ruby-on-rails nginx devise amazon-cloudfront amazon-elb

How to have an AWS ELB forward the actual host name to the target group instead of the ELB's host name?

We have a Ruby/Rails website we're migrating from Heroku to AWS. The original dev is not available. I'm now trying to complete the migration. My background is in the Windows / .NET world. This Linux / Ruby/Rails environment is quite foreign to me...

Here's the current environment I've set-up:

Route 53

Record Name	Record Type	Alias	Alias Route Traffic To
foo.example.com	A	yes	cloudfront: xyz.cloudfront.net

CloudFront

Domain Name	Alternate Domain Names	Origin Domain	Origin Protocol	Behavior Protocol
xyz.cloudfront.net	foo.example.com	foo.us-west-2.elb.amazonaws.com	HTTP only	Redirect HTTP to HTTPS

The CloudFront distribution:

uses an AWS issued SSL cert
handles the http to https redirect
forwards the request to the ELB over http (not https)

Load Balancer

DNS Name	Listener Rule	Forward To
foo.us-west-2.elb.amazonaws.com	HTTP 80: default action	Target Group: foo-ec2

Target Group: foo-ec2 contains a single Ubuntu ec2 instance running nginx/1.18.0 + Phusion Passenger 6.0.10 to serve up the Ruby/Rails site.

nginx config

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    # SSL Config - we should NEVER receive 443/https traffic; 
    # CloudFront manages https traffic => AWS ELB => http to this server
    #listen 443 ssl default_server;
    #listen [::]:443 ssl default_server;

    server_name foo.example.com
                foo.us-west-2.elb.amazonaws.com;

    # Tell Nginx and Passenger where the app's 'public' directory is
    root /var/www/foo_example/public;

    # Turn on Passenger
    passenger_enabled on;
    passenger_app_env production;
    passenger_ruby /home/ubuntu/.rbenv/versions/2.6.8/bin/ruby;
}

Issue

The rails app starts up without error and is served over https. However, when a user attempts to log in / authenticate, the Devise gem sends back a redirect using http and the ELB's DNS name.

Example

sign_in request

Request URL: https://foo.example.com/users/sign_in
Request Method: POST
Status Code: 302

sign_in response

location: http://foo.us-west-2.elb.amazonaws.com/users
server: nginx/1.18.0 + Phusion Passenger(R) 6.0.10
status: 302 Found

Notice the request was over https and our domain:

https://foo.example.com

But now we're over http and the ELB's domain:

http://foo.us-west-2.elb.amazonaws.com

My assumption

The devise gem is seeing the host from the ELB and then generates the URL from the ELB host, creating two issues:

we are now on http since the ELB communicates with the ec2 instance over http
we are now on the ELB's host name, foo.us-west-2.elb.amazonaws.com, instead of our name, foo.example.com

I've looked into the devise documentation to see if we can just pass in the http protocol and domain to use when creating the post back, but my ruby knowledge is limited. Plus, I think this would be the "bad" path; where the "good" path would be to have the AWS ELB forward the actual domain name, instead of it's own.

I've reviewed several SO and related stack sites with similar questions, but I've either ended up with an infinite loop redirect, or the various config changes have resulted in the same behavior of the devise gem creating the wrong URL post back.

These two questions seem to be the closest, but I'm not quite able to make the "connection" between the answers and my limited knowledge of this ecosystem.

Question

How can I get the AWS ELB to forward our domain, foo.example.com, to the ec2 target group and not the ELB's domain?

Solution

After more experimentation with AWS settings, the solution is actually rather simple. The other answers I posted in the question were vague in the actual settings, so here's the concrete solution.

In CloudFront, you need to create a new origin request policy, not a cache policy:

Open up CloudFront
go to Policies (left nav)
click the "Origin Request" tab
click the "create origin request policy" button
name the policy whatever you want, i.e., "my origin request policy"
under "Origin request settings" > Headers: select "Include the following headers"
under "Add header": check the "Host" option
click the "Create" button

The policy will look like this:

Once the new origin request policy has been created:

head back to the CloudFront distributions
click your distribution's Id so you can edit it
click the "Behaviors" tab
select your behavior and edit
scroll down to "Cache key and origin requests"
make sure the "Cache policy and origin request policy (recommended)" is selected
under the "Origin request policy - optional", select your new policy, i.e., "my origin request policy"
save changes

The behavior will look like this (I'm using no caching for now to verify the ec2 instance is getting all the requests):

That's it. The host header is now correctly passed through to the ELB and ec2 instance. Nothing else needs to be done with the ELB.

I verified the host header was being used in all requests by modifying the nginx logging option to include the $host variable in the log file (and did a bit more customization to the OOB format):

# prefixed log with '[my-log]', but it's not needed; remove.
log_format my-log '[my-log] $http_x_forwarded_for - $remote_user [$time_local] '
                  '"$request_method $scheme://$host$request_uri $server_protocol" '
                  '$status $body_bytes_sent "$http_referer" "$http_user_agent" $request_time';

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    # SSL Config - we should NEVER receive 443/https traffic; 
    # CloudFront manages https traffic => AWS ELB => http to this server
    #listen 443 ssl default_server;
    #listen [::]:443 ssl default_server;

    server_name foo.example.com
                foo.us-west-2.elb.amazonaws.com;

    # create the our log file
    access_log /var/log/nginx/my-log.access.log my-log;

    # Tell Nginx and Passenger where the app's 'public' directory is
    root /var/www/foo_example/public;

    # Turn on Passenger
    passenger_enabled on;
    passenger_app_env production;
    passenger_ruby /home/ubuntu/.rbenv/versions/2.6.8/bin/ruby;
}

Surely this will help future me as well as others.