200 lines
3.3 KiB
HTML
200 lines
3.3 KiB
HTML
<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
|
|
> |