They're sneaky. And stealthy. They're quiet and mostly unobtrusive, but once you've been visited by them, you'll know it. Because you'll be inundated with a seemingly never-ending stream of spam-mails.

They're email harvesting robots, and chances are you've been visited by one.

What these insidious creatures do is crawl your site, much like the search engine spiders do, and collect any and all email addresses they find there. Many of them crawl your entire site, following every link, gathering email addresses from your guestbook, your message boards, databases, and everywhere else they can get to.

What happens next is so sinister, so unthinkable; I can barely say it. They put your email addresses on CDRom and sell them- as opt-in lists. You've seen them, "20,000 targeted email addresses for only $29.95!", or my personal favorite, "Send 10 Bazillion emails- WITHOUT SPAMMING!!". What you didn't know was that it was YOUR email address they were selling.

To find out if your site has been visited by an email harvester, you only need to look at your logs. If your web host provides you with your stats, you can look in the Browser report for any of the following:

• EmailSiphon
• Crescent Internet Tool Pack v1.0
• Cherry Picker
• Email Collector
• Libwww-perl 1.0

If you don't have a stats program, you can examine your logs for visits from these agents. The easiest way to do this is to download them and open them in a program with a search function (like Wordpad). Then you can search for the names listed above.

So, what can you do to protect your site from these evil robots? Unfortunately, there's no single magic solution. There are, however steps you can take to discourage them.

The first thing you can do is create a Robots Exclusion file. This is simply a text file named robots.txt that you place in your root directory. What this file does is tells robots where they can and cannot go (as well as which robots can and cannot visit your site). The drawback of using this file to combat email harvesting robots is that as a rule, the robots.txt file is based on a sort of robot honor system. That is to say that you are assuming that any robot that visits will ask for and comply with the directives that you put there. Unfortunately, harvesting robots are typically ill-mannered robots that ignore this file. For more information on Robot Exclusion, visit the Robots Exclusion Standard

A really fun solution is to use a cgi-script that punishes bad robots. What these do is to direct the robot to a page full of fake email addresses- lots and lots of them. So, what the spammer gets is a whole lot of bounced email messages, which will discourage them from visiting you again. The downside of this method is that they do also collect the valid email addresses. Also, most scripts of this type have a little disclaimer attached to them stating that they won't be held responsible for any legal issues that arise from the use of their script- and that has to make you wonder.

There are other scripts that hide your email address from the robots, but not your site visitors. This is a great solution for smaller sites that don't have more than one or two addresses listed. You can find both types of scripts at the CGI Resource Index

Another handy script is one that will check to see if a robot is friendly, and if not it will put it to sleep for say, 10,000 minutes. This will cause the robot to terminate the request and move on to another victim.

$number = $ENV{REMOTE_ADDR};
($a,$b,$c,$d)=split(/\./,$number);
$ipadr=pack("C4",$a,$b,$c,$d);
($name,$aliases,$addrtype,$length,
@addrs)=(gethostbyaddr("$ipadr", 2));

if ($name =~ /foo.com/i) {
$ENV{HTTP_USER_AGENT} =~ /emailsiphon/i;
$access_denied++;
sleep(10000);
}

The last option is, in my humble opinion, the best option. If you have the ability to modify your .htaccess file, you can specify certain host agents that are not allowed to visit your site using the mod_rewrite file. This effectively blocks the offending robots from ever touching your site. You should definitely check with your hosting provider to see whether or not you can make such a modification. Most hosts will be more than happy to make the modification for you.

For those of you willing and able to make the changes yourself, just add the following to your.htaccess file:

RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} ^EmailSiphon [OR]
RewriteCond %{HTTP_USER_AGENT} ^EmailWolf [OR]
RewriteCond %{HTTP_USER_AGENT} ^ExtractorPro [OR]
RewriteCond %{HTTP_USER_AGENT} ^Mozilla.*NEWT [OR]
RewriteCond %{HTTP_USER_AGENT} ^Crescent [OR]
RewriteCond %{HTTP_USER_AGENT} ^CherryPicker [OR]
RewriteCond %{HTTP_USER_AGENT} ^[Ww]eb[Bb]andit [OR]
RewriteCond %{HTTP_USER_AGENT} ^WebEMailExtrac.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^NICErsPRO [OR]
RewriteCond %{HTTP_USER_AGENT} ^Telesoft [OR]
RewriteCond %{HTTP_USER_AGENT} ^Zeus.*Webster [OR]
RewriteCond %{HTTP_USER_AGENT} ^Microsoft.URL [OR]
RewriteCond %{HTTP_USER_AGENT} ^Mozilla/3.Mozilla/2.01 [OR]
RewriteCond %{HTTP_USER_AGENT} ^EmailCollector
RewriteRule ^.*$ /badspammer.html [L]

While these are all effective measures to fight the Email Snatchers, there are new robots evolving every day. It's important to stay informed with the latest tools that the spammers are using. Some excellent sources of information can be found at:

Search Engine World
http://searchengineworld.com/engine/denied.htm

Apache Today
"Restricting Access by Host"

SpiderHunter.com
http://www.spiderhunter.com/