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<TITLE>Special Edition Using Linux, Fourth Edition:Accessing the Network with telnet, ftp, and the r-Commands</TITLE>

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<H4 ALIGN="LEFT"><A NAME="Heading14"></A><FONT COLOR="#000077"><I>ssh</I>

</FONT></H4>

<P><TT>ssh</TT> (short for secure shell), like the <TT>rsh</TT> command, is a program for logging into a remote machine and executing commands on that remote machine. <TT>ssh</TT> is designed to replace both <TT>rsh</TT> and <TT>rlogin</TT> by providing the ability to define an encrypted session between two untrusted systems over an insecure network. One problem with <TT>telnet</TT> is that, when you log in to the remote system, the password is sent as ASCII over the network. By watching the Ethernet packets, someone could collect your login name and password for the remote system. <TT>ssh</TT> prevents this from happening by using RSA-based authentication. Because of its security, <TT>ssh</TT> is commonly used by system administrators today. <TT>ssh</TT> clients are available for other operating systems, including Macintosh and Windows.</P>

<P>The <TT>ssh</TT> command is very similar to the <TT>telnet</TT> command because it allows you to start an interactive command session on a remote system. The syntax of <TT>ssh</TT> is as follows:</P>

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<PRE>

ssh [-a] [-c idea|blowfish|des|3des|arcfour|tss|none] [-e escape_char]

[-I identity_file] [-l login-name] [-n] [-k] [-V] [=o option] [-p port]

[-q] [-P] [-t] [-v] [-x] [-C] [-L port&#148;host:hostport]

[-R port:host:hostport]

<I>hostname</I> [command]

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<P>However, the most common usage is this:

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<PRE>

ssh <I>hostname</I>

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<P>Table 30.4 explains the various options for ssh.

</P>

<TABLE WIDTH="100%"><CAPTION ALIGN=LEFT><B>Table 30.4</B> Command-Line Options for the <TT>ssh</TT> Command

<TR>

<TH COLSPAN="2"><HR>

<TR>

<TH WIDTH="15%" ALIGN="LEFT">Option

<TH WIDTH="85%" ALIGN="LEFT">Description

<TR>

<TD COLSPAN="2"><HR>

<TR>

<TD VALIGN="TOP"><TT>-a</TT>

<TD>Disables forwarding of the authentication agent.

<TR>

<TD VALIGN="TOP"><TT>-c</TT>

<TD>Selects the cipher to use for encrypting the session. Idea is the default, arcfour is the fastest, and none is the equivalent of using <TT>rlogin</TT> or <TT>rsh</TT> (no encryption).

<TR>

<TD VALIGN="TOP"><TT>-e</TT>

<TD>Sets the escape character for the session.

<TR>

<TD VALIGN="TOP"><TT>-f</TT>

<TD>Sets <TT>ssh</TT> in the background after authentication and forwardings are established.

<TR>

<TD VALIGN="TOP"><TT>-i</TT>

<TD>Selects the identity file from which the private key for RSA authentication is read.

<TR>

<TD><TT>-k</TT>

<TD>Disables forwarding of Kerberos tickets.

<TR>

<TD><TT>-l</TT>

<TD>Sets the login name for use to the remote machine.

<TR>

<TD><TT>-n</TT>

<TD>Redirects <TT>stdin</TT> from /dev/nulls used when <TT>ssh</TT> runs in the background.

<TR>

<TD VALIGN="TOP"><TT>-o</TT>

<TD>Used for user-defined options following the format in the configuration file.

<TR>

<TD><TT>-p</TT>

<TD>Sets the port to connect to on the remote host.

<TR>

<TD VALIGN="TOP"><TT>-q</TT>

<TD>Activates quiet mode, which suppresses all messages except fatal errors.

<TR>

<TD><TT>-P</TT>

<TD>Uses a nonprivileged port.

<TR>

<TD><TT>-t</TT>

<TD>Forces pseudo-tty allocation.

<TR>

<TD><TT>-v</TT>

<TD>Activates verbose mode (useful for debugging).

<TR>

<TD><TT>-x</TT>

<TD>Disables X11 forwarding.

<TR>

<TD><TT>-C</TT>

<TD>Requests compression of all data.

<TR>

<TD VALIGN="TOP"><TT>-L</TT>

<TD>Specifies the local port to forward to the designated remote host and port.

<TR>

<TD VALIGN="TOP"><TT>-R</TT>

<TD>Specifies the remote port to be forwarded to the local host and designated port.

<TR>

<TD COLSPAN="2"><HR>

</TABLE>

<H3><A NAME="Heading15"></A><FONT COLOR="#000077">From Here&#133;</FONT></H3>

<P>You can find more information about the Internet in the following chapters:

</P>

<DL>

<DD><B>&#149;</B>&nbsp;&nbsp;Chapter 31, &#147;Surfing the Internet with the World Wide Web,&#148; describes the various types of information available on the Internet and the tools for accessing this information.

<DD><B>&#149;</B>&nbsp;&nbsp;Chapter 33, &#147;Using Electronic Mail,&#148; shows how to send and receive e-mail over the Internet.

</DL>

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