<chapter id="callback"> <title >&kppp; instellen voor terugbellen</title> <para >Dit hoofdstuk is gebaseerd op materiaal geleverd door Martin Häfner, <email >mh@ap-dec717c.physik.uni-karlsruhe.de</email ></para> <sect1 id="unix-callback-server"> <title >&UNIX; of &Linux; terugbelserver</title> <para >Deze sectiw introduceert &UNIX; (&Linux;) terugbellen en hoe &kppp; geconfigureerd kan worden om te verbinden met een &UNIX; terugbelserver, speciaal naar een script dat is gebaseerd op de &Linux; <link linkend="callback-resources" >terugbelserver</link ></para> <sect2> <title >Een inleiding tot terugbellen</title> <para >Er zijn verschillende redenen om terugbellen te gebruiken. Enkele hiervan zijn:</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 >Dit vertelt het modem om de hoorn op te nemen na één <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 >De opzet van &kppp;</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 >De procedure is tamelijk recht-toe-recht-aan, als volgt.</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 its 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 >Probleemoplossing</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. </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>
