ACS Notes: SSH (Secure Shell)

## SSH Overview

## SSH (Secure Shell)

SSH is a widely-used suite of remote access programs which provides authentication (to protect your password) and encryption (to protect your data). It does not generally use Kerberos, although more recent versions include this as an option. For general information on SSH, including mailing lists and FAQs, see the SSH Users Group and SSH Home Page.

A free Unix version of SSH is available for most platforms from http://www.ssh.org/.

Mac and PC versions are available from various sources including the following. Please note that the Computing Helpdesk is not equipped to offer help with SSH on the level provided for IS supported software.

- F-Secure SSH (commercial clients for Mac and PC; trial versions are available)
- SecureCRT (Windows95/NT client including zmodem file transfer capabilities, available to the MIT community courtesy of the Media Lab)
- http://bmrc.berkeley.edu/people/chaffee/winntutil.html for PC (free, command-line only; can be configured with port forwarding to secure an FTP program such as WS_FTP)
- http://www.zip.com.au/~roca/ttssh.html for PC (free extension to the freeware Tera Term terminal emulator)
- http://www.doc.ic.ac.uk/~ci2/ssh/ for PC (free client)

## Using SSH on Athena

Generally, ssh involves users generating public/private key pairs and using them to authenticate, but the default behavior on Athena is to use either Kerberos authentication (with forwardable tickets), or simple password authentication. The reason for this is that you need to get Kerberos tickets to be able to do things like read your files and incorporate your mail, and you can't get tickets on the remote machine without either forwarding them from the machine you're already logged in to, or typing your password.

If you have forwardable Kerberos tickets on your machine (the default in Athena 8.2 and later), then ssh will use them to authenticate you and log you in. If you don't, it will fall back to password authentication (ssh always encrypts your connection, including any password you type).

### Using SSH for remote login

ssh is a secure remote login program, which can be used in place of regular telnet, or on systems where kerberized telnet is not readily available.

> Example of how to use ssh to login remotely to/from Athena

#### Using SSH with remote X Windows applications

You can run X windows applications remotely from ssh to secure the X connection.

### Using SSH for file transfers

scp is a secure alternative to the regular UNIX rcp command for copying files between hosts (scp uses ssh for authentication and data encryption). You can use it between any UNIX host with SSH installed and the Athena dialups (or a private workstation with remote-access enabled).

> Example of how to use scp to transfer files to/from Athena.

Most of the SSH clients currently available for Mac and PC do not include an scp feature, but some provide other means for making secure file transfers:

- SecureCRT for Windows supports zmodem file transfer, which can be used for secure file transfer as demonstrated below. Note: this example assumes you have already installed SecureCRT and set up an ssh profile as explained in the SecureCRT for Windows95/NT section below.

  > Example of how to use SecureCRT for file transfer

- Other ssh clients can be set up to secure the FTP command channel (this encrypts the commands you type, but not the files being transferred). For more information on specific clients, see the F-Secure SSH FAQ or vendor documentation. For general information on securing the FTP command channel in SSH, see the SSH FAQ section on FTP or man pages. See also:

  > Example of how to set up port fowarding to secure the FTP channel

### SecureCRT for Windows95/NT

SecureCRT is a terminal emulation program which includes ssh and has zmodem file transfer capabilities. The MIT Media Lab has extended its license of this program client to make it available to the MIT community. MIT's SecureCRT download site includes a README file which explains how to enter the MIT license data. Once you have downloaded the program and entered the necessary license information, you'll need to set up a session profile specifying ssh as the protocol:

- The Connect window may open automaticaly; otherwise, go to the File menu and select Connect.
- Under the Session List tab, select New.
- Under the Session tab, set the Protocol to ssh, and enter either the hostname for a private workstation or athena.dialup.mit.edu. The other default settings should be correct.

related ACS notes: Remote Access Overview | Remote Login Guide | File Transfer Guide

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Last modified: Thu Jul 8 16:59:33 1999