systemd/docs/GROUP_RECORD.md

title	category	layout
JSON Group Records	Users, Groups and Home Directories	default

JSON Group Records

Long story short: JSON Group Records are to struct group what JSON User Records are to struct passwd.

Conceptually, much of what applies to JSON user records also applies to JSON group records. They also consist of seven sections, with similar properties and they carry some identical (or at least very similar) fields.

Fields in the regular section

groupName → A string with the UNIX group name. Matches the gr_name field of UNIX/glibc NSS struct group, or the shadow structure struct sgrp's sg_namp field.

realm → The "realm" the group belongs to, conceptually identical to the same field of user records. A string in DNS domain name syntax.

description → A descriptive string for the group. This is similar to the realName field of user records, and accepts arbitrary strings, as long as they follow the same GECOS syntax requirements as realName.

disposition → The disposition of the group, conceptually identical to the same field of user records. A string.

service → A string, an identifier for the service managing this group record (this field is typically in reverse domain name syntax.)

lastChangeUSec → An unsigned 64bit integer, a timestamp (in µs since the UNIX epoch 1970) of the last time the group record has been modified. (Covers only the regular, perMachine and privileged sections).

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

members → An array of strings, listing user names that are members of this group. Note that JSON user records also contain a memberOf field, or in other words a group membership can either be denoted in the JSON user record or in the JSON group record, or in both. The list of memberships should be determined as the combination of both lists (plus optionally others). If a user is listed as member of a group and doesn't exist it should be ignored. This field corresponds to the gr_mem field of struct group and the sg_mem field of struct sgrp.

administrators → Similarly, an array of strings, listing user names that shall be considered "administrators" of this group. This field corresponds to the sg_adm field of struct sgrp.

privileged/perMachine/binding/status/signature/secret → The objects/arrays for the other six group record sections. These are organized the same way as for the JSON user records, and have the same semantics.

Fields in the privileged section

The following fields are defined:

hashedPassword → An array of strings with UNIX hashed passwords; see the matching field for user records for details. This field corresponds to the sg_passwd field of struct sgrp (and gr_passwd of struct group in a way).

Fields in the perMachine section

matchMachineId/matchHostname → Strings, match expressions similar as for user records, see the user record documentation for details.

The following fields are defined for the perMachine section and are defined equivalent to the fields of the same name in the regular section, and override those:

gid, members, administrators

Fields in the binding section

The following fields are defined for the binding section, and are equivalent to the fields of the same name in the regular and perMachine sections:

gid

Fields in the status section

The following fields are defined in the status section, and are mostly equivalent to the fields of the same name in the regular section, though with slightly different conceptual semantics, see the same fields in the user record documentation:

service

Fields in the signature section

The fields in this section are defined identically to those in the matching section in the user record.

Fields in the secret section

Currently no fields are defined in this section for group records.

Mapping to struct group and struct sgrp

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

Structure	Field	Section	Field	Condition
struct group	gr_name	regular	groupName
struct group	gr_passwd	privileged	password	(See notes below)
struct group	gr_gid	regular	gid
struct group	gr_mem	regular	members
struct sgrp	sg_namp	regular	groupName
struct sgrp	sg_passwd	privileged	password	(See notes below)
struct sgrp	sg_adm	regular	administrators
struct sgrp	sg_mem	regular	members

At this time almost all Linux machines employ shadow passwords, thus the gr_passwd field in struct group is set to "x", and the actual password is stored in the shadow entry struct sgrp's field sg_passwd.

Extending These Records

The same logic and recommendations apply as for JSON user records.

Examples

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

{
	"groupName" : "systemd-resolve",
	"gid" : 193,
	"status" : {
		"6b18704270e94aa896b003b4340978f1" : {
			"service" : "io.systemd.NameServiceSwitch"
		}
	}
}

And here's a more complete one for a regular group:

{
	"groupName" : "grobie",
	"binding" : {
		"6b18704270e94aa896b003b4340978f1" : {
			"gid" : 60232
		}
	},
	"disposition" : "regular",
	"status" : {
		"6b18704270e94aa896b003b4340978f1" : {
			"service" : "io.systemd.Home"
		}
	}
}