Automatically Host Content When Specific Nameservers - subdomain

Question

Basically, this is going to probably an incredibly generic and poorly crafted question. I do apologise in advance for that and hope you can look past that and potentially offer some solutions/help.
I am looking at starting a new project, which I guess functions similar to Shopify in a way. Users will pay a monthly fee and then get their own website which has a store-style thing on it.
I am comfortable with most aspects of making this, however, the one thing I'm not completely sure how to do is if they want to have a custom domain (which I assume most customers would). Based on my experience with services such as Spotify and Tictail, to do this I am going to have to get them to change their nameservers to my nameservers. After that, I'm not completely sure how it will function and how to set it up. All of the files for the sites are going to be pretty much the exact same so I don't need much to change there.
So basically my main question is, how would I develop it to automatically host certain content when someone sets their nameservers as my nameservers? I would like it to be completely automatic if possible, but I don't mind if there is a little manual input.
I'm super sorry if the question isn't worded properly or if it's confusing as I've never developed something like this. A simple point in the right direction would be much appreciated as I'm not too sure where to start with this.
Thanks

Answer 1

It depends on the server technology you use to provide such user related nameservers. If I understand you properly, you are looking to get something like:
# Your service runs here:
http://yourdomain.com
# For the user account (user specific application)
http://{unique_username}.yourdomain.com/
# Then you would have (for N users)
http://username0.yourdomain.com/
http://username1.yourdomain.com/
...
http://usernameN.yourdomain.com/
The way you have to avoid dns hijacking is to actually specify the nameserver in the server host configuration. There are many servers around which provide virtual host configurations to allow many different domain names in the same ip address.
As an example, in nginx this can be done using virtual hosts. In your case you would need to programatically create them. In order to do so, a file must be added to the folder /etc/nginx/sites-available. This could be a file called: /etc/nginx/sites-available/username0 with this content:
server {
listen 80
server_name username0.yourdomain.com;
root /path/to/app/;
...
}
So for your solution, you would create a file per customer user. In order to activate a new created virtualhost (server block), link it in the folder with path: /etc/nginx/sites-enabled
ln -s /etc/nginx/sites-avaible/username1 /etc/nginx/sites-enabled/username1
sudo service nginx reload
Read more about dns hijacking here and have a look to virtual hosts implementations, like the nginx server blocks shown above, or the apache virtual hosts.
Good luck!