98 lines
4.9 KiB
Text
98 lines
4.9 KiB
Text
SSH (Secure Shell) is a program to log into another computer over a
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network, to execute commands in a remote machine, and to move files
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from one machine to another. It provides strong authentication and
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secure communications over insecure channels. It is intended as a
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replacement for rlogin, rsh, rcp, and rdist.
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FEATURES
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o Strong authentication. Closes several security holes (e.g., IP,
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routing, and DNS spoofing). New authentication methods: .rhosts
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together with RSA based host authentication, and pure RSA
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authentication.
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o Improved privacy. All communications are automatically and
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transparently encrypted. RSA is used for key exchange, and a
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conventional cipher (normally IDEA, Blowfish, or triple-DES) for
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encrypting the session. Encryption is started before
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authentication, and no passwords or other information is
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transmitted in the clear. Encryption is also used to protect
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against spoofed packets.
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o Secure X11 sessions. The program automatically sets DISPLAY on
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the server machine, and forwards any X11 connections over the
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secure channel. Fake Xauthority information is automatically
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generated and forwarded to the remote machine; the local client
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automatically examines incoming X11 connections and replaces the
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fake authorization data with the real data (never telling the
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remote machine the real information).
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o Arbitrary TCP/IP ports can be redirected through the encrypted channel
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in both directions (e.g., for e-cash transactions).
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o No retraining needed for normal users; everything happens
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automatically, and old .rhosts files will work with strong
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authentication if administration installs host key files.
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o Never trusts the network. Minimal trust on the remote side of
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the connection. Minimal trust on domain name servers. Pure RSA
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authentication never trusts anything but the private key.
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o Client RSA-authenticates the server machine in the beginning of
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every connection to prevent trojan horses (by routing or DNS
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spoofing) and man-in-the-middle attacks, and the server
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RSA-authenticates the client machine before accepting .rhosts or
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/etc/hosts.equiv authentication (to prevent DNS, routing, or
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IP-spoofing).
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o Host authentication key distribution can be centrally by the
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administration, automatically when the first connection is made
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to a machine (the key obtained on the first connection will be
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recorded and used for authentication in the future), or manually
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by each user for his/her own use. The central and per-user host
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key repositories are both used and complement each other. Host
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keys can be generated centrally or automatically when the software
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is installed. Host authentication keys are typically 1024 bits.
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o Any user can create any number of user authentication RSA keys for
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his/her own use. Each user has a file which lists the RSA public
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keys for which proof of possession of the corresponding private
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key is accepted as authentication. User authentication keys are
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typically 1024 bits.
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o The server program has its own server RSA key which is
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automatically regenerated every hour. This key is never saved in
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any file. Exchanged session keys are encrypted using both the
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server key and the server host key. The purpose of the separate
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server key is to make it impossible to decipher a captured session by
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breaking into the server machine at a later time; one hour from
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the connection even the server machine cannot decipher the session
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key. The key regeneration interval is configurable. The server
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key is normally 768 bits.
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o An authentication agent, running in the user's laptop or local
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workstation, can be used to hold the user's RSA authentication
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keys. Ssh automatically forwards the connection to the
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authentication agent over any connections, and there is no need to
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store the RSA authentication keys on any machine in the network
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(except the user's own local machine). The authentication
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protocols never reveal the keys; they can only be used to verify
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that the user's agent has a certain key. Eventually the agent
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could rely on a smart card to perform all authentication
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computations.
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o The software can be installed and used (with restricted
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functionality) even without root privileges.
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o The client is customizable in system-wide and per-user
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configuration files. Most aspects of the client's operation can
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be configured. Different options can be specified on a per-host basis.
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o Automatically executes conventional rsh (after displaying a
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warning) if the server machine is not running sshd.
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o Optional compression of all data with gzip (including forwarded X11
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and TCP/IP port data), which may result in significant speedups on
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slow connections.
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o Complete replacement for rlogin, rsh, and rcp.
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