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User Accounts

  • Each user has a unique name, numeric ID and password
  • File ownership is determined by a numeric user ID
  • The owner is usually the user who created the file, but ownership can be transferred by root
  • Default users:

root  super  user
adm, sys bin…. IDs that own system files but cannot be used for login

Notes:The security of the system is based on a user being assigned a unique name, user ID (UID) and password. When the user logs in, the UID is used to validate all requests for file access. When a file is created, the UID associated with the process that created the file is assigned to the file. Only the owner or root  can change the access permissions.

There are several user accounts automatically created.root , for example, is one. Some user accounts are not made for login but only to own certain files.admsys, and are examples of that type of account.


  • A group is a set of users, all of whome need access to a given set of files.
  • Every user is a member of at least one group and can be a member of several groups.
  • The user has access to files in their groupset. To list the groupset use groups.
  • The user’s primary group is used for a file ownership on creation. To change the primary group use the newgrp.
  • Default groups:
  • System administrators
  • Staff ordinary users

Notes:Users that require shared access to a set of files are placed in groups. A user can belong to multiple groups. Each group has a unique name and Group ID (GID). The GID is also assigned to a file when it is created. There are several groups predefined on an AIX system. For example, the system group is root ‘s group and the staff group is for all ordinary users.

The creation of groups to organize and differentiate the users of a system or network is part of systems administration. The guidelines for forming groups should be part of the security policy. Defining groups for large systems can be quite complex and once a system is operational, it is very difficult to change the group structure. Investing time and effort in devising group definitions before your system arrives is recommended.
Groups should be defined as broadly as possible and be consistent with your security policy. Do not define too many groups because defining groups for every possible combination of data type and user type can lead to impossible extremes. A group administrator is a user who is allowed to assign the members and administrators of a group. It does not imply that the user has any “administrative” abilities for the system.

There are three types of groups on the system:

1.User Groups

User groups should be made for people who need to share files on the system, such as people who work in the same department, or people who are working on the same project.

2.System Administrator Groups

System administrators are automatically members of the system group. Membership of this group allows the administrators to perform some of the system tasks without having to be the root  user.

3.System Defined Groups

Several system-defined groups exist. staffis the default group for all non-administrative users created in the system. security is another system defined group having limited privileges for performing security administration. The system-defined groups are used to control certain subsystems.


Common groups on the system are:system For most configuration and standard hardware and software maintenanceprintq For managing queuing. Typical commands which can be run by members of this group are: enabledisableqadmqpri, and so forth.security To handle most passwords and limits control. Typical commands which can be run by members of this group are: mkuserrmuserpwdadm, chuser,chgroup, and so forth.adm Most monitoring functions such as performance, cron, accountingstaff Default group assigned to all new users. You may want to change this in


audit For auditors

shutdown Allows use of the shutdown command.

User Hierarchy:

  • To protect important users/groups from members of the security groups AIX has admin users and admin groups.
  • Only root  users can add/remove/change an admin user or admin group
  • Any user on the system can be defined as an admin user regardless of the group they are in


The ability to perform certain system tasks (like creating users) depends upon the standard AIX file permissions. Most system admin tasks can be performed by users other than root  by assigning those users to groups such as system, security, printq, cron, adm, audit orshutdown In particular, a user in thesecurity group can add/remove/change other users and groups.

To protect important users/groups from users in the security group, AIX has three levels of user hierarchy: root , admin users/groups and normal users/groups. Only root  can add/remove/change an admin user or admin group. Therefore, you can define a user with a high level of access, but who is protected from users in the security group.

Controls root ’s Access:

  • Restrict access to privileged logins.
  • root ’s passwords should be changed on an unannounced schedule by the system administrator.
  • Assign different root  passwords to different machines
  • System administrators should always login as themselves first and then su to root  instead of logging in as root . This helps provide an audit trail for root  usage.
  • Do not include unsecured directories in root ’s PATH


If the root  password is known by too many people, no one can be held accountable. The root  password should be limited to just two or three administrators. The fewer people who know root ‘s password the better.

The system administrator should ensure that distinct root  passwords are assigned to different machines. You may allow normal users to have the same passwords on different machines, but never do this for root .

Attempts to become root  through su can be investigated. Successful and unsuccessful attempts might be logged by the audit system.

root ‘s PATH is used by many implicit system functions, not just by a user logged in as root .
AIX Security configuration files:

/etc/security/environ : Contains the environment attributes for users.
/etc/security/lastlog : Contain the last login attributes for user.
/etc/security/limits: Contains process resources limits for users.
/etc/security/user: Contains extends attributes for user.
/usr/lib/security/mkuser.default: Contains the default attributes for new user.
/usr/lib/security/mkuser.sys: Customizes new user account.
/etc/passwd: Contain the basic attributes of user.
/etc/security/passwd: Contain password information.
/etc/security/login.cfg: Contain system default login parameters.
/etc/utmp: Contain a record of user logged into the system.
/var/adm/wtmp: Contain connect-time accounting records.
/etc/security/failedlogin: Records all failed login attempts.
/etc/motd: Contains the message to be displayed every time a user logs in to the system.
/etc/environment: Specifies the basic environment for all the process.
/etc/profile: Specifies environment settings for all users.
$HOME/.profile : Specifics environment settings for a specific user.
/etc/group: Contain the basic attributes of group.
/etc/security/group: Contains the extended attributes of group.



Depending on how the umask is set, when a user creates a file, the file permissions will be set to

  • read/write for the ownerof the file
  • read-only forgroup and other

If you look at the permissions using the ls -l command, you will see…
-rw-r–r– 1 roger staff 129 Oct 07 10:14 testfile

- r w - r - - r - - 1 roger staff 129 Oct 7 10:14 testfile
Fle Type Owner Flag Group Flag Other Flag Links Owner Group filename

l=symbolic link
B=remote block
c=character special
f l
owner of file group owner size of file in bytes date last written to

dash under the owner, group, and other fields means that particular attribute (either read,write or execute) isn’tpermitted. When a file is created, a mask is applied to the permission bits to determine what permissions are allowed. The mask can be user defined by theumask command.
File permission bits are verified when a file is opened. The commands mv (move) and rm(remove), for example, don’t open a file. Thus, it’s possible to remove a file that you don’t have permissions to open, as long as you have write (w) permission for the directory containing that file.

Sticky bit :


The Sticky bit is usually associated with world writeable directories, such as the /tmp directory. This prevents users from deleting files they don’t own. Users can write to those directories but only they can delete or update those files they own. It’s commonly used for scratch directories like /tmp to provide some security for otherwise world-writable directories.

/etc/security :

The directory /etc/security contains many files used to configure individual user or system-wide security defaults. The files listed below can be modified to enhance system security at your site:

/etc/security.ids :

Holds the value for the next assignment to a group/user id and group/user admin id. Used by mkuserandmkgroup commands.
ample contents:4 203 12 200


  • 4 = administrative user id (mkuser -a)
  • 203 = user id (mkuser)
  • 12 =  administrative group id (mkgroup -a)
  • 200 = group id (mkgroup)

/etc/security/limits :

Set one or more of the following stanzas for each user. All field attributes below default to 512 byte blocks.

Value What it does
fsize Largest file size that can be created or extended
core Largest core file size that can be created
cpu maximum Amount of cpu time to be used by each process. Must log out and back infor the changes to take affect.
data maximum Size of data segment for a process (malloc call) Data and stack are tied together. Combined, they can never be greater than 256 MB.
stack maximum Size of stack segment for a process.
rss maximum Set a process’s resident set size (working set) may grow. As long as real memory is available, system will give additional memory to the process instead of causing the process to page portions of it’s code.

/etc/security/login.cfg :

Sets system-wide password restrictions (pw_restrictions).

Value What it does
maxage Value of 4 forces users to change passwords every 4 weeks. Default=0 indicating no maximum age.
minage Defines the miniumum number of weeks that can pass before a password must be changed. This option can conflict with the flags field in /etc/security/passwd. If minage has a value greater than zero and flags = ADMCHG in the /etc/security/passwd file for that user, then the user will not be forced to change their password until the number of week(s) has elapsed.Default=0
minalpha Minimum number of alphabetic characters in password. Default=0
minother Minimum number of non-alphabetic characters in the password. Default=0
mindiff Number of characters that must be different from previous password. This option is recommended since by default, you can reuse previous password when prompted to ch ange to a new password. This is not positional. If the new password is xycd and the old password was abcd, then the number of different characters is one. Default=0
maxrepeats Maximum number of repeatable characters in a password. The repeatable characters don’t have to be consecutive. Default of 8 is used to indicate that there is no maximum number.

/etc/security/mkuser.default :

Default attributes for generating a new user.
group = staff

groups = staff

prog = /bin/ksh

home = /u/$USER


group = system

groups = system

prog = /bin/ksh

home = /u/$USER

/etc/security/passwd :

Holds the encrypted password for each user. Lets assume you have an unhappy worker that is asked to leave the company. Your afraid that person will attempt to get into the system. You ask everybody to change their password but aren’t sure if everyone will follow suite.
To force users to change their password, perform the following procedures.

  1. Edit /etc/security/login.cfg and change maxage=1. This will force users, who haven’t changed their password in the past week, to do so the next time they login.
  2. If some people have changed their password in the past week, review the lastupdate field for that user. Subtract 604800 (number of seconds in one week) from the value specified in the lastupdate field . The next time that user logs into the system, they will be forced to change their password. The lastupdate field represents the number of seconds passed since Jan 1, 1970.

A simpler way to force a user to change their password the next time they log into the system is to change that user‘s flag field to ADMCHG. Use the command pwdadm -f ADMCHG <username>to change the flag.

  • The next time a login or su command is executed for that user, they will be forced to change their password, based on the password guidelines specified in the /etc/security/login.cfg file.
  • If the user has a password, they will be asked for their original password before being prompted to change to a new password.
  • Make sure the mindiff attribute in the pw_restrictions stanza of /etc/security/login.cfgis nonzero to force the new password to be different from the old one.

If flag field for a user is set to llags = NOCHECK, then the /etc/security/login.cfg restrictions won’t apply to that user for new passwords that follow.

Note: If an ‘*’ appears in the password field ( password = * ), this indicates that no password has been set for that user and prevents the user from logging in until root  user assigns a password to that account.

If the /etc/passwd has an asterik (*) in the second field, this implies that the account has been defined to the system but is not an account that can be logged into yet. There will not be an entry placed into the /etc/security/passwd file until a passwd has been assigned to this account.

/etc/security/.profile :

System-wide default profile used to copy into the $HOME directory of each user as they are created on the system. Add to this file to acquire default capabilities that otherwise would have to be added manually to each $HOME/.profile.

Remember, the /etc/profile file contains commands executed by allusers at login, so if everyone needs to have certain environmental variables set, set them in that file.

/etc/security/user :

Lists attributes that have changed from the default values for individual users, like whether that user has rlogin or telnet access, login authentication method and so on. For example, if a particular user could only log into the system from a particular terminal, the stanza for that user would read:


admin = false

ttys = /dev/tty1

Where the only terminal user rogercould log in from would be tty1.

One could also specify that user roger could use any terminal directly connected to the system but terminal tty5 by stating the following in this file:


admin = false

ttys = !/dev/tty5, ALL

The chuser command modifies this file.

If the login field is set to false (login = false), that user will be locked out from logging into the system from a locally attached terminal. There are only two ways to login to that account. You could ‘su’ into that account, assumming the SU to user fieldis set to TRUE. The other way is to log into that system if the telnet or rlogin field is set to true and your system is connected to a TCP/IP network.

login = false

telnet = true

rlogin = true

acledit :


Combines aclget and aclput capabilities. Must set the EDITOR variable to the following:
export EDITOR=/usr/bin/vi

Must be owner or ROOT  to modify the base or extended permissions of file – regardless of what the extended permissions say. The use of an octal operand (chmod 755 <filename>) will disable the extended ACL parameters (if any) associated with the file.

Example: acledit mytest

aclget :


Gets the ACL values for a file

Example:aclget mytest(Displays base and extended permissions for file mytest)

aclput :


Sets the ACL for a file

Example:aclget mytest | aclput yourtest (Copies ACL attributes from file mytest to file yourtest)

chgrp :


Changes the group ownership of a file or directory.

Examples What it does
chgrp staff mgmt. Changes group from staff to mgmt
chgrp -R office /lisa Will recursively descend through the lisa directory and all its subdirectories to change the group permissions of all files to the group named office.

chmod :

Change permissioms of a file or directory. Without considering the umask value, when a directory is created, the permissions are set to ‘rwxrwxrwx’; and for a new file, the permissions are ‘rw-rw-rw’. When the umask is applied, directory settings are ‘rwxr-xr-x’ and files are ‘rw-r–r–’. To have access to another user‘s directory, that users directory must have the execute bit set for either group or other (eg., ‘rwxr-xr-x).

Be very careful with the files needed for booting. Other system files should be handled with care. Accidentally entering chmod 666 / usr/file instead of chmod 666 /usr/file can be disastrous. The former will instantly render the root  file system unusable and unbootable, since it takes execute (directory search) permission away from the entire file system except for references relative to the current directory that do not go through the root  directory.
Same analogy applies to the /u or /home directory. If execute permissions is removed for group and other (drwxr–r–) users other than ROOT  will not be able to log into the system.

Examples What it does
chmod go+wr testfile Add read/write to group/others
chmod u-x testfile Remove execute permission from owner
chmod 1777 ickfar Turns on sticky bit for file named ickfar

tcbck :


Examples What it does
tcbck -p ALL Checks files specified in /etc/security/tcbck.cfg. Use this command tocheck the file system anytime you suspect the integrity of the system may have been compromised
tcbck -t tree All files in the system are checked for correct installation (this may take several hours to complete)

lsgroup :


List attributes of a group

Example:lsgroup staff (Displays characteristics of the group named staff)

grpck :


Verifies that all users listed as group members are defined as users, that the GID is unique, and that the group name is correct.

Examples What it does
grpck -n ALL Reports errors but doesn’t fix them.
grpck -t ALL If problems are detected, will ask if they should be fixed

lsuser :


List characteristics of user accounts

Example:lsuser roger (Display attributes for the user roger)
usrck :


Verifies most parameters in the /etc/security/user database.

  • Each user name listed in the /etc/passwd file has a stanza in
    • /etc/security/user
    • /etc/security/limits
    • /etc/security/passwd
  • Stanzas will be added for each missing user name as needed.
  • Also verifies each group name listed in /etc/group has a corresponding stanza in /etc/security/group.
  • It may disable a userid by adding an expired expiration date to the /etc/security/user database.
  • The user‘s $HOME directory is not effected.
  • Re-enable account by removing the expiration date in the /etc/security/user database or via smit chuser
Examples What it does
usrck -n ALL Reports errors but doesn’t fix them.
usrck -t ALL if problems are detected, will ask if they should be fixed.

mkpasswd :


Creates a hashed version of the /etc/passwd for faster login capability. This command should be run every time the /etc/passwd and etc/security/passwd files have changed. If more than 40-50 user accounts are defined to the system, running this command will quicken the time it takes to login to your account.

Example: mkpasswd -v /etc/passwd(Creates the /etc/passwd.pag and /etc/passwd.dir files.)

pwdck :


Checks authentication stanzas in /etc/passwd and /etc/security/passwd. May not check for specified password rules such as minalpha, minother, and lastupdate in the /etc/security/login.cfg file. One drawback to pwdck is that it doesn’t report on root  equivalent accounts (accounts with a UID of 0 but a username other than root ).

Examples What it does
pwdck -n ALL Reports errors but doesn’t fix them.
pwdck -t ALL If problems are detected, will ask if they should be fixed.
1)pwdck -y ALL
Will import a BSD or SYSV password file. Use the usrck command to create default limit entries.

newgrp :


Switch to another group for users who are members of multiple groups.

Rmuser :

Removes a user from the system. The $HOME directory for this user is left intact. Must manually remove the directory.

su :

By changing the flag of Another user can SU to user? = no for root , one can disable users from acquiring ROOT  priviledge. The su command wont accept any user name greater than 8 characters.

sysck :


Audits the security state of the system. This command uses the /etc/security/sysck.cfg configuration file to provide it with the expected security state of the system. The effectiveness of sysck is directly related to the accuracy of this configuration file.Checks for file ownership, permissions, acess control lists, and checksums. System directories aren’t included, but this info can be easily added via a text editor of your choice.

Examples What it does
sysck -n ALL Errors are to be reported but not fixed
sysck -t ALL Errors are to be reported with a prompt asking whether the error should be fixed
tcbck -n ALL Check for inconsistencies in the system

umask :


Sets the default file permissions for files created by a user.

Type in theumask command by itself to show what the current defaults are. This can be changed by thesmit usercommand or by the umask command itself.
By default, the umask is set to 022 which means

  • Don’t allow write access for GROUP and OTHER.
  • When this mask is applied to the system file default of 666 (read/write for everyone), the result is 644 read/write for owner, and read for group and other).
  • The math works out as follows 666 – 022 = 644.
  • The system default for creating directories is 777 – 022 = 755, which means the owner has read/write/execute permissions on that directory, while group and others only have read and execute permissions.

ulimit :


Quota limits for individual users for memory usage and file size. Doesn’t effect the /etc/security/limits database. Only changes your environment for the current session (not a permanent change). This command allows the user to change their hard or soft limits.
To raise or lower the soft limit (within the bounds of the hard limit), use the -S option.

To raise or lower the hard limit (within the bounds of the default values specified in the /etc/security/limits (database), use the -H option.

Examples What it does
ulimit -a View current limits
time(seconds) 3600
file(blocks) 2097151
data(kbytes) 65536
stack(kbytes) 4096
memory(kbytes) 32768
coredump(blocks) 2048
ulimit -Sf 1000000 Increase file limit to 1000000
ulimit -d 100000 Data section is now increased to 50 MB. When compiling large C or FORTRAN programs, you may receive a MALLOC error during link phase. This is because there is not enough heap space allocated to this process.