Reserving Subdomains in Your Multi-Tenant Web Application May 7, 2012

It’s fairly common to segment the accounts in your multi-tenant application with subdomains. GitHub gives you a subdomain that matches your username (e.g. Freshbooks gives you a subdomain that matches your company name (e.g. At BusyConf, we give you a subdomain that matches your conference name (e.g.

Your customers usually get to choose their own subdomain during account sign-up, and when developing a subdomain-based multi-tenant application, it easy to forget to reserve some common subdomains for your own use and future growth. There’s nothing like trying to register a new service only to realize that one of your customers already has the subdomain that you had hoped to use.

It’s a good idea to make a list of subdomains that you don’t want your customers to use. Here’s a list of subdomains that I like to reserve in my multi-tenant applications:

admin, api, assets, blog, calendar, camo, dashboard, demo, developer, developers, docs, files, ftp, git, imap, lab, m, mail, manage, mx, pages, pop, sites, smtp, ssl, staging, status, support, www

What subdomains are missing from this list?

In terms of an ActiveModel validation in Rails, that looks something like this:

  admin api assets blog calendar camo dashboard demo developer developers docs files ftp git imap lab m mail manage mx pages pop sites smtp ssl staging status support www
validates_exclusion_of :subdomain, :in => RESERVED_SUBDOMAINS,
                       :message => "Subdomain %{value} is reserved."

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