forked-synapse/docs/MSC1711_certificates_FAQ.md

MSC1711 Certificates FAQ

Historical Note

This document was originally written to guide server admins through the upgrade path towards Synapse 1.0. Specifically, MSC1711 required that all servers present valid TLS certificates on their federation API. Admins were encouraged to achieve compliance from version 0.99.0 (released in February 2019) ahead of version 1.0 (released June 2019) enforcing the certificate checks.

Much of what follows is now outdated since most admins will have already upgraded, however it may be of use to those with old installs returning to the project.

If you are setting up a server from scratch you almost certainly should look at the installation guide instead.

Introduction

The goal of Synapse 0.99.0 is to act as a stepping stone to Synapse 1.0.0. It supports the r0.1 release of the server to server specification, but is compatible with both the legacy Matrix federation behaviour (pre-r0.1) as well as post-r0.1 behaviour, in order to allow for a smooth upgrade across the federation.

The most important thing to know is that Synapse 1.0.0 will require a valid TLS certificate on federation endpoints. Self signed certificates will not be sufficient.

Synapse 0.99.0 makes it easy to configure TLS certificates and will interoperate with both >= 1.0.0 servers as well as existing servers yet to upgrade.

It is critical that all admins upgrade to 0.99.0 and configure a valid TLS certificate. Admins will have 1 month to do so, after which 1.0.0 will be released and those servers without a valid certificate will not longer be able to federate with >= 1.0.0 servers.

Full details on how to carry out this configuration change is given below. A timeline and some frequently asked questions are also given below.

For more details and context on the release of the r0.1 Server/Server API and imminent Matrix 1.0 release, you can also see our main talk from FOSDEM 2019.

Timeline

5th Feb 2019 - Synapse 0.99.0 is released.

All server admins are encouraged to upgrade.

0.99.0:

• provides support for ACME to make setting up Let's Encrypt certs easy, as well as .well-known support.

• does not enforce that a valid CA cert is present on the federation API, but rather makes it easy to set one up.

• provides support for .well-known

Admins should upgrade and configure a valid CA cert. Homeservers that require a .well-known entry (see below), should retain their SRV record and use it alongside their .well-known record.

10th June 2019 - Synapse 1.0.0 is released

1.0.0 is scheduled for release on 10th June. In accordance with the the S2S spec 1.0.0 will enforce certificate validity. This means that any homeserver without a valid certificate after this point will no longer be able to federate with 1.0.0 servers.

Configuring certificates for compatibility with Synapse 1.0.0

If you do not currently have an SRV record

In this case, your server_name points to the host where your Synapse is running. There is no need to create a .well-known URI or an SRV record, but you will need to give Synapse a valid, signed, certificate.

If you do have an SRV record currently

If you are using an SRV record, your matrix domain (server_name) may not point to the same host that your Synapse is running on (the 'target domain'). (If it does, you can follow the recommendation above; otherwise, read on.)

Let's assume that your server_name is example.com, and your Synapse is hosted at a target domain of customer.example.net. Currently you should have an SRV record which looks like:

_matrix._tcp.example.com. IN SRV 10 5 8000 customer.example.net.

In this situation, you have three choices for how to proceed:

Option 1: give Synapse a certificate for your matrix domain

Synapse 1.0 will expect your server to present a TLS certificate for your server_name (example.com in the above example). You can achieve this by acquiring a certificate for the server_name yourself (for example, using certbot), and giving it and the key to Synapse via tls_certificate_path and tls_private_key_path.

Option 2: run Synapse behind a reverse proxy

If you have an existing reverse proxy set up with correct TLS certificates for your domain, you can simply route all traffic through the reverse proxy by updating the SRV record appropriately (or removing it, if the proxy listens on 8448).

See the reverse proxy documentation for information on setting up a reverse proxy.

Option 3: add a .well-known file to delegate your matrix traffic

This will allow you to keep Synapse on a separate domain, without having to give it a certificate for the matrix domain.

You can do this with a .well-known file as follows:

1. Keep the SRV record in place - it is needed for backwards compatibility with Synapse 0.34 and earlier.

2. Give Synapse a certificate corresponding to the target domain (customer.example.net in the above example). You can do this by acquire a certificate for the target domain and giving it to Synapse via tls_certificate_path and tls_private_key_path.

3. Restart Synapse to ensure the new certificate is loaded.

4. Arrange for a .well-known file at https://<server_name>/.well-known/matrix/server with contents:

   {"m.server": "<target server name>"}
   

   where the target server name is resolved as usual (i.e. SRV lookup, falling back to talking to port 8448).

   In the above example, where synapse is listening on port 8000, https://example.com/.well-known/matrix/server should have m.server set to one of:

   1. customer.example.net ─ with a SRV record on _matrix._tcp.customer.example.com pointing to port 8000, or:

   2. customer.example.net ─ updating synapse to listen on the default port 8448, or:

   3. customer.example.net:8000 ─ ensuring that if there is a reverse proxy on customer.example.net:8000 it correctly handles HTTP requests with Host header set to customer.example.net:8000.

FAQ

Synapse 0.99.0 has just been released, what do I need to do right now?

Upgrade as soon as you can in preparation for Synapse 1.0.0, and update your TLS certificates as above.

What will happen if I do not set up a valid federation certificate immediately?

Nothing initially, but once 1.0.0 is in the wild it will not be possible to federate with 1.0.0 servers.

What will happen if I do nothing at all?

If the admin takes no action at all, and remains on a Synapse < 0.99.0 then the homeserver will be unable to federate with those who have implemented .well-known. Then, as above, once the month upgrade window has expired the homeserver will not be able to federate with any Synapse >= 1.0.0

When do I need a SRV record or .well-known URI?

If your homeserver listens on the default federation port (8448), and your server_name points to the host that your homeserver runs on, you do not need an SRV record or .well-known/matrix/server URI.

For instance, if you registered example.com and pointed its DNS A record at a fresh Upcloud VPS or similar, you could install Synapse 0.99 on that host, giving it a server_name of example.com, and it would automatically generate a valid TLS certificate for you via Let's Encrypt and no SRV record or .well-known URI would be needed.

This is the common case, although you can add an SRV record or .well-known/matrix/server URI for completeness if you wish.

However, if your server does not listen on port 8448, or if your server_name does not point to the host that your homeserver runs on, you will need to let other servers know how to find it.

In this case, you should see "If you do have an SRV record currently" above.

Can I still use an SRV record?

Firstly, if you didn't need an SRV record before (because your server is listening on port 8448 of your server_name), you certainly don't need one now: the defaults are still the same.

If you previously had an SRV record, you can keep using it provided you are able to give Synapse a TLS certificate corresponding to your server name. For example, suppose you had the following SRV record, which directs matrix traffic for example.com to matrix.example.com:443:

_matrix._tcp.example.com. IN SRV 10 5 443 matrix.example.com

In this case, Synapse must be given a certificate for example.com - or be configured to acquire one from Let's Encrypt.

If you are unable to give Synapse a certificate for your server_name, you will also need to use a .well-known URI instead. However, see also "I have created a .well-known URI. Do I still need an SRV record?".

I have created a .well-known URI. Do I still need an SRV record?

As of Synapse 0.99, Synapse will first check for the existence of a .well-known URI and follow any delegation it suggests. It will only then check for the existence of an SRV record.

That means that the SRV record will often be redundant. However, you should remember that there may still be older versions of Synapse in the federation which do not understand .well-known URIs, so if you removed your SRV record you would no longer be able to federate with them.

It is therefore best to leave the SRV record in place for now. Synapse 0.34 and earlier will follow the SRV record (and not care about the invalid certificate). Synapse 0.99 and later will follow the .well-known URI, with the correct certificate chain.

It used to work just fine, why are you breaking everything?

We have always wanted Matrix servers to be as easy to set up as possible, and so back when we started federation in 2014 we didn't want admins to have to go through the cumbersome process of buying a valid TLS certificate to run a server. This was before Let's Encrypt came along and made getting a free and valid TLS certificate straightforward. So instead, we adopted a system based on Perspectives: an approach where you check a set of "notary servers" (in practice, homeservers) to vouch for the validity of a certificate rather than having it signed by a CA. As long as enough different notaries agree on the certificate's validity, then it is trusted.

However, in practice this has never worked properly. Most people only use the default notary server (matrix.org), leading to inadvertent centralisation which we want to eliminate. Meanwhile, we never implemented the full consensus algorithm to query the servers participating in a room to determine consensus on whether a given certificate is valid. This is fiddly to get right (especially in face of sybil attacks), and we found ourselves questioning whether it was worth the effort to finish the work and commit to maintaining a secure certificate validation system as opposed to focusing on core Matrix development.

Meanwhile, Let's Encrypt came along in 2016, and put the final nail in the coffin of the Perspectives project (which was already pretty dead). So, the Spec Core Team decided that a better approach would be to mandate valid TLS certificates for federation alongside the rest of the Web. More details can be found in MSC1711.

This results in a breaking change, which is disruptive, but absolutely critical for the security model. However, the existence of Let's Encrypt as a trivial way to replace the old self-signed certificates with valid CA-signed ones helps smooth things over massively, especially as Synapse can now automate Let's Encrypt certificate generation if needed.

Can I manage my own certificates rather than having Synapse renew certificates itself?

Yes, you are welcome to manage your certificates yourself. Synapse will only attempt to obtain certificates from Let's Encrypt if you configure it to do so.The only requirement is that there is a valid TLS cert present for federation end points.

Do you still recommend against using a reverse proxy on the federation port?

We no longer actively recommend against using a reverse proxy. Many admins will find it easier to direct federation traffic to a reverse proxy and manage their own TLS certificates, and this is a supported configuration.

See the reverse proxy documentation for information on setting up a reverse proxy.

Do I still need to give my TLS certificates to Synapse if I am using a reverse proxy?

Practically speaking, this is no longer necessary.

If you are using a reverse proxy for all of your TLS traffic, then you can set no_tls: True. In that case, the only reason Synapse needs the certificate is to populate a legacy 'tls_fingerprints' field in the federation API. This is ignored by Synapse 0.99.0 and later, and the only time pre-0.99 Synapses will check it is when attempting to fetch the server keys - and generally this is delegated via matrix.org, which is on 0.99.0.

However, there is a bug in Synapse 0.99.0 4554 which prevents Synapse from starting if you do not give it a TLS certificate. To work around this, you can give it any TLS certificate at all. This will be fixed soon.

Do I need the same certificate for the client and federation port?

No. There is nothing stopping you from using different certificates, particularly if you are using a reverse proxy. However, Synapse will use the same certificate on any ports where TLS is configured.

How do I tell Synapse to reload my keys/certificates after I replace them?

Synapse will reload the keys and certificates when it receives a SIGHUP - for example kill -HUP $(cat homeserver.pid). Alternatively, simply restart Synapse, though this will result in downtime while it restarts.