LetsEncrypt & Rails & Heroku

While Heroku now supports free automated SSL certificates for paid dynos, and you should use that instead of this gem unless your situation is covered by the known limitations of ACM, e.g. your app runs in Heroku Private Spaces. For wildcard certificates, this gem is quite useful, especially if you also use DNSimple to manage your DNS.

This gem is a complete solution for securing your Ruby on Rails application on Heroku using their free SNI-based SSL and LetsEncrypt. It will automatically handle renewals and keeping your certificate up to date.

With some extra steps, this gem can also be used with Sinatra. For an example of how to do this, see the letsencrypt-rails-heroku-sinatra-example repository.

Requirements

You must be using hobby or professional dynos to use free SNI-based SSL. Find out more on Heroku's documentation page about SSL.
You should have already configured your app DNS as per Heroku's documentation.

Installation

Add the gem to your Gemfile:

# Until the new API calls are generally available, you must manually specify my fork
# of the Heroku API gem:
gem 'platform-api', git: 'https://github.com/jalada/platform-api', branch: 'master'

gem 'letsencrypt-rails-heroku', group: 'production'

And add it as middleware in your config/environments/production.rb:

Rails.application.configure do
  <...>

  config.middleware.use Letsencrypt::Middleware

  <...>
end

If you have configured your app to enforce SSL with the configuration option config.force_ssl = true you will need to insert the middleware in front of the middleware performing that enforcement instead, as LetsEncrypt do not allow redirects on their verification requests:

Rails.application.configure do
  # <...>
  
  config.middleware.insert_before ActionDispatch::SSL, Letsencrypt::Middleware

  # <...>
end

Configuring

By default the gem will try to use the following set of configuration variables, which you should set.

ACME_DOMAIN: Comma separated list of domains for which you want certificates, e.g. example.com,www.example.com. Your Heroku app should be configured to answer to all these domains, because LetsEncrypt will make a request to verify ownership.

If you leave this blank, the gem will try and use the Heroku API to get a list of configured domains for your app, and verify all of them.
ACME_EMAIL: Your email address, should be valid.
ACME_TERMS_AGREED: Existence of envvar represents your agreement to the ACME provider Terms of Service. By default this is Let's Encrypt; you can find their TOS here.
HEROKU_TOKEN: An API token for this app. See below
HEROKU_APP: Name of Heroku app e.g. bottomless-cavern-7173
SSL_TYPE: Optional: One of sni or endpoint, defaults to sni. endpoint requires your app to have an SSL endpoint addon configured.
FORCE_DNS: Optional: set this to anything if you'd like to force the DNS validation method.
DNSIMPLE_ACCESS_TOKEN: Optional: Access token to the your DNSimple.com account that manages the domain(s) you want to renew. If set, the TXT record will automatically be updated if needed. This is particularly useful for wildcard domains which cannot use the HTTP validation method.

The gem itself will temporarily create additional environment variables during the challenge / validation process:

ACME_CHALLENGE_FILENAME: The path of the file LetsEncrypt will request.
ACME_CHALLENGE_FILE_CONTENT: The content of that challenge file.

Creating a Heroku token

Use the heroku-oauth toolbelt plugin to generate an access token suitable for accessing the Heroku API to update the certificates. From within your project directory:

> heroku plugins:install heroku-cli-oauth
> heroku authorizations:create -d "LetsEncrypt"
Created OAuth authorization.
  ID:          <heroku-client-id>
  Description: LetsEncrypt
  Scope:       global
  Token:       <heroku-token>

Use the output of that to set the token (HEROKU_TOKEN).

Using for the first time

After deploying, run heroku run rake letsencrypt:renew. Ensure that the output looks good:

$ heroku run rake letsencrypt:renew
Running rake letsencrypt:renew on ⬢ yourapp... ⣷ connecting, run.1234
Creating account key...Done!
Registering with LetsEncrypt...Done!
Setting config vars on Heroku...Done!
Giving config vars time to change...Done!
Testing filename works (to bring up app)...done!
Adding new certificate...Done!
$

If this is the first time you have used an SNI-based SSL certificate on your app, you may need to alter your DNS configuration as per Heroku's instructions.

You can see these details by typing heroku domains.

Adding a scheduled task

You should add a scheduled task on Heroku to renew the certificate. If you are unfamiliar with how to do this, take a look at Heroku's documentation on their scheduler addon.

The scheduled task should be configured to run rake letsencrypt:renew as often as you want to renew your certificate. Letsencrypt certificates are valid for 90 days, but there's no harm renewing them more frequently than that.

Heroku Scheduler only lets you run a task as infrequently as once a day, but you don't want to renew your SSL certificate every day (you will hit the rate limit). You can make it run less frequently using a shell control statement. For example to renew your certificate on the 1st day of every month:

if [ "$(date +%d)" = 01 ]; then bundle exec rake letsencrypt:renew; fi

Source: blog.dbrgn.ch

Security considerations

Suggestions and pull requests are welcome in improving the situation with the following security considerations:

When configuring this gem you must add a non-expiring Heroku API token into your application environment. Your collaborators could use this token to impersonate the account it was created with when accessing the Heroku API. This is important if your account has access to other apps that your collaborators don’t. Additionally, if your application environment was leaked this would give the attacker access to the Heroku API as your user account. More information about Heroku’s API and oAuth.

You should create the API token from a suitably locked-down account.
This gem uses two environment variables (ACME_CHALLENGE_FILENAME and ACME_CHALLENGE_FILE_CONTENT) to construct routes and responses in your app. These environment variables could be manipulated to spoof URLs on your application.

The gem performs some cursory checks to make sure the filename is roughly what is expected to try and mitigate this.

Troubleshooting

Common name invalid errors (security certificate is from *.herokuapp.com)

Your domain is still configured as a CNAME or ALIAS to your-app.herokuapp.com. Check the output of heroku domains matches your DNS configuration. When you add an SNI cert to an app for the first time the DNS target changes.

To-do list

Persist account key, or at least give the option of using an existing one, so we don’t register with LetsEncrypt over and over.
Stop using a fork of the platform-api gem once it supports the SNI endpoint API calls. See issue #49 of the platform-api gem.
Provide instructions for running the gem decoupled from the app it is securing, for the paranoid.

Contributing

Check out the latest master to make sure the feature hasn't been implemented or the bug hasn't been fixed yet.
Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it.
Fork the project.
Start a feature/bugfix branch.
Commit and push until you are happy with your contribution.
Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.

Generating a new release

Bump the version: rake version:bump:{major,minor,patch}.
Update CHANGELOG.md & commit.
Use rake release to regenerate gemspec, push a tag to git, and push a new .gem to rubygems.org

troya2/letsencrypt-rails-heroku