On 03/03/2015 03:33 PM, P. Larry Nelson wrote:
> That used to happen in the old days before
> system-config-users pretty much kept generated UIDs/GIDs well out
> of the range that an installed piece of software might use.
> I believe the rule is now that real people users get a UID > 500
> and installed apps (like ntop, UID:103, GID:160) use UIDs < 500,
> but I don't know if that's a hard and fast rule with apps or not.
> I do the same thing with any local group I create - give it a
> GID > 500. 

The authoritative source used by useradd (perhaps others) is /etc/login.defs:

grep ^UID_MIN /etc/login.defs
UID_MIN              500

Historically it was UID >= 500 (note 500 was the first), in recent Fedora's and EL7, it's now 1000:

grep ^UID_MIN /etc/login.defs
UID_MIN                  1000


Note new systems also have min/max values for system accounts in login.defs:

# Min/max values for automatic uid selection in useradd
#
UID_MIN                  1000
UID
# System accounts
SYS_UID
SYS_UID_MAX               999