path: root/en_US.ISO8859-1/books/handbook/serialcomms/chapter.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!--
     The FreeBSD Documentation Project

     $FreeBSD$
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<chapter xmlns="http://docbook.org/ns/docbook"
  xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
  xml:id="serialcomms">
  <title>Serial Communications</title>

  <sect1 xml:id="serial-synopsis">
    <title>Synopsis</title>

    <indexterm><primary>serial communications</primary></indexterm>

    <para>&unix; has always had support for serial communications as
      the very first &unix; machines relied on serial lines for user
      input and output.  Things have changed a lot from the days
      when the average terminal consisted of a 10-character-per-second
      serial printer and a keyboard.  This chapter covers some of the
      ways serial communications can be used on &os;.</para>

    <para>After reading this chapter, you will know:</para>

    <itemizedlist>
      <listitem>
	<para>How to connect terminals to a &os; system.</para>
      </listitem>
      <listitem>
	<para>How to use a modem to dial out to remote hosts.</para>
      </listitem>
      <listitem>
	<para>How to allow remote users to login to a &os; system
	  with a modem.</para>
      </listitem>
      <listitem>
	<para>How to boot a &os; system from a serial console.</para>
      </listitem>
    </itemizedlist>

    <para>Before reading this chapter, you should:</para>

    <itemizedlist>
      <listitem>
	<para>Know how to <link linkend="kernelconfig"> configure and
	    install a custom kernel</link>.</para>
      </listitem>
      <listitem>
	<para>Understand <link linkend="basics"> &os; permissions
	    and processes</link>.</para>
      </listitem>
      <listitem>
	<para>Have access to the technical manual for the serial
	  hardware to be used with &os;.</para>
      </listitem>
    </itemizedlist>
  </sect1>

  <sect1 xml:id="serial">
    <title>Serial Terminology and Hardware</title>

    <para>The following terms are often used in serial
      communications:</para>
    <variablelist>
      <varlistentry>
	<term><acronym>bps</acronym></term>
	<listitem>
	  <para>Bits per
	    Second<indexterm><primary>bits-per-second</primary></indexterm>
	    (<acronym>bps</acronym>) is the rate at which data is
	    transmitted.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><acronym>DTE</acronym></term>
	<listitem>
	  <para>Data Terminal
	    Equipment<indexterm><primary>DTE</primary></indexterm>
	    (<acronym>DTE</acronym>) is one of two endpoints in a
	    serial communication.  An example would be a
	    computer.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><acronym>DCE</acronym></term>
	<listitem>
	  <para>Data Communications
	    Equipment<indexterm><primary>DCE</primary></indexterm>
	    (<acronym>DTE</acronym>) is the other endpoint in a
	    serial communication.  Typically, it is a modem or serial
	    terminal.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><acronym>RS-232</acronym></term>
	<listitem>
	  <para>The original standard which defined hardware serial
	    communications.  It has since been renamed to
	    <acronym>TIA-232</acronym>.</para>
	</listitem>
      </varlistentry>
    </variablelist>

    <para>When referring to communication data rates, this section
      does not use the term <firstterm>baud</firstterm>.  Baud refers
      to the number of electrical state transitions made in a period
      of time, while <acronym>bps</acronym> is the correct term to
      use.</para>

    <para>To connect a serial terminal to a &os; system, a serial port
      on the computer and the proper cable to connect to the serial
      device are needed.  Users who are already familiar with serial
      hardware and cabling can safely skip this section.</para>

    <sect2 xml:id="term-cables-null">
      <title>Serial Cables and Ports</title>

      <para>There are several different kinds of serial cables.  The
	two most common types are null-modem cables and standard
	<acronym>RS-232</acronym> cables.  The documentation for the
	hardware should describe the type of cable required.</para>

      <para>These two types of cables differ in how the wires are
	connected to the connector.  Each wire represents a signal,
	with the defined signals summarized in <xref
	  linkend="serialcomms-signal-names"/>.  A standard serial
	cable passes all of the <acronym>RS-232C</acronym> signals
	straight through.  For example, the <quote>Transmitted
	  Data</quote> pin on one end of the cable goes to the
	<quote>Transmitted Data</quote> pin on the other end.  This is
	the type of cable used to connect a modem to the &os; system,
	and is also appropriate for some terminals.</para>

      <para>A null-modem cable switches the <quote>Transmitted
	  Data</quote> pin of the connector on one end with the
	<quote>Received Data</quote> pin on the other end.  The
	connector can be either a <acronym>DB-25</acronym> or a
	<acronym>DB-9</acronym>.</para>

      <para>A null-modem cable can be constructed using the pin
	connections summarized in <xref linkend="nullmodem-db25"/>,
	<xref linkend="nullmodem-db9"/>, and <xref
	  linkend="nullmodem-db9-25"/>.  While the standard calls for
	a straight-through pin 1 to pin 1 <quote>Protective
	  Ground</quote> line, it is often omitted.  Some terminals
	work using only pins 2, 3, and 7, while others require
	different configurations.  When in doubt, refer to the
	documentation for the hardware.</para>

      <indexterm>
	<primary>null-modem cable</primary>
      </indexterm>

      <table frame="none" pgwide="1"
	xml:id="serialcomms-signal-names">
	<title><acronym>RS-232C</acronym> Signal Names</title>

	<tgroup cols="2">
	  <thead>
	    <row>
	      <entry align="left">Acronyms</entry>
	      <entry align="left">Names</entry>
	    </row>
	  </thead>

	  <tbody>
	    <row>
	      <entry><acronym>RD</acronym></entry>
	      <entry>Received Data</entry>
	    </row>

	    <row>
	      <entry><acronym>TD</acronym></entry>
	      <entry>Transmitted Data</entry>
	    </row>

	    <row>
	      <entry><acronym>DTR</acronym></entry>
	      <entry>Data Terminal Ready</entry>
	    </row>

	    <row>
	      <entry><acronym>DSR</acronym></entry>
	      <entry>Data Set Ready</entry>
	    </row>

	    <row>
	      <entry><acronym>DCD</acronym></entry>
	      <entry>Data Carrier Detect</entry>
	    </row>

	    <row>
	      <entry><acronym>SG</acronym></entry>
	      <entry>Signal Ground</entry>
	    </row>

	    <row>
	      <entry><acronym>RTS</acronym></entry>
	      <entry>Request to Send</entry>
	    </row>

	    <row>
	      <entry><acronym>CTS</acronym></entry>
	      <entry>Clear to Send</entry>
	    </row>
	  </tbody>
	</tgroup>
      </table>

      <table frame="none" pgwide="1" xml:id="nullmodem-db25">
	<title>DB-25 to DB-25 Null-Modem Cable</title>

	<tgroup cols="5">
	  <thead>
	    <row>
	      <entry align="left">Signal</entry>
	      <entry align="left">Pin #</entry>
	      <entry></entry>
	      <entry align="left">Pin #</entry>
	      <entry align="left">Signal</entry>
	    </row>
	  </thead>

	  <tbody>
	    <row>
	      <entry>SG</entry>
	      <entry>7</entry>
	      <entry>connects to</entry>
	      <entry>7</entry>
	      <entry>SG</entry>
	    </row>

	    <row>
	      <entry>TD</entry>
	      <entry>2</entry>
	      <entry>connects to</entry>
	      <entry>3</entry>
	      <entry>RD</entry>
	    </row>

	    <row>
	      <entry>RD</entry>
	      <entry>3</entry>
	      <entry>connects to</entry>
	      <entry>2</entry>
	      <entry>TD</entry>
	    </row>

	    <row>
	      <entry>RTS</entry>
	      <entry>4</entry>
	      <entry>connects to</entry>
	      <entry>5</entry>
	      <entry>CTS</entry>
	    </row>

	    <row>
	      <entry>CTS</entry>
	      <entry>5</entry>
	      <entry>connects to</entry>
	      <entry>4</entry>
	      <entry>RTS</entry>
	    </row>

	    <row>
	      <entry>DTR</entry>
	      <entry>20</entry>
	      <entry>connects to</entry>
	      <entry>6</entry>
	      <entry>DSR</entry>
	    </row>

	    <row>
	      <entry>DTR</entry>
	      <entry>20</entry>
	      <entry>connects to</entry>
	      <entry>8</entry>
	      <entry>DCD</entry>
	    </row>

	    <row>
	      <entry>DSR</entry>
	      <entry>6</entry>
	      <entry>connects to</entry>
	      <entry>20</entry>
	      <entry>DTR</entry>
	    </row>

	    <row>
	      <entry>DCD</entry>
	      <entry>8</entry>
	      <entry>connects to</entry>
	      <entry>20</entry>
	      <entry>DTR</entry>
	    </row>
	  </tbody>
	</tgroup>
      </table>

      <table frame="none" pgwide="1" xml:id="nullmodem-db9">
	<title>DB-9 to DB-9 Null-Modem Cable</title>

	<tgroup cols="5">
	  <thead>
	    <row>
	      <entry align="left">Signal</entry>
	      <entry align="left">Pin #</entry>
	      <entry></entry>
	      <entry align="left">Pin #</entry>
	      <entry align="left">Signal</entry>
	    </row>
	  </thead>

	  <tbody>
	    <row>
	      <entry>RD</entry>
	      <entry>2</entry>
	      <entry>connects to</entry>
	      <entry>3</entry>
	      <entry>TD</entry>
	    </row>

	    <row>
	      <entry>TD</entry>
	      <entry>3</entry>
	      <entry>connects to</entry>
	      <entry>2</entry>
	      <entry>RD</entry>
	    </row>

	    <row>
	      <entry>DTR</entry>
	      <entry>4</entry>
	      <entry>connects to</entry>
	      <entry>6</entry>
	      <entry>DSR</entry>
	    </row>

	    <row>
	      <entry>DTR</entry>
	      <entry>4</entry>
	      <entry>connects to</entry>
	      <entry>1</entry>
	      <entry>DCD</entry>
	    </row>

	    <row>
	      <entry>SG</entry>
	      <entry>5</entry>
	      <entry>connects to</entry>
	      <entry>5</entry>
	      <entry>SG</entry>
	    </row>

	    <row>
	      <entry>DSR</entry>
	      <entry>6</entry>
	      <entry>connects to</entry>
	      <entry>4</entry>
	      <entry>DTR</entry>
	    </row>

	    <row>
	      <entry>DCD</entry>
	      <entry>1</entry>
	      <entry>connects to</entry>
	      <entry>4</entry>
	      <entry>DTR</entry>
	    </row>

	    <row>
	      <entry>RTS</entry>
	      <entry>7</entry>
	      <entry>connects to</entry>
	      <entry>8</entry>
	      <entry>CTS</entry>
	    </row>

	    <row>
	      <entry>CTS</entry>
	      <entry>8</entry>
	      <entry>connects to</entry>
	      <entry>7</entry>
	      <entry>RTS</entry>
	    </row>
	  </tbody>
	</tgroup>
      </table>

      <table frame="none" pgwide="1" xml:id="nullmodem-db9-25">
	<title>DB-9 to DB-25 Null-Modem Cable</title>

	<tgroup cols="5">
	  <thead>
	    <row>
	      <entry align="left">Signal</entry>
	      <entry align="left">Pin #</entry>
	      <entry></entry>
	      <entry align="left">Pin #</entry>
	      <entry align="left">Signal</entry>
	    </row>
	  </thead>

	  <tbody>
	    <row>
	      <entry>RD</entry>
	      <entry>2</entry>
	      <entry>connects to</entry>
	      <entry>2</entry>
	      <entry>TD</entry>
	    </row>

	    <row>
	      <entry>TD</entry>
	      <entry>3</entry>
	      <entry>connects to</entry>
	      <entry>3</entry>
	      <entry>RD</entry>
	    </row>

	    <row>
	      <entry>DTR</entry>
	      <entry>4</entry>
	      <entry>connects to</entry>
	      <entry>6</entry>
	      <entry>DSR</entry>
	    </row>

	    <row>
	      <entry>DTR</entry>
	      <entry>4</entry>
	      <entry>connects to</entry>
	      <entry>8</entry>
	      <entry>DCD</entry>
	    </row>

	    <row>
	      <entry>SG</entry>
	      <entry>5</entry>
	      <entry>connects to</entry>
	      <entry>7</entry>
	      <entry>SG</entry>
	    </row>

	    <row>
	      <entry>DSR</entry>
	      <entry>6</entry>
	      <entry>connects to</entry>
	      <entry>20</entry>
	      <entry>DTR</entry>
	    </row>

	    <row>
	      <entry>DCD</entry>
	      <entry>1</entry>
	      <entry>connects to</entry>
	      <entry>20</entry>
	      <entry>DTR</entry>
	    </row>

	    <row>
	      <entry>RTS</entry>
	      <entry>7</entry>
	      <entry>connects to</entry>
	      <entry>5</entry>
	      <entry>CTS</entry>
	    </row>

	    <row>
	      <entry>CTS</entry>
	      <entry>8</entry>
	      <entry>connects to</entry>
	      <entry>4</entry>
	      <entry>RTS</entry>
	    </row>
	  </tbody>
	</tgroup>
      </table>

      <note>
	<para>When one pin at one end connects to a pair of pins at
	  the other end, it is usually implemented with one short wire
	  between the pair of pins in their connector and a long wire
	  to the other single pin.</para>
      </note>

      <para>Serial ports are the devices through which data is
	transferred between the &os; host computer and the terminal.
	Several kinds of serial ports exist.  Before purchasing or
	constructing a cable, make sure it will fit the ports on the
	terminal and on the &os; system.</para>

      <para>Most terminals have <acronym>DB-25</acronym> ports.
	Personal computers may have <acronym>DB-25</acronym> or
	<acronym>DB-9</acronym> ports.  A multiport serial card may
	have <acronym>RJ-12</acronym> or <acronym>RJ-45/</acronym>
	ports.  See the documentation that accompanied the hardware
	for specifications on the kind of port or visually verify the
	type of port.</para>

      <para>In &os;, each serial port is accessed through an entry in
	<filename>/dev</filename>.  There are two different kinds of
	entries:</para>

      <itemizedlist>
	<listitem>
	  <para>Call-in ports are named
	    <filename>/dev/ttyu<replaceable>N</replaceable></filename>
	    where <replaceable>N</replaceable> is the port number,
	    starting from zero.  If a terminal is connected to the
	    first serial port (<filename>COM1</filename>), use
	    <filename>/dev/ttyu0</filename> to refer to the terminal.
	    If the terminal is on the second serial port
	    (<filename>COM2</filename>), use
	    <filename>/dev/ttyu1</filename>, and so forth.  Generally,
	    the call-in port is used for terminals.  Call-in ports
	    require that the serial line assert the <quote>Data
	      Carrier Detect</quote> signal to work correctly.</para>
	</listitem>

	<listitem>
	  <para>Call-out ports are named
	    <filename>/dev/cuau<replaceable>N</replaceable></filename>
	    on &os; versions 8.X and higher and
	    <filename>/dev/cuad<replaceable>N</replaceable></filename>
	    on &os; versions 7.X and lower.  Call-out ports are
	    usually not used for terminals, but are used for modems.
	    The call-out port can be used if the serial cable or the
	    terminal does not support the <quote>Data Carrier
	      Detect</quote> signal.</para>
	</listitem>
      </itemizedlist>

      <para>&os; also provides initialization devices
	(<filename>/dev/ttyu<replaceable>N</replaceable>.init</filename>
	and
	<filename>/dev/cuau<replaceable>N</replaceable>.init</filename>
	or
	<filename>/dev/cuad<replaceable>N</replaceable>.init</filename>)
	and locking devices
	(<filename>/dev/ttyu<replaceable>N</replaceable>.lock</filename>
	and
	<filename>/dev/cuau<replaceable>N</replaceable>.lock</filename>
	or
	<filename>/dev/cuad<replaceable>N</replaceable>.lock</filename>).
	The initialization devices are used to initialize
	communications port parameters each time a port is opened,
	such as <literal>crtscts</literal> for modems which use
	<literal>RTS/CTS</literal> signaling for flow control.  The
	locking devices are used to lock flags on ports to prevent
	users or programs changing certain parameters.  Refer to
	&man.termios.4;, &man.sio.4;, and &man.stty.1; for information
	on terminal settings, locking and initializing devices, and
	setting terminal options, respectively.</para>
    </sect2>

    <sect2 xml:id="serial-hw-config">
      <title>Serial Port Configuration</title>

      <para>By default, &os; supports four serial ports which are
	commonly known as <filename>COM1</filename>,
	<filename>COM2</filename>, <filename>COM3</filename>, and
	<filename>COM4</filename>.  &os; also supports dumb multi-port
	serial interface cards, such as the BocaBoard 1008 and 2016,
	as well as more intelligent multi-port cards such as those
	made by Digiboard.  However, the default kernel only looks for
	the standard <filename>COM</filename> ports.</para>

      <para>To see if the system recognizes the serial ports, look for
	system boot messages that start with
	<literal>uart</literal>:</para>

      <screen>&prompt.root; <userinput>grep uart /var/run/dmesg.boot</userinput></screen>

      <para>If the system does not recognize all of the needed serial
	ports, additional entries can be added to
	<filename>/boot/device.hints</filename>.  This file already
	contains <literal>hint.uart.0.*</literal> entries for
	<filename>COM1</filename> and <literal>hint.uart.1.*</literal>
	entries for <filename>COM2</filename>.  When adding a port
	entry for <filename>COM3</filename> use
	<literal>0x3E8</literal>, and for <filename>COM4</filename>
	use <literal>0x2E8</literal>.  Common <acronym>IRQ</acronym>
	addresses are <literal>5</literal> for
	<filename>COM3</filename> and <literal>9</literal> for
	<filename>COM4</filename>.</para>

      <indexterm><primary><filename>ttyu</filename></primary></indexterm>
      <indexterm><primary><filename>cuau</filename></primary></indexterm>

      <para>To determine the default set of terminal
	<acronym>I/O</acronym> settings used by the port, specify its
	device name.  This example determines the settings for the
	call-in port on <filename>COM2</filename>:</para>

      <screen>&prompt.root; <userinput>stty -a -f /dev/<replaceable>ttyu1</replaceable></userinput></screen>

      <para>System-wide initialization of serial devices is controlled
	by <filename>/etc/rc.d/serial</filename>.  This file affects
	the default settings of serial devices.  To change the
	settings for a device, use <command>stty</command>.  By
	default, the changed settings are in effect until the device
	is closed and when the device is reopened, it goes back to the
	default set.  To permanently change the default set, open and
	adjust the settings of the initialization device.  For
	example, to turn on <option>CLOCAL</option> mode, 8 bit
	communication, and <option>XON/XOFF</option> flow control for
	<filename>ttyu5</filename>, type:</para>

      <screen>&prompt.root; <userinput>stty -f /dev/ttyu5.init clocal cs8 ixon ixoff</userinput></screen>

      <indexterm>
	<primary>rc files</primary>
	<secondary><filename>rc.serial</filename></secondary>
      </indexterm>

      <para>To prevent certain settings from being changed by an
	application, make adjustments to the locking device.  For
	example, to lock the speed of <filename>ttyu5</filename> to
	57600&nbsp;bps, type:</para>

      <screen>&prompt.root; <userinput>stty -f /dev/ttyu5.lock 57600</userinput></screen>

      <para>Now, any application that opens <filename>ttyu5</filename>
	and tries to change the speed of the port will be stuck with
	57600&nbsp;bps.</para>
    </sect2>
  </sect1>

  <sect1 xml:id="term">
    <info>
      <title>Terminals</title>

      <authorgroup>
	<author>
	  <personname>
	    <firstname>Sean</firstname>
	    <surname>Kelly</surname>
	  </personname>
	  <contrib>Contributed by </contrib> <!--in July 1996 -->
	</author>
      </authorgroup>
    </info>

    <indexterm><primary>terminals</primary></indexterm>

    <para>Terminals provide a convenient and low-cost way to access
      a &os; system when not at the computer's console or on a
      connected network.  This section describes how to use terminals
      with &os;.</para>

    <para>The original &unix; systems did not have consoles.  Instead,
      users logged in and ran programs through terminals that were
      connected to the computer's serial ports.</para>

    <para>The ability to establish a login session on a serial port
      still exists in nearly every &unix;-like operating system
      today, including &os;.  By using a terminal attached to an
      unused serial port, a user can log in and run any text program
      that can normally be run on the console or in an
      <command>xterm</command> window.</para>

    <para>Many terminals can be attached to a &os; system.  An older
      spare computer can be used as a terminal wired into a more
      powerful computer running &os;.  This can turn what might
      otherwise be a single-user computer into a powerful
      multiple-user system.</para>

    <para>&os; supports three types of terminals:</para>

    <variablelist>
      <varlistentry>
	<term>Dumb terminals</term>
	<listitem>
	  <para>Dumb terminals are specialized hardware that connect
	    to computers over serial lines.  They are called
	    <quote>dumb</quote> because they have only enough
	    computational power to display, send, and receive text.
	    No programs can be run on these devices.  Instead, dumb
	    terminals connect to a computer that runs the needed
	    programs.</para>

	  <para>There are hundreds of kinds of dumb terminals made by
	    many manufacturers, and just about any kind will work with
	    &os;.  Some high-end terminals can even display graphics,
	    but only certain software packages can take advantage of
	    these advanced features.</para>

	  <para>Dumb terminals are popular in work environments where
	    workers do not need access to graphical
	    applications.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>Computers Acting as Terminals</term>
	<listitem>
	  <para>Since a dumb terminal has just enough ability to
	    display, send, and receive text, any spare computer can
	    be a dumb terminal.  All that is needed is the proper
	    cable and some <firstterm>terminal emulation</firstterm>
	    software to run on the computer.</para>

	  <para>This configuration can be useful.  For example, if one
	    user is busy working at the &os; system's console, another
	    user can do some text-only work at the same time from a
	    less powerful personal computer hooked up as a terminal to
	    the &os; system.</para>

	  <para>There are at least two utilities in the base-system of
	    &os; that can be used to work through a serial connection:
	    &man.cu.1; and &man.tip.1;.</para>

	  <para>For example, to connect from a client system that runs
	    &os; to the serial connection of another system:</para>

	  <screen>&prompt.root; <userinput>cu -l /dev/cuau<replaceable>N</replaceable></userinput></screen>

	  <para>Ports are numbered starting from zero.  This means that
	    <filename>COM1</filename> is
	    <filename>/dev/cuau0</filename>.</para>

	  <para>Additional programs are available through the Ports
	    Collection, such as
	    <package>comms/minicom</package>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>X Terminals</term>
	<listitem>
	  <para>X terminals are the most sophisticated kind of
	    terminal available.  Instead of connecting to a serial
	    port, they usually connect to a network like Ethernet.
	    Instead of being relegated to text-only applications, they
	    can display any <application>&xorg;</application>
	    application.</para>

	  <para>This chapter does not cover the setup, configuration,
	    or use of X terminals.</para>
	</listitem>
      </varlistentry>
    </variablelist>

    <sect2 xml:id="term-config">
      <title>Terminal Configuration</title>

      <para>This section describes how to configure a &os; system to
	enable a login session on a serial terminal.  It assumes that
	the system recognizes the serial port to which the terminal is
	connected and that the terminal is connected with the correct
	cable.</para>

      <para>In &os;, <command>init</command> reads
	<filename>/etc/ttys</filename> and starts a
	<command>getty</command> process on the available terminals.
	The <command>getty</command> process is responsible for
	reading a login name and starting the <command>login</command>
	program.  The ports on the &os; system which allow logins are
	listed in <filename>/etc/ttys</filename>.  For example, the
	first virtual console, <filename>ttyv0</filename>, has an
	entry in this file, allowing logins on the console.  This file
	also contains entries for the other virtual consoles, serial
	ports, and pseudo-ttys.  For a hardwired terminal, the serial
	port's <filename>/dev</filename> entry is listed without the
	<literal>/dev</literal> part.  For example,
	<filename>/dev/ttyv0</filename> is listed as
	<literal>ttyv0</literal>.</para>

      <para>The default <filename>/etc/ttys</filename> configures
	support for the first four serial ports,
	<filename>ttyu0</filename> through
	<filename>ttyu3</filename>:</para>

      <programlisting>ttyu0   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu1   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu2   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu3   "/usr/libexec/getty std.9600"   dialup  off secure</programlisting>

      <para>When attaching a terminal to one of those ports, modify
	the default entry to set the required speed and terminal type,
	to turn the device <literal>on</literal> and, if needed, to
	change the port's <literal>secure</literal> setting.  If the
	terminal is connected to another port, add an entry for the
	port.</para>

      <para><xref linkend="ex-etc-ttys"/> configures two terminals in
	<filename>/etc/ttys</filename>.  The first entry configures a
	Wyse-50 connected to <filename>COM2</filename>.  The second
	entry configures an old computer running
	<application>Procomm</application> terminal software emulating
	a VT-100 terminal.  The computer is connected to the sixth
	serial port on a multi-port serial card.</para>

      <example xml:id="ex-etc-ttys">
	<title>Configuring Terminal Entries</title>

	<programlisting>ttyu1<co xml:id="co-ttys-line1col1"/>  "/usr/libexec/getty std.38400"<co xml:id="co-ttys-line1col2"/>  wy50<co xml:id="co-ttys-line1col3"/>  on<co xml:id="co-ttys-line1col4"/>  insecure<co xml:id="co-ttys-line1col5"/>
ttyu5   "/usr/libexec/getty std.19200"  vt100  on insecure</programlisting>

	<calloutlist>
	  <callout arearefs="co-ttys-line1col1">
	    <para>The first field specifies the device name of the
	      serial terminal.</para>
	  </callout>

	  <callout arearefs="co-ttys-line1col2">
	    <para>The second field tells <command>getty</command> to
	      initialize and open the line, set the line speed, prompt
	      for a user name, and then execute the
	      <command>login</command> program.  The optional
	      <firstterm>getty type</firstterm> configures
	      characteristics on the terminal line, like
	      <acronym>bps</acronym> rate and parity.  The available
	      getty types are listed in
	      <filename>/etc/gettytab</filename>.  In almost all
	      cases, the getty types that start with
	      <literal>std</literal> will work for hardwired terminals
	      as these entries ignore parity.  There is a
	      <literal>std</literal> entry for each
	      <acronym>bps</acronym> rate from 110 to 115200.  Refer
	      to &man.gettytab.5; for more information.</para>

	    <para>When setting the getty type, make sure to match the
	      communications settings used by the terminal.  For this
	      example, the Wyse-50 uses no parity and connects at
	      38400&nbsp;bps.  The computer uses no parity and
	      connects at 19200&nbsp;bps.</para>
	  </callout>

	  <callout arearefs="co-ttys-line1col3">
	    <para>The third field is the type of terminal.  For
	      dial-up ports, <literal>unknown</literal> or
	      <literal>dialup</literal> is typically used since users
	      may dial up with practically any type of terminal or
	      software.  Since the terminal type does not change for
	      hardwired terminals, a real terminal type from
	      <filename>/etc/termcap</filename> can be specified.  For
	      this example, the Wyse-50 uses the real terminal type
	      while the computer running
	      <application>Procomm</application> is set to emulate a
	      VT-100.</para>
	  </callout>

	  <callout arearefs="co-ttys-line1col4">
	    <para>The fourth field specifies if the port should be
	      enabled.  To enable logins on this port, this field must
	      be set to <literal>on</literal>.</para>
	  </callout>

	  <callout arearefs="co-ttys-line1col5">
	    <para>The final field is used to specify whether the port
	      is secure.  Marking a port as <literal>secure</literal>
	      means that it is trusted enough to allow <systemitem
		class="username">root</systemitem> to login from that
	      port.  Insecure ports do not allow <systemitem
		class="username">root</systemitem> logins.  On an
	      insecure port, users must login from unprivileged
	      accounts and then use <command>su</command> or a similar
	      mechanism to gain superuser privileges, as described in
	      <xref linkend="users-superuser"/>.  For security
	      reasons, it is recommended to change this setting to
	      <literal>insecure</literal>.</para>
	  </callout>
	</calloutlist>
      </example>

      <para>After making any changes to
	<filename>/etc/ttys</filename>, send a SIGHUP (hangup) signal
	to the <command>init</command> process to force it to re-read
	its configuration file:</para>

      <screen>&prompt.root; <userinput>kill -HUP 1</userinput></screen>

      <para>Since <command>init</command> is always the first process
	run on a system, it always has a process <acronym>ID</acronym>
	of <literal>1</literal>.</para>

      <para>If everything is set up correctly, all cables are in
	place, and the terminals are powered up, a
	<command>getty</command> process should now be running on each
	terminal and login prompts should be available on each
	terminal.</para>
    </sect2>

    <sect2 xml:id="term-debug">
      <title>Troubleshooting the Connection</title>

      <para>Even with the most meticulous attention to detail,
	something could still go wrong while setting up a terminal.
	Here is a list of common symptoms and some suggested
	fixes.</para>

      <para>If no login prompt appears, make sure the terminal is
	plugged in and powered up.  If it is a personal computer
	acting as a terminal, make sure it is running terminal
	emulation software on the correct serial port.</para>

      <para>Make sure the cable is connected firmly to both the
	terminal and the &os; computer.  Make sure it is the right
	kind of cable.</para>

      <para>Make sure the terminal and &os; agree on the
	<acronym>bps</acronym> rate and parity settings.  For a video
	display terminal, make sure the contrast and brightness
	controls are turned up.  If it is a printing terminal, make
	sure paper and ink are in good supply.</para>

      <para>Use <command>ps</command> to make sure that a
	<command>getty</command> process is running and serving the
	terminal.  For example, the following listing shows that a
	<command>getty</command> is running on the second serial port,
	<filename>ttyu1</filename>, and is using the
	<literal>std.38400</literal> entry in
	<filename>/etc/gettytab</filename>:</para>

      <screen>&prompt.root; <userinput>ps -axww|grep ttyu</userinput>
22189  d1  Is+    0:00.03 /usr/libexec/getty std.38400 ttyu1</screen>

      <para>If no <command>getty</command> process is running, make
	sure the port is enabled in <filename>/etc/ttys</filename>.
	Remember to run <command>kill -HUP 1</command> after modifying
	<filename>/etc/ttys</filename>.</para>

      <para>If the <command>getty</command> process is running but the
	terminal still does not display a login prompt, or if it
	displays a prompt but will not accept typed input, the
	terminal or cable may not support hardware handshaking.  Try
	changing the entry in <filename>/etc/ttys</filename> from
	<literal>std.38400</literal> to
	<literal>3wire.38400</literal>, then run <command>kill -HUP
	  1</command> after modifying <filename>/etc/ttys</filename>.
	The <literal>3wire</literal> entry is similar to
	<literal>std</literal>, but ignores hardware handshaking.  The
	baud rate may need to be reduced or software flow control
	enabled when using <literal>3wire</literal> to prevent buffer
	overflows.</para>

      <para>If garbage appears instead of a login prompt, make sure
	the terminal and &os; agree on the <acronym>bps</acronym> rate
	and parity settings.  Check the <command>getty</command>
	processes to make sure the correct
	<replaceable>getty</replaceable> type is in use.  If not, edit
	<filename>/etc/ttys</filename> and run <command>kill
	  -HUP 1</command>.</para>

      <para>If characters appear doubled and the password appears when
	typed, switch the terminal, or the terminal emulation
	software, from <quote>half duplex</quote> or <quote>local
	  echo</quote> to <quote>full duplex.</quote></para>
    </sect2>
  </sect1>

  <sect1 xml:id="dialup">
    <info>
      <title>Dial-in Service</title>

      <authorgroup>
	<author>
	  <personname>
	    <firstname>Guy</firstname>
	    <surname>Helmer</surname>
	  </personname>
	  <contrib>Contributed by </contrib>
	</author>
      </authorgroup>

      <authorgroup>
	<author>
	  <personname>
	    <firstname>Sean</firstname>
	    <surname>Kelly</surname>
	  </personname>
	  <contrib>Additions by </contrib>
	</author>
      </authorgroup>
    </info>

    <indexterm><primary>dial-in service</primary></indexterm>

    <para>Configuring a &os; system for dial-in service is similar to
      configuring terminals, except that modems are used instead of
      terminal devices.  &os; supports both external and internal
      modems.</para>

    <para>External modems are more convenient because they often can
      be configured via parameters stored in non-volatile
      <acronym>RAM</acronym> and they usually provide lighted
      indicators that display the state of important
      <acronym>RS-232</acronym> signals, indicating whether the modem
      is operating properly.</para>

    <para>Internal modems usually lack non-volatile
      <acronym>RAM</acronym>, so their configuration may be limited to
      setting <acronym>DIP</acronym> switches.  If the internal modem
      has any signal indicator lights, they are difficult to view when
      the system's cover is in place.</para>

    <indexterm><primary>modem</primary></indexterm>

    <para>When using an external modem, a proper cable is needed.  A
      standard <acronym>RS-232C</acronym> serial cable should
      suffice.</para>

    <para>&os; needs the <acronym>RTS</acronym> and
      <acronym>CTS</acronym> signals for flow control at speeds above
      2400&nbsp;bps, the <acronym>CD</acronym> signal to detect when a
      call has been answered or the line has been hung up, and the
      <acronym>DTR</acronym> signal to reset the modem after a session
      is complete.  Some cables are wired without all of the needed
      signals, so if a login session does not go away when the line
      hangs up, there may be a problem with the cable.  Refer to <xref
	linkend="term-cables-null"/> for more information about these
      signals.</para>

    <para>Like other &unix;-like operating systems, &os; uses the
      hardware signals to find out when a call has been answered or a
      line has been hung up and to hangup and reset the modem after a
      call.  &os; avoids sending commands to the modem or watching for
      status reports from the modem.</para>

    <para>&os; supports the <acronym>NS8250</acronym>,
      <acronym>NS16450</acronym>, <acronym>NS16550</acronym>, and
      <acronym>NS16550A</acronym>-based <acronym>RS-232C</acronym>
      (<acronym>CCITT</acronym> V.24) communications interfaces.  The
      8250 and 16450 devices have single-character buffers.  The 16550
      device provides a 16-character buffer, which allows for better
      system performance.  Bugs in plain 16550 devices prevent the use
      of the 16-character buffer, so use 16550A devices if possible.
      Because single-character-buffer devices require more work by the
      operating system than the 16-character-buffer devices,
      16550A-based serial interface cards are preferred.  If the
      system has many active serial ports or will have a heavy load,
      16550A-based cards are better for low-error-rate
      communications.</para>

    <para>The rest of this section demonstrates how to configure a
      modem to receive incoming connections, how to communicate with
      the modem, and offers some troubleshooting tips.</para>

    <sect2 xml:id="dialup-ttys">
      <title>Modem Configuration</title>

      <indexterm><primary>getty</primary></indexterm>
      <para>As with terminals, <command>init</command> spawns a
	<command>getty</command> process for each configured serial
	port used for dial-in connections.  When a user dials the
	modem's line and the modems connect, the <quote>Carrier
	  Detect</quote> signal is reported by the modem.  The kernel
	notices that the carrier has been detected and instructs
	<command>getty</command> to open the port and display a
	<prompt>login:</prompt> prompt at the specified initial line
	speed.  In a typical configuration, if garbage characters are
	received, usually due to the modem's connection speed being
	different than the configured speed, <command>getty</command>
	tries adjusting the line speeds until it receives reasonable
	characters.  After the user enters their login name,
	<command>getty</command> executes <command>login</command>,
	which completes the login process by asking for the user's
	password and then starting the user's shell.</para>

      <indexterm>
	<primary><command>/usr/bin/login</command></primary>
      </indexterm>

      <para>There are two schools of thought regarding dial-up modems.
	One configuration method is to set the modems and systems so
	that no matter at what speed a remote user dials in, the
	dial-in <acronym>RS-232</acronym> interface runs at a locked
	speed.  The benefit of this configuration is that the remote
	user always sees a system login prompt immediately.  The
	downside is that the system does not know what a user's true
	data rate is, so full-screen programs like
	<application>Emacs</application> will not adjust their
	screen-painting methods to make their response better for
	slower connections.</para>

      <para>The second method is to configure the
	<acronym>RS-232</acronym> interface to vary its speed based on
	the remote user's connection speed.  Because
	<command>getty</command> does not understand any particular
	modem's connection speed reporting, it gives a
	<prompt>login:</prompt> message at an initial speed and
	watches the characters that come back in response.  If the
	user sees junk, they should press <keycap>Enter</keycap> until
	they see a recognizable prompt.  If the data rates do not
	match, <command>getty</command> sees anything the user types
	as junk, tries the next speed, and gives the
	<prompt>login:</prompt> prompt again.  This procedure normally
	only takes a keystroke or two before the user sees a good
	prompt.  This login sequence does not look as clean as the
	locked-speed method, but a user on a low-speed connection
	should receive better interactive response from full-screen
	programs.</para>

      <para>When locking a modem's data communications rate at a
	particular speed, no changes to
	<filename>/etc/gettytab</filename> should be needed.  However,
	for a matching-speed configuration, additional entries may be
	required in order to define the speeds to use for the modem.
	This example configures a 14.4&nbsp;Kbps modem with a top
	interface speed of 19.2&nbsp;Kbps using 8-bit, no parity
	connections.  It configures <command>getty</command> to start
	the communications rate for a V.32bis connection at
	19.2&nbsp;Kbps, then cycles through 9600&nbsp;bps,
	2400&nbsp;bps, 1200&nbsp;bps, 300&nbsp;bps, and back to
	19.2&nbsp;Kbps.  Communications rate cycling is implemented
	with the <literal>nx=</literal> (next table) capability.  Each
	line uses a <literal>tc=</literal> (table continuation) entry
	to pick up the rest of the settings for a particular data
	rate.</para>

      <programlisting>#
# Additions for a V.32bis Modem
#
um|V300|High Speed Modem at 300,8-bit:\
        :nx=V19200:tc=std.300:
un|V1200|High Speed Modem at 1200,8-bit:\
        :nx=V300:tc=std.1200:
uo|V2400|High Speed Modem at 2400,8-bit:\
        :nx=V1200:tc=std.2400:
up|V9600|High Speed Modem at 9600,8-bit:\
        :nx=V2400:tc=std.9600:
uq|V19200|High Speed Modem at 19200,8-bit:\
        :nx=V9600:tc=std.19200:</programlisting>

      <para>For a 28.8&nbsp;Kbps modem, or to take advantage of
	compression on a 14.4&nbsp;Kbps modem, use a higher
	communications rate, as seen in this example:</para>

      <programlisting>#
# Additions for a V.32bis or V.34 Modem
# Starting at 57.6 Kbps
#
vm|VH300|Very High Speed Modem at 300,8-bit:\
        :nx=VH57600:tc=std.300:
vn|VH1200|Very High Speed Modem at 1200,8-bit:\
        :nx=VH300:tc=std.1200:
vo|VH2400|Very High Speed Modem at 2400,8-bit:\
        :nx=VH1200:tc=std.2400:
vp|VH9600|Very High Speed Modem at 9600,8-bit:\
        :nx=VH2400:tc=std.9600:
vq|VH57600|Very High Speed Modem at 57600,8-bit:\
        :nx=VH9600:tc=std.57600:</programlisting>

      <para>For a slow <acronym>CPU</acronym> or a heavily loaded
	system without 16550A-based serial ports, this configuration
	may produce <errorname>sio</errorname>
	<quote>silo</quote> errors at 57.6&nbsp;Kbps.</para>

      <indexterm>
	<primary><filename>/etc/ttys</filename></primary>
      </indexterm>

      <para>The configuration of <filename>/etc/ttys</filename> is
	similar to <xref linkend="ex-etc-ttys"/>, but a different
	argument is passed to <command>getty</command> and
	<literal>dialup</literal> is used for the terminal type.
	Replace <replaceable>xxx</replaceable> with the process
	<command>init</command> will run on the device:</para>

      <programlisting>ttyu0   "/usr/libexec/getty <replaceable>xxx</replaceable>"   dialup on</programlisting>

      <para>The <literal>dialup</literal> terminal type can be
	changed.  For example, setting <literal>vt102</literal> as the
	default terminal type allows users to use
	<acronym>VT102</acronym> emulation on their remote
	systems.</para>

      <para>For a locked-speed configuration, specify the speed with
	a valid type listed in <filename>/etc/gettytab</filename>.
	This example is for a modem whose port speed is locked at
	19.2&nbsp;Kbps:</para>

      <programlisting>ttyu0   "/usr/libexec/getty std.<replaceable>19200</replaceable>"   dialup on</programlisting>

      <para>In a matching-speed configuration, the entry needs to
	reference the appropriate beginning <quote>auto-baud</quote>
	entry in <filename>/etc/gettytab</filename>.  To continue the
	example for a matching-speed modem that starts at
	19.2&nbsp;Kbps, use this entry:</para>

      <programlisting>ttyu0   "/usr/libexec/getty V19200"   dialup on</programlisting>

      <para>After editing <filename>/etc/ttys</filename>, wait until
	the modem is properly configured and connected before
	signaling <command>init</command>:</para>

      <screen>&prompt.root; <userinput>kill -HUP 1</userinput></screen>

      <indexterm>
	<primary>rc files</primary>
	<secondary><filename>rc.serial</filename></secondary>
      </indexterm>

      <para>High-speed modems, like <acronym>V.32</acronym>,
	<acronym>V.32bis</acronym>, and <acronym>V.34</acronym>
	modems, use hardware (<literal>RTS/CTS</literal>) flow
	control.  Use <command>stty</command> to set the hardware flow
	control flag for the modem port.  This example sets the
	<varname>crtscts</varname> flag on <filename>COM2</filename>'s
	dial-in and dial-out initialization devices:</para>

      <screen>&prompt.root; <userinput>stty -f /dev/ttyu1.init crtscts</userinput>
&prompt.root; <userinput>stty -f /dev/cuau1.init crtscts</userinput></screen>
    </sect2>

<!--
Comment out for now. If this is still needed, the example should
either be updated or the section modified to be more generic
e.g. refer to the modem's manual
    <sect2>
      <title>Modem Settings</title>

      <para>For a modem whose parameters may be permanently set in
	non-volatile RAM, a terminal program such as
	<command>tip</command> can be used to set the parameters.
	Connect to the modem using the same communications speed as
	the initial speed <command>getty</command> will use and
	configure the modem's non-volatile RAM to match these
	requirements:</para>

      <itemizedlist>
	<listitem>
	  <para><acronym>CD</acronym> asserted when connected.</para>
	</listitem>

	<listitem>
	  <para><acronym>DTR</acronym> asserted for operation and
	    dropping DTR hangs up the line and resets the
	    modem.</para>
	</listitem>

	<listitem>
	  <para><acronym>CTS</acronym> transmitted data flow
	    control.</para>
	</listitem>

	<listitem>
	  <para>Disable <acronym>XON/XOFF</acronym> flow
	    control.</para>
	</listitem>

	<listitem>
	  <para><acronym>RTS</acronym> received data flow
	    control.</para>
	</listitem>

	<listitem>
	  <para>Quiet mode (no result codes).</para>
	</listitem>

	<listitem>
	  <para>No command echo.</para>
	</listitem>
      </itemizedlist>

      <para>Read the documentation for the modem to find out
	which commands and/or DIP switch settings are needed.</para>

      <para>For example, to set the above parameters on a &usrobotics;
	&sportster; 14,400 external modem, give these commands to
	the modem:</para>

      <programlisting>ATZ
AT&amp;C1&amp;D2&amp;H1&amp;I0&amp;R2&amp;W</programlisting>

      <para>Other settings can be adjusted in the modem, such as
	whether it will use V.42bis and/or MNP5 compression.</para>

      <para>The &usrobotics; &sportster; 14,400 external modem also
	has some DIP switches that need to be set.  Other modems,
	may need these settings:</para>

      <itemizedlist>
	<listitem>
	  <para>Switch 1: UP &mdash; DTR Normal</para>
	</listitem>

	<listitem>
	  <para>Switch 2: N/A (Verbal Result Codes/Numeric Result
	    Codes)</para>
	</listitem>

	<listitem>
	  <para>Switch 3: UP &mdash; Suppress Result Codes</para>
	</listitem>

	<listitem>
	  <para>Switch 4: DOWN &mdash; No echo, offline
	    commands</para>
	</listitem>

	<listitem>
	  <para>Switch 5: UP &mdash; Auto Answer</para>
	</listitem>

	<listitem>
	  <para>Switch 6: UP &mdash; Carrier Detect Normal</para>
	</listitem>

	<listitem>
	  <para>Switch 7: UP &mdash; Load NVRAM Defaults</para>
	</listitem>

	<listitem>
	  <para>Switch 8: N/A (Smart Mode/Dumb Mode)</para>
	</listitem>
      </itemizedlist>

      <para>Result codes should be disabled/suppressed for dial-up
	modems to avoid problems that can occur if
	<command>getty</command> mistakenly gives a
	<prompt>login:</prompt> prompt to a modem that is in command
	mode and the modem echoes the command or returns a result
	code.  This sequence can result in an extended, silly
	conversation between <command>getty</command> and the
	modem.</para>

      <para>For a locked-speed configuration, configure the modem to
	maintain a constant modem-to-computer data rate independent
	of the communications rate.  On a &usrobotics; &sportster;
	14,400 external modem, these commands will lock the
	modem-to-computer data rate at the speed used to issue the
	commands:</para>

      <programlisting>ATZ
AT&amp;B1&amp;W</programlisting>

      <para>For a variable-speed configuration, configure the modem
	to adjust its serial port data rate to match the incoming
	call rate.  On a &usrobotics; &sportster; 14,400 external
	modem, these commands will lock the modem's error-corrected
	data rate to the speed used to issue the commands, while
	allowing the serial port rate to vary for
	non-error-corrected connections:</para>

      <programlisting>ATZ
AT&amp;B2&amp;W</programlisting>

      <para>Most high-speed modems provide commands to view the
	modem's current operating parameters in a somewhat
	human-readable fashion.  On the &usrobotics; &sportster;
	14,400 external modem, <command>ATI5</command> displays the
	settings that are stored in the non-volatile RAM.  To see the
	true operating parameters of the modem, as influenced by the
	modem's DIP switch settings, use <command>ATZ</command> and
	then <command>ATI4</command>.</para>

      <para>For a different brand of modem, check the modem's manual
	to see how to double-check the modem's configuration
	parameters.</para>
    </sect2>
    -->

    <sect2>
      <title>Troubleshooting</title>

      <para>This section provides a few tips for troubleshooting a
	dial-up modem that will not connect to a &os; system.</para>

      <para>Hook up the modem to the &os; system and boot the system.
	If the modem has status indication lights, watch to see
	whether the modem's <acronym>DTR</acronym> indicator lights
	when the <prompt>login:</prompt> prompt appears on the
	system's console.  If it lights up, that should mean that &os;
	has started a <command>getty</command> process on the
	appropriate communications port and is waiting for the modem
	to accept a call.</para>

      <para>If the <acronym>DTR</acronym> indicator does not light,
	login to the &os; system through the console and type
	<command>ps ax</command> to see if &os; is running a
	<command>getty</command> process on the correct port:</para>

      <screen>  114 ??  I      0:00.10 /usr/libexec/getty V19200 <replaceable>ttyu0</replaceable></screen>

      <para>If the second column contains a <literal>d0</literal>
	instead of a <literal>??</literal> and the modem has not
	accepted a call yet, this means that <command>getty</command>
	has completed its open on the communications port.  This could
	indicate a problem with the cabling or a misconfigured modem
	because <command>getty</command> should not be able to open
	the communications port until the carrier detect signal has
	been asserted by the modem.</para>

      <para>If no <command>getty</command> processes are waiting to
	open the port, double-check that the entry for the port is
	correct in <filename>/etc/ttys</filename>.  Also, check
	<filename>/var/log/messages</filename> to see if there are
	any log messages from <command>init</command> or
	<command>getty</command>.</para>

      <para>Next, try dialing into the system.  Be sure to use 8 bits,
	no parity, and 1 stop bit on the remote system.  If a prompt
	does not appear right away, or the prompt shows garbage, try
	pressing <keycap>Enter</keycap> about once per second.  If
	there is still no <prompt>login:</prompt> prompt,
	try sending a <command>BREAK</command>.  When using a
	high-speed modem, try dialing again after locking the
	dialing modem's interface speed.</para>

      <para>If there is still no <prompt>login:</prompt> prompt, check
	<filename>/etc/gettytab</filename> again and double-check
	that:</para>

      <itemizedlist>
	<listitem>
	  <para>The initial capability name specified in the entry in
	    <filename>/etc/ttys</filename> matches the name of a
	    capability in <filename>/etc/gettytab</filename>.</para>
	</listitem>

	<listitem>
	  <para>Each <literal>nx=</literal> entry matches another
	    <filename>gettytab</filename> capability name.</para>
	</listitem>

	<listitem>
	  <para>Each <literal>tc=</literal> entry matches another
	    <filename>gettytab</filename> capability name.</para>
	</listitem>
      </itemizedlist>

      <para>If the modem on the &os; system will not answer, make
	sure that the modem is configured to answer the phone when
	<acronym>DTR</acronym> is asserted.  If the modem seems to be
	configured correctly, verify that the
	<acronym>DTR</acronym> line is asserted by checking the
	modem's indicator lights.</para>

      <para>If it still does not work, try sending an email
	to the &a.questions; describing the modem and the
	problem.</para>
    </sect2>
  </sect1>

  <sect1 xml:id="dialout">
    <title>Dial-out Service</title>

    <indexterm><primary>dial-out service</primary></indexterm>

    <para>The following are tips for getting the host to connect over
      the modem to another computer.  This is appropriate for
      establishing a terminal session with a remote host.</para>

    <para>This kind of connection can be helpful to get a file on the
      Internet if there are problems using PPP.  If PPP is not
      working, use the terminal session to FTP the needed file.  Then
      use zmodem to transfer it to the machine.</para>

    <sect2 xml:id="hayes-unsupported">
      <title>Using a Stock Hayes Modem</title>

      <para>A generic Hayes dialer is built into
	<command>tip</command>.  Use <literal>at=hayes</literal> in
	<filename>/etc/remote</filename>.</para>

      <para>The Hayes driver is not smart enough to recognize some of
	the advanced features of newer modems messages like
	<literal>BUSY</literal>, <literal>NO DIALTONE</literal>, or
	<literal>CONNECT 115200</literal>.  Turn those messages off
	when using <command>tip</command> with
	<command>ATX0&amp;W</command>.</para>

      <para>The dial timeout for <command>tip</command> is 60
	seconds.  The modem should use something less, or else
	<command>tip</command> will think there is a communication
	problem.  Try <command>ATS7=45&amp;W</command>.</para>
    </sect2>

    <sect2 xml:id="direct-at">
      <title>Using <literal>AT</literal> Commands</title>

      <indexterm>
	<primary><filename>/etc/remote</filename></primary>
      </indexterm>
      <para>Create a <quote>direct</quote> entry in
	<filename>/etc/remote</filename>.  For example, if the modem
	is hooked up to the first serial port,
	<filename>/dev/cuau0</filename>, use the following
	line:</para>

      <programlisting>cuau0:dv=/dev/cuau0:br#19200:pa=none</programlisting>

      <para>Use the highest <acronym>bps</acronym> rate the modem
	supports in the <literal>br</literal> capability.  Then, type
	<command>tip cuau0</command> to connect to the modem.</para>

      <para>Or, use <command>cu</command> as <systemitem
	  class="username">root</systemitem> with the following
	      command:</para>

      <screen>&prompt.root; <userinput>cu -l<replaceable>line</replaceable> -s<replaceable>speed</replaceable></userinput></screen>

      <para><replaceable>line</replaceable> is the serial port, such
	as <filename>/dev/cuau0</filename>, and
	<replaceable>speed</replaceable> is the speed, such as
	<literal>57600</literal>.  When finished entering the AT
	commands, type <command>~.</command> to exit.</para>
    </sect2>

    <sect2 xml:id="gt-failure">
      <title>The <literal>@</literal> Sign Does Not Work</title>

      <para>The <literal>@</literal> sign in the phone number
	capability tells <command>tip</command> to look in
	<filename>/etc/phones</filename> for a phone number.  But, the
	<literal>@</literal> sign is also a special character in
	capability files like <filename>/etc/remote</filename>, so it
	needs to be escaped with a backslash:</para>

      <programlisting>pn=\@</programlisting>
    </sect2>

    <sect2 xml:id="dial-command-line">
      <title>Dialing from the Command Line</title>

      <para>Put a <quote>generic</quote> entry in
	<filename>/etc/remote</filename>.  For example:</para>

      <programlisting>tip115200|Dial any phone number at 115200 bps:\
        :dv=/dev/cuau0:br#115200:at=hayes:pa=none:du:
tip57600|Dial any phone number at 57600 bps:\
        :dv=/dev/cuau0:br#57600:at=hayes:pa=none:du:</programlisting>

      <para>This should now work:</para>

      <screen>&prompt.root; <userinput>tip -115200 5551234</userinput></screen>

      <para>Users who prefer <command>cu</command> over
	<command>tip</command>, can use a generic
	<literal>cu</literal> entry:</para>

      <programlisting>cu115200|Use cu to dial any number at 115200bps:\
        :dv=/dev/cuau1:br#57600:at=hayes:pa=none:du:</programlisting>

      <para>and type:</para>

      <screen>&prompt.root; <userinput>cu 5551234 -s 115200</userinput></screen>
    </sect2>

    <sect2 xml:id="set-bps">
      <title>Setting the <acronym>bps</acronym> Rate</title>

      <para>Put in an entry for <literal>tip1200</literal> or
	<literal>cu1200</literal>, but go ahead and use whatever
	<acronym>bps</acronym> rate is appropriate with the
	<literal>br</literal> capability.
	<command>tip</command> thinks a good default is 1200&nbsp;bps
	which is why it looks for a <literal>tip1200</literal> entry.
	1200&nbsp;bps does not have to be used, though.</para>
    </sect2>

    <sect2 xml:id="terminal-server">
      <title>Accessing a Number of Hosts Through a Terminal
	Server</title>

      <para>Rather than waiting until connected and typing
	<command>CONNECT <replaceable>host</replaceable></command>
	each time, use <command>tip</command>'s <literal>cm</literal>
	capability.  For example, these entries in
	<filename>/etc/remote</filename> will let you type
	<command>tip pain</command> or <command>tip muffin</command>
	to connect to the hosts <systemitem>pain</systemitem> or
	<systemitem>muffin</systemitem>, and <command>tip
	  deep13</command> to connect to the terminal server.</para>

      <programlisting>pain|pain.deep13.com|Forrester's machine:\
        :cm=CONNECT pain\n:tc=deep13:
muffin|muffin.deep13.com|Frank's machine:\
        :cm=CONNECT muffin\n:tc=deep13:
deep13:Gizmonics Institute terminal server:\
        :dv=/dev/cuau2:br#38400:at=hayes:du:pa=none:pn=5551234:</programlisting>

    </sect2>

    <sect2 xml:id="tip-multiline">
      <title>Using More Than One Line with
	<command>tip</command></title>

      <para>This is often a problem where a university has several
	modem lines and several thousand students trying to use
	them.</para>

      <para>Make an entry in <filename>/etc/remote</filename> and use
	<literal>@</literal> for the <literal>pn</literal>
	capability:</para>

      <programlisting>big-university:\
        :pn=\@:tc=dialout
dialout:\
        :dv=/dev/cuau3:br#9600:at=courier:du:pa=none:</programlisting>

      <para>Then, list the phone numbers in
	<filename>/etc/phones</filename>:</para>

      <programlisting>big-university 5551111
big-university 5551112
big-university 5551113
big-university 5551114</programlisting>

      <para><command>tip</command> will try each number in the listed
	order, then give up.  To keep retrying, run
	<command>tip</command> in a <literal>while</literal>
	loop.</para>
    </sect2>

    <sect2 xml:id="multi-controlp">
      <title>Using the Force Character</title>

      <para><keycombo action="simul">
	  <keycap>Ctrl</keycap>
	  <keycap>P</keycap>
	</keycombo> is the default <quote>force</quote> character,
	used to tell <command>tip</command> that the next character is
	literal data.  The force character can be set to any other
	character with the <command>~s</command> escape, which means
	<quote>set a variable.</quote></para>

      <para>Type
	<command>~sforce=<replaceable>single-char</replaceable></command>
	followed by a newline.  <replaceable>single-char</replaceable>
	is any single character.  If
	<replaceable>single-char</replaceable> is left out, then the
	force character is the null character, which is accessed by
	typing
	<keycombo action="simul">
	  <keycap>Ctrl</keycap><keycap>2</keycap>
	</keycombo>
	or <keycombo action="simul">
	  <keycap>Ctrl</keycap><keycap>Space</keycap>
	</keycombo>.  A pretty good value for
	<replaceable>single-char</replaceable> is
	<keycombo action="simul">
	  <keycap>Shift</keycap>
	  <keycap>Ctrl</keycap>
	  <keycap>6</keycap>
	</keycombo>, which is only used on some terminal
	servers.</para>

      <para>To change the force character, specify the following in
	<filename>~/.tiprc</filename>:</para>

      <programlisting>force=<replaceable>single-char</replaceable></programlisting>
    </sect2>

    <sect2 xml:id="uppercase">
      <title>Upper Case Characters</title>

      <para>This happens when
	<keycombo action="simul">
	  <keycap>Ctrl</keycap>
	  <keycap>A</keycap>
	</keycombo> is pressed, which is <command>tip</command>'s
	<quote>raise character</quote>, specially designed for people
	with broken caps-lock keys.  Use <command>~s</command> to set
	<literal>raisechar</literal> to something reasonable.  It can
	be set to be the same as the force character, if neither
	feature is used.</para>

      <para>Here is a sample <filename>~/.tiprc</filename> for
	<application>Emacs</application> users who need to type
	<keycombo action="simul">
	  <keycap>Ctrl</keycap>
	  <keycap>2</keycap>
	</keycombo> and <keycombo action="simul">
	  <keycap>Ctrl</keycap>
	  <keycap>A</keycap>
	</keycombo>:</para>

    <programlisting>force=^^
raisechar=^^</programlisting>

      <para>The <literal>^^</literal> is
	<keycombo action="simul">
	  <keycap>Shift</keycap><keycap>Ctrl</keycap><keycap>6</keycap>
	  </keycombo>.</para>

    </sect2>

    <sect2 xml:id="tip-filetransfer">
      <title>File Transfers with <command>tip</command></title>

      <para>When talking to another &unix;-like operating system,
	files can be sent and received using <command>~p</command>
	(put) and <command>~t</command> (take).  These commands run
	<command>cat</command> and <command>echo</command> on the
	remote system to accept and send files.  The syntax is:</para>

      <cmdsynopsis>
	<command>~p</command>
	<arg choice="plain">local-file</arg>
	<arg choice="opt">remote-file</arg>
      </cmdsynopsis>

      <cmdsynopsis>
	<command>~t</command>
	<arg choice="plain">remote-file</arg>
	<arg choice="opt">local-file</arg>
      </cmdsynopsis>

      <para>There is no error checking, so another protocol, like
	zmodem, should probably be used.</para>
    </sect2>

    <sect2 xml:id="zmodem-tip">
      <title>Using <application>zmodem</application> with
	<command>tip</command>?</title>

      <para>To receive files, start the sending program on the remote
	end.  Then, type <command>~C rz</command> to begin receiving
	them locally.</para>

      <para>To send files, start the receiving program on the remote
	end.  Then, type <command>~C sz
	<replaceable>files</replaceable></command> to send them to the
	remote system.</para>
    </sect2>
  </sect1>

  <sect1 xml:id="serialconsole-setup">
    <info>
      <title>Setting Up the Serial Console</title>

      <authorgroup>
	<author>
	  <personname>
	    <firstname>Kazutaka</firstname>
	    <surname>YOKOTA</surname>
	  </personname>
	  <contrib>Contributed by </contrib>
	</author>
      </authorgroup>

      <authorgroup>
	<author>
	  <personname>
	    <firstname>Bill</firstname>
	    <surname>Paul</surname>
	  </personname>
	  <contrib>Based on a document by </contrib>
	</author>
      </authorgroup>
    </info>

    <indexterm><primary>serial console</primary></indexterm>

    <para>&os; has the ability to boot a system with a dumb
      terminal on a serial port as a console.  This configuration is
      useful for system administrators who wish to install &os; on
      machines that have no keyboard or monitor attached, and
      developers who want to debug the kernel or device
      drivers.</para>

    <para>As described in <xref linkend="boot"/>, &os; employs a three
      stage bootstrap.  The first two stages are in the boot block
      code which is stored at the beginning of the &os; slice on the
      boot disk.  The boot block then loads and runs the boot loader
      as the third stage code.</para>

    <para>In order to set up booting from a serial console, the boot
      block code, the boot loader code, and the kernel need to be
      configured.</para>

    <sect2 xml:id="serialconsole-howto-fast">
      <title>Quick Serial Console Configuration</title>

      <para>This section provides a fast overview of setting up the
	serial console.  This procedure can be used when the dumb
	terminal is connected to <filename>COM1</filename>.</para>

      <procedure>
	<title>Configuring a Serial Console on
	  <filename>COM1</filename></title>

	<step>
	  <para>Connect the serial cable to
	    <filename>COM1</filename> and the controlling
	    terminal.</para>
	</step>

	<step>
	  <para>To configure boot messages to display on the serial
	    console, issue the following command as the
	    superuser:</para>

	  <screen>&prompt.root; echo 'console="comconsole"' &gt;&gt; /boot/loader.conf</screen>
	</step>

	<step>
	  <para>Edit <filename>/etc/ttys</filename> and change
	    <literal>off</literal> to <literal>on</literal> and
	    <literal>dialup</literal> to <literal>vt100</literal> for
	    the <filename>ttyu0</filename> entry.  Otherwise, a
	    password will not be required to connect via the serial
	    console, resulting in a potential security hole.</para>
	</step>

	<step>
	  <para>Reboot the system to see if the changes took
	    effect.</para>
	</step>

      </procedure>

      <para>If a different configuration is required, see the next
	section for a more in-depth configuration explanation.</para>
    </sect2>

    <sect2 xml:id="serialconsole-howto">
      <title>In-Depth Serial Console Configuration</title>

      <para>This section provides a more detailed explanation of the
	steps needed to setup a serial console in &os;.</para>

      <procedure>
	<title>Configuring a Serial Console</title>

	<step>
	  <para>Prepare a serial cable.</para>

	  <indexterm><primary>null-modem cable</primary></indexterm>

	  <para>Use either a null-modem cable or a standard serial
	    cable and a null-modem adapter.  See <xref
	      linkend="term-cables-null"/> for a discussion on serial
	    cables.</para>
	</step>

	<step>
	  <para>Unplug the keyboard.</para>

	  <para>Many systems probe for the keyboard during the
	    Power-On Self-Test (<acronym>POST</acronym>) and will
	    generate an error if the keyboard is not detected.  Some
	    machines will refuse to boot until the keyboard is plugged
	    in.</para>

	  <para>If the computer complains about the error, but boots
	    anyway, no further configuration is needed.</para>

	  <para>If the computer refuses to boot without a keyboard
	    attached, configure the <acronym>BIOS</acronym> so that it
	    ignores this error.  Consult the motherboard's manual for
	    details on how to do this.</para>

	  <tip>
	    <para>Try setting the keyboard to <quote>Not
		installed</quote> in the <acronym>BIOS</acronym>.
	      This setting tells the <acronym>BIOS</acronym> not to
	      probe for a keyboard at power-on so it should not
	      complain if the keyboard is absent.  If that option is
	      not present in the <acronym>BIOS</acronym>, look for an
	      <quote>Halt on Error</quote> option instead.  Setting
	      this to <quote>All but Keyboard</quote> or to <quote>No
		Errors</quote> will have the same effect.</para>
	  </tip>

	  <para>If the system has a &ps2; mouse, unplug it as well.
	    &ps2; mice share some hardware with the keyboard and
	    leaving the mouse plugged in can fool the keyboard probe
	    into thinking the keyboard is still there.</para>

	  <note>
	    <para>While most systems will boot without a keyboard,
	      quite a few will not boot without a graphics adapter.
	      Some systems can be configured to  boot with no graphics
	      adapter by changing the <quote>graphics adapter</quote>
	      setting in the <acronym>BIOS</acronym> configuration to
	      <quote>Not installed</quote>.  Other systems do not
	      support this option and will refuse to boot if there is
	      no display hardware in the system.  With these machines,
	      leave some kind of graphics card plugged in, even if it
	      is just a junky mono board.  A monitor does not need to
	      be attached.</para>
	  </note>
	</step>

	<step>
	  <para>Plug a dumb terminal, an old computer with a modem
	    program, or the serial port on another &unix; box into the
	    serial port.</para>
	</step>

	<step>
	  <para>Add the appropriate <literal>hint.sio.*</literal>
	    entries to <filename>/boot/device.hints</filename> for the
	    serial port.  Some multi-port cards also require kernel
	    configuration options.  Refer to &man.sio.4; for the
	    required options and device hints for each supported
	    serial port.</para>
	</step>

	<step>
	  <para>Create <filename>boot.config</filename> in the root
	    directory of the <literal>a</literal> partition on the
	    boot drive.</para>

	  <para>This file instructs the boot block code how to boot
	    the system.  In order to activate the serial console, one
	    or more of the following options are needed.  When using
	    multiple options, include them all on the same
	    line:</para>

	  <variablelist>
	    <varlistentry>
	      <term><option>-h</option></term>

	      <listitem>
		<para>Toggles between the internal and serial
		  consoles.  Use this to switch console devices.  For
		  instance, to boot from the internal (video) console,
		  use <option>-h</option> to direct the boot loader
		  and the kernel to use the serial port as its console
		  device.  Alternatively, to boot from the serial
		  port, use <option>-h</option> to tell the boot
		  loader and the kernel to use the video display as
		  the console instead.</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><option>-D</option></term>

	      <listitem>
		<para>Toggles between the single and dual console
		  configurations.  In the single configuration, the
		  console will be either the internal console (video
		  display) or the serial port, depending on the state
		  of <option>-h</option>.  In the dual console
		  configuration, both the video display  and the
		  serial port will become the console at the same
		  time, regardless of the state of
		  <option>-h</option>.  However, the dual console
		  configuration takes effect only while the boot
		  block is running.  Once the boot loader gets
		  control, the console specified by
		  <option>-h</option> becomes the only
		  console.</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><option>-P</option></term>

	      <listitem>
		<para>Makes the boot block probe the keyboard.  If no
		  keyboard is found, the <option>-D</option> and
		  <option>-h</option> options are automatically
		  set.</para>

		<note>
		  <para>Due to space constraints in the current
		    version of the boot blocks, <option>-P</option> is
		    capable of detecting extended keyboards only.
		    Keyboards with less than 101 keys and without F11
		    and F12 keys may not be detected.  Keyboards on
		    some laptops may not be properly found because of
		    this limitation.  If this is the case, do not use
		    <option>-P</option>.</para>
		</note>
	      </listitem>
	    </varlistentry>
	  </variablelist>

	  <para>Use either <option>-P</option> to select the console
	    automatically or <option>-h</option> to activate the
	    serial console.  Refer to &man.boot.8; and
	    &man.boot.config.5; for more details.</para>

	  <para>The options, except for <option>-P</option>, are
	    passed to the boot loader.  The boot loader will
	    determine whether the internal video or the serial port
	    should become the console by examining the state of
	    <option>-h</option>.  This means that if
	    <option>-D</option> is specified but <option>-h</option>
	    is not specified in <filename>/boot.config</filename>, the
	    serial port can be used as the console only during the
	    boot block as the boot loader will use the internal video
	    display as the console.</para>
	</step>

	<step>
	  <para>Boot the machine.</para>

	  <para>When &os; starts, the boot blocks echo the contents of
	    <filename>/boot.config</filename> to the console.  For
	    example:</para>

	  <screen>/boot.config: -P
Keyboard: no</screen>

	  <para>The second line appears only if <option>-P</option> is
	    in <filename>/boot.config</filename> and indicates the
	    presence or absence of the keyboard.  These messages go
	    to either the serial or internal console, or both,
	    depending on the option in
	    <filename>/boot.config</filename>:</para>

	  <informaltable frame="none" pgwide="1">
	    <tgroup cols="2">
	      <thead>
		<row>
		  <entry align="left">Options</entry>
		  <entry align="left">Message goes to</entry>
		</row>
	      </thead>

	      <tbody>
		<row>
		  <entry>none</entry>
		  <entry>internal console</entry>
		</row>

		<row>
		  <entry><option>-h</option></entry>
		  <entry>serial console</entry>
		</row>

		<row>
		  <entry><option>-D</option></entry>
		  <entry>serial and internal consoles</entry>
		</row>

		<row>
		  <entry><option>-Dh</option></entry>
		  <entry>serial and internal consoles</entry>
		</row>

		<row>
		  <entry><option>-P</option>, keyboard present</entry>
		  <entry>internal console</entry>
		</row>

		<row>
		  <entry><option>-P</option>, keyboard absent</entry>
		  <entry>serial console</entry>
		</row>
	      </tbody>
	    </tgroup>
	  </informaltable>

	  <para>After the message, there will be a small pause before
	    the boot blocks continue loading the boot loader and
	    before any further messages are printed to the console.
	    Under normal circumstances, there is no need to interrupt
	    the boot blocks, but one can do so in order to make sure
	    things are set up correctly.</para>

	  <para>Press any key, other than <keycap>Enter</keycap>, at
	    the console to interrupt the boot process.  The boot
	    blocks will then prompt for further action:</para>

	  <screen>&gt;&gt; FreeBSD/i386 BOOT
Default: 0:ad(0,a)/boot/loader
boot:</screen>

	  <para>Verify that the above message appears on either the
	    serial or internal console, or both, according to the
	    options in <filename>/boot.config</filename>.  If the
	    message appears in the correct console, press
	    <keycap>Enter</keycap> to continue the boot
	    process.</para>

	  <para>If there is no prompt on the serial terminal,
	    something is wrong with the settings.  Enter
	    <option>-h</option> then <keycap>Enter</keycap> or
	    <keycap>Return</keycap> to tell the boot block (and then
	    the boot loader and the kernel) to choose the serial port
	    for the console.  Once the system is up, go back and check
	    what went wrong.</para>
	</step>
      </procedure>

      <para>During the third stage of the boot process, one can still
	switch between the internal console and the serial console by
	setting appropriate environment variables in the boot loader.
	See &man.loader.8; for more
	information.</para>

      <note>
	<para>This line in <filename>/boot/loader.conf</filename> or
	  <filename>/boot/loader.conf.local</filename> configures the
	  boot loader and the kernel to send their boot messages to
	  the serial console, regardless of the options in
	  <filename>/boot.config</filename>:</para>

	<programlisting>console="comconsole"</programlisting>

	<para>That line should be the first line of
	  <filename>/boot/loader.conf</filename> so that boot messages
	  are displayed on the serial console as early as
	  possible.</para>

	<para>If that line does not exist, or if it is set to
	  <literal>console="vidconsole"</literal>, the boot loader and
	  the kernel will use whichever console is indicated by
	  <option>-h</option> in the boot block.  See
	  &man.loader.conf.5; for more information.</para>

	<para>At the moment, the boot loader has no option
	  equivalent to <option>-P</option> in the boot block, and
	  there is no provision to automatically select the internal
	  console and the serial console based on the presence of the
	  keyboard.</para>
      </note>

      <tip>
	<para>While it is not required, it is possible to provide a
	  <command>login</command> prompt over the serial line.  To
	  configure this, edit the entry for the serial port in
	  <filename>/etc/ttys</filename> using the instructions in
	  <xref linkend="term-config"/>.  If the speed of the serial
	  port has been changed, change <literal>std.9600</literal> to
	  match the new setting.</para>
      </tip>
    </sect2>

    <sect2>
      <title>Setting a Faster Serial Port Speed</title>

      <para>By default, the serial port settings are 9600 baud, 8
	bits, no parity, and 1 stop bit.  To change the default
	console speed, use one of the following options:</para>

      <itemizedlist>
	<listitem>
	  <para>Edit <filename>/etc/make.conf</filename> and set
	    <varname>BOOT_COMCONSOLE_SPEED</varname> to the new
	    console speed.  Then, recompile and install the boot
	    blocks and the boot loader:</para>

	  <screen>&prompt.root; <userinput>cd /sys/boot</userinput>
&prompt.root; <userinput>make clean</userinput>
&prompt.root; <userinput>make</userinput>
&prompt.root; <userinput>make install</userinput></screen>

	  <para>If the serial console is configured in some other way
	    than by booting with <option>-h</option>, or if the serial
	    console used by the kernel is different from the one used
	    by the boot blocks, add the following option, with the
	    desired speed, to a custom kernel configuration file and
	    compile a new kernel:</para>

	  <programlisting>options CONSPEED=<replaceable>19200</replaceable></programlisting>
	</listitem>

	<listitem>
	  <para>Add the
	    <option>-S<replaceable>19200</replaceable></option> boot
	    option to <filename>/boot.config</filename>, replacing
	    <replaceable>19200</replaceable> with the speed to
	    use.</para>
	</listitem>

	<listitem>
	  <para>Add the following options to
	    <filename>/boot/loader.conf</filename>.  Replace
	    <replaceable>115200</replaceable> with the speed to
	    use.</para>

	  <programlisting>boot_multicons="YES"
boot_serial="YES"
comconsole_speed="<replaceable>115200</replaceable>"
console="comconsole,vidconsole"</programlisting>
	</listitem>
      </itemizedlist>
    </sect2>

    <sect2 xml:id="serialconsole-ddb">
      <title>Entering the DDB Debugger from the Serial Line</title>

      <para>To configure the ability to drop into the kernel debugger
	from the serial console, add the following options to a custom
	kernel configuration file and compile the kernel using the
	instructions in <xref linkend="kernelconfig"/>.  Note that
	while this is useful for remote diagnostics, it is also
	dangerous if a spurious BREAK is generated on the serial port.
	Refer to &man.ddb.4; and &man.ddb.8; for more information
	about the kernel debugger.</para>

      <programlisting>options BREAK_TO_DEBUGGER
options DDB</programlisting>
    </sect2>
  </sect1>
</chapter>