MSC1708: .well-known support for server name resolution

proposals/1708-well-known-for-federation.md

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+# MSC1708: .well-known support for server name resolution
+
+Currently, mapping from a server name to a hostname for federation is done via
+`SRV` records. However,
+[MSC1711](https://github.com/matrix-org/matrix-doc/pull/1711) proposes
+requiring valid X.509 certificates on the federation endpoint. It will then be
+necessary for the homeserver to present a certificate which is valid for the
+server name. This presents difficulties for hosted server offerings: BigCorp
+may want to delegate responsibility for running its Matrix homeserver to an
+outside supplier, but it may be difficult for that supplier to obtain a TLS
+certificate for `bigcorp.com` (and BigCorp may be reluctant to let them have
+one).
+
+This MSC proposes to solve this problem by augmenting the current `SRV` record
+with a `.well-known` lookup.
+
+## Proposal
+
+For reference, the current [specification for resolving server
+names](https://matrix.org/docs/spec/server_server/unstable.html#resolving-server-names)
+is as follows:
+
+1. If the hostname is an IP literal, then that IP address should be used,
+   together with the given port number, or 8448 if no port is given.
+
+2. Otherwise, if the port is present, then an IP address is discovered by
+   looking up an AAAA or A record for the hostname, and the specified port is
+   used.
+
+3. If the hostname is not an IP literal and no port is given, the server is
+   discovered by first looking up a `_matrix._tcp` SRV record for the
+   hostname, which may give a hostname (to be looked up using AAAA or A queries)
+   and port.
+
+4. Finally, the server is discovered by looking up an AAAA or A record on the
+   hostname, and taking the default fallback port number of 8448.
+
+We insert the following between Steps 3 and 4.
+
+If the SRV record does not exist, the requesting server should make a `GET`
+request to `https://<server_name>/.well-known/matrix/server`, with normal X.509
+certificate validation, and following 30x redirects (being careful to avoid
+redirect loops). If the request does not return a 200, continue to step 4,
+otherwise:
+
+The response must have a `Content-Type` of `application/json`, and must be
+valid JSON which follows the structure documented below. Otherwise, the
+request is aborted.
+
+If the response is valid, the `m.server` property is parsed as
+`<delegated_server_name>[:<delegated_port>]`, and processed as follows:
+
+* If `<delegated_server_name>` is an IP literal, then that IP address should be
+  used, together with `<delegated_port>`, or 8448 if no port is given. The
+  server should present a valid TLS certificate for `<delegated_server_name>`.
+
+* If `<delegated_server_name>` is not an IP literal, and `<delegated_port>` is
+  present, then an IP address is discovered by looking up an AAAA or A record
+  for `<delegated_server_name>`, and the specified port is used. The server
+  should present a valid TLS certificate for `<delegated_server_name>`.
+
+  (In other words, the federation connection is made to
+  `https://<delegated_server_name>:<delegated_port>`).
+
+* If the hostname is not an IP literal and no port is given, a second SRV
+  record is looked up; this time for `_matrix._tcp.<delegated_server_name>`,
+  which may give yet another hostname (to be looked up using A/AAAA queries)
+  and port. The server must present a TLS cert for the
+  `<delegated_server_name>` from the .well-known.
+
+* If no SRV record is found, the server is discovered by looking up an AAAA
+  or A record on `<delegated_server_name>`, and taking the default fallback
+  port number of 8448.
+
+  (In other words, the federation connection is made to
+  `https://<delegated_server_name>:8448`).
+
+### Structure of the `.well-known` response
+
+The contents of the `.well-known` response should be structured as shown:
+
+```json
+{
+   "m.server": "<server>[:<port>]"
+}
+```
+
+If the response cannot be parsed as JSON, or lacks a valid `m.server` property,
+the request is considered to have failed, and no fallback to port 8448 takes
+place.
+
+The formal grammar for the `m.server` property is the same as that of a [server
+name](https://matrix.org/docs/spec/appendices.html#server-name): it is a
+hostname or IP address, followed by an optional port.
+
+### Caching
+
+Servers should not look up the `.well-known` file for every request, as this
+would impose an unacceptable overhead on both sides. Instead, the results of
+the `.well-known` request should be cached according to the HTTP response
+headers, as per [RFC7234](https://tools.ietf.org/html/rfc7234). If the response
+does not include an explicit expiry time, the requesting server should use a
+sensible default: 24 hours is suggested.
+
+Because there is no way to request a revalidation, it is also recommended that
+requesting servers cap the expiry time. 48 hours is suggested.
+
+A failure to retrieve the `.well-known` file should also be cached, though care
+must be taken that a single 500 error or connection failure should not break
+federation for an extended period. A short cache time of about an hour might be
+appropriate; alternatively, servers might use an exponential backoff.
+
+## Problems
+
+It will take a while for `.well-known` to be supported across the ecosystem;
+until it is, it will be difficult to deploy homeservers which rely on it for
+their routing: if Alice is using a current homeserver implementation, and Bob
+deploys a new implementation which relies on `.well-known` for routing, then
+Alice will be unable to send messages to Bob. (This is the same problem we have with
+[SNI](https://github.com/matrix-org/synapse/issues/1491#issuecomment-415153428).)
+
+The main defence against this seems to be to release support for `.well-known`
+as soon as possible, to maximise uptake in the ecosystem. It is likely that, as
+we approach Matrix 1.0, there will be sufficient other new features (such as
+new Room versions) that upgrading will be necessary anyway.
+
+## Security considerations
+
+The `.well-known` file potentially broadens the attack surface for an attacker
+wishing to intercept federation traffic to a particular server.
+
+## Dismissed alternatives
+
+For future reference, here are the alternative solutions which have been
+considered and dismissed.
+
+### Look up the `.well-known` file before the SRV record
+
+We could make the request for `.well-known` before looking up the `SRV`
+record. On the one hand this is maybe marginally simpler (and avoids the
+overhead of having to make *two* `SRV` lookups in the case that a `.well-known`
+is found. It might also open a future path for using `.well-known` for
+information other than delegation.
+
+Ultimately we decided to include the initial `SRV` lookup so that deployments
+have a mechanism to avoid the `.well-known` overhead in the common case that it
+is not required.
+
+### Subdomain hack
+
+As well as accepting TLS certs for `example.com`, we could also accept them for
+`delegated--matrix.example.com`. This would allow `example.com` to delegate its
+matrix hosting by (a) setting up the SRV record at `_matrix._tcp.example.com`
+and (b) setting up a CNAME at `delegated--matrix.example.com`. The latter would
+enable the delegatee to obtain an acceptable TLS certificate.
+
+This was certainly an interesting idea, but we dismissed it for the following
+reasons:
+
+* There's a security trap for anybody who lets people sign up for subdomains
+  (which is certainly not an uncommon business model): if you can register for
+  delegated--matrix.example.com, you get to intercept all the matrix traffic
+  for example.com.
+
+* Generally it feels quite unintuitive and violates the principle of least
+  surprise.
+
+* The fact that we can't find any prior art for this sets off alarm bells too.
+
+### Rely on DNS/DNSSEC
+
+If we could trust SRV records, we would be able to accept TLS certs for the
+*target* of the SRV record, which avoids this whole problem.
+
+Such trust could come from assuming that plain DNS is "good enough". However,
+DNS cache poisoning attacks are a real thing, and the fact that the designers
+of TLS chose to implement a server-name check specifically to deal with this
+case suggests we would be foolish to make this assumption.
+
+The alternative is to rely on DNSSEC to provide security for SRV records. The
+problem here is simply that DNSSEC is not that widely deployed currently. A
+number of large organisations are actively avoiding enabling it on their
+domains, so requiring DNSSEC would be a direct impediment to the uptake of
+Matrix. Furthermore, if we required DNSSEC-authenticated SRV records for
+domains doing delegation, we would end up with a significant number of
+homeservers unable to talk to such domains, because their local DNS
+infrastructure may not implement DNSSEC.
+
+Finally, if we're expecting servers to present the cert for the *target* of the
+SRV record, then we'll have to change the Host and SNI fields, and that will
+break backwards compat everywhere (and it's hard to see how to mitigate that).
+
+### Stick with perspectives
+
+The final option is to double-down on the Perspectives approach, ie to skip
+[MSC1711](https://github.com/matrix-org/matrix-doc/pull/1711). MSC1711
+discusses the reasons we do not believe this to be a viable option.
+
+## Conclusion
+
+This proposal adds a new mechanism, alongside the existing `SRV` record lookup
+for finding the server responsible for a particular matrix server_name, which
+will allow greater flexibility in deploying homeservers.