When you add listeners to a NLB, you specify port and protocols only. Let's say I create a VPC connection to a NLB, of which I have a bunch of different listeners. In the API Gateway integration request I now put in a request using this VPC link, and specify an Endpoint URL of: http://foobar:yolo/api.
Is the host of foobar actually used or could I write anything there? As it is only the http protocol and the yolo port which routes the request to the listener. Is the host still important? Or is the host only used if your endpoint would understand tcp and therefore read the host in the request?
Is the host of foobar actually used or could I write anything there?
Yes, it is used, and you can't write anything. API gateway is going to send your request to the Endpoint URL you are going to specify. If you specify http://foobar.com/api your requests will fail. They will only succeed if you actually own a domain foobar.com that is associated with your NLB.
Thus, if you don't have a domain, you have to use default DNS name provided for your NLB by AWS:, e.g.:
http://yyyy-fd434e91.elb.us-east-1.amazonaws.com
In contrast, if you own a domain foobar.com and you want to use http you are going to use:
http://foobar.com
If you want HTTPS, and have proper SSL certificate deployed on the NLB along with TLS listener, then you can write:
https://foobar.com
In addition, your application behind the NLB is going to receive the host from Endpoint URL in a request from the API in a Host header, e.g.:'
Host: yyyy-fd434e91.elb.us-east-1.amazonaws.com