Neil Blakey-Milner Contributed by FreeBSD allows multiple users to use the computer at the same time. Obviously, only one of those users can be sitting in front of the screen and keyboard at any one time Well, unless you hook up multiple terminals, but we will save that for . , but any number of users can log in through the network to get their work done. To use the system every user must have an account. After reading this chapter, you will know: The differences between the various user accounts on a FreeBSD system. How to add user accounts. How to remove user accounts. How to change account details, such as the user's full name, or preferred shell. How to set limits on a per-account basis, to control the resources such as memory and CPU time that accounts and groups of accounts are allowed to access. How to use groups to make account management easier. Before reading this chapter, you should: Understand the basics of &unix; and FreeBSD (). All access to the system is achieved via accounts, and all processes are run by users, so user and account management are of integral importance on FreeBSD systems. Every account on a FreeBSD system has certain information associated with it to identify the account. User name The user name as it would be typed at the login: prompt. User names must be unique across the computer; you may not have two users with the same user name. There are a number of rules for creating valid user names, documented in &man.passwd.5;; you would typically use user names that consist of eight or fewer all lower case characters. Password Each account has a password associated with it. The password may be blank, in which case no password will be required to access the system. This is normally a very bad idea; every account should have a password. User ID (UID) The UID is a number, traditionally from 0 to 65535 It is possible to use UID/GIDs as large as 4294967295, but such IDs can cause serious problems with software that makes assumptions about the values of IDs. , used to uniquely identify the user to the system. Internally, FreeBSD uses the UID to identify users—any FreeBSD commands that allow you to specify a user name will convert it to the UID before working with it. This means that you can have several accounts with different user names but the same UID. As far as FreeBSD is concerned these accounts are one user. It is unlikely you will ever need to do this. Group ID (GID) The GID is a number, traditionally from 0 to 65535, used to uniquely identify the primary group that the user belongs to. Groups are a mechanism for controlling access to resources based on a user's GID rather than their UID. This can significantly reduce the size of some configuration files. A user may also be in more than one group. Login class Login classes are an extension to the group mechanism that provide additional flexibility when tailoring the system to different users. Password change time By default FreeBSD does not force users to change their passwords periodically. You can enforce this on a per-user basis, forcing some or all of your users to change their passwords after a certain amount of time has elapsed. Account expiry time By default FreeBSD does not expire accounts. If you are creating accounts that you know have a limited lifespan, for example, in a school where you have accounts for the students, then you can specify when the account expires. After the expiry time has elapsed the account cannot be used to log in to the system, although the account's directories and files will remain. User's full name The user name uniquely identifies the account to FreeBSD, but does not necessarily reflect the user's real name. This information can be associated with the account. Home directory The home directory is the full path to a directory on the system in which the user will start when logging on to the system. A common convention is to put all user home directories under /home/username or /usr/home/username. The user would store their personal files in their home directory, and any directories they may create in there. User shell The shell provides the default environment users use to interact with the system. There are many different kinds of shells, and experienced users will have their own preferences, which can be reflected in their account settings. There are three main types of accounts: the Superuser, system users, and user accounts. The Superuser account, usually called root, is used to manage the system with no limitations on privileges. System users run services. Finally, user accounts are used by real people, who log on, read mail, and so forth. accounts superuser (root) The superuser account, usually called root, comes preconfigured to facilitate system administration, and should not be used for day-to-day tasks like sending and receiving mail, general exploration of the system, or programming. This is because the superuser, unlike normal user accounts, can operate without limits, and misuse of the superuser account may result in spectacular disasters. User accounts are unable to destroy the system by mistake, so it is generally best to use normal user accounts whenever possible, unless you especially need the extra privilege. You should always double and triple-check commands you issue as the superuser, since an extra space or missing character can mean irreparable data loss. So, the first thing you should do after reading this chapter is to create an unprivileged user account for yourself for general usage if you have not already. This applies equally whether you are running a multi-user or single-user machine. Later in this chapter, we discuss how to create additional accounts, and how to change between the normal user and superuser. accounts system System users are those used to run services such as DNS, mail, web servers, and so forth. The reason for this is security; if all services ran as the superuser, they could act without restriction. accounts daemon accounts operator Examples of system users are daemon, operator, bind (for the Domain Name Service), news, and www. accounts nobody nobody is the generic unprivileged system user. However, it is important to keep in mind that the more services that use nobody, the more files and processes that user will become associated with, and hence the more privileged that user becomes. accounts user User accounts are the primary means of access for real people to the system, and these accounts insulate the user and the environment, preventing the users from damaging the system or other users, and allowing users to customize their environment without affecting others. Every person accessing your system should have a unique user account. This allows you to find out who is doing what, prevent people from clobbering each others' settings or reading each others' mail, and so forth. Each user can set up their own environment to accommodate their use of the system, by using alternate shells, editors, key bindings, and language. accounts modifying There are a variety of different commands available in the &unix; environment to manipulate user accounts. The most common commands are summarized below, followed by more detailed examples of their usage. Command Summary &man.adduser.8; The recommended command-line application for adding new users. &man.rmuser.8; The recommended command-line application for removing users. &man.chpass.1; A flexible tool to change user database information. &man.passwd.1; The simple command-line tool to change user passwords. &man.pw.8; A powerful and flexible tool to modify all aspects of user accounts. accounts adding adduser /usr/share/skel skeleton directory &man.adduser.8; is a simple program for adding new users. It creates entries in the system passwd and group files. It will also create a home directory for the new user, copy in the default configuration files (dotfiles) from /usr/share/skel, and can optionally mail the new user a welcome message. &prompt.root; adduser Username: jru Full name: J. Random User Uid (Leave empty for default): Login group [jru]: Login group is jru. Invite jru into other groups? []: wheel Login class [default]: Shell (sh csh tcsh zsh nologin) [sh]: zsh Home directory [/home/jru]: Home directory permissions (Leave empty for default): Use password-based authentication? [yes]: Use an empty password? (yes/no) [no]: Use a random password? (yes/no) [no]: Enter password: Enter password again: Lock out the account after creation? [no]: Username : jru Password : **** Full Name : J. Random User Uid : 1001 Class : Groups : jru wheel Home : /home/jru Shell : /usr/local/bin/zsh Locked : no OK? (yes/no): yes adduser: INFO: Successfully added (jru) to the user database. Add another user? (yes/no): no Goodbye! &prompt.root; The password you type in is not echoed, nor are asterisks displayed. Make sure that you do not mistype the password. rmuser accounts removing You can use &man.rmuser.8; to completely remove a user from the system. &man.rmuser.8; performs the following steps: Removes the user's &man.crontab.1; entry (if any). Removes any &man.at.1; jobs belonging to the user. Kills all processes owned by the user. Removes the user from the system's local password file. Removes the user's home directory (if it is owned by the user). Removes the incoming mail files belonging to the user from /var/mail. Removes all files owned by the user from temporary file storage areas such as /tmp. Finally, removes the username from all groups to which it belongs in /etc/group. If a group becomes empty and the group name is the same as the username, the group is removed; this complements the per-user unique groups created by &man.adduser.8;. &man.rmuser.8; cannot be used to remove superuser accounts, since that is almost always an indication of massive destruction. By default, an interactive mode is used, which attempts to make sure you know what you are doing. &prompt.root; rmuser jru Matching password entry: jru:*:1001:1001::0:0:J. Random User:/home/jru:/usr/local/bin/zsh Is this the entry you wish to remove? y Remove user's home directory (/home/jru)? y Updating password file, updating databases, done. Updating group file: trusted (removing group jru -- personal group is empty) done. Removing user's incoming mail file /var/mail/jru: done. Removing files belonging to jru from /tmp: done. Removing files belonging to jru from /var/tmp: done. Removing files belonging to jru from /var/tmp/vi.recover: done. &prompt.root; chpass &man.chpass.1; changes user database information such as passwords, shells, and personal information. Only system administrators, as the superuser, may change other users' information and passwords with &man.chpass.1;. When passed no options, aside from an optional username, &man.chpass.1; displays an editor containing user information. When the user exists from the editor, the user database is updated with the new information. You will be asked for your password after exiting the editor if you are not the superuser. #Changing user database information for jru. Login: jru Password: * Uid [#]: 1001 Gid [# or name]: 1001 Change [month day year]: Expire [month day year]: Class: Home directory: /home/jru Shell: /usr/local/bin/zsh Full Name: J. Random User Office Location: Office Phone: Home Phone: Other information: The normal user can change only a small subset of this information, and only for themselves. #Changing user database information for jru. Shell: /usr/local/bin/zsh Full Name: J. Random User Office Location: Office Phone: Home Phone: Other information: &man.chfn.1; and &man.chsh.1; are just links to &man.chpass.1;, as are &man.ypchpass.1;, &man.ypchfn.1;, and &man.ypchsh.1;. NIS support is automatic, so specifying the yp before the command is not necessary. If this is confusing to you, do not worry, NIS will be covered in . passwd accounts changing password &man.passwd.1; is the usual way to change your own password as a user, or another user's password as the superuser. To prevent accidental or unauthorized changes, the original password must be entered before a new password can be set. &prompt.user; passwd Changing local password for jru. Old password: New password: Retype new password: passwd: updating the database... passwd: done &prompt.root; passwd jru Changing local password for jru. New password: Retype new password: passwd: updating the database... passwd: done As with &man.chpass.1;, &man.yppasswd.1; is just a link to &man.passwd.1;, so NIS works with either command. pw &man.pw.8; is a command line utility to create, remove, modify, and display users and groups. It functions as a front end to the system user and group files. &man.pw.8; has a very powerful set of command line options that make it suitable for use in shell scripts, but new users may find it more complicated than the other commands presented here. limiting users accounts limiting If you have users, the ability to limit their system use may have come to mind. FreeBSD provides several ways an administrator can limit the amount of system resources an individual may use. These limits are divided into two sections: disk quotas, and other resource limits. quotas limiting users quotas disk quotas Disk quotas limit disk usage to users, and they provide a way to quickly check that usage without calculating it every time. Quotas are discussed in . The other resource limits include ways to limit the amount of CPU, memory, and other resources a user may consume. These are defined using login classes and are discussed here. /etc/login.conf Login classes are defined in /etc/login.conf. The precise semantics are beyond the scope of this section, but are described in detail in the &man.login.conf.5; manual page. It is sufficient to say that each user is assigned to a login class (default by default), and that each login class has a set of login capabilities associated with it. A login capability is a name=value pair, where name is a well-known identifier and value is an arbitrary string processed accordingly depending on the name. Setting up login classes and capabilities is rather straight-forward and is also described in &man.login.conf.5;. The system does not normally read the configuration in /etc/login.conf directly, but reads the database file /etc/login.conf.db which provides faster lookups. To generate /etc/login.conf.db from /etc/login.conf, execute the following command: &prompt.root; cap_mkdb /etc/login.conf Resource limits are different from plain vanilla login capabilities in two ways. First, for every limit, there is a soft (current) and hard limit. A soft limit may be adjusted by the user or application, but may be no higher than the hard limit. The latter may be lowered by the user, but never raised. Second, most resource limits apply per process to a specific user, not the user as a whole. Note, however, that these differences are mandated by the specific handling of the limits, not by the implementation of the login capability framework (i.e., they are not really a special case of login capabilities). And so, without further ado, below are the most commonly used resource limits (the rest, along with all the other login capabilities, may be found in &man.login.conf.5;). coredumpsize coredumpsize limiting users coredumpsize The limit on the size of a core file generated by a program is, for obvious reasons, subordinate to other limits on disk usage (e.g., filesize, or disk quotas). Nevertheless, it is often used as a less-severe method of controlling disk space consumption: since users do not generate core files themselves, and often do not delete them, setting this may save them from running out of disk space should a large program (e.g., emacs) crash. cputime cputime limiting users cputime This is the maximum amount of CPU time a user's process may consume. Offending processes will be killed by the kernel. This is a limit on CPU time consumed, not percentage of the CPU as displayed in some fields by &man.top.1; and &man.ps.1;. A limit on the latter is, at the time of this writing, not possible, and would be rather useless: a compiler—probably a legitimate task—can easily use almost 100% of a CPU for some time. filesize filesize limiting users filesize This is the maximum size of a file the user may possess. Unlike disk quotas, this limit is enforced on individual files, not the set of all files a user owns. maxproc maxproc limiting users maxproc This is the maximum number of processes a user may be running. This includes foreground and background processes alike. For obvious reasons, this may not be larger than the system limit specified by the kern.maxproc &man.sysctl.8;. Also note that setting this too small may hinder a user's productivity: it is often useful to be logged in multiple times or execute pipelines. Some tasks, such as compiling a large program, also spawn multiple processes (e.g., &man.make.1;, &man.cc.1;, and other intermediate preprocessors). memorylocked memorylocked limiting users memorylocked This is the maximum amount a memory a process may have requested to be locked into main memory (e.g., see &man.mlock.2;). Some system-critical programs, such as &man.amd.8;, lock into main memory such that in the event of being swapped out, they do not contribute to a system's trashing in time of trouble. memoryuse memoryuse limiting users memoryuse This is the maximum amount of memory a process may consume at any given time. It includes both core memory and swap usage. This is not a catch-all limit for restricting memory consumption, but it is a good start. openfiles openfiles limiting users openfiles This is the maximum amount of files a process may have open. In FreeBSD, files are also used to represent sockets and IPC channels; thus, be careful not to set this too low. The system-wide limit for this is defined by the kern.maxfiles &man.sysctl.8;. sbsize sbsize limiting users sbsize This is the limit on the amount of network memory, and thus mbufs, a user may consume. This originated as a response to an old DoS attack by creating a lot of sockets, but can be generally used to limit network communications. stacksize stacksize limiting users stacksize This is the maximum size a process' stack may grow to. This alone is not sufficient to limit the amount of memory a program may use; consequently, it should be used in conjunction with other limits. There are a few other things to remember when setting resource limits. Following are some general tips, suggestions, and miscellaneous comments. Processes started at system startup by /etc/rc are assigned to the daemon login class. Although the /etc/login.conf that comes with the system is a good source of reasonable values for most limits, only you, the administrator, can know what is appropriate for your system. Setting a limit too high may open your system up to abuse, while setting it too low may put a strain on productivity. Users of the X Window System (X11) should probably be granted more resources than other users. X11 by itself takes a lot of resources, but it also encourages users to run more programs simultaneously. Remember that many limits apply to individual processes, not the user as a whole. For example, setting openfiles to 50 means that each process the user runs may open up to 50 files. Thus, the gross amount of files a user may open is the value of openfiles multiplied by the value of maxproc. This also applies to memory consumption. For further information on resource limits and login classes and capabilities in general, please consult the relevant manual pages: &man.cap.mkdb.1;, &man.getrlimit.2;, &man.login.conf.5;. groups /etc/groups accounts groups A group is simply a list of users. Groups are identified by their group name and GID (Group ID). In FreeBSD (and most other &unix; like systems), the two factors the kernel uses to decide whether a process is allowed to do something is its user ID and list of groups it belongs to. Unlike a user ID, a process has a list of groups associated with it. You may hear some things refer to the group ID of a user or process; most of the time, this just means the first group in the list. The group name to group ID map is in /etc/group. This is a plain text file with four colon-delimited fields. The first field is the group name, the second is the encrypted password, the third the group ID, and the fourth the comma-delimited list of members. It can safely be edited by hand (assuming, of course, that you do not make any syntax errors!). For a more complete description of the syntax, see the &man.group.5; manual page. If you do not want to edit /etc/group manually, you can use the &man.pw.8; command to add and edit groups. For example, to add a group called teamtwo and then confirm that it exists you can use: &prompt.root; pw groupadd teamtwo &prompt.root; pw groupshow teamtwo teamtwo:*:1100: The number 1100 above is the group ID of the group teamtwo. Right now, teamtwo has no members, and is thus rather useless. Let's change that by inviting jru to the teamtwo group. &prompt.root; pw groupmod teamtwo -M jru &prompt.root; pw groupshow teamtwo teamtwo:*:1100:jru The argument to the -M option is a comma-delimited list of users who are to be in the group. From the preceding sections, we know that the password file also contains a group for each user. The latter (the user) is automatically added to the group list by the system; the user will not show up as a member when using the groupshow command to &man.pw.8;, but will show up when the information is queried via &man.id.1; or similar tool. In other words, &man.pw.8; only manipulates the /etc/group file; it will never attempt to read additionally data from /etc/passwd. &prompt.root; pw groupmod teamtwo -m db &prompt.root; pw groupshow teamtwo teamtwo:*:1100:jru,db The argument to the -m option is a comma-delimited list of users who are to be added to the group. Unlike the previous example, these users are added to the group and do not replace the list of users in the group. &prompt.user; id jru uid=1001(jru) gid=1001(jru) groups=1001(jru), 1100(teamtwo) As you can see, jru is a member of the groups jru and teamtwo. For more information about &man.pw.8;, see its manual page, and for more information on the format of /etc/group, consult the &man.group.5; manual page.