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ACME

From version 1.0 (June 2019) onwards, Synapse requires valid TLS certificates for communication between servers (by default on port 8448) in addition to those that are client-facing (port 443). To help homeserver admins fulfil this new requirement, Synapse v0.99.0 introduced support for automatically provisioning certificates through Let's Encrypt using the ACME protocol.

Deprecation of ACME v1

In March 2019, Let's Encrypt announced that they were deprecating version 1 of the ACME protocol, with the plan to disable the use of it for new accounts in November 2019, for new domains in June 2020, and for existing accounts and domains in June 2021.

Synapse doesn't currently support version 2 of the ACME protocol, which means that:

  • for existing installs, Synapse's built-in ACME support will continue to work until June 2021.
  • for new installs, this feature will not work at all.

Either way, it is recommended to move from Synapse's ACME support feature to an external automated tool such as certbot (or browse this list for an alternative ACME client).

It's also recommended to use a reverse proxy for the server-facing communications (more documentation about this can be found here) as well as the client-facing ones and have it serve the certificates.

In case you can't do that and need Synapse to serve them itself, make sure to set the tls_certificate_path configuration setting to the path of the certificate (make sure to use the certificate containing the full certification chain, e.g. fullchain.pem if using certbot) and tls_private_key_path to the path of the matching private key. Note that in this case you will need to restart Synapse after each certificate renewal so that Synapse stops using the old certificate.

If you still want to use Synapse's built-in ACME support, the rest of this document explains how to set it up.

Initial setup

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.

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

Make sure to restart/reload your webserver after making changes.

Now make the relevant changes in homeserver.yaml to enable ACME support:

acme:
    enabled: true
    port: 8009

Authbind

authbind allows a program which does not run as root to bind to low-numbered ports in a controlled way. The setup is simpler, but requires a webserver not to already be running on port 80. This includes every time Synapse renews a certificate, which may be cumbersome if you usually run a web server on port 80. Nevertheless, if you're sure port 80 is not being used for any other purpose then all that is necessary is the following:

Install authbind. For example, on Debian/Ubuntu:

sudo apt-get install authbind

Allow authbind to bind port 80:

sudo touch /etc/authbind/byport/80
sudo chmod 777 /etc/authbind/byport/80

When Synapse is started, use the following syntax:

authbind --deep <synapse start command>

Make the relevant changes in homeserver.yaml to enable ACME support:

acme:
    enabled: true

(Re)starting synapse

Ensure that the certificate paths specified in homeserver.yaml (tls_certificate_path and tls_private_key_path) do not currently point to any files. Synapse will not provision certificates if files exist, as it does not want to overwrite existing certificates.

Finally, start/restart Synapse.