Re: [SLUG] script to parse and compare an IP address

From: Dylan Hardison (
Date: Sat Apr 01 2006 - 17:07:37 EST

On 4/1/06, Sick Twist <> wrote:
> >From: Paul M Foster <>
> >Reply-To:
> >To:
> >Subject: Re: [SLUG] script to parse and compare an IP address
> >Date: Sat, 01 Apr 2006 12:56:18 -0500
> >
> >Sick Twist wrote:
> >
> >>I am trying to write a script that parses an unknown IP address from a
> >>page of HTML, compares the IP address to a file and overwrites the file if
> >>the saved IP address is different. My question is, what language should I
> >>investigate to accomplish this? Is this able to be done with a bash script
> >>and command line utilities or is there another language like perl or
> >>python that would be better suited for this task? C has always been my
> >>game so I'm less familiar with the strengths and capabilities of some of
> >>the other tools out there. Thanks in advance!
> >>
> >
> >Let the language wars begin! ;-}
> >
> >If this is something that you could run from a web page, I'd suggest PHP.
> >Its syntax is the most similar to C, of the languages you mentioned.
> >
> >I've written code in Perl, but I no longer do. Perl is hard to read, and
> >has odd conventions for naming variables, depending on the type of the
> >variable and the context. However, for doing regexp work, Perl has the
> >simplest interface. It's built in to the language; completely integrated.
> >
> >Python is my choice for this kind of work. However, Python has some quirks,
> >too. First, it has almost no end-of-line delimiters and requires strict
> >indentation, a fact that causes jeers from its detractors. In addition, to
> >do regexp work you must include a statement at the beginning of the file to
> >pull in the regexp module; like an #include directive in C. The method of
> >doing regexp work is more complicated than Perl. But the language is
> >more... orthogonal?... than Perl. Perl's kinda down-and-dirty
> >it'll-figure-out-what-you-mean. Python is more rigorous.
> >Larry Wall (Perl creator) built Perl so that there many many ways to do the
> >same thing, some quite counter-intuitive. Guido Van Rossum (Python's
> >creator) wanted a language with the power of Perl, but with a more
> >regular/orthogonal/rigorous syntax. Fortunately, both these languages don't
> >require the same pre-declaration of variables that C does, and they do
> >garbage collection that C can't.
> >
> >These scripting languages drive me crazy because their syntax for certain
> >common tasks is just different enough from each other and C that you often
> >make stupid mistakes while coding. (else if, elseif, elif, elsif...) This
> >is one of the reasons I prefer PHP when I can use it-- it's close enough to
> >C syntax that I seldom use the wrong keyword, and I still have to terminate
> >lines with a semicolon (unlike Python).
> >
> >If this is a one-shot deal, you might just consider using grep/sed and
> >bash. That's got to be easier than mastering the intricacies of a whole new
> >language.
> >
> >Paul
> >
> >--
> >Paul M. Foster
> Yep, one-shot deal. Basically I want a script that will automatically log-in
> to my router (via curl or wget) and download the status page. Then I want to
> parse the page to obtain the router's external IP address and compare it
> with one that was saved previously to determine if it has changed. If so,
> the script should send me an e-mail with the current IP address. This is a
> poor man's substitute for a static IP :)

Aaah, you need passwords? Here, is updated version of script that
handles passwords and usernames (for http basic authentication).
Only thing I changed was fetch_ip.

use strict;
use warnings;
use LWP::UserAgent;
use HTTP::Request;
use HTTP::Response;
use HTTP::Status;
use IO::File;

my $url = '';
my $username = 'foobar';
my $password = 'password';
my $file = 'myipfile';

my $ip = fetch_ip($url, $username, $password);
my $old_ip = fetch_old_ip($file);

update_ipfile($file, $old_ip, $ip);

sub fetch_ip {
        my ($url, $username, $password) = @_;
        my $ua = new LWP::UserAgent;
        my $req = new HTTP::Request (
                GET => $url,
        $req->authorization_basic($username, $password);
        my $resp = $ua->request($req);

        if ($resp->code != RC_OK) {
                die "Request for $url failed: ", $resp->message, " (code: ",
$resp->code, ")\n";

        my $html = $resp->content;
        if ($html =~ /(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})/s) {
                my $ip = $1;
                print "Found IP: $ip\n";
                return $ip;
        } else {
                die "Cannot find ip address on $url\n";

sub fetch_old_ip {
        my ($file) = @_;

        # If $file exists, we open it and read the first line.
        if (-e $file) {
                my $fh = new IO::File($file, 'r') or die "Cannot open $file: $!";
                my $line = $fh->getline;
                chomp $line;
                return $line;
        } else {
                # Return something that cannot be an IP.
                return "(nada)";

sub update_ipfile {
        my ($file, $old_ip, $ip) = @_;
        if ($old_ip ne $ip) {
                print "Changing IP to $ip (was $old_ip)\n";
                my $fh = new IO::File($file, 'w') or die "Cannot open $file for writing: $!";
        } else {
                print "Old IP and new IP are the same ($ip).\n";

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