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diff --git a/doc/kppp/callback.docbook b/doc/kppp/callback.docbook new file mode 100644 index 00000000..93f79238 --- /dev/null +++ b/doc/kppp/callback.docbook @@ -0,0 +1,268 @@ +<chapter id="callback"> +<title>Configuring &kppp; for callback</title> + +<para>This chapter is based on material provided by Martin Häfner, +<email>[email protected]</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&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&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&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 > <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, ⪚ 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>[email protected]</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>[email protected]</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> |