login.defs(5)

This parameter specifies which values in the gecos field of the /etc/passwd file may be changed by regular users using the chfn program. It can be any combination of letters f, r, w, h, for Full name, Room number, Work phone, and Home phone, respectively. For backward compatibility, yes is equivalent to rwh and no is equivalent to frwh. If not specified, only the superuser can make any changes. The most restrictive setting is better achieved by not installing chfn SUID.

Indicate if a home directory should be created by default for new users.

This setting does not apply to system users, and can be overridden on the command line.

Indicate if login is allowed if we can't cd to the home directory. Default is no.

If set to yes, the user will login in the root (/) directory if it is not possible to cd to her home directory.

This defines the system default encryption algorithm for encrypting passwords (if no algorithm is specified on the command line).

It can take one of these values: SHA256, SHA512 (default), YESCRYPT. See crypt(5) for recommendations.

Note: this only affects the generation of group passwords. The generation of user passwords is done by PAM and subject to the PAM configuration. It is recommended to set this variable consistently with the PAM configuration.

History: This variable is used by other programs, which may still default to unsafe algorithms such as DES. To avoid using unsafe algorithms, the variable should always be specified.

If set, it will be used to define the PATH environment variable when a regular user login. The value is a colon separated list of paths (for example /bin:/usr/bin) and can be preceded by PATH=. The default value is PATH=/bin:/usr/bin.

If set, it will be used to define the PATH environment variable when the superuser login. The value is a colon separated list of paths (for example /sbin:/bin:/usr/sbin:/usr/bin) and can be preceded by PATH=. The default value is PATH=/sbin:/bin:/usr/sbin:/usr/bin.

Terminal ERASE character (010 = backspace, 0177 = DEL).

The value can be prefixed "0" for an octal value, or "0x" for an hexadecimal value.

Delay in seconds before being allowed another attempt after a login failure.

If set, login will execute this shell instead of the users' shell specified in /etc/passwd.

Range of group IDs used for the creation of regular groups by useradd, groupadd, or newusers.

The default value for GID_MIN (resp. GID_MAX) is 1000 (resp. 60000).

The mode for new home directories. If not specified, the UMASK is used to create the mode.

useradd and newusers use this to set the mode of the home directory they create.

If defined, this file can inhibit all the usual chatter during the login sequence. If a full pathname is specified, then hushed mode will be enabled if the user's name or shell are found in the file. If not a full pathname, then hushed mode will be enabled if the file exists in the user's home directory.

Terminal KILL character (025 = CTRL/U).

The value can be prefixed "0" for an octal value, or "0x" for an hexadecimal value.

Highest user ID number for which the lastlog entries should be updated. As higher user IDs are usually tracked by remote user identity and authentication services there is no need to create a huge sparse lastlog file for them.

No LASTLOG_UID_MAX option present in the configuration means that there is no user ID limit for writing lastlog entries.

Enable logging of successful logins.

Enable display of unknown usernames when login failures are recorded.

Note: logging unknown usernames may be a security issue if an user enter her password instead of her login name.

Maximum number of login retries in case of bad password.

This will most likely be overridden by PAM, since the default pam_unix module has its own built in of 3 retries. However, this is a safe fallback in case you are using an authentication module that does not enforce PAM_MAXTRIES.

Max time in seconds for login.

The mail spool directory. This is needed to manipulate the mailbox when its corresponding user account is modified or deleted. If not specified, a compile-time default is used. The parameter CREATE_MAIL_SPOOL in /etc/default/useradd determines whether the mail spool should be created.

Defines the location of the users mail spool files relatively to their home directory.

Maximum members per group entry. When the maximum is reached, a new group entry (line) is started in /etc/group (with the same name, same password, and same GID).

The default value is 0, meaning that there are no limits in the number of members in a group.

This feature (split group) permits to limit the length of lines in the group file. This is useful to make sure that lines for NIS groups are not larger than 1024 characters.

If you need to enforce such limit, you can use 25.

Note: split groups may not be supported by all tools (even in the Shadow toolsuite). You should not use this variable unless you really need it.

If a system account intentionally does not have a home directory that exists, this string can be provided in the /etc/passwd entry for the account to indicate this. The result is that pwck will not emit a spurious warning for this account.

The maximum number of days a password may be used. If the password is older than this, a password change will be forced. If not specified, -1 will be assumed (which disables the restriction).

The number of days warning given before a password expires. A zero means warning is given only upon the day of expiration, a value of -1 means no warning is given. If not specified, no warning will be provided.

When ENCRYPT_METHOD is set to SHA256 or SHA512, this defines the number of SHA rounds used by the encryption algorithm by default (when the number of rounds is not specified on the command line).

With a lot of rounds, it is more difficult to brute force the password. But note also that more CPU resources will be needed to authenticate users.

If not specified, the libc will choose the default number of rounds (5000), which is orders of magnitude too low for modern hardware.

The values must be inside the 1000-999,999,999 range.

If only one of the SHA_CRYPT_MIN_ROUNDS or SHA_CRYPT_MAX_ROUNDS values is set, then this value will be used.

If SHA_CRYPT_MIN_ROUNDS > SHA_CRYPT_MAX_ROUNDS, the highest value will be used.

Note: this only affect the generation of group passwords. The generation of user passwords is done by PAM and subject to the PAM configuration. It is recommended to set this variable consistently with the PAM configuration.

If defined, all su activity is logged to this file.

If defined, the command name to display when running "su -". For example, if this is defined as "su" then a "ps" will display the command is "-su". If not defined, then "ps" would display the name of the shell actually being run, e.g. something like "-sh".

If /etc/subuid exists, the commands useradd and newusers (unless the user already have subordinate group IDs) allocate SUB_GID_COUNT unused group IDs from the range SUB_GID_MIN to SUB_GID_MAX for each new user.

The default values for SUB_GID_MIN, SUB_GID_MAX, SUB_GID_COUNT are respectively 100000, 600100000 and 65536.

If set to yes, the commands useradd, usermod, and newusers will calculate subordinate GID ranges deterministically based on the user's UID instead of searching for the next free range. The formula used is:

      start = SUB_GID_MIN + ((UID - UID_MIN) * SUB_GID_COUNT)


      end   = start + SUB_GID_COUNT - 1


    

This ensures the same UID always receives the same subordinate GID range on every system, making it suitable for environments with centralized user management (LDAP, NIS, etc.) or synchronized UIDs across systems.

If SUB_GID_DETERMINISTIC is enabled, you can use usermod --add-subgids -S to produce deterministic subgids.

WARNING: Because UID_MIN is used to calculate the ranges, any change of UID_MIN will change the ranges calculated. SUB_GID_COUNT is used to calculate the ranges, any change of SUB_GID_COUNT will change the ranges calculated. Users with identities less than UID_MIN are incompatible with SUB_GID_DETERMINISTIC, but can still be set manually.

WARNING: Do not mix deterministic and linear (default) allocation on the same system or across systems sharing /etc/subgid via network storage (NFS, etc.). Mixing allocation methods will cause subordinate ID range conflicts and overlaps .

The default value for SUB_GID_DETERMINISTIC is no.

.SS "Range Calculation Examples"

With default configuration (UID_MIN=1000, SUB_GID_MIN=100000, SUB_GID_COUNT=65536):

.TS allbox tab(:); lB lB lB. T{ UID T}:T{ Calculation T}:T{ Subordinate GID Range T} l l l l l l l l l l l l. T{ 1000 T}:T{ 100000 + ((1000-1000) * 65536) T}:T{ 100000-165535 T} T{ 1001 T}:T{ 100000 + ((1001-1000) * 65536) T}:T{ 165536-231071 T} T{ 1002 T}:T{ 100000 + ((1002-1000) * 65536) T}:T{ 231072-296607 T} T{ 1100 T}:T{ 100000 + ((1100-1000) * 65536) T}:T{ 6653600-6719135 T}

.SS "Subordinate ID Space Planning"

When planning subordinate ID allocation, calculate the maximum number of users the space can accommodate:

      capacity = (SUB_GID_MAX - SUB_GID_MIN) / SUB_GID_COUNT


      

With default values: (600100000 - 100000) / 65536 ≈ 9155 users.

For high-density environments with many users and smaller allocations:

        SUB_GID_COUNT  4096


        SUB_GID_MIN    100000


        SUB_GID_MAX    10000000


      

This gives: (10000000 - 100000) / 4096 ≈ 2417 users.

WARNING: SECURITY RISK - MAY CAUSE RANGE OVERLAPS AND PRIVILEGE ESCALATION!

Only effective when SUB_GID_DETERMINISTIC is set to yes.

When set to yes (WRAP MODE), allows the deterministic range calculation to wrap around using modulo arithmetic when a UID would overflow the configured subordinate ID space. The subordinate ID space is treated as a ring buffer.

WARNING: Range overlaps can lead to container escapes and privilege escalation. For example, with SUB_GID_MIN=100000, SUB_GID_MAX=200000, SUB_GID_COUNT=65536, User A (UID 1000) gets range [100000, 165535] and User B (UID 1001) wraps and overlaps with User A. User B's container can now access files from User A's containers.

Use only in development, testing, or tightly constrained lab environments.

When set to no (default), any arithmetic overflow or range exceeding SUB_GID_MAX is a hard error. This guarantees non-overlapping, monotonic allocation.

The default value for UNSAFE_SUB_GID_DETERMINISTIC_WRAP is no.

If set to yes, subordinate group ID entries in /etc/subgid are stored using the numeric user ID rather than the username. The default value is no.

If /etc/subuid exists, the commands useradd and newusers (unless the user already have subordinate user IDs) allocate SUB_UID_COUNT unused user IDs from the range SUB_UID_MIN to SUB_UID_MAX for each new user.

The default values for SUB_UID_MIN, SUB_UID_MAX, SUB_UID_COUNT are respectively 100000, 600100000 and 65536.

If set to yes, the commands useradd, usermod, and newusers will calculate subordinate UID ranges deterministically based on the user's UID instead of searching for the next free range. The formula used is:

      start = SUB_UID_MIN + ((UID - UID_MIN) * SUB_UID_COUNT)


      end   = start + SUB_UID_COUNT - 1


    

This ensures the same UID always receives the same subordinate UID range on every system, making it suitable for environments with centralized user management (LDAP, NIS, etc.) or synchronized UIDs across systems.

If SUB_UID_DETERMINISTIC is enabled, you can use usermod --add-subuids -S to produce deterministic subuids.

WARNING: Because UID_MIN is used to calculate the ranges, any change of UID_MIN will change the ranges calculated. SUB_UID_COUNT is used to calculate the ranges, any change of SUB_UID_COUNT will change the ranges calculated. Users with identities less than UID_MIN are incompatible with SUB_UID_DETERMINISTIC, but can still be set manually.

WARNING: Do not mix deterministic and linear (default) allocation on the same system or across systems sharing /etc/subuid via network storage (NFS, etc.). Mixing allocation methods will cause subordinate ID range conflicts and overlaps .

The default value for SUB_UID_DETERMINISTIC is no.

.SS "Range Calculation Examples"

With default configuration (UID_MIN=1000, SUB_UID_MIN=100000, SUB_UID_COUNT=65536):

.TS allbox tab(:); lB lB lB. T{ UID T}:T{ Calculation T}:T{ Subordinate UID Range T} l l l l l l l l l l l l. T{ 1000 T}:T{ 100000 + ((1000-1000) * 65536) T}:T{ 100000-165535 T} T{ 1001 T}:T{ 100000 + ((1001-1000) * 65536) T}:T{ 165536-231071 T} T{ 1002 T}:T{ 100000 + ((1002-1000) * 65536) T}:T{ 231072-296607 T} T{ 1100 T}:T{ 100000 + ((1100-1000) * 65536) T}:T{ 6653600-6719135 T}

.SS "Subordinate ID Space Planning"

When planning subordinate ID allocation, calculate the maximum number of users the space can accommodate:

      capacity = (SUB_UID_MAX - SUB_UID_MIN) / SUB_UID_COUNT


      

With default values: (600100000 - 100000) / 65536 ≈ 9155 users.

For high-density environments with many users and smaller allocations:

        SUB_UID_COUNT  4096


        SUB_UID_MIN    100000


        SUB_UID_MAX    10000000


      

This gives: (10000000 - 100000) / 4096 ≈ 2417 users.

WARNING: SECURITY RISK - MAY CAUSE RANGE OVERLAPS AND PRIVILEGE ESCALATION!

Only effective when SUB_UID_DETERMINISTIC is set to yes.

When set to yes (WRAP MODE), allows the deterministic range calculation to wrap around using modulo arithmetic when a UID would overflow the configured subordinate ID space. The subordinate ID space is treated as a ring buffer.

WARNING: Range overlaps can lead to container escapes and privilege escalation. For example, with SUB_UID_MIN=100000, SUB_UID_MAX=200000, SUB_UID_COUNT=65536, User A (UID 1000) gets range [100000, 165535] and User B (UID 1001) wraps and overlaps with User A. User B's container can now access files from User A's containers.

Use only in development, testing, or tightly constrained lab environments.

When set to no (default), any arithmetic overflow or range exceeding SUB_UID_MAX is a hard error. This guarantees non-overlapping, monotonic allocation.

The default value for UNSAFE_SUB_UID_DETERMINISTIC_WRAP is no.

If set to yes, subordinate user ID entries in /etc/subuid are stored using the numeric user ID rather than the username. The default value is no.

Range of group IDs used for the creation of system groups by useradd, groupadd, or newusers.

The default value for SYS_GID_MIN (resp. SYS_GID_MAX) is 101 (resp. GID_MIN-1).

Range of user IDs used for the creation of system users by useradd or newusers.

The default value for SYS_UID_MIN (resp. SYS_UID_MAX) is 101 (resp. UID_MIN-1).

Enable "syslog" logging of sg activity.

Enable "syslog" logging of su activity - in addition to sulog file logging.

The terminal permissions: the login tty will be owned by the TTYGROUP group, and the permissions will be set to TTYPERM.

TTYGROUP can be either the name of a group or a numeric group identifier.

If TTYGROUP is not defined, then the group ownership of the terminal is set to the user's primary group. If TTYPERM is not defined, then the permissions are set to 0600.

If you have a write program which is "setgid" to a special group which owns the terminals, define TTYGROUP to the group number and TTYPERM to 0620. Otherwise leave TTYGROUP commented out and assign TTYPERM to either 622 or 600.

If defined, file which maps tty line to TERM environment parameter. Each line of the file is in a format something like "vt100 tty01".

Range of user IDs used for the creation of regular users by useradd or newusers.

The default value for UID_MIN (resp. UID_MAX) is 1000 (resp. 60000).

The file mode creation mask is initialized to this value. If not specified, the mask will be initialized to 022.

useradd and newusers use this mask to set the mode of the home directory they create if HOME_MODE is not set.

It is also used by pam_umask as the default umask value.

If defined, this command is run when removing a user. It should remove any at/cron/print jobs etc. owned by the user to be removed (passed as the first argument).

The return code of the script is not taken into account.

Here is an example script, which removes the user's cron, at and print jobs:

#! /bin/sh
# Check for the required argument.
if [ $# != 1 ]; then
	echo "Usage: $0 username"
	exit 1
fi
# Remove cron jobs.
crontab -r -u $1
# Remove at jobs.
# Note that it will remove any jobs owned by the same UID,
# even if it was shared by a different username.
AT_SPOOL_DIR=/var/spool/cron/atjobs
find $AT_SPOOL_DIR -name "[^.]*" -type f -user $1 -delete \;
# Remove print jobs.
lprm $1
# All done.
exit 0


      

If set to yes, userdel will remove the user's group if it contains no more members, and useradd will create by default a group with the name of the user.

When ENCRYPT_METHOD is set to YESCRYPT, this defines the cost factor used by the encryption algorithm by default (when the cost factor is not specified on the command line).

With a high cost factor, it is more difficult to brute force the password. But note also that more CPU resources will be needed to authenticate users.

The value must be inside the 1-11 range.

Note: this only affect the generation of group passwords. The generation of user passwords is done by PAM and subject to the PAM configuration. It is recommended to set this variable consistently with the PAM configuration.

CHFN_RESTRICT

.RE
.PP chgpasswd