Merge pull request #6905 from matrix-org/babolivier/acme.md

Update ACME.md to mention ACME v1 deprecation
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Mention in `ACME.md` that ACMEv1 is deprecated and explain what it means for Synapse admins.

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# 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](https://letsencrypt.org/) if you tell it to.
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](https://letsencrypt.org/) using the ACME protocol.
## Deprecation of ACME v1
In [March 2019](https://community.letsencrypt.org/t/end-of-life-plan-for-acmev1/88430),
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, and for existing accounts in June 2020.
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 2020.
* 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](https://github.com/certbot/certbot)
(or browse [this list](https://letsencrypt.org/fr/docs/client-options/)
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](/docs/reverse_proxy.md)) 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
@ -32,11 +68,6 @@ 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: