Shell security

[Manixat]

Hello, I'm concerned about the security of this code because I am not really familiar with shell and OS stuff. I needed to add this function because filter_var wasn't enough to filter bullshit emails out like john@crack.house. I need to verify domain existence and I grabbed a hold of this function from eHow, which is a pretty trustworthy site, but I need to be 100% sure before using it. Are there any possible issues with this function:

function checkDomainAvailability($domain) {
    if(preg_match('/[;\&\|\>\<]/', $domain)) exit; //Could be a hack attempt
    exec("whois " . escapeshellarg($domain), $output); //:CAREFUL:
    $result = implode("\n", $output);
    return (strpos($result, 'No match') !== false);
}

Thank you in advance!

 

EDIT: I just noticed that this won't  work on windows. Is there any cross-OS command I can use?

Edited June 8, 2013 by Manixat

[boompa]

Instead of shelling out, you could use getmxrr to see if there's an MX record for the domain.

[Manixat]

Really useful, thanks! Even though in the comments section it is said that this function can return various values which may evaluate to true even if the domain is nonexistent, I tried it myself and didn't experience this issue!