systemd optionally processes user records that go beyond the classic UNIX (or glibc NSS) struct passwd. Various components of systemd are able to provide and consume records in a more extensible format of a dictionary of key/value pairs, encoded as JSON. Specifically:

1. systemd-homed.service manages human user home directories and embeds these JSON records directly in the home directory images (see Home Directories for details).

2. pam_systemd processes these JSON records for users that log in, and applies various settings to the activated session, including environment variables, nice levels and more.

3. systemd-logind.service processes these JSON records of users that log in, and applies various resource management settings to the per-user slice units it manages. This allows setting global limits on resource consumption by a specific user.

4. nss-systemd is a glibc NSS module that synthesizes classic NSS records from these JSON records, providing full backwards compatibility with the classic UNIX APIs both for look-up and enumeration.

5. The service manager (PID 1) exposes dynamic users (i.e. users synthesized as effect of DynamicUser= in service unit files) as these advanced JSON records, making them discoverable to the rest of the system.

6. systemd-userdbd.service is a small service that can translate UNIX/glibc NSS records to these JSON user records. It also provides a unified Varlink API for querying and enumerating records of this type, optionally acquiring them from various other services.

JSON user records may contain various fields that are not available in struct passwd, and are extensible for other applications. For example, the record may contain information about:

1. Additional security credentials (PKCS#11 security token information, biometrical authentication information, SSH public key information)

2. Additional user metadata, such as a picture, email address, location string, preferred language or timezone

3. Resource Management settings (such as CPU/IO weights, memory and tasks limits, classic UNIX resource limits or nice levels)

4. Runtime parameters such as environment variables or the nodev, noexec, nosuid flags to use for the home directory

5. Information about where to mount the home directory from

And various other things. The record is intended to be extensible, for example the following extensions are envisioned:

1. Windows network credential information

2. Information about default IMAP, SMTP servers to use for this user

3. Parental control information to enforce on this user

4. Default parameters for backup applications and similar

Similar to JSON User Records there are also JSON Group Records that encapsulate UNIX groups.

JSON User Records may be transferred or written to disk in various protocols and formats. To inquire about such records defined on the local system use the User/Group Lookup API via Varlink. User/group records may also be dropped in number of drop-in directories as files. See nss-systemd(8) for details.

JSON is nicely extensible and widely used. In particular it's easy to synthesize and process with numerous programming languages. It's particularly popular in the web communities, which hopefully should make it easy to link user credential data from the web and from local systems more closely together.

Please note that this specification assumes that JSON numbers may cover the full integer range of -2^63 … 2^64-1 without loss of precision (i.e. INT64_MIN … UINT64_MAX). Please read, write and process user records as defined by this specification only with JSON implementations that provide this number range.

The JSON user records generated and processed by systemd follow a general structure, consisting of seven distinct "sections". Specifically:

1. Various fields are placed at the top-level of user record (the regular section). These are generally fields that shall apply unconditionally to the user in all contexts, are portable and not security sensitive.

2. A number of fields are located in the privileged section (a sub-object of the user record). Fields contained in this object are security sensitive, i.e. contain information that the user and the administrator should be able to see, but other users should not. In many ways this matches the data stored in /etc/shadow in classic Linux user accounts, i.e. includes password hashes and more. Algorithmically, when a user record is passed to an untrusted client, by monopolizing such sensitive records in a single object field we can easily remove it from view.

3. A number of fields are located in objects inside the perMachine section (an array field of the user record). Primarily these are resource management-related fields, as those tend to make sense on a specific system only, e.g. limiting a user's memory use to 1G only makes sense on a specific system that has more than 1G of memory. Each object inside the perMachine array comes with a matchMachineId or matchHostname field which indicate which systems to apply the listed settings to. Note that many fields accepted in the perMachine section can also be set at the top level (the regular section), where they define the fallback if no matching object in perMachine is found.

4. Various fields are located in the binding section (a sub-sub-object of the user record; an intermediary object is inserted which is keyed by the machine ID of the host). Fields included in this section "bind" the object to a specific system. They generally include non-portable information about paths or UID assignments, that are true on a specific system, but not necessarily on others, and which are managed automatically by some user record manager (such as systemd-homed). Data in this section is considered part of the user record only in the local context, and is generally not ported to other systems. Due to that it is not included in the reduced user record the cryptographic signature defined in the signature section is calculated on. In systemd-homed this section is also removed when the user's record is stored in the ~/.identity file in the home directory, so that every system with access to the home directory can manage these binding fields individually. Typically, the binding section is persisted to the local disk.

5. Various fields are located in the status section (a sub-sub-object of the user record, also with an intermediary object between that is keyed by the machine ID, similar to the way the binding section is organized). This section is augmented during runtime only, and never persisted to disk. The idea is that this section contains information about current runtime resource usage (for example: currently used disk space of the user), that changes dynamically but is otherwise immediately associated with the user record and for many purposes should be considered to be part of the user record.

6. The signature section contains one or more cryptographic signatures of a reduced version of the user record. This is used to ensure that only user records defined by a specific source are accepted on a system, by validating the signature against the set of locally accepted signature public keys. The signature is calculated from the JSON user record with all sections removed, except for regular, privileged, perMachine. Specifically, binding, status, signature itself and secret are removed first and thus not covered by the signature. This section is optional, and is only used when cryptographic validation of user records is required (as it is by systemd-homed.service for example).

7. The secret section contains secret user credentials, such as password or PIN information. This data is never persisted, and never returned when user records are inquired by a client, privileged or not. This data should only be included in a user record very briefly, for example when certain very specific operations are executed. For example, in tools such as systemd-homed this section may be included in user records, when creating a new home directory, as passwords and similar credentials need to be provided to encrypt the home directory with.

Here's a tabular overview of the sections and their properties:

Note that services providing user records to the local system are free to manage only a subset of these sections and never include the others in them. For example, a service that has no concept of signed records (for example because the records it manages are inherently trusted anyway) does not have to bother with the signature section. A service that only defines records in a strictly local context and without signatures doesn't have to deal with the perMachine or binding sections and can include its data exclusively in the regular section. A service that uses a separate, private channel for authenticating users (or that doesn't have a concept of authentication at all) does not need to be concerned with the secret section of user records, as the fields included therein are only useful when executing authentication operations natively against JSON user records.

The systemd-homed manager uses all seven sections for various purposes. Inside the home directories (and if the LUKS2 backend is used, also in the LUKS2 header) a user record containing the regular, privileged, perMachine and signature sections is stored. systemd-homed also stores a version of the record on the host, with the same four sections and augmented with an additional, fifth binding section. When a local client enquires about a user record managed by systemd-homed the service will add in some additional information about the user and home directory in the status section — this version is only transferred via IPC and never written to disk. Finally the secret section is used during authentication operations via IPC to transfer the user record along with its authentication tokens in one go.

As mentioned, the regular section's fields are placed at the top level object. The following fields are currently defined:

userName → The UNIX user name for this record. Takes a string with a valid UNIX user name. This field is the only mandatory field, all others are optional. Corresponds with the pw_name field of struct passwd and the sp_namp field of struct spwd (i.e. the shadow user record stored in /etc/shadow). See User/Group Name Syntax for the (relaxed) rules the various systemd components enforce on user/group names.

realm → The "realm" a user is defined in. This concept allows distinguishing users with the same name that originate in different organizations or installations. This should take a string in DNS domain syntax, but doesn't have to refer to an actual DNS domain (though it is recommended to use one for this). The idea is that the user lpoetter in the redhat.com realm might be distinct from the same user in the poettering.hq realm. User records for the same user name that have different realm fields are considered referring to different users. When updating a user record it is required that any new version has to match in both userName and realm field. This field is optional, when unset the user should not be considered part of any realm. A user record with a realm set is never compatible (for the purpose of updates, see above) with a user record without one set, even if the userName field matches.

realName → The real name of the user, a string. This should contain the user's real ("human") name, and corresponds loosely to the GECOS field of classic UNIX user records. When converting a struct passwd to a JSON user record this field is initialized from GECOS (i.e. the pw_gecos field), and vice versa when converting back. That said, unlike GECOS this field is supposed to contain only the real name and no other information. This field must not contain control characters (such as \n) or colons (:), since those are used as record separators in classic /etc/passwd files and similar formats.

emailAddress → The email address of the user, formatted as string. pam_systemd initializes the $EMAIL environment variable from this value for all login sessions.

iconName → The name of an icon picked by the user, for example for the purpose of an avatar. This must be a string, and should follow the semantics defined in the Icon Naming Specification.

location → A free-form location string describing the location of the user, if that is applicable. It's probably wise to use a location string processable by geo-location subsystems, but this is not enforced nor required. Example: Berlin, Germany or Basement, Room 3a.

disposition → A string, one of intrinsic, system, dynamic, regular, container, reserved. If specified clarifies the disposition of the user, i.e. the context it is defined in. For regular, "human" users this should be regular, for system users (i.e. users that system services run under, and similar) this should be system. The intrinsic disposition should be used only for the two users that have special meaning to the OS kernel itself, i.e. the root and nobody users. The container string should be used for users that are used by an OS container, and hence will show up in ps listings and such, but are only defined in container context. Finally reserved should be used for any users outside of these use-cases. Note that this property is entirely optional and applications are assumed to be able to derive the disposition of a user automatically from a record even in absence of this field, based on other fields, for example the numeric UID. By setting this field explicitly applications can override this default determination.

lastChangeUSec → An unsigned 64bit integer value, referring to a timestamp in µs since the epoch 1970, indicating when the user record (specifically, any of the regular, privileged, perMachine sections) was last changed. This field is used when comparing two records of the same user to identify the newer one, and is used for example for automatic updating of user records, where appropriate.

lastPasswordChangeUSec → Similar, also an unsigned 64bit integer value, indicating the point in time the password (or any authentication token) of the user was last changed. This corresponds to the sp_lstchg field of struct spwd, i.e. the matching field in the user shadow database /etc/shadow, though provides finer resolution.

shell → A string, referring to the shell binary to use for terminal logins of this user. This corresponds with the pw_shell field of struct passwd, and should contain an absolute file system path. For system users not suitable for terminal log-in this field should not be set.

umask → The umask to set for the user's login sessions. Takes an integer. Note that usually on UNIX the umask is noted in octal, but JSON's integers are generally written in decimal, hence in this context we denote it umask in decimal too. The specified value should be in the valid range for umasks, i.e. 0000…0777 (in octal as typical in UNIX), or 0…511 (in decimal, how it actually appears in the JSON record). This umask is automatically set by pam_systemd for all login sessions of the user.

environment → An array of strings, each containing an environment variable and its value to set for the user's login session, in a format compatible with putenv(). Any environment variable listed here is automatically set by pam_systemd for all login sessions of the user.

timeZone → A string indicating a preferred timezone to use for the user. When logging in pam_systemd will automatically initialize the $TZ environment variable from this string. The string should be a tzdata compatible location string, for example: Europe/Berlin.

preferredLanguage → A string indicating the preferred language/locale for the user. When logging in pam_systemd will automatically initialize the $LANG environment variable from this string. The string hence should be in a format compatible with this environment variable, for example: de_DE.UTF8.

niceLevel → An integer value in the range -20…19. When logging in pam_systemd will automatically initialize the login process' nice level to this value with, which is then inherited by all the user's processes, see setpriority() for more information.

resourceLimits → An object, where each key refers to a Linux resource limit (such as RLIMIT_NOFILE and similar). Their values should be an object with two keys cur and max for the soft and hard resource limit. When logging in pam_systemd will automatically initialize the login process' resource limits to these values, which is then inherited by all the user's processes, see setrlimit() for more information.

locked → A boolean value. If true, the user account is locked, the user may not log in. If this field is missing it should be assumed to be false, i.e. logins are permitted. This field corresponds to the sp_expire field of struct spwd (i.e. the /etc/shadow data for a user) being set to zero or one.

notBeforeUSec → An unsigned 64bit integer value, indicating a time in µs since the UNIX epoch (1970) before which the record should be considered invalid for the purpose of logging in.

notAfterUSec → Similar, but indicates the point in time after which logins shall not be permitted anymore. This corresponds to the sp_expire field of struct spwd, when it is set to a value larger than one, but provides finer granularity.

storage → A string, one of classic, luks, directory, subvolume, fscrypt, cifs. Indicates the storage mechanism for the user's home directory. If classic the home directory is a plain directory as in classic UNIX. When directory, the home directory is a regular directory, but the ~/.identity file in it contains the user's user record, so that the directory is self-contained. Similar, subvolume is a btrfs subvolume that also contains a ~/.identity user record; fscrypt is an fscrypt-encrypted directory, also containing the ~/.identity user record; luks is a per-user LUKS volume that is mounted as home directory, and cifs a home directory mounted from a Windows File Share. The five latter types are primarily used by systemd-homed when managing home directories, but may be used if other managers are used too. If this is not set, classic is the implied default.

diskSize → An unsigned 64bit integer, indicating the intended home directory disk space in bytes to assign to the user. Depending on the selected storage type this might be implemented differently: for luks this is the intended size of the file system and LUKS volume, while for the others this likely translates to classic file system quota settings.

diskSizeRelative → Similar to diskSize but takes a relative value, but specifies a fraction of the available disk space on the selected storage medium to assign to the user. This unsigned integer value is normalized to 2^32 = 100%.

skeletonDirectory → Takes a string with the absolute path to the skeleton directory to populate a new home directory from. This is only used when a home directory is first created, and defaults to /etc/skel if not defined.

accessMode → Takes an unsigned integer in the range 0…511 indicating the UNIX access mask for the home directory when it is first created.

tasksMax → Takes an unsigned 64bit integer indicating the maximum number of tasks the user may start in parallel during system runtime. This counts all tasks (i.e. threads, where each process is at least one thread) the user starts or that are forked from these processes even if the user identity is changed (for example by setuid binaries/su/sudo and similar). systemd-logind.service enforces this by setting the TasksMax slice property for the user's slice user-$UID.slice.

memoryHigh/memoryMax → These take unsigned 64bit integers indicating upper memory limits for all processes of the user (plus all processes forked off them that might have changed user identity), in bytes. Enforced by systemd-logind.service, similar to tasksMax.

cpuWeight/ioWeight → These take unsigned integers in the range 1…10000 (defaults to 100) and configure the CPU and IO scheduling weights for the user's processes as a whole. Also enforced by systemd-logind.service, similar to tasksMax, memoryHigh and memoryMax.

mountNoDevices/mountNoSuid/mountNoExecute → Three booleans that control the nodev, nosuid, noexec mount flags of the user's home directories. Note that these booleans are only honored if the home directory is managed by a subsystem such as systemd-homed.service that automatically mounts home directories on login.

cifsDomain → A string indicating the Windows File Sharing domain (CIFS) to use. This is generally useful, but particularly when cifs is used as storage mechanism for the user's home directory, see above.

cifsUserName → A string indicating the Windows File Sharing user name (CIFS) to associate this user record with. This is generally useful, but particularly useful when cifs is used as storage mechanism for the user's home directory, see above.

cifsService → A string indicating the Windows File Share service (CIFS) to mount as home directory of the user on login. Should be in format //<host>/<service>/<directory/…>. The directory part is optional. If missing the top-level directory of the CIFS share is used.

cifsExtraMountOptions → A string with additional mount options to pass to mount.cifs when mounting the home directory CIFS share.

imagePath → A string with an absolute file system path to the file, directory or block device to use for storage backing the home directory. If the luks storage is used, this refers to the loopback file or block device node to store the LUKS volume on. For fscrypt, directory, subvolume this refers to the directory to bind mount as home directory on login. Not defined for classic or cifs.

homeDirectory → A string with an absolute file system path to the home directory. This is where the image indicated in imagePath is mounted to on login and thus indicates the application facing home directory while the home directory is active, and is what the user's $HOME environment variable is set to during log-in. It corresponds to the pw_dir field of struct passwd.

uid → An unsigned integer in the range 0…4294967295: the numeric UNIX user ID (UID) to use for the user. This corresponds to the pw_uid field of struct passwd.

gid → An unsigned integer in the range 0…4294967295: the numeric UNIX group ID (GID) to use for the user. This corresponds to the pw_gid field of struct passwd.

memberOf → An array of strings, each indicating a UNIX group this user shall be a member of. The listed strings must be valid group names, but it is not required that all groups listed exist in all contexts: any entry for which no group exists should be silently ignored.

fileSystemType → A string, one of ext4, xfs, btrfs (possibly others) to use as file system for the user's home directory. This is primarily relevant when the storage mechanism used is luks as a file system to use inside the LUKS container must be selected.

partitionUuid → A string containing a lower-case, text-formatted UUID, referencing the GPT partition UUID the home directory is located in. This is primarily relevant when the storage mechanism used is luks.

luksUuid → A string containing a lower-case, text-formatted UUID, referencing the LUKS volume UUID the home directory is located in. This is primarily relevant when the storage mechanism used is luks.

fileSystemUuid → A string containing a lower-case, text-formatted UUID, referencing the file system UUID the home directory is located in. This is primarily relevant when the storage mechanism used is luks.

luksDiscard → A boolean. If true and luks storage is used, controls whether the loopback block devices, LUKS and the file system on top shall be used in discard mode, i.e. erased sectors should always be returned to the underlying storage. If false and luks storage is used turns this behavior off. In addition, depending on this setting an FITRIM or fallocate() operation is executed to make sure the image matches the selected option.

luksOfflineDiscard → A boolean. Similar to luksDiscard, it controls whether to trim/allocate the file system/backing file when deactivating the home directory.

luksExtraMountOptions → A string with additional mount options to append to the default mount options for the file system in the LUKS volume.

luksCipher → A string, indicating the cipher to use for the LUKS storage mechanism.

luksCipherMode → A string, selecting the cipher mode to use for the LUKS storage mechanism.

luksVolumeKeySize → An unsigned integer, indicating the volume key length in bytes to use for the LUKS storage mechanism.

luksPbkdfHashAlgorithm → A string, selecting the hash algorithm to use for the PBKDF operation for the LUKS storage mechanism.

luksPbkdfType → A string, indicating the PBKDF type to use for the LUKS storage mechanism.

luksPbkdfForceIterations → An unsigned 64bit integer, indicating the intended number of iterations for the PBKDF operation, when LUKS storage is used.

luksPbkdfTimeCostUSec → An unsigned 64bit integer, indicating the intended time cost for the PBKDF operation, when the LUKS storage mechanism is used, in µs. Ignored when luksPbkdfForceIterations is set.

luksPbkdfMemoryCost → An unsigned 64bit integer, indicating the intended memory cost for the PBKDF operation, when LUKS storage is used, in bytes.

luksPbkdfParallelThreads → An unsigned 64bit integer, indicating the intended required parallel threads for the PBKDF operation, when LUKS storage is used.

luksSectorSize → An unsigned 64bit integer, indicating the sector size to use for the LUKS storage mechanism, in bytes. Must be a power of two between 512 and 4096.

autoResizeMode → A string, one of off, grow, shrink-and-grow. Unless set to off, controls whether the home area shall be grown automatically to the size configured in diskSize automatically at login time. If set to shrink-and-grown the home area is also shrunk to the minimal size possible (as dictated by used disk space and file system constraints) on logout.

rebalanceWeight → An unsigned integer, null or a boolean. Configures the free disk space rebalancing weight for the home area. The integer must be in the range 1…10000 to configure an explicit weight. If unset, or set to null or true the default weight of 100 is implied. If set to 0 or false rebalancing is turned off for this home area.

service → A string declaring the service that defines or manages this user record. It is recommended to use reverse domain name notation for this. For example, if systemd-homed manages a user a string of io.systemd.Home is used for this.

rateLimitIntervalUSec → An unsigned 64bit integer that configures the authentication rate limiting enforced on the user account. This specifies a timer interval (in µs) within which to count authentication attempts. When the counter goes above the value configured n rateLimitIntervalBurst log-ins are temporarily refused until the interval passes.

rateLimitIntervalBurst → An unsigned 64bit integer, closely related to rateLimitIntervalUSec, that puts a limit on authentication attempts within the configured time interval.

enforcePasswordPolicy → A boolean. Configures whether to enforce the system's password policy when creating the home directory for the user or changing the user's password. By default the policy is enforced, but if this field is false it is bypassed.

autoLogin → A boolean. If true the user record is marked as suitable for auto-login. Systems are supposed to automatically log in a user marked this way during boot, if there's exactly one user on it defined this way.

stopDelayUSec → An unsigned 64bit integer, indicating the time in µs the per-user service manager is kept around after the user fully logged out. This value is honored by systemd-logind.service. If set to zero the per-user service manager is immediately terminated when the user logs out, and longer values optimize high-frequency log-ins as the necessary work to set up and tear down a log-in is reduced if the service manager stays running.

killProcesses → A boolean. If true all processes of the user are automatically killed when the user logs out. This is enforced by systemd-logind.service. If false any processes left around when the user logs out are left running.

passwordChangeMinUSec/passwordChangeMaxUSec → An unsigned 64bit integer, encoding how much time has to pass at least/at most between password changes of the user. This corresponds with the sp_min and sp_max fields of struct spwd (i.e. the /etc/shadow entries of the user), but offers finer granularity.

passwordChangeWarnUSec → An unsigned 64bit integer, encoding how much time to warn the user before their password expires, in µs. This corresponds with the sp_warn field of struct spwd.

passwordChangeInactiveUSec → An unsigned 64bit integer, encoding how much time has to pass after the password expired that the account is deactivated. This corresponds with the sp_inact field of struct spwd.

passwordChangeNow → A boolean. If true the user has to change their password on next login. This corresponds with the sp_lstchg field of struct spwd being set to zero.

pkcs11TokenUri → An array of strings, each with an RFC 7512 compliant PKCS#11 URI referring to security token (or smart card) of some form, that shall be associated with the user and may be used for authentication. The URI is used to search for an X.509 certificate and associated private key that may be used to decrypt an encrypted secret key that is used to unlock the user's account (see below). It's undefined how precise the URI is: during log-in it is tested against all plugged in security tokens and if there's exactly one matching private key found with it it is used.

fido2HmacCredential → An array of strings, each with a Base64-encoded FIDO2 credential ID that shall be used for authentication with FIDO2 devices that implement the hmac-secret extension. The salt to pass to the FIDO2 device is found in fido2HmacSalt.

recoveryKeyType → An array of strings, each indicating the type of one recovery key. The only supported recovery key type at the moment is modhex64, for details see the description of recoveryKey below. An account may have any number of recovery keys defined, and the array should have one entry for each.

privileged → An object, which contains the fields of the privileged section of the user record, see below.

perMachine → An array of objects, which contain the perMachine section of the user record, and thus fields to apply on specific systems only, see below.

binding → An object, keyed by machine IDs formatted as strings, pointing to objects that contain the binding section of the user record, i.e. additional fields that bind the user record to a specific machine, see below.

status → An object, keyed by machine IDs formatted as strings, pointing to objects that contain the status section of the user record, i.e. additional runtime fields that expose the current status of the user record on a specific system, see below.

signature → An array of objects, which contain cryptographic signatures of the user record, i.e. the fields of the signature section of the user record, see below.

secret → An object, which contains the fields of the secret section of the user record, see below.

As mentioned, the privileged section is encoded in a sub-object of the user record top-level object, in the privileged field. Any data included in this object shall only be visible to the administrator and the user themselves, and be suppressed implicitly when other users get access to a user record. It thus takes the role of the /etc/shadow records for each user, which has similarly restrictive access semantics. The following fields are currently defined:

passwordHint → A user-selected password hint in free-form text. This should be a string like "What's the name of your first pet?", but is entirely for the user to choose.

hashedPassword → An array of strings, each containing a hashed UNIX password string, in the format crypt(3) generates. This corresponds with sp_pwdp field of struct spwd (and in a way the pw_passwd field of struct passwd).

sshAuthorizedKeys → An array of strings, each listing an SSH public key that is authorized to access the account. The strings should follow the same format as the lines in the traditional ~/.ssh/authorized_key file.

pkcs11EncryptedKey → An array of objects. Each element of the array should be an object consisting of three string fields: uri shall contain a PKCS#11 security token URI, data shall contain a Base64-encoded encrypted key and hashedPassword shall contain a UNIX password hash to test the key against. Authenticating with a security token against this account shall work as follows: the encrypted secret key is converted from its Base64 representation into binary, then decrypted with the PKCS#11 C_Decrypt() function of the PKCS#11 module referenced by the specified URI, using the private key found on the same token. The resulting decrypted key is then Base64-encoded and tested against the specified UNIX hashed password. The Base64-encoded decrypted key may also be used to unlock further resources during log-in, for example the LUKS or fscrypt storage backend. It is generally recommended that for each entry in pkcs11EncryptedKey there's also a matching one in pkcs11TokenUri and vice versa, with the same URI, appearing in the same order, but this should not be required by applications processing user records.

fido2HmacSalt → An array of objects, implementing authentication support with FIDO2 devices that implement the hmac-secret extension. Each element of the array should be an object consisting of three string fields: credential, salt, hashedPassword, and three boolean fields: up, uv and clientPin. The first two string fields shall contain Base64-encoded binary data: the FIDO2 credential ID and the salt value to pass to the FIDO2 device. During authentication this salt along with the credential ID is sent to the FIDO2 token, which will HMAC hash the salt with its internal secret key and return the result. This resulting binary key should then be Base64-encoded and used as string password for the further layers of the stack. The hashedPassword field of the fido2HmacSalt field shall be a UNIX password hash to test this derived secret key against for authentication. The up, uv and clientPin booleans map to the FIDO2 concepts of the same name and encode whether the uv/up options are enabled during the authentication, and whether a PIN shall be required. It is generally recommended that for each entry in fido2HmacSalt there's also a matching one in fido2HmacCredential, and vice versa, with the same credential ID, appearing in the same order, but this should not be required by applications processing user records.

recoveryKey→ An array of objects, each defining a recovery key. The object has two mandatory fields: type indicates the type of recovery key. The only currently permitted value is the string modhex64. The hashedPassword field contains a UNIX password hash of the normalized recovery key. Recovery keys are in most ways similar to regular passwords, except that they are generated by the computer, not chosen by the user, and are longer. Currently, the only supported recovery key format is modhex64, which consists of 64 modhex characters (i.e. 256bit of information), in groups of 8 chars separated by dashes, e.g. lhkbicdj-trbuftjv-tviijfck-dfvbknrh-uiulbhui-higltier-kecfhkbk-egrirkui. Recovery keys should be accepted wherever regular passwords are. The recoveryKey field should always be accompanied by a recoveryKeyType field (see above), and each entry in either should map 1:1 to an entry in the other, in the same order and matching the type. When accepting a recovery key it should be brought automatically into normalized form, i.e. the dashes inserted when missing, and converted into lowercase before tested against the UNIX password hash, so that recovery keys are effectively case-insensitive.

As mentioned, the perMachine section contains settings that shall apply to specific systems only. This is primarily interesting for resource management properties as they tend to require a per-system focus, however they may be used for other purposes too.

The perMachine field in the top-level object is an array of objects. When processing the user record first the various fields on the top-level object should be used. Then this array should be iterated in order, and the various settings be applied that match either the indicated machine ID or host name. There may be multiple array entries that match a specific system, in which case all the object's setting should be applied. If the same option is set in the top-level object as in a per-machine object the latter wins and entirely undoes the setting in the top-level object (i.e. no merging of properties that are arrays themselves is done). If the same option is set in multiple per-machine objects the one specified later in the array wins (and here too no merging of individual fields is done, the later field always wins in full).

The following fields are defined in this section:

matchMachineId → An array of strings that are formatted 128bit IDs in hex. If any of the specified IDs match the system's local machine ID (i.e. matches /etc/machine-id) the fields in this object are honored.

matchHostname → An array of strings that are valid hostnames. If any of the specified hostnames match the system's local hostname, the fields in this object are honored. If both matchHostname and matchMachineId are used within the same array entry, the object is honored when either match succeeds, i.e. the two match types are combined in OR, not in AND.

These two are the only two fields specific to this section. All other fields that may be used in this section are identical to the equally named ones in the regular section (i.e. at the top-level object). Specifically, these are:

iconName, location, shell, umask, environment, timeZone, preferredLanguage, niceLevel, resourceLimits, locked, notBeforeUSec, notAfterUSec, storage, diskSize, diskSizeRelative, skeletonDirectory, accessMode, tasksMax, memoryHigh, memoryMax, cpuWeight, ioWeight, mountNoDevices, mountNoSuid, mountNoExecute, cifsDomain, cifsUserName, cifsService, cifsExtraMountOptions, imagePath, uid, gid, memberOf, fileSystemType, partitionUuid, luksUuid, fileSystemUuid, luksDiscard, luksOfflineDiscard, luksCipher, luksCipherMode, luksVolumeKeySize, luksPbkdfHashAlgorithm, luksPbkdfType, luksPbkdfForceIterations, luksPbkdfTimeCostUSec, luksPbkdfMemoryCost, luksPbkdfParallelThreads, luksSectorSize, autoResizeMode, rebalanceWeight, rateLimitIntervalUSec, rateLimitBurst, enforcePasswordPolicy, autoLogin, stopDelayUSec, killProcesses, passwordChangeMinUSec, passwordChangeMaxUSec, passwordChangeWarnUSec, passwordChangeInactiveUSec, passwordChangeNow, pkcs11TokenUri, fido2HmacCredential.

As mentioned, the binding section contains additional fields about the user record, that bind it to the local system. These fields are generally used by a local user manager (such as systemd-homed.service) to add in fields that make sense in a local context but not necessarily in a global one. For example, a user record that contains no uid field in the regular section is likely extended with one in the binding section to assign a local UID if no global UID is defined.

All fields in the binding section only make sense in a local context and are suppressed when the user record is ported between systems. The binding section is generally persisted on the system but not in the home directories themselves and the home directory is supposed to be fully portable and thus not contain the information that binding is supposed to contain that binds the portable record to a specific system.

The binding sub-object on the top-level user record object is keyed by the machine ID the binding is intended for, which point to an object with the fields of the bindings. These fields generally match fields that may also be defined in the regular and perMachine sections, however override both. Usually, the binding value should not contain settings different from those set via regular or perMachine, however this might happen if some settings are not supported locally (think: fscrypt is recorded as intended storage mechanism in the regular section, but the local kernel does not support fscrypt, hence directory was chosen as implicit fallback), or have been changed in the regular section through updates (e.g. a home directory was created with luks as storage mechanism but later the user record was updated to prefer subvolume, which however doesn't change the actual storage used already which is pinned in the binding section).

The following fields are defined in the binding section. They all have an identical format and override their equally named counterparts in the regular and perMachine sections:

imagePath, homeDirectory, partitionUuid, luksUuid, fileSystemUuid, uid, gid, storage, fileSystemType, luksCipher, luksCipherMode, luksVolumeKeySize.

As mentioned, the status section contains additional fields about the user record that are exclusively acquired during runtime, and that expose runtime metrics of the user and similar metadata that shall not be persisted but are only acquired "on-the-fly" when requested.

This section is arranged similarly to the binding section: the status sub-object of the top-level user record object is keyed by the machine ID, which points to the object with the fields defined here. The following fields are defined:

diskUsage → An unsigned 64bit integer. The currently used disk space of the home directory in bytes. This value might be determined in different ways, depending on the selected storage mechanism. For LUKS storage this is the file size of the loopback file or block device size. For the directory/subvolume/fscrypt storage this is the current disk space used as reported by the file system quota subsystem.

diskFree → An unsigned 64bit integer, denoting the number of "free" bytes in the disk space allotment, i.e. usually the difference between the disk size as reported by diskSize and the used already as reported in diskFree, but possibly skewed by metadata sizes, disk compression and similar.

diskSize → An unsigned 64bit integer, denoting the disk space currently allotted to the user, in bytes. Depending on the storage mechanism this can mean different things (see above). In contrast to the top-level field of the same (or the one in the perMachine section), this field reports the current size allotted to the user, not the intended one. The values may differ when user records are updated without the home directory being re-sized.

diskCeiling/diskFloor → Unsigned 64bit integers indicating upper and lower bounds when changing the diskSize value, in bytes. These values are typically derived from the underlying data storage, and indicate in which range the home directory may be re-sized in, i.e. in which sensible range the diskSize value should be kept.

state → A string indicating the current state of the home directory. The precise set of values exposed here are up to the service managing the home directory to define (i.e. are up to the service identified with the service field below). However, it is recommended to stick to a basic vocabulary here: inactive for a home directory currently not mounted, absent for a home directory that cannot be mounted currently because it does not exist on the local system, active for a home directory that is currently mounted and accessible.

service → A string identifying the service that manages this user record. For example systemd-homed.service sets this to io.systemd.Home to all user records it manages. This is particularly relevant to define clearly the context in which state lives, see above. Note that this field also exists on the top-level object (i.e. in the regular section), which it overrides. The regular field should be used if conceptually the user record can only be managed by the specified service, and this status field if it can conceptually be managed by different managers, but currently is managed by the specified one.

signedLocally → A boolean. If true indicates that the user record is signed by a public key for which the private key is available locally. This means that the user record may be modified locally as it can be re-signed with the private key. If false indicates that the user record is signed by a public key recognized by the local manager but whose private key is not available locally. This means the user record cannot be modified locally as it couldn't be signed afterwards.

goodAuthenticationCounter → An unsigned 64bit integer. This counter is increased by one on every successful authentication attempt, i.e. an authentication attempt where a security token of some form was presented and it was correct.

badAuthenticationCounter → An unsigned 64bit integer. This counter is increased by one on every unsuccessfully authentication attempt, i.e. an authentication attempt where a security token of some form was presented and it was incorrect.

lastGoodAuthenticationUSec → An unsigned 64bit integer, indicating the time of the last successful authentication attempt in µs since the UNIX epoch (1970).

lastBadAuthenticationUSec → Similar, but the timestamp of the last unsuccessfully authentication attempt.

rateLimitBeginUSec → An unsigned 64bit integer: the µs timestamp since the UNIX epoch (1970) where the most recent rate limiting interval has been started, as configured with rateLimitIntervalUSec.

rateLimitCount → An unsigned 64bit integer, counting the authentication attempts in the current rate limiting interval, see above. If this counter grows beyond the value configured in rateLimitBurst authentication attempts are temporarily refused.

removable → A boolean value. If true the manager of this user record determined the home directory being on removable media. If false it was determined the home directory is in internal built-in media. (This is used by systemd-logind.service to automatically pick the right default value for stopDelayUSec if the field is not explicitly specified: for home directories on removable media the delay is selected very low to minimize the chance the home directory remains in unclean state if the storage device is removed from the system by the user).

accessMode → The access mode currently in effect for the home directory itself.

fileSystemType → The file system type backing the home directory: a short string, such as "btrfs", "ext4", "xfs".

As mentioned, the signature section of the user record may contain one or more cryptographic signatures of the user record. Like all others, this section is optional, and only used when cryptographic validation of user records shall be used. Specifically, all user records managed by systemd-homed.service will carry such signatures and the service refuses managing user records that come without signature or with signatures not recognized by any locally defined public key.

The signature field in the top-level user record object is an array of objects. Each object encapsulates one signature and has two fields: data and key (both are strings). The data field contains the actual signature, encoded in Base64, the key field contains a copy of the public key whose private key was used to make the signature, in PEM format. Currently only signatures with Ed25519 keys are defined.

Before signing the user record should be brought into "normalized" form, i.e. the keys in all objects should be sorted alphabetically. All redundant white-space and newlines should be removed and the JSON text then signed.

The signatures only cover the regular, perMachine and privileged sections of the user records, all other sections (include signature itself), are removed before the signature is calculated.

Rationale for signing and threat model: while a multi-user operating system like Linux strives for being sufficiently secure even after a user acquired a local login session reality tells us this is not the case. Hence it is essential to restrict carefully which users may gain access to a system and which ones shall not. A minimal level of trust must be established between system, user record and the user themselves before a log-in request may be permitted. In particular if the home directory is provided in its own LUKS2 encapsulated file system it is essential this trust is established before the user logs in (and hence the file system mounted), since file system implementations on Linux are well known to be relatively vulnerable to rogue disk images. User records and home directories in many context are expected to be something shareable between multiple systems, and the transfer between them might not happen via exclusively trusted channels. Hence it's essential that the user record is not manipulated between uses. Finally, resource management (which may be done by the various fields of the user record) is security sensitive, since it should forcefully lock the user into the assigned resource usage and not allow them to use more. The requirement of being able to trust the user record data combined with the potential transfer over untrusted channels suggest a cryptographic signature mechanism where only user records signed by a recognized key are permitted to log in locally.

Note that other mechanisms for establishing sufficient trust exist too, and are perfectly valid as well. For example, systems like LDAP/ActiveDirectory generally insist on user record transfer from trusted servers via encrypted TLS channels only. Or traditional UNIX users created locally in /etc/passwd never exist outside of the local trusted system, hence transfer and trust in the source are not an issue. The major benefit of operating with signed user records is that they are self-sufficiently trusted, not relying on a secure channel for transfer, and thus being compatible with a more distributed model of home directory transfer, including on USB sticks and such.

As mentioned, the secret section of the user record should never be persisted nor transferred across machines. It is only defined in short-lived operations, for example when a user record is first created or registered, as the secret key data needs to be available to derive encryption keys from and similar.

The secret field of the top-level user record contains the following fields:

password → an array of strings, each containing a plain text password.

tokenPin → an array of strings, each containing a plain text PIN, suitable for unlocking security tokens that require that. (The field pkcs11Pin should be considered a compatibility alias for this field, and merged with tokenPin in case both are set.)

pkcs11ProtectedAuthenticationPathPermitted → a boolean. If set to true allows the receiver to use the PKCS#11 "protected authentication path" (i.e. a physical button/touch element on the security token) for authenticating the user. If false or unset, authentication this way shall not be attempted.

fido2UserPresencePermitted → a boolean. If set to true allows the receiver to use the FIDO2 "user presence" flag. This is similar to the concept of pkcs11ProtectedAuthenticationPathPermitted, but exposes the FIDO2 "up" concept behind it. If false or unset authentication this way shall not be attempted.

fido2UserVerificationPermitted → a boolean. If set to true allows the receiver to use the FIDO2 "user verification" flag. This is similar to the concept of pkcs11ProtectedAuthenticationPathPermitted, but exposes the FIDO2 "uv" concept behind it. If false or unset authentication this way shall not be attempted.

When mapping classic UNIX user records (i.e. struct passwd and struct spwd) to JSON user records the following mappings should be applied:

At this time almost all Linux machines employ shadow passwords, thus the pw_passwd field in struct passwd is set to "x", and the actual password is stored in the shadow entry struct spwd's field sp_pwdp.

User records following this specifications are supposed to be extendable for various applications. In general, subsystems are free to introduce their own keys, as long as:

• Care should be taken to place the keys in the right section, i.e. the most appropriate for the data field.

• Care should be taken to avoid namespace clashes. Please prefix your fields with a short identifier of your project to avoid ambiguities and incompatibilities.

• This specification is supposed to be a living specification. If you need additional fields, please consider submitting them upstream for inclusion in this specification. If they are reasonably universally useful, it would be best to list them here.

The shortest valid user record looks like this:

{
        "userName" : "u"
}

A reasonable user record for a system user might look like this:

{
        "userName" : "httpd",
        "uid" : 473,
        "gid" : 473,
        "disposition" : "system",
        "locked" : true
}

A fully featured user record associated with a home directory managed by systemd-homed.service might look like this:

{
        "autoLogin" : true,
        "binding" : {
                "15e19cf24e004b949ddaac60c74aa165" : {
                        "fileSystemType" : "ext4",
                        "fileSystemUuid" : "758e88c8-5851-4a2a-b88f-e7474279c111",
                        "gid" : 60232,
                        "homeDirectory" : "/home/grobie",
                        "imagePath" : "/home/grobie.home",
                        "luksCipher" : "aes",
                        "luksCipherMode" : "xts-plain64",
                        "luksUuid" : "e63581ba-79fb-4226-b9de-1888393f7573",
                        "luksVolumeKeySize" : 32,
                        "partitionUuid" : "41f9ce04-c827-4b74-a981-c669f93eb4dc",
                        "storage" : "luks",
                        "uid" : 60232
                }
        },
        "disposition" : "regular",
        "enforcePasswordPolicy" : false,
        "lastChangeUSec" : 1565950024279735,
        "memberOf" : [
                "wheel"
        ],
        "privileged" : {
                "hashedPassword" : [
                        "$6$WHBKvAFFT9jKPA4k$OPY4D4TczKN/jOnJzy54DDuOOagCcvxxybrwMbe1SVdm.Bbr.zOmBdATp.QrwZmvqyr8/SafbbQu.QZ2rRvDs/"
                ]
        },
        "signature" : [
                {
                        "data" : "LU/HeVrPZSzi3MJ0PVHwD5m/xf51XDYCrSpbDRNBdtF4fDVhrN0t2I2OqH/1yXiBidXlV0ptMuQVq8KVICdEDw==",
                        "key" : "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEA/QT6kQWOAMhDJf56jBmszEQQpJHqDsGDMZOdiptBgRk=\n-----END PUBLIC KEY-----\n"
                }
        ],
        "userName" : "grobie",
        "status" : {
                "15e19cf24e004b949ddaac60c74aa165" : {
                        "goodAuthenticationCounter" : 16,
                        "lastGoodAuthenticationUSec" : 1566309343044322,
                        "rateLimitBeginUSec" : 1566309342340723,
                        "rateLimitCount" : 1,
                        "state" : "inactive",
                        "service" : "io.systemd.Home",
                        "diskSize" : 161118667776,
                        "diskCeiling" : 190371729408,
                        "diskFloor" : 5242880,
                        "signedLocally" : true
                }
        }
}

When systemd-homed.service manages a home directory it will also include a version of the user record in the home directory itself in the ~/.identity file. This version lacks the binding and status sections which are used for local management of the user, but are not intended to be portable between systems. It would hence look like this:

{
        "autoLogin" : true,
        "disposition" : "regular",
        "enforcePasswordPolicy" : false,
        "lastChangeUSec" : 1565950024279735,
        "memberOf" : [
                "wheel"
        ],
        "privileged" : {
                "hashedPassword" : [
                        "$6$WHBKvAFFT9jKPA4k$OPY4D4TczKN/jOnJzy54DDuOOagCcvxxybrwMbe1SVdm.Bbr.zOmBdATp.QrwZmvqyr8/SafbbQu.QZ2rRvDs/"
                ]
        },
        "signature" : [
                {
                        "data" : "LU/HeVrPZSzi3MJ0PVHwD5m/xf51XDYCrSpbDRNBdtF4fDVhrN0t2I2OqH/1yXiBidXlV0ptMuQVq8KVICdEDw==",
                        "key" : "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEA/QT6kQWOAMhDJf56jBmszEQQpJHqDsGDMZOdiptBgRk=\n-----END PUBLIC KEY-----\n"
                }
        ],
        "userName" : "grobie",
}