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How add-on domains work - and their ramifications on privacy and security

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Many websites

On a low-cost shared hosting package, your extra domains will typically be handled server-side as Add On Domains to your original domain. What does this mean?

What is an add-on domain?

Using lots of domains in your shared hosting package does not mean a bunch of totally separate hosting accounts. Instead, your extra domains will each “hang off” your one original domain that you signed up with. See the diagram below:

Schematic of domains

In this picture, the website content of Domains 2, 3, and 4 all reside in separate subdirectories within the Domain 1 account. This is all done server-side: viewers of your website won’t have any clue that this is how it’s structured. Say your main domain name associated with your hosting account is Domain1.com , and you then add on Domain2.com as an addon domain to your first one, viewers will still be taken to the correct site. In other words, visitors typing in http://Domain1.com will take them to the Domain1 site, and http://Domain2.com will take them to your Domain2 site. So far so good, right? Well, mostly, but there are a few issues you should be aware of.

Beware privacy issues

When I said no-one would know, that is not 100% true in all cases. If someone wanted to check the domain structure, they could, albeit in a “brute force” manner. If someone suspected that Domain1.com was your main domain, and Domain2.com was an addon domain, they could easily check that by typing into their browser http://Domain1.com/Domain2.com. If this correctly displays the Domain2 website (which it should), then the viewer has confirmed their hunch. Likewise, http://Domain1.com/Domain4.com should correctly display the Domain 4 website if they are add-on domains as shown in the picture above.

In most cases, this situation of addon domains should not be a problem. But if you don’t want it coming out that those two domains are co-hosted then you won’t want to set them up as addon domains. For example, if your Domain1 site was about ways to stop smoking, and Domain2 was about how to buy cigarettes online, and you didn’t want your audience to know you owned both, then yeah, you theoretically have a problem. I say “theoretically” because the person would still have to guess what your first domain and your second domain are, and string them together, and then check that the content that’s being displayed is the same is the second domain.

The odds of someone guessing these details may be low, plus they’d have to be willing to test all out the various combinations and permutations to figure out which of your sites is Domain1.

Overall, I personally would not be concerned about these privacy issues unless you have a specific reason for it, such as the example given above. But it is something to be aware of when purchasing a shared hosting package to develop a bunch of your domains.

A slightly bigger concern of addon domains other than the privacy issue is that of security.

Security issues of add-on domains

Because your add-on domains are linked back to your main domain, this means if someone hacks into any one of your sites, in theory it would be trivial for all the domains in your hosting account to become compromised. So, it’s especially important to maintain good security measures across all your sites.

Regardless of what CMS you use (or which mix of CMS’s you use), each of your sites are only as secure as the least secure site that you have in your hosting account. In other words, don’t let a site become “the weakest link” in your development portfolio.

Although security breaches can occur on any domain on your hosting account, they most commonly occur in the least up-to-date site – often one which the owner has almost forgotten exists. If you have a bunch of sites, the domain that doesn’t need frequent content updates can often be the most at risk of being hacked, for example due to an out-of-date WordPress core, or an out-of-date plugin with a serious security vulnerability. The same is true for cores or extensions/modules in other CMS’s, like Drupal, Joomla, and others.

It helps if your main domain is your long-term domain

When setting up an account with addon domains that are subdirectories of your main domain, it makes life just a little bit easier to make your main domain the one you plan to keep around for longest. This is because when you eventually part ways with your main domain name (for example if you sell it), it will pose big problems for the add-on domains.

Of course, in the situation of selling or of a name drop, any reputable web host will be able to re-configure things so that another of your domains becomes the main domain. Still, if you’re someone looking to start from scratch with a web host for a bunch of domains, you’d be well-served to consider which domain you’d like to keep the longest and use that for your main domain. If you do sell it or let it expire, your hosting company should be able to change things around for you, but it’s obviously easier on you if that would be an option you may never need, as opposed to something you plan to have to do in within a year’s time.

Are add-on domains a bad idea?

Add-on domains do have some significant drawbacks as outlined above. The decision of whether to go with add-on domains is up to you. If any of the above issues are dealbreakers, then add-on domains may not be right for you.

On the plus side, addon domains are cheap: a hosting account with add-on domains is almost always significantly cheaper than alternative ways of developing multiple domains. It can be an affordable way to start out when developing a bunch of sites.

Addon domains may be right for you if you are prepared to keep all of your sites fully up to date from a functional and security standpoint, and if you are not handling any sensitive information or any e-commerce.

Ultimately it comes down to what decisions you want to make and how much risk you want to take on. If you’re not a fan of add-on domains, rest assured there are alternatives for you.

Alternatives to add-on domains

An ideal alternative if you're starting out is reseller hosting. As its name suggests, you can even white-label it and sell hosting accounts yourself. But today, we're not actually looking to do that. Instead, you can sign up for a reseller hosting account and set up each of your sites in its own separate account within your reseller account. This means your websites are isolated from each other; there's much less risk of a hacker accessing your other sites too if one of your sites is breached. A2 Hosting and SiteGround are two reputable hosts who offer cheap reseller hosting.

In a separate article, we cover further alternatives to add-on domains; you may like to read that.


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