Lambda URL CloudFront Origin Host

CloudFront Function

Jets creates a CloudFront Function as part of the CloudFront Distribution. This function sets an Origin host header.

This allows your app to see the host used by the user.

CloudFront      -> Lambda URL                      -> Rails App
demo.domain.com -> abc.lambda-url.us-west-2.on.aws -> Your App

The Rails app will see demo.domain.com. Whatever cloudfront.aliases you configure is what the Rails app sees. Multiple CloudFront aliases also work.

config/jets/deploy.rb

Jets.deploy.configure do
  # config.lambda.url.cloudfront.function.enable = true # default: true
  config.lambda.url.cloudfront.aliases = [
    "demo.example.com",
    "demo2.domain.com
  ]
  config.lambda.url.cloudfront.route53.enable = true
end

If you are using Route53 to manage the DNS, than you can have Jets automatically also add the Route53 DNS record.

JETS_SHIM_HOST

If you turn off the CloudFront function, then Jets will set the JETS_SHIM_HOST env variable with the first CloudFront aliases item in config.lambda.url.cloudfront.aliases = ["name1.example.com", "name2.example.com"] and sets it as the JETS_SHIM_HOST env var, IE: JETS_SHIM_HOST = "name1.example.com". The Jets Shim will use JETS_SHIM_HOST when set instead of the Lambda URL. This also means that only one host can be used. It’s the first of config.lambda.url.cloudfront.aliases.

The JETS_SHIM_HOST env var override can also be useful if your own manually managed CloudFront distribution in front your Jets app.

Rails config.hosts

Remember, for Rails, you’ll need to set config.hosts to allow for the host. Example:

module Demo
  class Application < Rails::Application
    config.hosts += [/.*\.amazonaws\.com/, /.*\.on\.aws/, "demo.example.com", "demo2.example.com"]
  # ...

Otherwise you’ll get a Rails HostAuthorization error. Example:

START RequestId: b6614a37-2f42-4cde-87a3-c913a91583a4 Version: $LATEST
E, [2024-04-14T03:20:33.825228 #8] ERROR -- : [ActionDispatch::HostAuthorization::DefaultResponseApp] Blocked hosts: demo.dev.boltops.com
END RequestId: b6614a37-2f42-4cde-87a3-c913a91583a4
REPORT RequestId: b6614a37-2f42-4cde-87a3-c913a91583a4  Duration: 3.45 ms       Billed Duration: 4 ms   Memory Size: 1536 MB    Max Memory Used: 347 MB

CloudFront Host Note

Normally, with CloudFront, your app only sees the Lambda URL. For example:

CloudFront      -> Lambda URL                      -> Rails App
demo.domain.com -> abc.lambda-url.us-west-2.on.aws -> Your App

CloudFront does not forward any host info to the headers to the Origin. Thus Rails apps only sees the Lambda URL abc.lambda-url.us-west-2.on.aws not the CloudFront demo.domain.com.

Jets addresses the issue with a CloudFront Function or JETS_SHIM_HOST, as noted previously.

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