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"Linux Gazette...<I>making Linux just a little more fun!</I>"
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<H1><font color="maroon">Connecting to Your Home Computer</font></H1>
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<H4>By <a href="mailto:restagner@netscape.net">Robert Stagner</a></H4>
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A couple of weeks back, I was at work typing away on my NT box (please,
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no flames on this, I have no other choice : -), when I suddenly realized that
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a Perl script that I had been updating on my home machine (a Linux box running
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Red Hat 6.0), needed to be transferred to my computer at work. Since I do not
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have a 24/7 connection to the Internet for my Linux box, I was out of luck. I
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could not just FTP to my home machine and download the script. Or could I?
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<P>
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I suppose I could set up a DSL or cable modem connection, but I personally
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find that a 56K PPP connection is a lot less financially burdensome -- at least
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for the time being. Anyhow, I thought about how I might somehow get the Linux
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box on the Internet while unattended (i.e. just after I left for work each
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morning). My solution, although involving several scripts and a cronjob, can
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be set up for a PPP connection rather quickly. Once tested and configured
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properly, the end result will provide you with a means of automatically
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connecting to the Internet and then sending an e-mail off to your work address
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containing the dynamically generated IP Address assigned to your home computer
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by your ISP. The only assumptions to get this up and running are the following:
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<UL>
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<LI>you have root permissions (usually the case, if your running this from home).
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<LI>you have copies of <A HREF="misc/stagner/ppp-on.txt">ppp-on</A>, <A HREF="misc/stagner/ppp-off.txt">ppp-off</A>, and <A HREF="misc/stagner/ppp-on-dialer.txt">ppp-on-dialer</A> shell scripts. These can
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usually be found in the /usr/doc/ppp-2.3.7/scripts/ directory.
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<LI>you have successfully connected and disconnected to the Internet using the
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ppp-on and ppp-off scripts.
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<LI>your ISP has provided you with access to a local telephone number (I suppose
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this is not really required, but who wants to pay phone charges).
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</UL>
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<P>Other than these assumptions, configuring your system to automatically dial-up
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your ISP are pretty straight forward. The first step to get this up and running is to copy over (as root) the ppp-on and ppp-off shell scripts to the
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/usr/sbin/ directory. Make sure the permissions are set to 0755 for each file.
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Then, copy over ppp-on-dialer to the /etc/ppp/ directory. The "dialer" script
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is the second part of a two tier step in establishing a connection to your
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ISP. The first, is the ppp-on script.
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<P>Next, edit the ppp-on script by assigning the appropriate telephone number,
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account and password information. Also, remember to assign the appropriate
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device and speed of your modem. Mine read /dev/ttyS3 and 115200. It should
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resemble the following once your done:
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<BR>
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<BR>
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<CENTER>
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<TABLE BORDER=0 CELLASPACING=0 CELLPADDING=4 WIDTH="80%">
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<TR>
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<TD BGCOLOR="FFFFCC">
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<PRE>
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<CODE>
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#!/bin/sh
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#
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# These are the parameters. Change as needed.
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TELEPHONE=<FONT COLOR="red"><B>your phonenum </B></FONT># The telephone number for the connection
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ACCOUNT=<FONT COLOR="red"><B>your account </B></FONT> # The account name for logon (as in 'George Burns')
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PASSWORD=<FONT COLOR="red"><B>your password </B></FONT> # The password for this account (and 'Gracie Allen')
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LOCAL_IP=0.0.0.0 # Local IP address if known. Dynamic = 0.0.0.0
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REMOTE_IP=0.0.0.0 # Remote IP address if desired. Normally 0.0.0.0
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NETMASK=255.255.255.0 # The proper netmask if needed
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# Export them so that they will be available at 'ppp-on-dialer' time.
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export TELEPHONE ACCOUNT PASSWORD
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#
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# This is the location of the script which dials the phone and logs
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# in. Please use the absolute file name as the $PATH variable is not
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# used on the connect option. (To do so on a 'root' account would be
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# a security hole so don't ask.)
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#
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DIALER_SCRIPT=/etc/ppp/ppp-on-dialer
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#
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# Initiate the connection
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exec /usr/sbin/pppd debug lock modem crtscts <FONT COLOR="red"><B>/dev/ttyS3 115200</B></FONT> \
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asyncmap 20A0000 escape FF kdebug 0 $LOCAL_IP:$REMOTE_IP \
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noipdefault netmask $NETMASK defaultroute connect $DIALER_SCRIPT
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</CODE>
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</PRE>
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</TD>
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</TR>
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<TR>
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<TD BGCOLOR="#FFFFFF">
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<FONT COLOR="red"><B>Red Text</B></FONT> - code that needs to be altered to meet your needs.
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</TD>
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</TR>
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</TABLE>
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</CENTER>
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<P>Now that you have all the shell scripts in the appropriate places, it's time
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to look at a little Perl script I wrote that calls the connection or disconnection process, as well as send off an e-mail with the IP Address of my
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Linux box to my NT box at work. Place a copy (as root) of this
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<a href="misc/stagner/sendMeIP.pl.txt">script</a> in your /usr/local/bin directory with a
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permissions setting of 0755. The purpose of this script is to:
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<UL>
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<LI>Provide a connection/disconnection to the Internet via a
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shell program (/usr/sbin/ppp-on, or ppp-off).
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<LI>Capture the dynamically generated IP address from the ISP service.
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<LI>Send the returned IP Address to a predefined e-mail address so that
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a remote connection can be made to your home computer.
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</UL>
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<P>The Perl script begins with the following lines:
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<BR>
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<BR>
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<CENTER>
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<TABLE BORDER=0 CELLASPACING=0 CELLPADDING=4 WIDTH="80%">
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<TR>
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<TD BGCOLOR="FFFFCC">
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<PRE>
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<CODE>
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# Counter used to provide a delay while the ppp connection is established
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$COUNT = 1;
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# Check for the existence of options
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if (!$ARGV[0] || $ARGV[0] !~ m/(connect|disconnect)/i) {
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print "Usage: sendMeIP.pl <connect | disconnect>\n";
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exit -1;
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}
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# Call and execute connection/disconnection to the Internet
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&pppConnection($ARGV[0]);
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# Caputre the dynamically created remote IP Address
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$assignedIPAddress = &captureIPAddress;
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# Send the IP Address via e-mail
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&sendMail($assignedIPAddress);
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</CODE>
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</PRE>
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</TD>
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</TR>
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</TABLE>
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</CENTER>
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<BR>
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<BR>
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<P>Essentially, all this does is first set a counter, then check to see if the
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necessary arguments have been passed to the program. If so, establish a
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connection (or, disconnection -- &pppConnection) to the Internet. Capture
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the IP Address (&captureIPAddress) by searching the output of a call to
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/sbin/ifconfig and finally send off the e-mail (&sendMail). The call to execute
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ifconfig actually happens in the &captureIPAddress subroutine. Let us have a
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look at this code:
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<BR>
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<BR>
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<CENTER>
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<TABLE BORDER=0 CELLASPACING=0 CELLPADDING=4 WIDTH="80%">
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<TR>
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<TD BGCOLOR="FFFFCC">
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<PRE>
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<CODE>
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sub captureIPAddress {
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my $captureIFCONFIG;
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$captureIFCONFIG = qx#/sbin/ifconfig#;
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if ($captureIFCONFIG =~ /inet addr:(\d+\.\d+.\d+\.\d+)\s*P-t-P.*/) {
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my $assignedIPAddress = "$1";
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print "IP Address capture successful: $assignedIPAddress.\n";
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return ($assignedIPAddress);
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}
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elsif (($captureIFCONFIG !~ /inet addr:(\d+\.\d+.\d+\.\d+)\s*P-t-P.*/)
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&& ($COUNT < 60)) {
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$COUNT++;
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sleep 1;
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&captureIPAddress;
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}
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else {
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print "Houston, we have a problem in capturing the IP Address.\n";
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exit -2;
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}
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}
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... [code removed for clarity]
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sub sendMail {
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my $assignedIPAddress = shift;
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my $mailService = <FONT COLOR="red"><B>location of your mail program: mine is "/usr/sbin/sendmail"</B></FONT>;
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my $from = <FONT COLOR="red"><B>"Your Name <yourLocalEmail\@host.com>"</B></FONT>;
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my $to = <FONT COLOR="red"><B>"Your Name <yourWorkEmail\@host.com>"</B></FONT>;
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die "Trouble sending mail: $!" unless (-e $mailService);
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open(SENDMAIL, "|$mailService -oi -t") or die "Trouble sending mail:
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$!";
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print SENDMAIL <<"EOMAIL";
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From: $from
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To: $to
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Subject: An Important Message About <FONT COLOR="red"><B>your.computer</B></FONT>
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Use the following IP Address to establish a connection with <FONT COLOR="red">
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<B>your.computer</B></FONT> : $assignedIPAddress
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EOMAIL
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close(SENDMAIL);
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print "The e-mail message has been sent.\n";
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}
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</CODE>
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</PRE>
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</TD>
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</TR>
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<TR>
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<TD BGCOLOR="#FFFFFF">
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<FONT COLOR="red"><B>Red Text</B></FONT> - code that needs to be altered to meet your needs.
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</TD>
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</TR>
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</TABLE>
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</CENTER>
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<BR>
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<BR>
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<P>First, the /sbin/ifconfig command is executed using Perl's equivalence of a
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backtick (qx function). The output of this command is captured in the
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$captureIFCONFIG variable. We then attempt to look for a pattern that
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contains "inet addr:" followed by an IP Address and then followed by "P-t-P".
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Since it's the address we're concerned with, we store this in
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$assignedIPAddress. However, if we have not matched the IP Address and the
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count is less than 60, then we add one to the count variable and put the
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program to sleep for one second. At this point, we call the same subroutine
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again and again until we have a pattern match or we've exceeded the count.
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This gives the /sbin/ifconfig command a chance to write out the appropriate
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interface information for a PPP connection. Actually, we give the program about
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60 seconds to find the IP Address. If we can't find an IP Address, we print out
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an error message and exit the program.
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<P>Finally, you'll need to configure the Perl script to point to your email
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program, as well as set up the "From" and "To" e-mail addresses contained in
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the &sendMail subroutine. Simply reassign the $mailService, $from, and $to
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variables to match your case.
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<P>Now we've come to the final step -- setting up your system to automatically
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execute the Perl script. We will accomplish this through the use of the
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crontab command. As root, execute the crontab command with the -e option. The
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-e option puts you in the edit mode. From here add the following two lines
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and save the file:
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<BR>
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<BR>
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<PRE>
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<CODE>
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00 8 * * 1,2,3,4,5 perl /usr/local/bin/sendMeIP.pl connect
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15 12 * * 1,2,3,4,5 perl /usr/local/bin/sendMeIP.pl disconnect
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</CODE>
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</PRE>
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<P>The first of these two lines calls the Perl script at 8:00am, Monday thru
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Friday. The second line calls the Perl script to disconnect from the
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Internet at 12:15pm, Monday thru Friday. You can check your work in crontab by
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executing the crontab command again, but this time passing it the -l option.
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For further information on crontab, check out the man pages.
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<P>Once you have received the e-mail message at work, you should now be able to connect to your machine via telnet and ftp by using the given IP Address.
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<P> <hr> <!-- P -->
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<H5 ALIGN=center>
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Copyright © 2000, Robert Stagner<BR>
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Published in Issue 53 of <i>Linux Gazette</i>, May 2000</H5>
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