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Server-Server API	20	docs

Matrix homeservers use the Federation APIs (also known as server-server APIs) to communicate with each other. Homeservers use these APIs to push messages to each other in real-time, to retrieve historic messages from each other, and to query profile and presence information about users on each other's servers.

The APIs are implemented using HTTPS requests between each of the servers. These HTTPS requests are strongly authenticated using public key signatures at the TLS transport layer and using public key signatures in HTTP Authorization headers at the HTTP layer.

There are three main kinds of communication that occur between homeservers:

Persisted Data Units (PDUs): These events are broadcast from one homeserver to any others that have joined the same room (identified by Room ID). They are persisted in long-term storage and record the history of messages and state for a room.

Like email, it is the responsibility of the originating server of a PDU to deliver that event to its recipient servers. However PDUs are signed using the originating server's private key so that it is possible to deliver them through third-party servers.

Ephemeral Data Units (EDUs): These events are pushed between pairs of homeservers. They are not persisted and are not part of the history of a room, nor does the receiving homeserver have to reply to them.

Queries: These are single request/response interactions between a given pair of servers, initiated by one side sending an HTTPS GET request to obtain some information, and responded by the other. They are not persisted and contain no long-term significant history. They simply request a snapshot state at the instant the query is made.

EDUs and PDUs are further wrapped in an envelope called a Transaction, which is transferred from the origin to the destination homeserver using an HTTPS PUT request.

API standards

The mandatory baseline for server-server communication in Matrix is exchanging JSON objects over HTTPS APIs. More efficient transports may be specified in future as optional extensions.

All POST and PUT endpoints require the requesting server to supply a request body containing a (potentially empty) JSON object. Requesting servers should supply a Content-Type header of application/json for all requests with JSON bodies, but this is not required.

Similarly, all endpoints in this specification require the destination server to return a JSON object. Servers must include a Content-Type header of application/json for all JSON responses.

All JSON data, in requests or responses, must be encoded using UTF-8.

TLS

Server-server communication must take place over HTTPS.

The destination server must provide a TLS certificate signed by a known Certificate Authority.

Requesting servers are ultimately responsible for determining the trusted Certificate Authorities, however are strongly encouraged to rely on the operating system's judgement. Servers can offer administrators a means to override the trusted authorities list. Servers can additionally skip the certificate validation for a given whitelist of domains or netmasks for the purposes of testing or in networks where verification is done elsewhere, such as with .onion addresses.

Servers should respect SNI when making requests where possible: a SNI should be sent for the certificate which is expected, unless that certificate is expected to be an IP address in which case SNI is not supported and should not be sent.

Servers are encouraged to make use of the Certificate Transparency project.

Unsupported endpoints

If a request for an unsupported (or unknown) endpoint is received then the server must respond with a 404 M_UNRECOGNIZED error.

Similarly, a 405 M_UNRECOGNIZED error is used to denote an unsupported method to a known endpoint.

Server discovery

Resolving server names

Each Matrix homeserver is identified by a server name consisting of a hostname and an optional port, as described by the grammar. Where applicable, a delegated server name uses the same grammar.

Server names are resolved to an IP address and port to connect to, and have various conditions affecting which certificates and Host headers to send. The process overall is as follows:

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. The target server must present a valid certificate for the IP address. The Host header in the request should be set to the server name, including the port if the server name included one.
If the hostname is not an IP literal, and the server name includes an explicit port, resolve the IP address using AAAA or A records. Requests are made to the resolved IP address and given port with a Host header of the original server name (with port). The target server must present a valid certificate for the hostname.
If the hostname is not an IP literal, a regular HTTPS request is made to https://<hostname>/.well-known/matrix/server, expecting the schema defined later in this section. 30x redirects should be followed, however redirection loops should be avoided. Responses (successful or otherwise) to the /.well-known endpoint should be cached by the requesting server. Servers should respect the cache control headers present on the response, or use a sensible default when headers are not present. The recommended sensible default is 24 hours. Servers should additionally impose a maximum cache time for responses: 48 hours is recommended. Errors are recommended to be cached for up to an hour, and servers are encouraged to exponentially back off for repeated failures. The schema of the /.well-known request is later in this section. If the response is invalid (bad JSON, missing properties, non-200 response, etc), skip to step 4. If the response is valid, the m.server property is parsed as <delegated_hostname>[:<delegated_port>] and processed as follows:
- If <delegated_hostname> is an IP literal, then that IP address should be used together with the <delegated_port> or 8448 if no port is provided. The target server must present a valid TLS certificate for the IP address. Requests must be made with a Host header containing the IP address, including the port if one was provided.
- If <delegated_hostname> is not an IP literal, and <delegated_port> is present, an IP address is discovered by looking up an AAAA or A record for <delegated_hostname>. The resulting IP address is used, alongside the <delegated_port>. Requests must be made with a Host header of <delegated_hostname>:<delegated_port>. The target server must present a valid certificate for <delegated_hostname>.
- If <delegated_hostname> is not an IP literal and no <delegated_port> is present, an SRV record is looked up for _matrix._tcp.<delegated_hostname>. This may result in another hostname (to be resolved using AAAA or A records) and port. Requests should be made to the resolved IP address and port with a Host header containing the <delegated_hostname>. The target server must present a valid certificate for <delegated_hostname>.
- If no SRV record is found, an IP address is resolved using AAAA or A records. Requests are then made to the resolve IP address and a port of 8448, using a Host header of <delegated_hostname>. The target server must present a valid certificate for <delegated_hostname>.
If the /.well-known request resulted in an error response, a server is found by resolving an SRV record for _matrix._tcp.<hostname>. This may result in a hostname (to be resolved using AAAA or A records) and port. Requests are made to the resolved IP address and port, using 8448 as a default port, with a Host header of <hostname>. The target server must present a valid certificate for <hostname>.
If the /.well-known request returned an error response, and the SRV record was not found, an IP address is resolved using AAAA and A records. Requests are made to the resolved IP address using port 8448 and a Host header containing the <hostname>. The target server must present a valid certificate for <hostname>.

{{% boxes/note %}} The reasons we require <hostname> rather than <delegated_hostname> for SRV delegation are:

DNS is insecure (not all domains have DNSSEC), so the target of the delegation must prove that it is a valid delegate for <hostname> via TLS.
Consistency with the recommendations in RFC6125 and other applications using SRV records such XMPP. {{% /boxes/note %}}

Server implementation

Retrieving server keys

{{% boxes/note %}} There was once a "version 1" of the key exchange. It has been removed from the specification due to lack of significance. It may be reviewed from the historical draft. {{% /boxes/note %}}

Each homeserver publishes its public keys under /_matrix/key/v2/server/{keyId}. Homeservers query for keys by either getting /_matrix/key/v2/server/{keyId} directly or by querying an intermediate notary server using a /_matrix/key/v2/query/{serverName}/{keyId} API. Intermediate notary servers query the /_matrix/key/v2/server/{keyId} API on behalf of another server and sign the response with their own key. A server may query multiple notary servers to ensure that they all report the same public keys.

This approach is borrowed from the Perspectives Project, but modified to include the NACL keys and to use JSON instead of XML. It has the advantage of avoiding a single trust-root since each server is free to pick which notary servers they trust and can corroborate the keys returned by a given notary server by querying other servers.

Publishing Keys

Homeservers publish their signing keys in a JSON object at /_matrix/key/v2/server/{key_id}. The response contains a list of verify_keys that are valid for signing federation requests made by the homeserver and for signing events. It contains a list of old_verify_keys which are only valid for signing events.

Querying Keys Through Another Server

Servers may query another server's keys through a notary server. The notary server may be another homeserver. The notary server will retrieve keys from the queried servers through use of the /_matrix/key/v2/server/{keyId} API. The notary server will additionally sign the response from the queried server before returning the results.

Notary servers can return keys for servers that are offline or having issues serving their own keys by using cached responses. Keys can be queried from multiple servers to mitigate against DNS spoofing.

Authentication

Request Authentication

Every HTTP request made by a homeserver is authenticated using public key digital signatures. The request method, target and body are signed by wrapping them in a JSON object and signing it using the JSON signing algorithm. The resulting signatures are added as an Authorization header with an auth scheme of X-Matrix. Note that the target field should include the full path starting with /_matrix/..., including the ? and any query parameters if present, but should not include the leading https:, nor the destination server's hostname.

Step 1 sign JSON:

{
    "method": "GET",
    "uri": "/target",
    "origin": "origin.hs.example.com",
    "destination": "destination.hs.example.com",
    "content": <JSON-parsed request body>,
    "signatures": {
        "origin.hs.example.com": {
            "ed25519:key1": "ABCDEF..."
        }
    }
}

The server names in the JSON above are the server names for each homeserver involved. Delegation from the server name resolution section above do not affect these - the server names from before delegation would take place are used. This same condition applies throughout the request signing process.

Step 2 add Authorization header:

GET /target HTTP/1.1
Authorization: X-Matrix origin="origin.hs.example.com",destination="destination.hs.example.com",key="ed25519:key1",sig="ABCDEF..."
Content-Type: application/json

<JSON-encoded request body>

Example python code:

def authorization_headers(origin_name, origin_signing_key,
                          destination_name, request_method, request_target,
                          content=None):
    request_json = {
         "method": request_method,
         "uri": request_target,
         "origin": origin_name,
         "destination": destination_name,
    }

    if content is not None:
        # Assuming content is already parsed as JSON
        request_json["content"] = content

    signed_json = sign_json(request_json, origin_name, origin_signing_key)

    authorization_headers = []

    for key, sig in signed_json["signatures"][origin_name].items():
        authorization_headers.append(bytes(
            "X-Matrix origin=\"%s\",destination=\"%s\",key=\"%s\",sig=\"%s\"" % (
                origin_name, destination_name, key, sig,
            )
        ))

    return ("Authorization", authorization_headers[0])

The format of the Authorization header is given in RFC 7235. In summary, the header begins with authorization scheme X-Matrix, followed by one or more spaces, followed by a comma-separated list of parameters written as name=value pairs. The names are case insensitive and order does not matter. The values must be enclosed in quotes if they contain characters that are not allowed in tokens, as defined in RFC 7230; if a value is a valid token, it may or may not be enclosed in quotes. Quoted values may include backslash-escaped characters. When parsing the header, the recipient must unescape the characters. That is, a backslash-character pair is replaced by the character that follows the backslash.

For compatibility with older servers, the sender should

only include one space after X-Matrix,
only use lower-case names, and
avoid using backslashes in parameter values.

For compatibility with older servers, the recipient should allow colons to be included in values without requiring the value to be enclosed in quotes.

The authorization parameters to include are:

origin: the server name of the sending server. This is the same as the origin field from JSON described in step 1.
destination: {{< added-in v="1.3" >}} the server name of the receiving sender. This is the same as the destination field from the JSON described in step 1. For compatibility with older servers, recipients should accept requests without this parameter, but MUST always send it. If this property is included, but the value does not match the receiving server's name, the receiving server must deny the request with an HTTP status code 401 Unauthorized.
key: the ID, including the algorithm name, of the sending server's key used to sign the request.
signature: the signature of the JSON as calculated in step 1.

Unknown parameters are ignored.

Response Authentication

Responses are authenticated by the TLS server certificate. A homeserver should not send a request until it has authenticated the connected server to avoid leaking messages to eavesdroppers.

Client TLS Certificates

Requests are authenticated at the HTTP layer rather than at the TLS layer because HTTP services like Matrix are often deployed behind load balancers that handle the TLS and these load balancers make it difficult to check TLS client certificates.

A homeserver may provide a TLS client certificate and the receiving homeserver may check that the client certificate matches the certificate of the origin homeserver.

Transactions

The transfer of EDUs and PDUs between homeservers is performed by an exchange of Transaction messages, which are encoded as JSON objects, passed over an HTTP PUT request. A Transaction is meaningful only to the pair of homeservers that exchanged it; they are not globally-meaningful.

Transactions are limited in size; they can have at most 50 PDUs and 100 EDUs.

PDUs

Each PDU contains a single Room Event which the origin server wants to send to the destination.

The prev_events field of a PDU identifies the "parents" of the event, and thus establishes a partial ordering on events within the room by linking them into a Directed Acyclic Graph (DAG). The sending server should populate this field with all of the events in the room for which it has not yet seen a child - thus demonstrating that the event comes after all other known events.

For example, consider a room whose events form the DAG shown below. A server creating a new event in this room should populate the new event's prev_events field with both E4 and E6, since neither event yet has a child:

E1
^
|
E2 <--- E5
^       ^
|       |
E3      E6
^
|
E4

For a full schema of what a PDU looks like, see the room version specification.

Checks performed on receipt of a PDU

Whenever a server receives an event from a remote server, the receiving server must ensure that the event:

Is a valid event, otherwise it is dropped. For an event to be valid, it must contain a room_id, and it must comply with the event format of that room version.
Passes signature checks, otherwise it is dropped.
Passes hash checks, otherwise it is redacted before being processed further.
Passes authorization rules based on the event's auth events, otherwise it is rejected.
Passes authorization rules based on the state before the event, otherwise it is rejected.
Passes authorization rules based on the current state of the room, otherwise it is "soft failed".

Further details of these checks, and how to handle failures, are described below.

The Signing Events section has more information on which hashes and signatures are expected on events, and how to calculate them.

Definitions

Required Power Level: A given event type has an associated required power level. This is given by the current m.room.power_levels event. The event type is either listed explicitly in the events section or given by either state_default or events_default depending on if the event is a state event or not.
Invite Level, Kick Level, Ban Level, Redact Level: The levels given by the invite, kick, ban, and redact properties in the current m.room.power_levels state. The invite level defaults to 0 if unspecified. The kick level, ban level and redact level each default to 50 if unspecified.
Target User: For an m.room.member state event, the user given by the state_key of the event.

{{% boxes/warning %}} Some room versions accept power level values to be represented as strings rather than integers. This is strictly for backwards compatibility. A homeserver should take reasonable precautions to prevent users from sending new power level events with string values (eg: by rejecting the API request), and must never populate the default power levels in a room as string values.

See the room version specification for more information. {{% /boxes/warning %}}

Authorization rules

The rules governing whether an event is authorized depends on a set of state. A given event is checked multiple times against different sets of state, as specified above. Each room version can have a different algorithm for how the rules work, and which rules are applied. For more detailed information, please see the room version specification.

Auth events selection

The auth_events field of a PDU identifies the set of events which give the sender permission to send the event. The auth_events for the m.room.create event in a room is empty; for other events, it should be the following subset of the room state:

The m.room.create event.
The current m.room.power_levels event, if any.
The sender's current m.room.member event, if any.
If type is m.room.member:
- The target's current m.room.member event, if any.
- If membership is join or invite, the current m.room.join_rules event, if any.
- If membership is invite and content contains a third_party_invite property, the current m.room.third_party_invite event with state_key matching content.third_party_invite.signed.token, if any.
- If content.join_authorised_via_users_server is present, and the room version supports restricted rooms, then the m.room.member event with state_key matching content.join_authorised_via_users_server.

Rejection

If an event is rejected it should neither be relayed to clients nor be included as a prev event in any new events generated by the server. Subsequent events from other servers that reference rejected events should be allowed if they still pass the auth rules. The state used in the checks should be calculated as normal, except not updating with the rejected event where it is a state event.

If an event in an incoming transaction is rejected, this should not cause the transaction request to be responded to with an error response.

{{% boxes/note %}} This means that events may be included in the room DAG even though they should be rejected. {{% /boxes/note %}}

{{% boxes/note %}} This is in contrast to redacted events which can still affect the state of the room. For example, a redacted join event will still result in the user being considered joined. {{% /boxes/note %}}

Soft failure

{{% boxes/rationale %}} It is important that we prevent users from evading bans (or other power restrictions) by creating events which reference old parts of the DAG. For example, a banned user could continue to send messages to a room by having their server send events which reference the event before they were banned. Note that such events are entirely valid, and we cannot simply reject them, as it is impossible to distinguish such an event from a legitimate one which has been delayed. We must therefore accept such events and let them participate in state resolution and the federation protocol as normal. However, servers may choose not to send such events on to their clients, so that end users won't actually see the events.

When this happens it is often fairly obvious to servers, as they can see that the new event doesn't actually pass auth based on the "current state" (i.e. the resolved state across all forward extremities). While the event is technically valid, the server can choose to not notify clients about the new event.

This discourages servers from sending events that evade bans etc. in this way, as end users won't actually see the events. {{% /boxes/rationale %}}

When the homeserver receives a new event over federation it should also check whether the event passes auth checks based on the current state of the room (as well as based on the state at the event). If the event does not pass the auth checks based on the current state of the room (but does pass the auth checks based on the state at that event) it should be "soft failed".

When an event is "soft failed" it should not be relayed to the client nor be referenced by new events created by the homeserver (i.e. they should not be added to the server's list of forward extremities of the room). Soft failed events are otherwise handled as usual.

{{% boxes/note %}} Soft failed events participate in state resolution as normal if further events are received which reference it. It is the job of the state resolution algorithm to ensure that malicious events cannot be injected into the room state via this mechanism. {{% /boxes/note %}}

{{% boxes/note %}} Because soft failed state events participate in state resolution as normal, it is possible for such events to appear in the current state of the room. In that case the client should be told about the soft failed event in the usual way (e.g. by sending it down in the state section of a sync response). {{% /boxes/note %}}

{{% boxes/note %}} A soft failed event should be returned in response to federation requests where appropriate (e.g. in /event/<event_id>). Note that soft failed events are returned in /backfill and /get_missing_events responses only if the requests include events referencing the soft failed events. {{% /boxes/note %}}

Example

As an example consider the event graph:

  A
 /
B

where B is a ban of a user X. If the user X tries to set the topic by sending an event C while evading the ban:

  A
 / \
B   C

servers that receive C after B should soft fail event C, and so will neither relay C to its clients nor send any events referencing C.

If later another server sends an event D that references both B and C (this can happen if it received C before B):

  A
 / \
B   C
 \ /
  D

then servers will handle D as normal. D is sent to the servers' clients (assuming D passes auth checks). The state at D may resolve to a state that includes C, in which case clients should also to be told that the state has changed to include C. (Note: This depends on the exact state resolution algorithm used. In the original version of the algorithm C would be in the resolved state, whereas in latter versions the algorithm tries to prioritise the ban over the topic change.)

Note that this is essentially equivalent to the situation where one server doesn't receive C at all, and so asks another server for the state of the C branch.

Let's go back to the graph before D was sent:

  A
 / \
B   C

If all the servers in the room saw B before C and so soft fail C, then any new event D' will not reference C:

  A
 / \
B   C
|
D'

Retrieving event authorization information

The homeserver may be missing event authorization information, or wish to check with other servers to ensure it is receiving the correct auth chain. These APIs give the homeserver an avenue for getting the information it needs.

EDUs

EDUs, by comparison to PDUs, do not have an ID, a room ID, or a list of "previous" IDs. They are intended to be non-persistent data such as user presence, typing notifications, etc.

Room State Resolution

The state of a room is a map of (event_type, state_key) to event_id. Each room starts with an empty state, and each state event which is accepted into the room updates the state of that room.

Where each event has a single prev_event, it is clear what the state of the room after each event should be. However, when two branches in the event graph merge, the state of those branches might differ, so a state resolution algorithm must be used to determine the resultant state.

For example, consider the following event graph (where the oldest event, E0, is at the top):

  E0
  |
  E1
 /  \
E2  E4
|    |
E3   |
 \  /
  E5

Suppose E3 and E4 are both m.room.name events which set the name of the room. What should the name of the room be at E5?

The algorithm to be used for state resolution depends on the room version. For a description of each room version's algorithm, please see the room version specification.

Backfilling and retrieving missing events

Once a homeserver has joined a room, it receives all the events emitted by other homeservers in that room, and is thus aware of the entire history of the room from that moment onwards. Since users in that room are able to request the history by the /messages client API endpoint, it's possible that they might step backwards far enough into history before the homeserver itself was a member of that room.

To cover this case, the federation API provides a server-to-server analog of the /messages client API, allowing one homeserver to fetch history from another. This is the /backfill API.

To request more history, the requesting homeserver picks another homeserver that it thinks may have more (most likely this should be a homeserver for some of the existing users in the room at the earliest point in history it has currently), and makes a /backfill request.

Similar to backfilling a room's history, a server may not have all the events in the graph. That server may use the /get_missing_events API to acquire the events it is missing.

Retrieving events

In some circumstances, a homeserver may be missing a particular event or information about the room which cannot be easily determined from backfilling. These APIs provide homeservers with the option of getting events and the state of the room at a given point in the timeline.

Joining Rooms

When a new user wishes to join a room that the user's homeserver already knows about, the homeserver can immediately determine if this is allowable by inspecting the state of the room. If it is acceptable, it can generate, sign, and emit a new m.room.member state event adding the user into that room. When the homeserver does not yet know about the room it cannot do this directly. Instead, it must take a longer multi-stage handshaking process by which it first selects a remote homeserver which is already participating in that room, and use it to assist in the joining process. This is the remote join handshake.

This handshake involves the homeserver of the new member wishing to join (referred to here as the "joining" server), the directory server hosting the room alias the user is requesting to join with, and a homeserver where existing room members are already present (referred to as the "resident" server).

In summary, the remote join handshake consists of the joining server querying the directory server for information about the room alias; receiving a room ID and a list of join candidates. The joining server then requests information about the room from one of the residents. It uses this information to construct an m.room.member event which it finally sends to a resident server.

Conceptually these are three different roles of homeserver. In practice the directory server is likely to be resident in the room, and so may be selected by the joining server to be the assisting resident. Likewise, it is likely that the joining server picks the same candidate resident for both phases of event construction, though in principle any valid candidate may be used at each time. Thus, any join handshake can potentially involve anywhere from two to four homeservers, though most in practice will use just two.

+---------+          +---------------+            +-----------------+ +-----------------+
| Client  |          | JoiningServer |            | DirectoryServer | | ResidentServer  |
+---------+          +---------------+            +-----------------+ +-----------------+
     |                       |                             |                   |
     | join request          |                             |                   |
     |---------------------->|                             |                   |
     |                       |                             |                   |
     |                       | directory request           |                   |
     |                       |---------------------------->|                   |
     |                       |                             |                   |
     |                       |          directory response |                   |
     |                       |<----------------------------|                   |
     |                       |                             |                   |
     |                       | make_join request           |                   |
     |                       |------------------------------------------------>|
     |                       |                             |                   |
     |                       |                             |make_join response |
     |                       |<------------------------------------------------|
     |                       |                             |                   |
     |                       | send_join request           |                   |
     |                       |------------------------------------------------>|
     |                       |                             |                   |
     |                       |                             |send_join response |
     |                       |<------------------------------------------------|
     |                       |                             |                   |
     |         join response |                             |                   |
     |<----------------------|                             |                   |
     |                       |                             |                   |

The first part of the handshake usually involves using the directory server to request the room ID and join candidates through the /query/directory API endpoint. In the case of a new user joining a room as a result of a received invite, the joining user's homeserver could optimise this step away by picking the origin server of that invite message as the join candidate. However, the joining server should be aware that the origin server of the invite might since have left the room, so should be prepared to fall back on the regular join flow if this optimisation fails.

Once the joining server has the room ID and the join candidates, it then needs to obtain enough information about the room to fill in the required fields of the m.room.member event. It obtains this by selecting a resident from the candidate list, and using the GET /make_join endpoint. The resident server will then reply with enough information for the joining server to fill in the event.

The joining server is expected to add or replace the origin, origin_server_ts, and event_id on the templated event received by the resident server. This event is then signed by the joining server.

To complete the join handshake, the joining server submits this new event to the resident server it used for GET /make_join, using the PUT /send_join endpoint.

The resident homeserver then adds its signature to this event and accepts it into the room's event graph. The joining server receives the full set of state for the newly-joined room as well as the freshly signed membership event. The resident server must also send the event to other servers participating in the room.

Restricted rooms

Restricted rooms are described in detail in the client-server API and are available in room versions which support restricted join rules.

A resident server processing a request to join a restricted room must ensure that the joining server satisfies at least one of the conditions specified by m.room.join_rules. If no conditions are available, or none match the required schema, then the joining server is considered to have failed all conditions.

The resident server uses a 400 M_UNABLE_TO_AUTHORISE_JOIN error on /make_join and /send_join to denote that the resident server is unable to validate any of the conditions, usually because the resident server does not have state information about rooms required by the conditions.

The resident server uses a 400 M_UNABLE_TO_GRANT_JOIN error on /make_join and /send_join to denote that the joining server should try a different server. This is typically because the resident server can see that the joining user satisfies one of the conditions, though the resident server would be unable to meet the auth rules governing join_authorised_via_users_server on the resulting m.room.member event.

If the joining server fails all conditions then a 403 M_FORBIDDEN error is used by the resident server.

Knocking upon a room

Rooms can permit knocking through the join rules, and if permitted this gives users a way to request to join (be invited) to the room. Users who knock on a room where the server is already a resident of the room can just send the knock event directly without using this process, however much like joining rooms the server must handshake their way into having the knock sent on its behalf.

The handshake is largely the same as the joining rooms handshake, where instead of a "joining server" there is a "knocking server", and the APIs to be called are different (/make_knock and /send_knock).

Servers can retract knocks over federation by leaving the room, as described below for rejecting invites.

Inviting to a room

When a user on a given homeserver invites another user on the same homeserver, the homeserver may sign the membership event itself and skip the process defined here. However, when a user invites another user on a different homeserver, a request to that homeserver to have the event signed and verified must be made.

Note that invites are used to indicate that knocks were accepted. As such, receiving servers should be prepared to manually link up a previous knock to an invite if the invite event does not directly reference the knock.

Leaving Rooms (Rejecting Invites)

Normally homeservers can send appropriate m.room.member events to have users leave the room, to reject local invites, or to retract a knock. Remote invites/knocks from other homeservers do not involve the server in the graph and therefore need another approach to reject the invite. Joining the room and promptly leaving is not recommended as clients and servers will interpret that as accepting the invite, then leaving the room rather than rejecting the invite.

Similar to the Joining Rooms handshake, the server which wishes to leave the room starts with sending a /make_leave request to a resident server. In the case of rejecting invites, the resident server may be the server which sent the invite. After receiving a template event from /make_leave, the leaving server signs the event and replaces the event_id with its own. This is then sent to the resident server via /send_leave. The resident server will then send the event to other servers in the room.

Third-party invites

{{% boxes/note %}} More information about third party invites is available in the Client-Server API under the Third Party Invites module. {{% /boxes/note %}}

When a user wants to invite another user in a room but doesn't know the Matrix ID to invite, they can do so using a third-party identifier (e.g. an e-mail or a phone number).

This identifier and its bindings to Matrix IDs are verified by an identity server implementing the Identity Service API.

Cases where an association exists for a third-party identifier

If the third-party identifier is already bound to a Matrix ID, a lookup request on the identity server will return it. The invite is then processed by the inviting homeserver as a standard m.room.member invite event. This is the simplest case.

Cases where an association doesn't exist for a third-party identifier

If the third-party identifier isn't bound to any Matrix ID, the inviting homeserver will request the identity server to store an invite for this identifier and to deliver it to whoever binds it to its Matrix ID. It will also send an m.room.third_party_invite event in the room to specify a display name, a token and public keys the identity server provided as a response to the invite storage request.

When a third-party identifier with pending invites gets bound to a Matrix ID, the identity server will send a POST request to the ID's homeserver as described in the Invitation Storage section of the Identity Service API.

The following process applies for each invite sent by the identity server:

The invited homeserver will create an m.room.member invite event containing a special third_party_invite section containing the token and a signed object, both provided by the identity server.

If the invited homeserver is in the room the invite came from, it can auth the event and send it.

However, if the invited homeserver isn't in the room the invite came from, it will need to request the room's homeserver to auth the event.

Verifying the invite

When a homeserver receives an m.room.member invite event for a room it's in with a third_party_invite object, it must verify that the association between the third-party identifier initially invited to the room and the Matrix ID that claims to be bound to it has been verified without having to rely on a third-party server.

To do so, it will fetch from the room's state events the m.room.third_party_invite event for which the state key matches with the value for the token key in the third_party_invite object from the m.room.member event's content to fetch the public keys initially delivered by the identity server that stored the invite.

It will then use these keys to verify that the signed object (in the third_party_invite object from the m.room.member event's content) was signed by the same identity server.

Since this signed object can only be delivered once in the POST request emitted by the identity server upon binding between the third-party identifier and the Matrix ID, and contains the invited user's Matrix ID and the token delivered when the invite was stored, this verification will prove that the m.room.member invite event comes from the user owning the invited third-party identifier.

Public Room Directory

To complement the Client-Server API's room directory, homeservers need a way to query the public rooms for another server. This can be done by making a request to the /publicRooms endpoint for the server the room directory should be retrieved for.

Spaces

To complement the Client-Server API's Spaces module, homeservers need a way to query information about spaces from other servers.

Typing Notifications

When a server's users send typing notifications, those notifications need to be sent to other servers in the room so their users are aware of the same state. Receiving servers should verify that the user is in the room, and is a user belonging to the sending server.

{{% definition path="api/server-server/definitions/event-schemas/m.typing" %}}

Presence

The server API for presence is based entirely on exchange of the following EDUs. There are no PDUs or Federation Queries involved.

Servers should only send presence updates for users that the receiving server would be interested in. Such as the receiving server sharing a room with a given user.

{{% definition path="api/server-server/definitions/event-schemas/m.presence" %}}

Receipts

Receipts are EDUs used to communicate a marker for a given event. Currently the only kind of receipt supported is a "read receipt", or where in the event graph the user has read up to.

Read receipts for events that a user sent do not need to be sent. It is implied that by sending the event the user has read up to the event.

{{% definition path="api/server-server/definitions/event-schemas/m.receipt" %}}

Querying for information

Queries are a way to retrieve information from a homeserver about a resource, such as a user or room. The endpoints here are often called in conjunction with a request from a client on the client-server API in order to complete the call.

There are several types of queries that can be made. The generic endpoint to represent all queries is described first, followed by the more specific queries that can be made.

OpenID

Third party services can exchange an access token previously generated by the Client-Server API for information about a user. This can help verify that a user is who they say they are without granting full access to the user's account.

Access tokens generated by the OpenID API are only good for the OpenID API and nothing else.

Device Management

Details of a user's devices must be efficiently published to other users and kept up-to-date. This is critical for reliable end-to-end encryption, in order for users to know which devices are participating in a room. It's also required for to-device messaging to work. This section is intended to complement the Device Management module of the Client-Server API.

Matrix currently uses a custom pubsub system for synchronising information about the list of devices for a given user over federation. When a server wishes to determine a remote user's device list for the first time, it should populate a local cache from the result of a /user/keys/query API on the remote server. However, subsequent updates to the cache should be applied by consuming m.device_list_update EDUs. Each new m.device_list_update EDU describes an incremental change to one device for a given user which should replace any existing entry in the local server's cache of that device list. Servers must send m.device_list_update EDUs to all the servers who share a room with a given local user, and must be sent whenever that user's device list changes (i.e. for new or deleted devices, when that user joins a room which contains servers which are not already receiving updates for that user's device list, or changes in device information such as the device's human-readable name).

Servers send m.device_list_update EDUs in a sequence per origin user, each with a unique stream_id. They also include a pointer to the most recent previous EDU(s) that this update is relative to in the prev_id field. To simplify implementation for clustered servers which could send multiple EDUs at the same time, the prev_id field should include all m.device_list_update EDUs which have not been yet been referenced in a EDU. If EDUs are emitted in series by a server, there should only ever be one prev_id in the EDU.

This forms a simple directed acyclic graph of m.device_list_update EDUs, showing which EDUs a server needs to have received in order to apply an update to its local copy of the remote user's device list. If a server receives an EDU which refers to a prev_id it does not recognise, it must resynchronise its list by calling the /user/keys/query API and resume the process. The response contains a stream_id which should be used to correlate with subsequent m.device_list_update EDUs.

{{% definition path="api/server-server/definitions/event-schemas/m.device_list_update" %}}

End-to-End Encryption

This section complements the End-to-End Encryption module of the Client-Server API. For detailed information about end-to-end encryption, please see that module.

The APIs defined here are designed to be able to proxy much of the client's request through to federation, and have the response also be proxied through to the client.

{{% definition path="api/server-server/definitions/event-schemas/m.signing_key_update" %}}

Send-to-device messaging

The server API for send-to-device messaging is based on the m.direct_to_device EDU. There are no PDUs or Federation Queries involved.

Each send-to-device message should be sent to the destination server using the following EDU:

{{% definition path="api/server-server/definitions/event-schemas/m.direct_to_device" %}}

Content Repository

Attachments to events (images, files, etc) are uploaded to a homeserver via the Content Repository described in the Client-Server API. When a server wishes to serve content originating from a remote server, it needs to ask the remote server for the media.

Servers should use the server described in the Matrix Content URI, which has the format mxc://{ServerName}/{MediaID}. Servers should use the download endpoint described in the Client-Server API, being sure to use the allow_remote parameter (set to false).

Server Access Control Lists (ACLs)

Server ACLs and their purpose are described in the Server ACLs section of the Client-Server API.

When a remote server makes a request, it MUST be verified to be allowed by the server ACLs. If the server is denied access to a room, the receiving server MUST reply with a 403 HTTP status code and an errcode of M_FORBIDDEN.

The following endpoint prefixes MUST be protected:

/_matrix/federation/v1/send (on a per-PDU basis)
/_matrix/federation/v1/make_join
/_matrix/federation/v1/make_leave
/_matrix/federation/v1/send_join
/_matrix/federation/v2/send_join
/_matrix/federation/v1/send_leave
/_matrix/federation/v2/send_leave
/_matrix/federation/v1/invite
/_matrix/federation/v2/invite
/_matrix/federation/v1/make_knock
/_matrix/federation/v1/send_knock
/_matrix/federation/v1/state
/_matrix/federation/v1/state_ids
/_matrix/federation/v1/backfill
/_matrix/federation/v1/event_auth
/_matrix/federation/v1/get_missing_events

Signing Events

Signing events is complicated by the fact that servers can choose to redact non-essential parts of an event.

Adding hashes and signatures to outgoing events

Before signing the event, the content hash of the event is calculated as described below. The hash is encoded using Unpadded Base64 and stored in the event object, in a hashes object, under a sha256 key.

The event object is then redacted, following the redaction algorithm. Finally it is signed as described in Signing JSON, using the server's signing key (see also Retrieving server keys).

The signature is then copied back to the original event object.

For an example of a signed event, see the room version specification.

Validating hashes and signatures on received events

When a server receives an event over federation from another server, the receiving server should check the hashes and signatures on that event.

First the signature is checked. The event is redacted following the redaction algorithm, and the resultant object is checked for a signature from the originating server, following the algorithm described in Checking for a signature. Note that this step should succeed whether we have been sent the full event or a redacted copy.

The signatures expected on an event are:

The sender's server, unless the invite was created as a result of 3rd party invite. The sender must already match the 3rd party invite, and the server which actually sends the event may be a different server.
For room versions 1 and 2, the server which created the event_id. Other room versions do not track the event_id over federation and therefore do not need a signature from those servers.

If the signature is found to be valid, the expected content hash is calculated as described below. The content hash in the hashes property of the received event is base64-decoded, and the two are compared for equality.

If the hash check fails, then it is assumed that this is because we have only been given a redacted version of the event. To enforce this, the receiving server should use the redacted copy it calculated rather than the full copy it received.

Calculating the reference hash for an event

The reference hash of an event covers the essential fields of an event, including content hashes. It is used for event identifiers in some room versions. See the room version specification for more information. It is calculated as follows.

The event is put through the redaction algorithm.
The signatures, age_ts, and unsigned properties are removed from the event, if present.
The event is converted into Canonical JSON.
A sha256 hash is calculated on the resulting JSON object.

Calculating the content hash for an event

The content hash of an event covers the complete event including the unredacted contents. It is calculated as follows.

First, any existing unsigned, signature, and hashes members are removed. The resulting object is then encoded as Canonical JSON, and the JSON is hashed using SHA-256.

Example code

def hash_and_sign_event(event_object, signing_key, signing_name):
    # First we need to hash the event object.
    content_hash = compute_content_hash(event_object)
    event_object["hashes"] = {"sha256": encode_unpadded_base64(content_hash)}

    # Strip all the keys that would be removed if the event was redacted.
    # The hashes are not stripped and cover all the keys in the event.
    # This means that we can tell if any of the non-essential keys are
    # modified or removed.
    stripped_object = strip_non_essential_keys(event_object)

    # Sign the stripped JSON object. The signature only covers the
    # essential keys and the hashes. This means that we can check the
    # signature even if the event is redacted.
    signed_object = sign_json(stripped_object, signing_key, signing_name)

    # Copy the signatures from the stripped event to the original event.
    event_object["signatures"] = signed_object["signatures"]

def compute_content_hash(event_object):
    # take a copy of the event before we remove any keys.
    event_object = dict(event_object)

    # Keys under "unsigned" can be modified by other servers.
    # They are useful for conveying information like the age of an
    # event that will change in transit.
    # Since they can be modified we need to exclude them from the hash.
    event_object.pop("unsigned", None)

    # Signatures will depend on the current value of the "hashes" key.
    # We cannot add new hashes without invalidating existing signatures.
    event_object.pop("signatures", None)

    # The "hashes" key might contain multiple algorithms if we decide to
    # migrate away from SHA-2. We don't want to include an existing hash
    # output in our hash so we exclude the "hashes" dict from the hash.
    event_object.pop("hashes", None)

    # Encode the JSON using a canonical encoding so that we get the same
    # bytes on every server for the same JSON object.
    event_json_bytes = encode_canonical_json(event_object)

    return hashlib.sha256(event_json_bytes)

Security considerations

When a domain's ownership changes, the new controller of the domain can masquerade as the previous owner, receiving messages (similarly to email) and request past messages from other servers. In the future, proposals like MSC1228 will address this issue.

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