<HTML ><HEAD ><TITLE >Obey Data Terminal Ready and Data Carrier Detect</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="Use good passwords" HREF="security-password.html"><LINK REL="NEXT" TITLE="Use or configure a dumb modem" HREF="security-dumb.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-password.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-dumb.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECTION" ><H1 CLASS="SECTION" ><A NAME="SECURITY-DTR" ></A >9.2. Obey Data Terminal Ready and Data Carrier Detect</H1 ><P >The <SPAN CLASS="ACRONYM" >RS-232</SPAN > Data Terminal Ready signal is lowered when the computer wishes the modem to hang up. The computer wishes to hang up when people have ended their login session ends or when they fail to respond to the <TT CLASS="PROMPT" >login:</TT > prompt.</P ><P >Using a modem cable that has <SPAN CLASS="ACRONYM" >DTR</SPAN > wired and a modem that is configured to obey <SPAN CLASS="ACRONYM" >DTR</SPAN > is essential to prevent denial of service attacks upon the access to the console.</P ><P >Without <SPAN CLASS="ACRONYM" >DTR</SPAN > a caller can simply hold the modem line open, denying system administrators access to the console.</P ><P >The <SPAN CLASS="ACRONYM" >RS-232</SPAN > Data Carrier Detect signal is lowered when the user hangs up.</P ><P >Using a modem cable that has <SPAN CLASS="ACRONYM" >DCD</SPAN > wired and a modem that is configured to assert <SPAN CLASS="ACRONYM" >DCD</SPAN > is essential to prevent people dialling in after a user has hang up and from carrying on their session.</P ><P >Without <SPAN CLASS="ACRONYM" >DCD</SPAN > the session is not cleared when an accidental disconnection occurs. This allows any subsequent caller to resume the previous session. The machine is totally compromised if the previous user was <SPAN CLASS="SYSTEMITEM" >root</SPAN >.</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-password.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-dumb.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Use good passwords</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="security.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Use or configure a dumb modem</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
