Reverse DNS Lookup with PHP

And whatever other tools are required

[MatthewHSE]

I just read How To Verify Googlebot [googlewebmastercentral.blogspot.com], and immediately thought how useful this would be to prevent Google from getting session ID's in links. The problem is, I don't know how to do DNS lookups that way.

PHP is the only programming language I really know. So how can one use PHP (and probably some other tools/services) to do the reverse DNS and forward DNS lookups as suggested in the article referenced above?

Moderators, I posted this thread here because I'm guessing the technique involves several technologies, not just PHP. But please feel free to move this if it belongs in the PHP forum...

[Receptional Andy]

Reverse DNS is easy with PHP - you get the visitor's IP address stored in the $_SERVER['REMOTE_ADDR'] variable and then convert it to a hostname with gethostbyaddr [php.net]

E.g.

$first_ip=$_SERVER['REMOTE_ADDR'];
$hostname=gethostbyaddr($first_ip);

You reverse the lookup with gethostbyname [php.net]

$second_ip=gethostbyname($hostname);
if ($second_ip = $first_ip) {
// IPs matched
}

Added: there will be a bit of overhead time in doing this (depending on your server) so you would want to minimise how many lookups you do (i.e. only check visitors with Googlebot's UA)

[edited by: Receptional_Andy at 4:40 pm (utc) on Dec. 8, 2006]