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269 lines
12 KiB
269 lines
12 KiB
<chapter id="callback">
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<title>Configuring &kppp; for callback</title>
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<para>This chapter is based on material provided by Martin Häfner,
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<email>mh@ap-dec717c.physik.uni-karlsruhe.de</email></para>
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<sect1 id="unix-callback-server">
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<title>&UNIX; or &Linux; callback server</title>
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<para>This section introduces &UNIX; (&Linux;) callback, and how &kppp; can be
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configured to connect to a &UNIX; callback server, especially to a script based
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&Linux; <link linkend="callback-resources">callback server</link></para>
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<sect2>
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<title>An Introduction to callback</title>
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<para>There are several reasons to consider using callback. Some of these are:</para>
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<itemizedlist>
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<listitem>
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<para>To increase the security of your local network</para>
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</listitem>
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<listitem>
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<para>To reduce expenses of external co-workers</para>
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</listitem>
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<listitem>
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<para>To control telephone costs where calls are claimed as business
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expenses</para>
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</listitem>
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</itemizedlist>
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<para>Think about someone calling the number of your dial in server, and then
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cracking a password. Why bother to maintain a firewall for your internet
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connection, if access to your network is that easy?.</para>
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<para>Callback software generally asks for your name, and then hangs up the
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line. It then calls you back, usually at a number that is stored
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<emphasis>on the server</emphasis> in a database. The client then picks up the
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phone line and continues with the dial-in as if nothing had happened. The
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server now requests your username and password, knowing that you are who you
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said you were when you first dialled in, or at the least, you are where you said
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you were. The connection is established normally, and the
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<application>pppd</application> is started.</para>
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<para>Now the big question is, how to tell the client to pick up the phone, when
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the server calls you back. Do you need a special program, such as
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<application>mgetty</application>? The answer is, <emphasis>no</emphasis>, you
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don't need a special client program. In general, any client can be used for
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callback connections, you could even use an ordinary terminal program such as
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<application>minicom</application> to connect.</para>
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<para>The only thing you have to do is tell your modem to
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<command>AutoAnswer</command> the phone when a
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<computeroutput>RING</computeroutput> is detected by the modem. This is done
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with the following modem command:</para>
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<screen>
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<userinput><command>AT&SO=1</command></userinput>
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</screen>
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<para>This tells the modem to pick the phone up after one
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<computeroutput>RING</computeroutput>.</para>
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<para>Like a lot of other client programs, &kppp; checks to see if the
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connection is closed by the server, and then stops the current session if a
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<computeroutput>NO CARRIER</computeroutput> is detected. This, then, is the
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real problem when setting up callback. <computeroutput>NO
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CARRIER</computeroutput> will of course be detected the moment the callback
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server hangs up the line. Some servers therefore use a special login program.
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So how do you solve this problem? You tell your modem to show
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<computeroutput>CARRIER UP</computeroutput> at all times (which causes no
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problems if you tell the client to hang up the line.) You can do this with the
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following modem command:</para>
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<screen>
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<userinput><command>AT&C0</command></userinput>
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</screen>
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<para>If you want to test this, you can first use an ordinary terminal program
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such as <application>minicom</application>, and call your callback server, to
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see what hapens.</para>
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</sect2>
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<sect2>
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<title>The &kppp; setup</title>
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<para>So, now that you've seen the theory in action, how do you go about setting
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up &kppp; to handle the connection? </para>
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<para>The procedure is quite straightforward, as follows.</para>
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<procedure>
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<step>
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<para>First tell the modem to accept connections, and to not stop the
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negotiation when the callback server hangs up the line for the first time. You
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can add both these options in the <guilabel>Modem</guilabel> tab of the &kppp;
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configuration, by adding to the option <guilabel>Dial String</guilabel> the
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string <command>AT&C0S0=1DT</command></para>
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<para>There are no other changes with configuration for &kppp;. If you meet
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trouble with modem init and reset, check the <link
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linkend="callback-troubleshooting">Troubleshooting</link> section for more
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information.</para>
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</step>
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<step>
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<para>Think about your server for a moment. Remember that &UNIX;, &Windows; and
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Macintosh operating systems have differing opinions about how to end a line in a
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text file, and therefore, in login procedures too. If you are connecting to a
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&Windows; server, use <userinput>CR/LF</userinput>, if you are connecting to a
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&UNIX; server, use <userinput>CR</userinput>, and if you are connecting to a
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Macintosh server, use <userinput>LF</userinput>
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</para>
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</step>
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<step>
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<para>We are assuming for these instructions that you are calling a &Linux;
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callback package which uses ordinary login (not <acronym>PAP</acronym> or
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such).</para>
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<para>Set the <guilabel>Authentication</guilabel> style in the
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<guilabel>Dial</guilabel> tab of the account configuration to
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<guilabel>Script-based</guilabel></para>
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</step>
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<step>
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<para>Now you have to build the login script. Editing of login scripts is one
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of the very cool features of &kppp; You can find it in the <guilabel>Login
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Script</guilabel> tab of the <guilabel>Edit Account</guilabel> dialog.</para>
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<para>In this example, the user <systemitem>userxyz</systemitem> needs the
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following script to be called. The callback server already knows the table of
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names and their applicable phone numbers, so you select the phone number to be
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used with an alias, for security purposes.</para>
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<para>For each line, choose the criteria from the drop down list on the left of
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the dialog, and type in the action in the text box on it's right. Choose the
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<guibutton>Add</guibutton> to add each line to the script. You can use
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<guibutton>Insert</guibutton> to add a line into the middle of the script, and
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<guibutton>Remove</guibutton> to delete a line if you made a mistake.</para>
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<para>The entire script should look something like this (without the comments,
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shown here starting with a #)</para>
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<screen>
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Expect ogin: <lineannotation># remember, we do ordinary terminal login</lineannotation>
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ID "" <lineannotation># kppp sends the id you configured in
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the main dialog</lineannotation>
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Expect for userxyz: <lineannotation># a list of available numbers is
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shown, the user should choose one</lineannotation>
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Send userxyz-home <lineannotation># the user wants to be called back
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on their home number</lineannotation>
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Expect ogin: <lineannotation># The callback process is now
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running, a new connection, and so a new login.</lineannotation>
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ID
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Expect assword: <lineannotation># Now send your password</lineannotation>
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Expect > <lineannotation># Wait for the command prompt (the
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prompt may vary)</lineannotation>
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Send start_ppp <lineannotation># this command starts the pppd</lineannotation>
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</screen>
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<para>After waiting for the login request, the user sends his ID and waits for a
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list of available phone numbers for that username. Then he tells the server
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which of the numbers offered he would like to be called back on. &kppp; can
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open a dialog for this, if your location changes often, ⪚ you are a sales
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representative and move from hotel to hotel. Now the server is expecting login
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and password for authentication, but in the meantime, the server hangs up and
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calls the user back. The authentication information is sent, and &kppp; waits
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for a command prompt, and then starts a small script (here called
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<filename>start_ppp</filename> which fires up <application>pppd</application> on
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the server.</para>
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<para>The <filename>start_ppp</filename> script might look something like the
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following:</para>
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<programlisting>
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#!/bin/sh
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stty -echo
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exec /usr/sbin/pppd -detach silent modem
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</programlisting>
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<para>Of course, setting up a <acronym>PPP</acronym> server is not within the
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scope of this document. For detailed information, see the
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<application>pppd</application> man pages. An excellent description of a
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callback server can be found at <ulink
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url="http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback">
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http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback</ulink></para>
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</step>
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</procedure>
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<para>All other configuration issues, such as <application>pppd</application>
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configuration or <acronym>IP</acronym> settings work as normal, and no special
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software is required to pick up the line.</para>
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<note>
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<para>&kppp; callback and other programs such as
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<application>mgetty</application> or any other faxgetty can be run on the same
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serial port. There are no problems with the dial in, as &kppp; creates a lock
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file which will tell the getty program that another application (in this case,
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&kppp; of course,) is using the line at that time.</para>
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</note>
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</sect2>
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<sect2 id="callback-troubleshooting">
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<title>Troubleshooting</title>
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<para>There are some known problems with &kppp; in callback mode:</para>
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<itemizedlist>
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<listitem>
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<para>As you initialize the modem to auto answer, you need to reset the modem
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after your connection is closed. Otherwise, your modem will continue to pick up
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the line for you, which is not a good idea if the line in question is your main
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phone line.</para>
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</listitem>
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<listitem>
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<para>&kppp; has some small problems when sharing a line with another program,
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such as <application>mgetty</application>. If <application>mgetty</application>
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is running on the same modem line, &kppp; is not able to initialize the modem
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correctly. <!-- This happens on the second try (does this mean it can initialize -->
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<!-- on the second try, or it fails on the second time? --> </para>
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</listitem>
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<listitem>
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<para>&kppp; is unable to prompt for certain user input during a scripting based
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login. Unfortunately, when using the example script above, &kppp; also asks for
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the user name the second time the callback server requests it. You can get rid
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of this by hardcoding your userid into the login script (not very portable or
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nice, but it works.</para>
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</listitem>
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</itemizedlist>
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</sect2>
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<sect2 id="callback-resources">
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<title>Internet Resources for server software</title>
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<para>&Linux; callback server software bundles are available in many
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places.</para>
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<para>The well known <application>mgetty</application> is a very powerful
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program, and is also able to handle callback connections. A description of how
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to set up <application>mgetty</application> for this purpose is maintained at
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<ulink url="http://www.dyer.demon.co.uk/slug/tipscrip.htm">
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http://www.dyer.demon.co.uk/slug/tipscrip.htm</ulink>, by Colin McKinnon,
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<email>colin@wew.co.uk</email>.</para>
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<para>There is also a ready to use package for &Linux; at <ulink
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url="http://www.icce.rug.nl/docs/programs/callback/callback.html">
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http://www.icce.rug.nl/docs/programs/callback/callback.html</ulink>. This
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package is maintained by Frank B. Brokken, <email>frank@icce.rug.nl</email>. As
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the setup, although straightforward, is not very easy, I have written a short
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introduction for it at <ulink
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url="http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback">http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback/</ulink>,
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which also contains a more general introduction to callback.</para>
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</sect2>
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</sect1>
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<sect1 id="nt-callback">
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<title>&Windows; NT <acronym>RAS</acronym> callback</title>
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<para>&Windows; NT uses a completely different approach than the one described
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above. NT requires an extension to the <acronym>PPP</acronym> protocol itself,
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called <acronym>CBCP</acronym> (Call Back Control Protocol).
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<application>pppd</application> has support for this protocol, but you must
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recompile <application>pppd</application>. If anybody has experience with
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successfully connecting to an NT callback server, please let us know.</para>
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</sect1>
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</chapter>
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