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System Groups

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The standard groups as set up by the installation process (this is essentially the default the /etc/group  file). 

System groups are groups with GID less then 100 (this is a distribution defined limit; in some distributions it set higher to 500 (yolinux).  They are reserved for user IDs employed by the operating system or its services. Generally the concept of system groups is similar to the concept of privileged ports. Similar partitioning of numberic space is used for system accounts. See System Accounts and UID policy

Some of them such as bin, daemon, sys, adm are used just for partitioning of the ownership space. The others such as root and wheel are used to provide additional access capabilities. For example you can limit the ability to change your identity to root via su to the members of wheel group.

BSD Unix invented wheel group as a special system group for users who can assume the role of root (switch to root using su or sudo). AIX has similar notion of sugroups.  Solaris has sysadmin group (group 14) that is allowed to change permissions of files that user does no own. Generally Solaris 10  RBAC permits much flexible arrangement then wheel group and can be considered as a generalization of this concept.   In Linux wheel group is typically controlled by PAM. See Wheel Group

System groups are subset of a larger set called standard groups. Those are groups created by particular distribution installer if you install all packages ("everything").  Here is how RHEL defines them in the manual

Table 32.5, “Standard Groups” lists the standard groups configured by an Everything installation. Groups are stored in the /etc/group file.

Table 32.5. Standard Groups
Group GID Members
root 0 root
bin 1 root, bin, daemon
daemon 2 root, bin, daemon
sys 3 root, bin, adm
adm 4 root, adm, daemon
tty 5  
disk 6 root
lp 7 daemon, lp
mem 8  
kmem 9  
wheel 10 root
mail 12 mail, postfix, exim
news 13 news
uucp 14 uucp
man 15  
games 20  
gopher 30  
dip 40  
ftp 50  
lock 54  
nobody 99  
users 100  
rpm 37  
utmp 22  
floppy 19  
vcsa 69  
dbus 81  
ntp 38  
canna 39  
nscd 28  
rpc 32  
postdrop 90  
postfix 89  
mailman 41  
exim 93  
named 25  
postgres 26  
sshd 74  
rpcuser 29  
nfsnobody 65534  
pvm 24  
apache 48  
xfs 43  
gdm 42  
htt 101  
mysql 27  
webalizer 67  
mailnull 47  
smmsp 51  
squid 23  
ldap 55  
netdump 34  
pcap 77  
quaggavt 102  
quagga 92  
radvd 75  
slocate 21  
wnn 49  
dovecot 97  
radiusd 95  

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Users, Groups and Passwords

Some UID numbers are assigned to system accounts during the installation of the operating system. What numbers are assigned to what accounts will vary between types of Unix. Some typical system accounts and UID's are listed below.


For Solaris:

For HP-UX:

It is recommended that UID numbers not be reused, even if the user has left the system. If system files are ever restored from tape reusing UID numbers can cause problems as users are identified by UID number on tape.

... ... ...

GID's, like UID's, must be distinct integers between 0 and 32767. GID's of less then 10 are reserved for system groups. These default GID's are assigned during the installation of the operating system. Typical system groups and GID's are listed below.


For Solaris:

For HP-UX:

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