<HTML ><HEAD ><TITLE >Use or configure a dumb modem</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="Remote Serial Console HOWTO" HREF="index.html"><LINK REL="UP" TITLE="Security" HREF="security.html"><LINK REL="PREVIOUS" TITLE="Obey Data Terminal Ready and Data Carrier Detect" HREF="security-dtr.html"><LINK REL="NEXT" TITLE="Restrict console messages" HREF="security-messages.html"></HEAD ><BODY CLASS="SECTION" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Remote Serial Console HOWTO</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="security-dtr.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 9. Security</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="security-messages.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECTION" ><H1 CLASS="SECTION" ><A NAME="SECURITY-DUMB" ></A >9.3. Use or configure a dumb modem</H1 ><P >Most modems use the Hayes <SPAN CLASS="ACRONYM" >AT</SPAN > command set. The modem's attention is gained by sending <TT CLASS="LITERAL" >+++</TT > surrounded by some idle time. Commands are then sent prefixed by <TT CLASS="LITERAL" >AT</TT >.</P ><P >Unfortunately, if the modem sees <TT CLASS="LITERAL" >+++</TT > during a call it may revert to command mode. The modem can then be configured by the caller. For example, the modem could be set to permit incoming calls only from the number <SPAN CLASS="QUOTE" >"0"</SPAN >, this would deny the system administrators access to the modem.</P ><P >The attention command can be removed using <B CLASS="COMMAND" >AT S2=255</B >. Of course once that is done no more <SPAN CLASS="ACRONYM" >AT</SPAN > commands can be given to the modem, so any other configuration of the modem needs to be done prior to that command.</P ><P >Unfortunately, when power to the modem is applied the modem starts in command mode. So a carefully chosen console message could disable the modem.</P ><P >The best solution is to select a modem that has a <SPAN CLASS="QUOTE" >"dumb"</SPAN > or <SPAN CLASS="QUOTE" >"select profile"</SPAN > <SPAN CLASS="ACRONYM" >DIP</SPAN > switch or jumper. These switches disable command mode and load the modem's saved configuration when they start.</P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="security-dtr.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="security-messages.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Obey Data Terminal Ready and Data Carrier Detect</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="security.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Restrict console messages</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
