Custom domains for Multi-tenant web app - heroku

Question

I am developing an app (RoR + Heroku) which allows users create their own websites either using my subdomain (pagename.myapp.com) or using their own domain (pagename.com).
An important point of this is that this option is the key of my business: subdomains are the free plans and custom domains are the paid ones. So I have a table where I store the custom domains of each user and check if this page is active (exists and has paid the quota).
For that I need to give users the capability of point their domain to my servers. All we know that Heroku don't recommend the use of DNS A-Records.
Also I would like to abstract as much as possible this feature to being able to switch my infrastructure (Heroku to AWS) in the future without having to ask all my users to change their DNS Zone. Taking this into account, I think that the best option would be run something like an EC2 proxy (using AWS Elastic IP) which give me the ownership of this IP. This proxy I think that should redirect to proxy.myapp.com, and I would resolve the request in the app level.
Due to I didn't find clear information about that, I am not sure if this hypotesis is the best solution and how to setup the proxy (which type of proxy use? Nginx maybe?).
Said that, I would like to ask recommendations/best practices to solve this "common" feature.
Thanks

Answer 1

What you are wanting to do is fairly straight forward to implement. Your assumptions are correct about setting up the proxy. Nginx or haproxy will both work great for this (I personally would use haproxy). Here are some of the gotchas that you will run into though:
Changing the host header at a proxy server can cause the end web application to generate incorrect links. You can use relative paths to fix this, but it requires that the web application developer to be aware of the environment that they are running in.
user connects to www.example.com (proxy server)
proxy server connects to www.realdomain.com (web app)
the web app has a link for a shopping cart. www.realdomain.com/shoppingcart
the end user clicks on the link but the link is www.realdomain.com/shoppingcart instead of www.example.com/shoppingcart
The cost of the host acting as the proxy server. This can spiral out of control really quickly. For example, do you want redundancy, if so how are you planning on implementing that? Do you plan on having ssl termination? If so you will have to increase the CPU count to accommodate the additional load. Do you want to have a secure connection to heroku from your proxy? If you do then you will need to increase the CPU count for that as well. You may have to add additional ram as well depending on the number of concurrent connections.
Heroku also changes their load balancers regularly. This is important because your proxy service will need to reload the config / update the ip addresses of the heroku instances every 60 seconds. In my experience they may change once or twice a day, but the DNS entry that they use has a 60 second TTL. That means that you should make sure that you are capable of updating your config up to every 60 seconds.
My company has been doing something very similar to this for almost a year now. We use haproxy and simply have it reload the config regularly. We have never had an outage or an interruption to our end users. Nginx is also a very good product. It has built in DNS caching so if you go that route you will need to make sure that you configure it correctly so that the DNS cache TTL is 60 seconds.

Answer 2

Will many of your clients want to use your app on their domain apex? E.g. example.com rather than theapp.example.cpm? If not, I would recommend having them CNAME to proxy.myapp.com which CNAMEs to myapp.herokuapp.com. Then, you can update proxy.myapp.com without customer interruption.
If you do need apex or A record support, you would want to set up Nginx as a reverse proxy for your Heroku app. Keep in mind that if you need HTTPS support for client domains, you will need to do some sort of certificate management on your proxy.

Answer 3

I like the answer dtorgo gave and that he mentioned the TLS termination, which many online tutorials on custom domains don't touch at all.
I'll go into more detail on how to implement the custom domains feature for your SaaS while also handling the TLS/HTTPS.
If your customers just CNAME to your domain or create the A record to your IP and you don't handle TLS termination for these custom domains, your app will not support HTTPS, and without it, your app won't work in modern browsers on these custom domains.
You need to set up a TLS termination reverse proxy in front of your webserver. This proxy can be run on a separate machine but you can run it on the same machine as the webserver.
CNAME vs A record
If your customers want to have your app on their subdomain, e.g. app.customer.com they can create a CNAME app.customer.com pointing to your proxy.
If they want to have your app on their root domain, e.g. customer.com then they'll have to create an A record on customer.com pointing to your proxy's IP. Make sure this IP doesn't change, ever!
How to handle TLS termination?
To make TLS termination work, you'll have to issue TLS certificates for these custom domains. You can use Let's Encrypt for that. Your proxy will see the Host header of the incoming request, e.g. app.customer1.com or customer2.com etc., and then it will decide which TLS certificate to use by checking the SNI.
The proxy can be set up to automatically issue and renew certificates for these custom domains. On the first request from a new custom domain, the proxy will see it doesn't have the appropriate certificate. It will ask Let's Encrypt for a new certificate. Let's Encrypt will first issue a challenge to see if you manage the domain, and since the customer already created a CNAME or A record pointing to your proxy, that tells Let's Encrypt you indeed manage the domain, and it will let you issue a certificate for it.
To issue and renew certificates automatically, I'd recommend using Caddyserver, greenlock.js, OpenResty (Nginx).
tl;dr on what happens here;
Caddyserver listens on 443 and 80, it receives requests, issues, and renews certificates automatically, proxies traffic to your backend.
How to handle it on my backend
Your proxy is terminating TLS and proxying requests to your backend. However, your backend doesn't know who is the original customer behind the request. This is why you need to tell your proxy to include additional headers in proxied requests to identify the customer. Just add X-Serve-For: app.customer.com or X-Serve-For: customer2.com or whatever the Host header is of the original request.
Now when you receive the proxied request on the backend, you can read this custom header and you know who is the customer behind the request. You can implement your logic based on that, show data belonging to this customer, etc.
More
Put a load balancer in front of your fleet of proxies for higher availability. You'll also have to use distributed storage for certificates and Let's Encrypt challenges. Use AWS ECS or EBS for automated recovery if something fails, otherwise, you may be waking up in the middle of the night restarting machines, or your proxy manually.
If you need more detail you can DM me on Twitter #dragocrnjac