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<H4 ALIGN="LEFT"><A NAME="Heading5"></A><FONT COLOR="#000077">Using <I>dip</I> with Static IP Addresses
</FONT></H4>
<P>Assigning individual IP addresses to each machine that uses a SLIP provider is very common. When your machine initiates a SLIP link to the remote host, <TT>dip</TT> configures the SLIP interface with this known address. Listing 29.1 is a <TT>dip</TT> script using static IP addresses for initiating a SLIP link from linux2.burwell.com to linux1.burwell.com.</P>
<P><B>Listing 29.1 A Sample <TT>dip</TT> Script for Using Static IP Addresses over SLIP</B></P>
<!-- CODE //-->
<PRE>
# Connect linux2 to linux1 using static IP Addresses
# Configure Communication Parameters
port /dev/cua1 # use modem on /dev/cua1 serial line
speed 38400
modem HAYES
reset # Send initialization string to modem
flush # Throw away modem response
get $local linux2 # Set local IP address
get $remote linux1 # Set remote IP address
# Dial number for linux1 modem
dial 555-1234
if $errlvl != 0 goto error # If the dial command fails, error out
wait CONNECT 75
if $errlvl != 0 goto error # If we don’t get a CONNECT string
# from the modem, error out
send \r\n # Wake up login program
wait ogin: 30 # Wait 30 seconds for login prompt
if $errlvl != 0 goto error # Error out if we don’t get login prompt
send Slinux2\n # Send SLIP login name for linux2
wait ssword: 5 # Wait 5 seconds for password prompt
if $errlvl != 0 goto error # Error out if we don’t get password
send be4me\n # Send password
wait running 30 # Wait for indication that SLIP is up
if $errlvl != 0 goto error # Otherwise error out
# We’re in, print out useful information
print Connected to $remote with address $rmtip
default # Make this link our default route
mode SLIP # Turn on SLIP mode on our end
# Error routine in case things don’t work
error:
print SLIP to $remote failed.
</PRE>
<!-- END CODE //-->
<BLOCKQUOTE>
<P><FONT SIZE="-1"><HR><B>TIP: </B>Tracking SLIP accounts can be difficult. Traditionally, UNIX user accounts are assigned login names with all lowercase letters. Using the client machine name with a capital S added to the front as the login name for that machine’s SLIP account makes tracking it easier and avoids login name collisions with normal user accounts.<HR></FONT>
</BLOCKQUOTE>
<P>The script in Listing 29.1 initializes the modem and sets the local and remote IP addresses for the SLIP link. If you use host names here, <TT>dip</TT> resolves them to their IP address equivalents. The script then dials the modem and works its way through the login sequence. When logged in and sure that the SLIP link is up on the remote host, the script has <TT>dip</TT> configure the routing table and then switch the serial line into SLIP mode.</P>
<P>If an error occurs, the error routine at the end of the script prints a warning message and aborts the script. <TT>dip</TT> is excellent about leaving the serial line in a reasonable state when it’s done with it.</P>
<H4 ALIGN="LEFT"><A NAME="Heading6"></A><FONT COLOR="#000077">Using <I>dip</I> with Dynamic IP Addresses
</FONT></H4>
<P>As SLIP became more popular, the task of managing IP addressees for SLIP clients got more and more difficult. This problem got worse when terminal servers supporting SLIP came into use. Now, you might be assigned any one of a range of IP addresses, depending on which port the terminal server received your call. This led to changes in <TT>dip</TT> that captured IP address information from the incoming data on the serial line. Listing 29.2 is a <TT>dip</TT> script that captures the local and remote IP addresses from the serial line.</P>
<P><B>Listing 29.2 A Sample <TT>dip</TT> Script for Dynamic IP Addresses</B></P>
<!-- CODE //-->
<PRE>
# Connection script for SLIP to server with dynamic IP address
# assignment. The terminal server prints out:
#
# remote address is XXX.XXX.XXX.XXX the local address is YYY.YYY.YYY.YYY
# Set the desired serial port and speed.
port /dev/cua1
speed 38400
# Reset the modem and terminal line.
Reset
flush
# Prepare for dialing.
dial 555-1234
if $errlvl != 0 goto error
wait CONNECT 60
if $errlvl != 0 goto error
# We are connected. Login to the system.
login:
wait name: 10 # Log in to system
if $errlvl != 0 goto error
send Slinux2\n # Send user ID
wait ord: 10
if $errlvl != 0 goto error
send be4me\n # Send password
if $errlvl != 0 goto error
get $remote remote 10 # Get remote IP address
if $errlvl != 0 goto error
get $local remote 10 # Get local IP address
if $errlvl != 0 goto error
done:
print CONNECTED to $remote with address $rmtip we are $local
default # Set routing
mode SLIP # Go to SLIP mode
goto exit
error:
print SLIP to $host failed.
exit
</PRE>
<!-- END CODE //-->
<P>The script in Listing 29.2 uses <TT>get $remote remote 10</TT> to watch the serial line and to capture the first thing that looks like an IP address in the <TT>$remote</TT> variable. The command times out in 10 seconds with an error if it doesn’t see an IP address.</P>
<H3><A NAME="Heading7"></A><FONT COLOR="#000077">Using <I>diplogin</I> to Provide SLIP Service
</FONT></H3>
<P>The <TT>dip</TT> program automates starting SLIP links from the client machine. Linux also supports incoming dial-up SLIP links. A few packages are available for doing this as well. You use the <TT>diplogin</TT> program here, which is really just another name for <TT>dip</TT>.</P>
<P>Providing SLIP service to others requires that you create a specific account for each person on your Linux box and configure that account correctly. You also need to write an /etc/diphosts file with appropriate information for each host that you’re providing SLIP service for.</P>
<H4 ALIGN="LEFT"><A NAME="Heading8"></A><FONT COLOR="#000077">Creating SLIP Accounts</FONT></H4>
<P>You can manually create the SLIP account or use the <TT>adduser</TT> script with appropriate responses to each question. Here’s an example /etc/passwd entry for linux2.burwell.com in the passwd file on linux1.burwell.com:</P>
<!-- CODE SNIP //-->
<PRE>
Slinux2:IdR4gDZ7K7D82:505:100:linux2 SLIP Account:/tmp:/sbin/diplogin
</PRE>
<!-- END CODE SNIP //-->
<P>It’s recommended that /tmp be used as the home directory for SLIP accounts to minimize security risks by preventing SLIP users from writing files into sensitive areas of your file system by default. Make sure that you use the correct path to the <TT>diplogin</TT> program.</P><P><BR></P>
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