Automatic SSL certs from Let's Encrypt for your Phoenix applications. This is based on the ZeroSSL library which is used for the ACME handshake. Plugging into the sni_fun and the name is inspired by similar functionality of the golang certmagic library.

This is used in the real world for example on https://tcpbin.net.

For Cowboy add to your prod.exs:

config <your_app>, <your_endpoint>,
  https: [port: 443, sni_fun: &CertMagex.sni_fun/1],
  # ATTENTION: Ensure you comment http: out and port 80 is free!
  ...

For Bandit add to your prod.exs:

config <your_app>, <your_endpoint>,
  https: [port: 443, thousand_island_options: [transport_options: [sni_fun: &CertMagex.sni_fun/1]]],
  # ATTENTION: Ensure you comment http: out and port 80 is free!
  ...

And add this to your deps:

def deps do
  [
    {:certmagex, "~> 1.0"}
  ]
end

You're done!

The following configuration values are optional and can be set in your config.exs file.

• user_email: The email to use for the ACME handshake. Let's encrypt might send informational emails to this address.
• provider: The provider to use for the ACME handshake. Can be :letsencrypt or :zerossl. Defaults to :letsencrypt.
• account_key: The account key to use for the ACME handshake. Required only for :zerossl provider.
• addr: The address to bind to for the ACME handshake. Defaults to 0.0.0.0 on IPv4 and :: on IPv6.
• storage_module: The module to use for storage. Defaults to CertMagex.Storage.Acmev2Adapter. Changing the module allows storing retrieved certificates in a different storage location.
• storage_backend: The low-level key/value store for all CertMagex persisted data. Defaults to CertMagex.Storage.Dets (on-disk DetsPlus store). Configure with config :certmagex, :storage_backend, MyApp.CertStorage. Unlike storage_module — the ACME adapter Zerossl calls for account keys and EAB credentials — the backend provides generic insert, lookup, and delete for arbitrary Erlang-term keys and values (e.g. {:acmev2, key}, {:cache, domain}).
• renewal_threshold: The threshold for certificate renewal. Defaults to renewing certificates if they have 86_400 seconds (1 day) of validity left.
• sni_allowed_hosts: If set to a non-empty list of hostnames, only those names (compared case-insensitively) will trigger certificate handling; any other TLS SNI value yields no certificate and does not run ACME. Useful to avoid spurious Let’s Encrypt requests from internet scanners. If unset or [], every SNI is considered (the previous default).

A backend implements the CertMagex.Storage.Backend behaviour:

defmodule MyApp.CertStorage do
  @behaviour CertMagex.Storage.Backend

  @impl true
  def child, do: :ignore

  @impl true
  def insert(_key, _value), do: :ok

  @impl true
  def delete(_key), do: :ok

  @impl true
  def lookup(_key), do: nil
end

Return a supervisor child spec from child/0 when the backend needs its own process (as CertMagex.Storage.Dets does with DetsPlus); return :ignore if it does not.

When using Let's Encrypt (provider: :letsencrypt or :letsencrypt_test), certmagex can issue certificates for IP addresses (IPv4 and IPv6). IP certificates are short-lived (~6 days) and require reliable automated renewal. They are not supported with the :zerossl provider. If the SNI callback receives an IP and the provider does not support IP certs, no certificate is generated (and a warning is logged).

Example config.exs

config :certmagex,
  provider: :zerossl,
  account_key: System.get_env("ZEROSSL_ACCOUNT_KEY"),
  addr: "0.0.0.0",
  user_email: "your@email.com",
  storage_module: CertMagex.Storage.Acmev2Adapter

Generated certificates are by default stored in $HOME/.local/share/certmagex but the XDG_DATA_HOME variable is respected.

This wouldn't be possible without the Acmev2 module from zerossl https://hex.pm/packages/zerossl