We have a table with custom domains in it, so when a request comes in, we check that table first, then the subdomains. We're currently setting up DNS manually

We have a table with custom domains in it, so when a request comes in, we check that table first, then the subdomains. We're currently setting up DNS manually because we don't have that many people using their own domains, but we could easily automate it if it ever became burdensome. We give users the choice to let us host their DNS (and then help them update the nameservers on the domain) or just tell them what IP they need to point their domain to.