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diff --git a/sag-0.6.1-www/sag-0.6.1.html/x1944.html b/sag-0.6.1-www/sag-0.6.1.html/x1944.html new file mode 100644 index 0000000..e4169cf --- /dev/null +++ b/sag-0.6.1-www/sag-0.6.1.html/x1944.html @@ -0,0 +1,199 @@ +<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN"> +<HTML +><HEAD +><TITLE +>Logins via the network</TITLE +><META +NAME="GENERATOR" +CONTENT="Modular DocBook HTML Stylesheet"><LINK +REL="HOME" +TITLE="The Linux System Administrators' Guide" +HREF="book1.html"><LINK +REL="UP" +TITLE="Logging In And Out" +HREF="c1905.html"><LINK +REL="PREVIOUS" +TITLE="Logging In And Out" +HREF="c1905.html"><LINK +REL="NEXT" +TITLE="What login does" +HREF="x1956.html"></HEAD +><BODY +BGCOLOR="#FFFFFF" +TEXT="#000000" +><DIV +CLASS="NAVHEADER" +><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TH +COLSPAN="3" +ALIGN="center" +>The Linux System Administrators' Guide</TH +></TR +><TR +><TD +WIDTH="10%" +ALIGN="left" +VALIGN="bottom" +><A +HREF="c1905.html" +>Prev</A +></TD +><TD +WIDTH="80%" +ALIGN="center" +VALIGN="bottom" +>Chapter 8. Logging In And Out</TD +><TD +WIDTH="10%" +ALIGN="right" +VALIGN="bottom" +><A +HREF="x1956.html" +>Next</A +></TD +></TR +></TABLE +><HR +ALIGN="LEFT" +WIDTH="100%"></DIV +><DIV +CLASS="SECT1" +><H1 +CLASS="SECT1" +><A +NAME="AEN1944" +>Logins via the network</A +></H1 +><P +>Two computers in the same network are usually linked via a + single physical cable. When they communicate over the network, + the programs in each computer that take part in the communication + are linked via a <I +CLASS="GLOSSTERM" +>virtual connection</I +>, a sort + of imaginary cable. As far as the programs at either end of the + virtual connection are concerned, they have a monopoly on their + own cable. However, since the cable is not real, only imaginary, + the operating systems of both computers can have several virtual + connections share the same physical cable. This way, using just + a single cable, several programs can communicate without having + to know of or care about the other communications. It is even + possible to have several computers use the same cable; the virtual + connections exist between two computers, and the other computers + ignore those connections that they don't take part in. </P +><P +> That's a complicated and over-abstracted description of + the reality. It might, however, be good enough to understand + the important reason why network logins are somewhat different + from normal logins. The virtual connections are established + when there are two programs on different computers that wish + to communicate. Since it is in principle possible to login + from any computer in a network to any other computer, there is + a huge number of potential virtual communications. Because of + this, it is not practical to start a <B +CLASS="COMMAND" +>getty</B +> + for each potential login. </P +><P +> There is a single process inetd (corresponding to + <B +CLASS="COMMAND" +>getty</B +>) that handles all network logins. + When it notices an incoming network login (i.e., it notices + that it gets a new virtual connection to some other computer), + it starts a new process to handle that single login. The original + process remains and continues to listen for new logins. </P +><P +> To make things a bit more complicated, there is + more than one communication protocol for network logins. + The two most important ones are <B +CLASS="COMMAND" +>telnet</B +> and + <B +CLASS="COMMAND" +>rlogin</B +>. In addition to logins, there are many + other virtual connections that may be made (for FTP, Gopher, HTTP, + and other network services). It would be ineffective to have a + separate process listening for a particular type of connection, + so instead there is only one listener that can recognize the type + of the connection and can start the correct type of program to + provide the service. This single listener is called \cmd{inetd}; + see the <I +CLASS="CITETITLE" +>Linux Network Administrators' Guide</I +> + for more information. </P +></DIV +><DIV +CLASS="NAVFOOTER" +><HR +ALIGN="LEFT" +WIDTH="100%"><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +><A +HREF="c1905.html" +>Prev</A +></TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="book1.html" +>Home</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +><A +HREF="x1956.html" +>Next</A +></TD +></TR +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +>Logging In And Out</TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="c1905.html" +>Up</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +>What <B +CLASS="COMMAND" +>login</B +> does</TD +></TR +></TABLE +></DIV +></BODY +></HTML +>
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