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+# MSC1711 Certificates FAQ
+
+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](#configuring-certificates-for-compatibility-with-synapse-100). 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](https://matrix.org/blog/2019/02/04/matrix-at-fosdem-2019/).
+
+## Contents
+* Timeline
+* Configuring certificates for compatibility with Synapse 1.0
+* FAQ
+  * Synapse 0.99.0 has just been released, what do I need to do right now?
+  * How do I upgrade?
+  * What will happen if I do not set up a valid federation certificate
+    immediately?
+  * What will happen if I do nothing at all?
+  * When do I need a SRV record or .well-known URI?
+  * Can I still use an SRV record?
+  * I have created a .well-known URI. Do I still need an SRV record?
+  * It used to work just fine, why are you breaking everything?
+  * Can I manage my own certificates rather than having Synapse renew
+    certificates itself?
+  * Do you still recommend against using a reverse proxy on the federation port?
+  * Do I still need to give my TLS certificates to Synapse if I am using a
+    reverse proxy?
+  * Do I need the same certificate for the client and federation port?
+  * How do I tell Synapse to reload my keys/certificates after I replace them?
+
+## 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.
+
+**>= 5th March 2019  - Synapse 1.0.0 is released**
+
+1.0.0 will land no sooner than 1 month after 0.99.0, leaving server admins one
+month after 5th February to upgrade to 0.99.0 and deploy their certificates. In
+accordance with the the [S2S spec](https://matrix.org/docs/spec/server_server/r0.1.0.html)
+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.
+
+The easiest way to do that is with Synapse's built-in ACME (Let's Encrypt)
+support. Full details are in [ACME.md](./ACME.md) but, in a nutshell:
+
+ 1. Allow Synapse to listen on port 80 with `authbind`, or forward it from a
+    reverse proxy.
+ 2. Enable acme support in `homeserver.yaml`.
+ 3. Move your old certificates out of the way.
+ 4. Restart Synapse.
+
+### 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
+doing one of the following:
+
+ * Acquire a certificate for the `server_name` yourself (for example, using
+   `certbot`), and give it and the key to Synapse via `tls_certificate_path`
+   and `tls_private_key_path`, or:
+
+ * Use Synapse's [ACME support](./ACME.md), and forward port 80 on the
+   `server_name` domain to your Synapse instance.
+
+#### 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 [reverse_proxy.rst](reverse_proxy.rst) 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). Currently Synapse's ACME
+    support [does not support
+    this](https://github.com/matrix-org/synapse/issues/4552), so you will have
+    to acquire a certificate yourself and give 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:
+
+    ```json
+    {"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](#configuring-certificates-for-compatibility-with-synapse-100).
+
+### 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"](#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](https://en.wikipedia.org/wiki/Convergence_(SSL)): 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](https://github.com/matrix-org/matrix-doc/blob/master/proposals/1711-x509-for-federation.md#background-the-failure-of-the-perspectives-approach).
+
+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 [reverse_proxy.rst](reverse_proxy.rst) 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](<https://github.com/matrix-org/synapse/issues/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.