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<TITLE>Special Edition Using Linux, Fourth Edition:Using SLIP and PPP</TITLE>
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<!--TITLE=Special Edition Using Linux, Fourth Edition//-->
<!--AUTHOR=Jack Tackett//-->
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<P><BR></P>
<P>This <TT>chat</TT> script makes <TT>chat</TT> fail if it receives <TT>NO CARRIER</TT> or <TT>BUSY</TT> at any point during the script. <TT>chat</TT> recognizes a number of character and escape sequences, as outlined in Table 29.5.</P>
<TABLE WIDTH="100%"><CAPTION ALIGN=LEFT><B>Table 29.5</B> Character and Escape Sequences Recognized by <TT>chat</TT>
<TR>
<TH COLSPAN="2"><HR>
<TR>
<TH WIDTH="25%" ALIGN="LEFT">Sequence
<TH WIDTH="75%" ALIGN="LEFT">Description
<TR>
<TH COLSPAN="2"><HR>
<TR>
<TD VALIGN="TOP"><TT>BREAK</TT>
<TD>Used as a reply string, this makes <TT>chat</TT> send a break to the modem. This special signal normally causes the remote host to change its transmission speed.
<TR>
<TD><TT>‘’</TT>
<TD>Sends a null string with a single carriage return.
<TR>
<TD><TT>\b</TT>
<TD>The backspace character.
<TR>
<TD VALIGN="TOP"><TT>\c</TT>
<TD>Suppresses the newline sent after a reply string and must be at the end of the reply string.
<TR>
<TD><TT>\d</TT>
<TD>Makes <TT>chat</TT> wait for one second.
<TR>
<TD><TT>\K</TT>
<TD>Another means of specifying a break signal.
<TR>
<TD><TT>\n</TT>
<TD>Sends a newline character.
<TR>
<TD><TT>\N</TT>
<TD>Sends a null character.
<TR>
<TD><TT>\p</TT>
<TD>Pauses for 1/10th of one second.
<TR>
<TD><TT>\q</TT>
<TD>Prevents the string it’s included in from showing in the syslog file.
<TR>
<TD><TT>\r</TT>
<TD>Sends or expects a carriage return.
<TR>
<TD><TT>\s</TT>
<TD>Sends or expects a space character.
<TR>
<TD><TT>\t</TT>
<TD>Sends or expects a tab character.
<TR>
<TD><TT>\\</TT>
<TD>Sends or expects a backslash character.
<TR>
<TD><TT>\<I>ddd</I></TT>
<TD>Specifies an ASCII character in octal.
<TR>
<TD><TT>^<I>C</I></TT>
<TD>Specifies the control character represented by C.
<TR>
<TD COLSPAN="2"><HR>
</TABLE>
<BLOCKQUOTE>
<P><FONT SIZE="-1"><HR><B>TIP: </B>You can use the <TT>abort</TT> string to prevent low-speed calls on your high-speed modem. Configure your modem to return a <TT>CARRIER 14400</TT> string when it makes a connection and add <TT>abort CARRIER 2400</TT> to your <TT>chat</TT> script. This way, <TT>chat</TT> fails if your modem connects at 2400 bps instead of 14400 bps.<HR></FONT>
</BLOCKQUOTE>
<P><FONT SIZE="+1"><B>Using PPP with >chat</B></FONT></P>
<P>The <TT>pppd</TT> program has command-line options that control all aspects of the PPP link. The syntax for the <TT>pppd</TT> command is as follows:</P>
<!-- CODE SNIP //-->
<PRE>
pppd [<I>options</I>] [<I>tty_name</I>] [<I>speed</I>]
</PRE>
<!-- END CODE SNIP //-->
<P>Table 29.6 describes the most commonly used options.
</P>
<TABLE WIDTH="100%"><CAPTION ALIGN=LEFT><B>Table 29.6</B> Frequently Used <TT>pppd</TT> Command-Line Options
<TR>
<TH COLSPAN="2"><HR>
<TR>
<TH WIDTH="30%" ALIGN="LEFT">Option
<TH WIDTH="70%" ALIGN="LEFT">Description
<TR>
<TH COLSPAN="2"><HR>
<TR>
<TD VALIGN="TOP"><TT>device</TT>
<TD>Uses the specified device. <TT>pppd</TT> adds /dev/ to the string if needed. When no device is given, <TT>pppd</TT> uses the controlling terminal.
<TR>
<TD><TT>speed</TT>
<TD>Sets the modem speed.
<TR>
<TD VALIGN="TOP"><TT>asyncmap <I>map</I></TT>
<TD>Sets the async character map. This map specifies which control characters can’t be sent through the connection and need to be escaped. The map is a 32-bit hex number where each bit represents a character. The 0th bit (00000001) represents character 0 × 00.
<TR>
<TD><TT>auth</TT>
<TD>Requires the remote host to authenticate itself.
<TR>
<TD VALIGN="TOP"><TT>connect <I>program</I></TT>
<TD>Uses the program or shell command to set up the connection. This is where <TT>chat</TT> is used.
<TR>
<TD><TT>crtscts</TT>
<TD>Uses hardware flow control.
<TR>
<TD><TT>xonxoff</TT>
<TD>Uses software flow control.
<TR>
<TD VALIGN="TOP"><TT>defaultroute</TT>
<TD>Makes <TT>pppd</TT> set a default route to the remote host in your kernel routing table.
<TR>
<TD><TT>disconnect <I>program</I></TT>
<TD>Runs the specified program after <TT>pppd</TT> terminates its link.
<TR>
<TD VALIGN="TOP"><TT>escape <I>c1</I> ,<I>c2</I> ,...</TT>
<TD>Causes the specified characters to be escaped when transmitted. The characters are specified by using the ASCII hex equivalent.
<TR>
<TD><TT>file <I>filename</I></TT>
<TD>Reads <TT>pppd</TT> options from the specified file.
<TR>
<TD><TT>lock</TT>
<TD>Uses UUCP-style locking on the serial device.
<TR>
<TD><TT>mru <I>num</I></TT>
<TD>Sets the maximum receive unit to the specified number.
<TR>
<TD><TT>netmask <I>mask</I></TT>
<TD>Sets the PPP network interface netmask.
<TR>
<TD VALIGN="TOP"><TT>passive</TT>
<TD>Makes <TT>pppd</TT> wait for a valid connection rather than fail when it can’t initiate a connection immediately.
<TR>
<TD VALIGN="TOP"><TT>silent</TT>
<TD>Keeps <TT>pppd</TT> from initiating a connection. <TT>pppd</TT> waits for a connection attempt from the remote host instead.
<TR>
<TD COLSPAN="2"><HR>
</TABLE>
<P>More than 40 other command-line arguments control all aspects of PPP at all levels. Refer to the main page for information about them.
</P>
<BLOCKQUOTE>
<P><FONT SIZE="-1"><HR><B>NOTE: </B>The <TT>pppd</TT> program demands that the file <TT>/etc/ppp/options</TT> exist, even if it’s empty. This file is read by <TT>pppd</TT> and is an excellent place to put options you want <TT>pppd</TT> to use every time it runs.<HR></FONT>
</BLOCKQUOTE>
<P>You can combine <TT>pppd</TT> and <TT>chat</TT> in a number of ways. You can specify all the command-line arguments for both programs on the command line, put the <TT>pppd</TT> options in a file, or put the <TT>chat</TT> script in a file. The following is a simple example with everything on the command line:</P>
<!-- CODE SNIP //-->
<PRE>
$ <B>pppd connect ‘chat “” ATDT5551234 ogin: linux2 word: be4me’ \</B>
<B>/dev/cua1 38400 mru 296 lock debug crtscts modem defaultroute</B>
</PRE>
<!-- END CODE SNIP //-->
<P>This runs <TT>pppd</TT> with a simple <TT>chat</TT> script that dials a phone number and logs the user linux2 in to the remote host. The device, speed, MRU, and a number of other options are included.</P>
<P>At the other extreme, you can place most of the options for <TT>pppd</TT> in a file and have <TT>chat</TT> read a script file. The following is the call to <TT>pppd</TT> :</P>
<!-- CODE SNIP //-->
<PRE>
pppd /dev/cua1 38400 connect ‘chat -f linux1.chat’
</PRE>
<!-- END CODE SNIP //-->
<P>The following lines display the contents of the reference file:
</P>
<!-- CODE SNIP //-->
<PRE>
# Global PPP Options File
mru 296 # Set MRU value
lock # Use UUCP locking
crtscts # Use hardware handshaking
modem # Use modem control lines
defaultroute # Make PPP set up default route
</PRE>
<!-- END CODE SNIP //-->
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