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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+---
+title: User/Group Record Lookup API via Varlink
+category: Users, Groups and Home Directories
+layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
+---
+
+# User/Group Record Lookup API via Varlink
+
+JSON User/Group Records (as described in the [JSON User Records](USER_RECORD.md)
+and [JSON Group Records](GROUP_RECORD.md) documents) that are defined on the
+local system may be queried with a [Varlink](https://varlink.org/) API. This
+API takes both the role of what
+[`getpwnam(3)`](https://man7.org/linux/man-pages/man3/getpwnam.3.html) and
+related calls are for `struct passwd`, as well as the interfaces modules
+implementing the [glibc Name Service Switch
+(NSS)](https://www.gnu.org/software/libc/manual/html_node/Name-Service-Switch.html)
+expose. Or in other words, it both allows applications to efficiently query
+user/group records from local services, and allows local subsystems to provide
+user/group records efficiently to local applications.
+
+The concepts described here define an IPC interface. Alternatively, user/group
+records may be dropped in number of drop-in directories as files where they are
+picked up in addition to the users/groups defined by this IPC logic. See
+[`nss-systemd(8)`](https://www.freedesktop.org/software/systemd/man/nss-systemd.html)
+for details.
+
+This simple API only exposes only three method calls, and requires only a small
+subset of the Varlink functionality.
+
+## Why Varlink?
+
+The API described in this document is based on a simple subset of the
+mechanisms described by [Varlink](https://varlink.org/). The choice of
+preferring Varlink over D-Bus and other IPCs in this context was made for three
+reasons:
+
+1. User/Group record resolution should work during early boot and late shutdown
+   without special handling. This is very hard to do with D-Bus, as the broker
+   service for D-Bus generally runs as regular system daemon and is hence only
+   available at the latest boot stage.
+
+2. The JSON user/group records are native JSON data, hence picking an IPC
+   system that natively operates with JSON data is natural and clean.
+
+3. IPC systems such as D-Bus do not provide flow control and are thus unusable
+   for streaming data. They are useful to pass around short control messages,
+   but as soon as potentially many and large objects shall be transferred,
+   D-Bus is not suitable, as any such streaming of messages would be considered
+   flooding in D-Bus' logic, and thus possibly result in termination of
+   communication. Since the APIs defined in this document need to support
+   enumerating potentially large numbers of users and groups, D-Bus is simply
+   not an appropriate option.
+
+## Concepts
+
+Each subsystem that needs to define users and groups on the local system is
+supposed to implement this API, and offer its interfaces on a Varlink
+`AF_UNIX`/`SOCK_STREAM` file system socket bound into the
+`/run/systemd/userdb/` directory. When a client wants to look up a user or
+group record, it contacts all sockets bound in this directory in parallel, and
+enqueues the same query to each. The first positive reply is then returned to
+the application, or if all fail the last seen error is returned
+instead. (Alternatively a special Varlink service is available,
+`io.systemd.Multiplexer` which acts as frontend and will do the parallel
+queries on behalf of the client, drastically simplifying client
+development. This service is not available during earliest boot and final
+shutdown phases.)
+
+Unlike with glibc NSS there's no order or programmatic expression language
+defined in which queries are issued to the various services. Instead, all
+queries are always enqueued in parallel to all defined services, in order to
+make look-ups efficient, and the simple rule of "first successful lookup wins"
+is unconditionally followed for user and group look-ups (though not for
+membership lookups, see below).
+
+This simple scheme only works safely as long as every service providing
+user/group records carefully makes sure not to answer with conflicting
+records. This API does not define any mechanisms for dealing with user/group
+name/ID collisions during look-up nor during record registration. It assumes
+the various subsystems that want to offer user and group records to the rest of
+the system have made sufficiently sure in advance that their definitions do not
+collide with those of other services. Clients are not expected to merge
+multiple definitions for the same user or group, and will also not be able to
+detect conflicts and suppress such conflicting records.
+
+It is recommended to name the sockets in the directory in reverse domain name
+notation, but this is neither required nor enforced.
+
+## Well-Known Services
+
+Any subsystem that wants to provide user/group records can do so, simply by
+binding a socket in the aforementioned directory. By default two
+services are listening there, that have special relevance:
+
+1. `io.systemd.NameServiceSwitch` → This service makes the classic UNIX/glibc
+   NSS user/group records available as JSON User/Group records. Any such
+   records are automatically converted as needed, and possibly augmented with
+   information from the shadow databases.
+
+2. `io.systemd.Multiplexer` → This service multiplexes client queries to all
+   other running services. It's supposed to simplify client development: in
+   order to look up or enumerate user/group records it's sufficient to talk to
+   one service instead of all of them in parallel. Note that it is not available
+   during earliest boot and final shutdown phases, hence for programs running
+   in that context it is preferable to implement the parallel lookup
+   themselves.
+
+Both these services are implemented by the same daemon
+`systemd-userdbd.service`.
+
+Note that these services currently implement a subset of Varlink only. For
+example, introspection is not available, and the resolver logic is not used.
+
+## Other Services
+
+The `systemd` project provides three other services implementing this
+interface. Specifically:
+
+1. `io.systemd.DynamicUser` → This service is implemented by the service
+   manager itself, and provides records for the users and groups synthesized
+   via `DynamicUser=` in unit files.
+
+2. `io.systemd.Home` → This service is implemented by `systemd-homed.service`
+   and provides records for the users and groups defined by the home
+   directories it manages.
+
+3. `io.systemd.Machine` → This service is implemented by
+   `systemd-machined.service` and provides records for the users and groups used
+   by local containers that use user namespacing.
+
+Other projects are invited to implement these services too. For example it
+would make sense for LDAP/ActiveDirectory projects to implement these
+interfaces, which would provide them a way to do per-user resource management
+enforced by systemd and defined directly in LDAP directories.
+
+## Compatibility with NSS
+
+Two-way compatibility with classic UNIX/glibc NSS user/group records is
+provided. When using the Varlink API, lookups into databases provided only via
+NSS (and not natively via Varlink) are handled by the
+`io.systemd.NameServiceSwitch` service (see above). When using the NSS API
+(i.e. `getpwnam()` and friends) the `nss-systemd` module will automatically
+synthesize NSS records for users/groups natively defined via a Varlink
+API. Special care is taken to avoid recursion between these two compatibility
+mechanisms.
+
+Subsystems that shall provide user/group records to the system may choose
+between offering them via an NSS module or via a this Varlink API, either way
+all records are accessible via both APIs, due to the bidirectional
+forwarding. It is also possible to provide the same records via both APIs
+directly, but in that case the compatibility logic must be turned off. There
+are mechanisms in place for this, please contact the systemd project for
+details, as these are currently not documented.
+
+## Caching of User Records
+
+This API defines no concepts for caching records. If caching is desired it
+should be implemented in the subsystems that provide the user records, not in
+the clients consuming them.
+
+## Method Calls
+
+```
+interface io.systemd.UserDatabase
+
+method GetUserRecord(
+        uid : ?int,
+        userName : ?string,
+        service : string
+) -> (
+        record : object,
+        incomplete : bool
+)
+
+method GetGroupRecord(
+        gid : ?int,
+        groupName : ?string,
+        service : string
+) -> (
+        record : object,
+        incomplete : bool
+)
+
+method GetMemberships(
+        userName : ?string,
+        groupName : ?string,
+        service : string
+) -> (
+        userName : string,
+        groupName : string
+)
+
+error NoRecordFound()
+error BadService()
+error ServiceNotAvailable()
+error ConflictingRecordFound()
+error EnumerationNotSupported()
+```
+
+The `GetUserRecord` method looks up or enumerates a user record. If the `uid`
+parameter is set it specifies the numeric UNIX UID to search for. If the
+`userName` parameter is set it specifies the name of the user to search
+for. Typically, only one of the two parameters are set, depending whether a
+look-up by UID or by name is desired. However, clients may also specify both
+parameters, in which case a record matching both will be returned, and if only
+one exists that matches one of the two parameters but not the other an error of
+`ConflictingRecordFound` is returned. If neither of the two parameters are set
+the whole user database is enumerated. In this case the method call needs to be
+made with `more` set, so that multiple method call replies may be generated as
+effect, each carrying one user record.
+
+The `service` parameter is mandatory and should be set to the service name
+being talked to (i.e. to the same name as the `AF_UNIX` socket path, with the
+`/run/systemd/userdb/` prefix removed). This is useful to allow implementation
+of multiple services on the same socket (which is used by
+`systemd-userdbd.service`).
+
+The method call returns one or more user records, depending which type of query is
+used (see above). The record is returned in the `record` field. The
+`incomplete` field indicates whether the record is complete. Services providing
+user record lookup should only pass the `privileged` section of user records to
+clients that either match the user the record is about or to sufficiently
+privileged clients, for all others the section must be removed so that no
+sensitive data is leaked this way. The `incomplete` parameter should indicate
+whether the record has been modified like this or not (i.e. it is `true` if a
+`privileged` section existed in the user record and was removed, and `false` if
+no `privileged` section existed or one existed but hasn't been removed).
+
+If no user record matching the specified UID or name is known the error
+`NoRecordFound` is returned (this is also returned if neither UID nor name are
+specified, and hence enumeration requested but the subsystem currently has no
+users defined).
+
+If a method call with an incorrectly set `service` field is received
+(i.e. either not set at all, or not to the service's own name) a `BadService`
+error is generated. Finally, `ServiceNotAvailable` should be returned when the
+backing subsystem is not operational for some reason and hence no information
+about existence or non-existence of a record can be returned nor any user
+record at all. (The `service` field is defined in order to allow implementation
+of daemons that provide multiple distinct user/group services over the same
+`AF_UNIX` socket: in order to correctly determine which service a client wants
+to talk to, the client needs to provide the name in each request.)
+
+The `GetGroupRecord` method call works analogously but for groups.
+
+The `GetMemberships` method call may be used to inquire about group
+memberships. The `userName` and `groupName` arguments take what the name
+suggests. If one of the two is specified all matching memberships are returned,
+if neither is specified all known memberships of any user and any group are
+returned. The return value is a pair of user name and group name, where the
+user is a member of the group. If both arguments are specified the specified
+membership will be tested for, but no others, and the pair is returned if it is
+defined. Unless both arguments are specified the method call needs to be made
+with `more` set, so that multiple replies can be returned (since typically
+there are multiple members per group and also multiple groups a user is
+member of). As with `GetUserRecord` and `GetGroupRecord` the `service`
+parameter needs to contain the name of the service being talked to, in order to
+allow implementation of multiple services within the same IPC socket. In case no
+matching membership is known `NoRecordFound` is returned. The other two errors
+are also generated in the same cases as for `GetUserRecord` and
+`GetGroupRecord`.
+
+Unlike with `GetUserRecord` and `GetGroupRecord` the lists of memberships
+returned by services are always combined. Thus unlike the other two calls a
+membership lookup query has to wait for the last simultaneous query to complete
+before the complete list is acquired.
+
+Note that only the `GetMemberships` call is authoritative about memberships of
+users in groups. i.e. it should not be considered sufficient to check the
+`memberOf` field of user records and the `members` field of group records to
+acquire the full list of memberships. The full list can only be determined by
+`GetMemberships`, and as mentioned requires merging of these lists of all local
+services. Result of this is that it can be one service that defines a user A,
+and another service that defines a group B, and a third service that declares
+that A is a member of B.
+
+Looking up explicit users/groups by their name or UID/GID, or querying
+user/group memberships must be supported by all services implementing these
+interfaces. However, supporting enumeration (i.e. user/group lookups that may
+result in more than one reply, because neither UID/GID nor name is specified)
+is optional. Services which are asked for enumeration may return the
+`EnumerationNotSupported` error in this case.
+
+And that's really all there is to it.