tdenetwork/doc/kppp/callback.docbook

<chapter id="callback">
<title>Configuring &kppp; for callback</title>

<para>This chapter is based on material provided by Martin H&auml;fner,
<email>mh@ap-dec717c.physik.uni-karlsruhe.de</email></para>

<sect1 id="unix-callback-server">
<title>&UNIX; or &Linux; callback server</title>

<para>This section introduces &UNIX; (&Linux;) callback, and how &kppp; can be
configured to connect to a &UNIX; callback server, especially to a script based
&Linux; <link linkend="callback-resources">callback server</link></para>

<sect2>
<title>An Introduction to callback</title>

<para>There are several reasons to consider using callback.  Some of these are:</para>

<itemizedlist>
<listitem>
<para>To increase the security of your local network</para>
</listitem>
<listitem>
<para>To reduce expenses of external co-workers</para>
</listitem>
<listitem>
<para>To control telephone costs where calls are claimed as business
expenses</para>
</listitem>
</itemizedlist>

<para>Think about someone calling the number of your dial in server, and then
cracking a password.  Why bother to maintain a firewall for your internet
connection, if access to your network is that easy?.</para>

<para>Callback software generally asks for your name, and then hangs up the
line.  It then calls you back, usually at a number that is stored
<emphasis>on the server</emphasis> in a database.  The client then picks up the
phone line and continues with the dial-in as if nothing had happened.  The
server now requests your username and password, knowing that you are who you
said you were when you first dialled in, or at the least, you are where you said
you were.  The connection is established normally, and the
<application>pppd</application> is started.</para>

<para>Now the big question is, how to tell the client to pick up the phone, when
the server calls you back.  Do you need a special program, such as
<application>mgetty</application>?  The answer is, <emphasis>no</emphasis>, you
don't need a special client program.  In general, any client can be used for
callback connections, you could even use an ordinary terminal program such as
<application>minicom</application> to connect.</para>

<para>The only thing you have to do is tell your modem to
<command>AutoAnswer</command> the phone when a
<computeroutput>RING</computeroutput> is detected by the modem.  This is done
with the following modem command:</para>

<screen>
<userinput><command>AT&amp;SO=1</command></userinput>
</screen>

<para>This tells the modem to pick the phone up after one
<computeroutput>RING</computeroutput>.</para>

<para>Like a lot of other client programs, &kppp; checks to see if the
connection is closed by the server, and then stops the current session if a
<computeroutput>NO CARRIER</computeroutput> is detected.  This, then, is the
real problem when setting up callback.  <computeroutput>NO
CARRIER</computeroutput> will of course be detected the moment the callback
server hangs up the line.  Some servers therefore use a special login program.
So how do you solve this problem?  You tell your modem to show
<computeroutput>CARRIER UP</computeroutput> at all times (which causes no
problems if you tell the client to hang up the line.)  You can do this with the
following modem command:</para>

<screen>
<userinput><command>AT&amp;C0</command></userinput>
</screen>

<para>If you want to test this, you can first use an ordinary terminal program
such as <application>minicom</application>, and call your callback server, to
see what hapens.</para>

</sect2>

<sect2>
<title>The &kppp; setup</title>

<para>So, now that you've seen the theory in action, how do you go about setting
up &kppp; to handle the connection? </para>

<para>The procedure is quite straightforward, as follows.</para>

<procedure>
<step>
<para>First tell the modem to accept connections, and to not stop the
negotiation when the callback server hangs up the line for the first time.  You
can add both these options in the <guilabel>Modem</guilabel> tab of the &kppp;
configuration, by adding to the option <guilabel>Dial String</guilabel> the
string <command>AT&amp;C0S0=1DT</command></para>
<para>There are no other changes with configuration for &kppp;.  If you meet
trouble with modem init and reset, check the <link
linkend="callback-troubleshooting">Troubleshooting</link> section for more
information.</para>
</step>
<step>
<para>Think about your server for a moment.  Remember that &UNIX;, &Windows; and
Macintosh operating systems have differing opinions about how to end a line in a
text file, and therefore, in login procedures too.  If you are connecting to a
&Windows; server, use <userinput>CR/LF</userinput>, if you are connecting to a
&UNIX; server, use <userinput>CR</userinput>, and if you are connecting to a
Macintosh server, use <userinput>LF</userinput>

</para>
</step>
<step>
<para>We are assuming for these instructions that you are calling a &Linux;
callback package which uses ordinary login (not <acronym>PAP</acronym> or
such).</para>
<para>Set the <guilabel>Authentication</guilabel> style in the
<guilabel>Dial</guilabel> tab of the account configuration to
<guilabel>Script-based</guilabel></para>
</step>
<step>
<para>Now you have to build the login script.  Editing of login scripts is one
of the very cool features of &kppp; You can find it in the <guilabel>Login
Script</guilabel> tab of the <guilabel>Edit Account</guilabel> dialog.</para>

<para>In this example, the user <systemitem>userxyz</systemitem> needs the
following script to be called.  The callback server already knows the table of
names and their applicable phone numbers, so you select the phone number to be
used with an alias, for security purposes.</para>

<para>For each line, choose the criteria from the drop down list on the left of
the dialog, and type in the action in the text box on it's right.  Choose the
<guibutton>Add</guibutton> to add each line to the script.  You can use
<guibutton>Insert</guibutton> to add a line into the middle of the script, and
<guibutton>Remove</guibutton> to delete a line if you made a mistake.</para>

<para>The entire script should look something like this (without the comments,
shown here starting with a #)</para>

<screen> 
Expect       ogin:           <lineannotation># remember, we do ordinary terminal login</lineannotation>
ID           ""              <lineannotation># kppp sends the id you configured in
the main dialog</lineannotation>
Expect       for userxyz:    <lineannotation># a list of available numbers is
shown, the user should choose one</lineannotation> 
Send         userxyz-home    <lineannotation># the user wants to be called back
on their home number</lineannotation>
Expect       ogin:           <lineannotation># The callback process is now
running, a new connection, and so a new login.</lineannotation>
ID
Expect       assword:        <lineannotation># Now send your password</lineannotation>
Expect       &gt;            <lineannotation># Wait for the command prompt (the
prompt may vary)</lineannotation>
Send         start_ppp       <lineannotation># this command starts the pppd</lineannotation>
</screen>

<para>After waiting for the login request, the user sends his ID and waits for a
list of available phone numbers for that username.  Then he tells the server
which of the numbers offered he would like to be called back on.  &kppp; can
open a dialog for this, if your location changes often, &eg; you are a sales
representative and move from hotel to hotel.  Now the server is expecting login
and password for authentication, but in the meantime, the server hangs up and
calls the user back.  The authentication information is sent, and &kppp; waits
for a command prompt, and then starts a small script (here called
<filename>start_ppp</filename> which fires up <application>pppd</application> on
the server.</para>
					      
<para>The <filename>start_ppp</filename> script might look something like the
following:</para>
					      
<programlisting>
#!/bin/sh
stty -echo
exec /usr/sbin/pppd -detach silent modem
</programlisting>
					      
<para>Of course, setting up a <acronym>PPP</acronym> server is not within the
scope of this document.  For detailed information, see the
<application>pppd</application> man pages.  An excellent description of a
callback server can be found at <ulink
url="http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback">
http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback</ulink></para>
</step>
</procedure>

<para>All other configuration issues, such as <application>pppd</application>
configuration or <acronym>IP</acronym> settings work as normal, and no special
software is required to pick up the line.</para>

<note>
<para>&kppp; callback and other programs such as
<application>mgetty</application> or any other faxgetty can be run on the same
serial port.  There are no problems with the dial in, as &kppp; creates a lock
file which will tell the getty program that another application (in this case,
&kppp; of course,) is using the line at that time.</para>
</note>

</sect2>

<sect2 id="callback-troubleshooting">
<title>Troubleshooting</title>

<para>There are some known problems with &kppp; in callback mode:</para>

<itemizedlist>
<listitem>
<para>As you initialize the modem to auto answer, you need to reset the modem
after your connection is closed.  Otherwise, your modem will continue to pick up
the line for you, which is not a good idea if the line in question is your main
phone line.</para>
</listitem>
<listitem>
<para>&kppp; has some small problems when sharing a line with another program,
such as <application>mgetty</application>.  If <application>mgetty</application>
is running on the same modem line, &kppp; is not able to initialize the modem
correctly. <!-- This happens on the second try (does this mean it can initialize -->
<!-- on the second try, or it fails on the second time? --> </para>
</listitem>
<listitem>
<para>&kppp; is unable to prompt for certain user input during a scripting based
login.  Unfortunately, when using the example script above, &kppp; also asks for
the user name the second time the callback server requests it.  You can get rid
of this by hardcoding your userid into the login script (not very portable or
nice, but it works.</para>
</listitem>
</itemizedlist>

</sect2>

<sect2 id="callback-resources">
<title>Internet Resources for server software</title>

<para>&Linux; callback server software bundles are available in many
places.</para>

<para>The well known <application>mgetty</application> is a very powerful
program, and is also able to handle callback connections.  A description of how
to set up <application>mgetty</application> for this purpose is maintained at 
<ulink url="http://www.dyer.demon.co.uk/slug/tipscrip.htm">
http://www.dyer.demon.co.uk/slug/tipscrip.htm</ulink>, by Colin McKinnon,
<email>colin@wew.co.uk</email>.</para>

<para>There is also a ready to use package for &Linux; at <ulink
url="http://www.icce.rug.nl/docs/programs/callback/callback.html">
http://www.icce.rug.nl/docs/programs/callback/callback.html</ulink>.  This
package is maintained by Frank B. Brokken, <email>frank@icce.rug.nl</email>.  As
the setup, although straightforward, is not very easy, I have written a short
introduction for it at <ulink
url="http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback">http://ap-dec717c.physik.uni-karlsruhe.de/~mh/callback/</ulink>,
which also contains a more general introduction to callback.</para>

</sect2>
</sect1>

<sect1 id="nt-callback">
<title>&Windows; NT <acronym>RAS</acronym> callback</title>

<para>&Windows; NT uses a completely different approach than the one described
above.  NT requires an extension to the <acronym>PPP</acronym> protocol itself,
called <acronym>CBCP</acronym> (Call Back Control Protocol).
<application>pppd</application> has support for this protocol, but you must
recompile <application>pppd</application>.  If anybody has experience with
successfully connecting to an NT callback server, please let us know.</para>

</sect1>
</chapter>