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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 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 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 be valid JSON which follows the structure documented below. Otherwise, continue to the next step in the discovery process. It is NOT necessary for the response to have a Content-Type of application/json.

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:

{
   "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: 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. 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.)

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. 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.