ACME.md

ACME

Synapse v1.0 will require valid TLS certificates for communication between servers (port 8448 by default) in addition to those that are client-facing (port 443). If you do not already have a valid certificate for your domain, the easiest way to get one is with Synapse's new ACME support, which will use the ACME protocol to provision a certificate automatically. Synapse v0.99.0+ will provision server-to-server certificates automatically for you for free through Let's Encrypt if you tell it to.

In the case that your server_name config variable is the same as the hostname that the client connects to, then the same certificate can be used between client and federation ports without issue.

If your configuration file does not already have an acme section, you can generate an example config by running the generate_config executable. For example:

~/synapse/env3/bin/generate_config

You will need to provide Let's Encrypt (or another ACME provider) access to your Synapse ACME challenge responder on port 80, at the domain of your homeserver. This requires you to either change the port of the ACME listener provided by Synapse to a high port and reverse proxy to it, or use a tool like authbind to allow Synapse to listen on port 80 without root access. (Do not run Synapse with root permissions!) Detailed instructions are available under "ACME setup" below.

If you already have certificates, you will need to back up or delete them (files example.com.tls.crt and example.com.tls.key in Synapse's root directory), Synapse's ACME implementation will not overwrite them.

You may wish to use alternate methods such as Certbot to obtain a certificate from Let's Encrypt, depending on your server configuration. Of course, if you already have a valid certificate for your homeserver's domain, that can be placed in Synapse's config directory without the need for any ACME setup.

ACME setup

The main steps for enabling ACME support in short summary are:

1. Allow Synapse to listen for incoming ACME challenges.
2. Enable ACME support in homeserver.yaml.
3. Move your old certificates (files example.com.tls.crt and example.com.tls.key out of the way if they currently exist at the paths specified in homeserver.yaml.
4. Restart Synapse.

Detailed instructions for each step are provided below.

Listening on port 80

In order for Synapse to complete the ACME challenge to provision a certificate, it needs access to port 80. Typically listening on port 80 is only granted to applications running as root. There are thus two solutions to this problem.

Using a reverse proxy

A reverse proxy such as Apache or nginx allows a single process (the web server) to listen on port 80 and proxy traffic to the appropriate program running on your server. It is the recommended method for setting up ACME as it allows you to use your existing webserver while also allowing Synapse to provision certificates as needed.

For nginx users, add the following line to your existing server block:

location /.well-known/acme-challenge {
    proxy_pass http://localhost:8009;
}

For Apache, add the following to your existing webserver config:

ProxyPass /.well-known/acme-challenge http://localhost:8009/.well-known/acme-challenge