knows the pri-
     vate key but without disclosing it to the server.

     ssh implements the RSA authentication protocol automatically.  The user
     creates his/her RSA key pair by running ssh-keygen(1).  This stores the
     private key in $HOME/.ssh/identity and stores the public key in
     $HOME/.ssh/identity.pub in the user's home directory.  The user should
     then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home
     directory on the remote machine (the authorized_keys file corresponds to
     the conventional $HOME/.rhosts file, and has one key per line, though the
     lines can be very long).  After this, the user can log in without giving
     the password.

     The most convenient way to use RSA authentication may be with an authen-
     tication agent.  See ssh-agent(1) for more information.

     If other authentication methods fail, ssh prompts the user for a pass-
     word.  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 protocol version 2
     When a user connects using protocol version 2, similar authentication
     methods are available.  Using the default values for
     PreferredAuthentications, the client will try to authenticate first using
     the hostbased method; if this method fails, public key authentication is
     attempted, and finally if this method fails, keyboard-interactive and
     password authentication are tried.

     The public key method is similar to RSA authentication described in the
     previous section and allows the RSA or DSA algorithm to be used: The
     client uses his private key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to
     sign the session identifier and sends the result to the server.  The
     server checks whether the matching public key is listed in
     $HOME/.ssh/authorized_keys and grants access if both the key is found and
     the signature is correct.  The session identifier is derived from a
     shared Diffie-Hellman value and is only known to the client and the serv-
     er.

     If public key authentication fails or is not available, a password can be
     sent encrypted to the remote host to prove the user's identity.

     Additionally, ssh supports hostbased or challenge response authentica-
     tion.

     Protocol 2 provides additional mechanisms for confidentiality (the traf-
     fic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour) and in-
     tegrity (hmac-md5, hmac-sha1, hmac-ripemd160).  Note that protocol 1
     lacks a strong mechanism for ensuring the integrity of the connection.

   Login session and remote execution
     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/IP connections have been closed.  The exit sta-
     tus of the remote program is returned as the exit status of ssh.

   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 below).  It also al-
             lows the cancellation of existing remote port-forwardings using
             -KR hostport.  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).

   X11 and TCP forwarding
     If the ForwardX11 variable is set to ``yes'' (or see the description of
     the -X and -x options described later) and the user is using X11 (the
     DISPLAY environment variable is set), the connection to the X11 display
     is automatically forwarded to the remote side in such a way that any X11
     programs started from the shell (or command) will go through the encrypt-
     ed 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 described later) and the user is using an authenti-
     cation agent, the connection to the agent is automatically forwarded to
     the remote side.

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

   Server authentication
     ssh automatically maintains and checks a database containing identifica-
     tions for all hosts it has ever been used with.  Host keys are stored in
     $HOME/.ssh/known_hosts in the user's home directory.  Additionally, the
     file /etc/ssh/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 a trojan horse from getting the user's pass-
     word.  Another purpose of this mechanism is to prevent man-in-the-middle
     attacks which could otherwise be used to circumvent the encryption.  The
     StrictHostKeyChecking option can be used to prevent logins to machines
     whose host key is not known or has changed.

     ssh can be configured to verify host identification using fingerprint re-
     source records (SSHFP) published in DNS.  The VerifyHostKeyDNS option can
     be used to control how DNS lookups are performed.  SSHFP resource records
     can be generated using ssh-keygen(1).

     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
             Specify the interface to transmit from on machines with multiple
             interfaces or aliased addresses.

     -C      Requests compression of all data (including stdin, stdout,
             stderr, and data for forwarded X11 and TCP/IP connections).  The
             compression algorithm is the same used by gzip(1), and the
             ``level'' can be controlled by the CompressionLevel option for