System Accounts

The /etc/passwd  file can be thought of as a simple flat database that lists all of the users on the system. It is possible to configure a Unix system to use other directory services, such as NIS, LDAP, and/or Kerberos. When these systems are used, the Unix operating system is often modified so that the system programs and applications are can still think that all of the account information still reside in a /etc/passwd  file. In a way they create virtual /etc/passwd file. If LDAP became unavailable the authentication goes back to passwd authentication. 

On most Unix systems  the passwords for these accounts are kept in a file named /etc/shadow, or /etc/security/passwd.  T

• root Superuser account. Performs accounting and low-level system functions. The root user account, UID 0, through which you can perform system maintenance tasks and troubleshoot system problems.
• adm  The adm user account owns the following basic system functions:
  • Diagnostics, the tools for which are stored in the /usr/sbin/perf/diag_tool directory.
  • Accounting, the tools for which are stored in the following directories:
    • /usr/sbin/acct
    • /usr/lib/acct
    • /var/adm
    • /var/adm/acct/fiscal
    • /var/adm/acct/nite
    • /var/adm/acct/sum
• bin The bin user account typically owns the executable files for most user commands. Has ownership of system files. Ii is not used for execution of  programs.  This account's primary purpose is to help distribute the ownership of important system directories and files so that everything is not owned solely by the root and sys user accounts.
• sys The sys user owns the default mounting point for the Distributed File Service (DFS) cache, which must exist before you can install or configure DFS on a client. The /usr/sys directory can also store installation images.
• daemon The daemon user account exists only to own and run system server processes and their associated files. This account guarantees that such processes run with the appropriate file access permissions.  Handles some aspects of the network. This username is also associated with other utility systems, such as the print spoolers, on some versions of Unix.
• mail  Handles aspects of electronic mail.  Used by sendmail and postfix daemons.
• guest Used (infrequently) for site visitors to access the system.
• ftp Used for anonymous FTP access.
• uucp Controls ownership of the Unix serial ports. (uucp traditionally managed the UUCP system, which is now deprecated.). Now this is an outdated user, which can be removed.
• news  Used for Usenet news. Now this is an outdated user, which can be removed.
• lp  Used for the printer system. lp stands for line printer, although these days most people seem to be using laser printers. Most Unix system are not connected to printer so this account can be removed.
• nobody Owns no files and is sometimes used as a default user for unprivileged operations.  The nobody user account is used by the Network File System (NFS) to enable remote printing. This account exists so that a program can permit temporary root access to root users. For example, before enabling Secure RPC or Secure NFS, check the /etc/public key on the master NIS server to find a user who has not been assigned a public key and a secret key. As root user, you can create an entry in the database for each unassigned user by entering:

  newkey -u username

  Or, you can create an entry in the database for the nobody user account, and then any user can run the chkey program to create their own entries in the database without logging in as root. 
• www or http Runs the web server.
• named Runs the BIND name server.
• sshd Performs unprivileged operations for the OpenSSH Secure Shell daemon.
• operator Used for creating backups and (sometimes) for printer operation.
• games Allowed to access high-score files.
• amanda Used for the Amanda remote backup system.

Each flavor of Unix provides a default set of system special user accounts that prevents the root and system accounts from owning all operating system files and file systems.

Attention: Use caution when removing a system special user account. You can disable a specific account by inserting an asterisk (*) at the beginning of its corresponding line of the /etc/shadow file. However, be careful not to disable the root user account. If you remove system special user accounts or disable the root account, the operating system will not function.

Removing Unnecessary Default User Accounts

During installation of the operating system, a number of default user and group IDs are created. Depending on the applications you are running on your system and where your system is located in the network, some of these user and group IDs can become security weaknesses, vulnerable to exploitation. If these users and group IDs are not needed, you can remove them to minimize security risks associated with them.

The following table lists the most common default user IDs that you might be able to remove:

The following table lists common groups that might not be needed:

Analyze your system to determine which IDs are indeed not needed. There might also be additional user and group IDs that you might not need. Before your system goes into production, perform a thorough evaluation of available IDs.

NEWS CONTENTS

• 20110903 : RHEL Standard Users ( Sep 3, 2011 )
• 20110902 : An interesting inter-nonoperability issue. ( Sep 2, 2011 )

Old News ;-)

[Sep 3, 2011] RHEL Standard Users

In Table 2-1, you'll find the standard users set up by the installation process (this is essentially the /etc/passwd file). The Group ID (GID) in this table is the primary group for the user. See the section called User Private Groups for details on how groups are used.

Table 2-1. Standard Users

User	UID	GID	Home Directory	Shell
root	0	0	/root	/bin/bash
bin	1	1	/bin
daemon	2	2	/sbin
adm	3	4	/var/adm
lp	4	7	/var/spool/lpd
sync	5	0	/sbin	/bin/sync
shutdown	6	0	/sbin	/sbin/shutdown
halt	7	0	/sbin	/sbin/halt
mail	8	12	/var/spool/mail
news	9	13	/var/spool/news
uucp	10	14	/var/spool/uucp
operator	11	0	/root
games	12	100	/usr/games
gopher	13	30	/usr/lib/gopher-data
ftp	14	50	/home/ftp
nobody	99	99	/

[Sep 2, 2011] An interesting inter-nonoperability issue.

Hi,

This is not strictly openldap, but it should be a common problem. I am
suprised nobody ran into this earlier.

Here is an interesting inter-nonoperability issue. So I used the migrate scripts from PADL to dump NIS+ user and group maps into an LDAP directory.
Well and good. However, Solaris stores the primary group membership information in the gid field in the passwd map. As Unix usually does. :)

Enter auth_ldap, authentication module for Apache. When I use the "require valid group" directive, the code searches the group entry for the "member"
attribute with the value of the current user. The trouble is, there usually are none, because the group map on NIS+ did not define it, except in the
case of secondary groups.

Not that it's very diffucult to manually hack this, but there has got to be an official solution!

Thanks in advance,
Simon
--
Simon (Vsevolod ILyushchenko) [email protected]
http://www.simonf.com [email protected]

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