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System Accounts

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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

 

 

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:

User ID Description
uucp, nuucp Owner of hidden files used by uucp protocol. The uucp user account is used for the UNIX-to-UNIX Copy Program, which is a group of commands, programs, and files, present on most AIX systems, that allows the user to communicate with another AIX system over a dedicated line or a telephone line.
lpd Owner of files used by printing subsystem
guest Allows access to users who do not have access to accounts

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

Group ID

Description

uucp Group to which uucp and nuucp users belong
printq Group to which lpd user belongs

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.

User Name User ID Description
root 0 Superuser account.
daemon 1 Umbrella system daemon associated with routine system tasks.
bin 2 Administrative daemon associated with running system binaries to perform some routine system task.
sys 3 Administrative daemon associated with system logging or updating files in temporary directories.
adm 4 Administrative daemon associated with system logging.
lp 71 Line printer daemon.
uucp 5 Daemon associated with uucp functions.
nuucp 6 Daemon associated with uucp functions.
smmsp 25 Sendmail message submission program daemon.
listen 37 Network listener daemon.
nobody 60001 Assigned to users or software processes that do not need nor should have any special permissions. 
noaccess 60002 Assigned to a user or a process that needs access to a system through some application but without actually logging in.
nobody4 65534 SunOS 4.0 or 4.1 version of the nobody  user account.

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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)   simonf@cshl.edu  
http://www.simonf.com          simonf@simonf.com
 

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Last modified: August 05, 2013