Recently I had a bit of a shock when I discovered my website that was hosted on a dedicated IP was being indexed by Google using subdomains that I had previously deleted and no longer used. The subdomains were all pointing to my main website, which was disastrous as this would be flagged as duplicate contents by search engines.
For example, I used to have a subdomain called blog.viraltech.co.uk that had a few post on it many months ago, but I decided to delete it because it wasn't being updated. However, because there was a few links pointing to that particular subdomain and Google previously knew about it, it was inevitable that Google bots would crawl it and check if there was any updates.
This wasn't a problem with my old web hosting company, and if you typed in blog.viraltech.co.uk or any random subdomain into a web browser, it would just return a "server not found" message, but after migrating to a new web host (ipage.com), several months later I discovered that any random subdomain that I hadn't created or had previously deleted would load and point to my main website rather than return an error message.
I could type absolutely anything such as jimbo-jones-is-greatest.viraltech.co.uk. or w1w2w3w4w5.viraltech.co.uk and they would all point to the main website. Random subdomains in themselves weren't a major issue, but it was subdomains that I had previously registered and used because search engines knew about these and would inevitably start indexing them.
So, What Was Causing the Random Subdomain Problem?
After contacting ipage.com, I was told it was a due to wildcard entry ( * record) set up on my domain and that it had been removed by a technician and my website would take between 1 and 12 hours to resolve itself. I'd never heard of a wildcard entry before, but after a little bit of research, it allows your domain name to be accessed by any random subdomain such as 12345.viraltech.co.uk.
Okay, that sounded absolutely fine, so I waited the recommended time, but the random subdomain problem still persisted. ipage.com hadn't done a particularly good job and only removed the wildcard entry from one of my domains when in fact, I had 4 domains hosted with them. After several more emails to ipage.com support explaining the situation they still couldn't fix the problem so rather waste more time going round and round in circles with their rather unhelpful support I decided to try to fix the problem myself.
It was a fairly quick and simple job. All I had to do was go into "DomainCentral" click on the "More Info" button on the affected domain, and select DNS. After that, you will see Modify: with a drop-down box where you need to choose "A Record".
After that, you simply scroll down and look for * (asterisk) and remove it by using the box on the right-hand side. Once you have removed the wildcard entry, it will take anywhere from a few seconds to several hours for the random subdomain problem to resolve itself.
I have no idea whether iPage.com. apply the * (asterisk) wildcard entry as standard or if it was caused by a technical error. Either way, it was extremely annoying and could have easily triggered search engine penalty because of duplicate contents.
If you do find that your website can be accessed by any random subdomain then this is almost certainly the problem. Some Webmasters may want to enable this feature, but you have to be careful that Google (or any other search engine) doesn't start indexing those subdomains because it will cause a duplicate contents problem. If you do have a wildcard entry enabled make sure search engine robots cannot index those random subdomain pages.
Whatever web host or domain company you are with the process will be pretty much the same. Simply log into your Control Panel and access your domain details and look for the DNS and A Record option and delete that * (asterisk)