215 lines
4.6 KiB
HTML
215 lines
4.6 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Logins via the network</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Linux System Administrators Guide"
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TITLE="Logging In And Out"
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HREF="log-in-and-out.html"><LINK
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TITLE="Logins via terminals"
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HREF="login-via-terminal.html"><LINK
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TITLE="What login does"
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HREF="what-login-does.html"></HEAD
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><TR
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><TH
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>Linux System Administrators Guide: </TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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><A
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HREF="login-via-terminal.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 10. Logging In And Out</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="LOGIN-VIA-NETWORK"
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></A
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>10.2. Logins via the network</H1
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><P
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>Two computers in the same network are usually linked via a
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single physical cable. When they communicate over the network,
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the programs in each computer that take part in the communication
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are linked via a <I
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CLASS="GLOSSTERM"
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>virtual connection</I
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>, a sort
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of imaginary cable. As far as the programs at either end of the
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virtual connection are concerned, they have a monopoly on their
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own cable. However, since the cable is not real, only imaginary,
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the operating systems of both computers can have several virtual
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connections share the same physical cable. This way, using just
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a single cable, several programs can communicate without having
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to know of or care about the other communications. It is even
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possible to have several computers use the same cable; the virtual
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connections exist between two computers, and the other computers
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ignore those connections that they don't take part in. </P
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><P
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> That's a complicated and over-abstracted description of
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the reality. It might, however, be good enough to understand
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the important reason why network logins are somewhat different
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from normal logins. The virtual connections are established
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when there are two programs on different computers that wish
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to communicate. Since it is in principle possible to login
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from any computer in a network to any other computer, there is
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a huge number of potential virtual communications. Because of
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this, it is not practical to start a <B
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CLASS="COMMAND"
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>getty</B
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>
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for each potential login. </P
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><P
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> There is a single process inetd (corresponding to
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<B
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CLASS="COMMAND"
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>getty</B
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>) that handles all network logins.
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When it notices an incoming network login (i.e., it notices
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that it gets a new virtual connection to some other computer),
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it starts a new process to handle that single login. The original
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process remains and continues to listen for new logins. </P
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><P
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> To make things a bit more complicated, there is
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more than one communication protocol for network logins.
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The two most important ones are <B
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CLASS="COMMAND"
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>telnet</B
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> and
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<B
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CLASS="COMMAND"
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>rlogin</B
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>. In addition to logins, there are many
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other virtual connections that may be made (for FTP, Gopher, HTTP,
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and other network services). It would be ineffective to have a
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separate process listening for a particular type of connection,
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so instead there is only one listener that can recognize the type
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of the connection and can start the correct type of program to
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provide the service. This single listener is called
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<B
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CLASS="COMMAND"
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>inetd</B
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>;
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see the <I
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CLASS="CITETITLE"
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>Linux Network Administrators' Guide</I
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>
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for more information. </P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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SUMMARY="Footer navigation table"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="login-via-terminal.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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VALIGN="top"
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HREF="index.html"
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>Home</A
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></TD
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><TD
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VALIGN="top"
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><A
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HREF="what-login-does.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Logins via terminals</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="log-in-and-out.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>What <B
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CLASS="COMMAND"
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>login</B
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> does</TD
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></TR
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></TABLE
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></DIV
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></HTML
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> |