Hosting companies have all kinds of tactics to keep spam away from their customers, but one very common complaint I get is the amount of spam coming in through contact forms. Even though we don’t allow ‘nobody’ mail through the php mail() function and we provide the best server-side spam filters available, local mail cannot be filtered or limited. In other words, no spam filter in the world is going to save you from your shitty contact form.

I started recommending to our customers to implement MX checks in their forms as spam bots nowadays can easily get past things like captcha and textual confirmations. Spammers rarely send email from valid mail hosts so it’s very easy to filter these out with just a few lines of code:

<?php
list($user, $domain) = split(“@”, $email);
if (checkdnsrr($domain, “MX”)) {
} else {
}

?>

To explain the code a little bit, you’re basically taking your stored email address variable ($email) and using the split() function to single out the domain name into one variable, $domain. When you pass the domain through the checkdnsrr() function, PHP will return either a ‘1′ or ‘0′ result, which is interpreted as either true or false. The above is just the basic code, but you can have it spit out errors as well:

if (checkdnsrr($domain, “MX”)) {
} else {

echo "Invalid email";
}

The checkdnsrr() function can also be used to check for other records as well, like A, CNAME, NS, etc.

A recent topic in the webhosting business is wildcard DNS.  Setting up a wildcard record will allow you to essentially have infinite subdomains all pointing to the same place…. you can try this on my site if you’d like by typing in any random subdomain (like lkjairl.v-nessa.net) and then see if point back to my primary, unless it’s one that I’ve actually created.  Now if you combine this ‘technology’ with the power of .htaccess rewrites, then congratulations.  You’ve just cheated your host and obtained unlimited subdomains.

The first thing you would need to do is have a wildcard DNS record set up.  If you have access to your own zone files, that’s great, otherwise you’ll need to ask your host to set it up for you.  Fair warning though, your hosting company is not stupid (unless it’s Dreamhost) so don’t be surprised if they decline your request.  To set up the DNS zone you simple need to add an a-record “*” like so:

* 14400 IN A 205.134.252.71

*Note: Some argue that you need to use the full  *.domain.com. as the first field…this is actually not required at all, nor recommended!

Depending on your platform, you’ll likely find your domain’s zone file in /var/named/domain.com.db unless you have this feature in your host’s control panel.  Once you’ve added the record simply reload named/BIND with the /etc/init.d/named reload command if needed.   If you’re using an interface it will probably do this for you automatically.

Now you need to add ServerAlias line to your httpd.conf for the wildcard, if your host does not already have Apache configured this way.  Inside the <virtualhost> tags for your domain, add this line:

ServerAlias *.domain.com

Then restart Apache as usual (/etc/init.d/httpd restart) and test a random subdomain to see if it’s working.

So now you will notice that any non-distinguished subdomain shared the same document root as your primary….that’s great and all but it really serves no purpose, so that is where mod_rewrite comes in.  You can add and modify the follow code in your .htaccess to have Apache direct these subdomains to where they need to go:

RewriteEngine On
RewriteRule ^\.htaccess$ – [F]
RewriteCond %{HTTP_HOST} !^www\.example\.com?$
RewriteCond %{HTTP_HOST} ^([^.]+)\.example\.com?$
RewriteRule ^$ /folder/page.html [L]

Using your imagination you can write your own script that can create subdomains and the appropriate .htaccess entries on-demand (assuming that this is not blocked by mod_security).  The end result is seemingly unlimited subdomains on your account!  Also, the changes are usually immediate and do not require propagation.