I am now installing a new Linux server with CentOS-7. Before, I used CentOS-6 on all machines and used uid = 555 for my account. However, on CentOS-7, it seems that uid <= 999 are reserved for system (accoding to some articles on the net). For testing purpose, I have tried to make an account with uid = 555, such that
# useradd -u 555 {my-login-name}
Then, a new account was created even on CentOS-7 with no warning etc from the command.
I understand that uid <= 999 is "reserved for system", but practically speaking, is there any serious problem to keep using the above uid (555)? Or, considering the future possibility that a new service may use 555, should I avoid using it? I appreciate any advice for this!
EDIT
The /etc/login.defs on my new machine (Centos-7) shows
# Min/max values for automatic uid selection in useradd
#
UID_MIN 1000
UID_MAX 60000
while that on my old machine (Centos-6) is
# Min/max values for automatic uid selection in useradd
#
UID_MIN 500
UID_MAX 60000
so having different UID_MIN. Also, /etc/passwd on my new machine looks like
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
pegasus:x:66:65:tog-pegasus OpenPegasus WBEM/CIM services:/var/lib/Pegasus:/sbin/nologin
ods:x:999:998:softhsm private keys owner:/var/lib/softhsm:/sbin/nologin
systemd-bus-proxy:x:998:996:systemd Bus Proxy:/:/sbin/nologin
systemd-network:x:192:192:systemd Network Management:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
polkitd:x:997:995:User for polkitd:/:/sbin/nologin
apache:x:48:48:Apache:/usr/share/httpd:/sbin/nologin
tss:x:59:59:Account used by the trousers package to sandbox the tcsd daemon:/dev/null:/sbin/nologin
colord:x:996:993:User for colord:/var/lib/colord:/sbin/nologin
abrt:x:173:173::/etc/abrt:/sbin/nologin
unbound:x:995:992:Unbound DNS resolver:/etc/unbound:/sbin/nologin
usbmuxd:x:113:113:usbmuxd user:/:/sbin/nologin
libstoragemgmt:x:994:991:daemon account for libstoragemgmt:/var/run/lsm:/sbin/nologin
saslauth:x:993:76:Saslauthd user:/run/saslauthd:/sbin/nologin
dirsrv:x:389:389:389-ds-base:/usr/share/dirsrv:/sbin/nologin
rpc:x:32:32:Rpcbind Daemon:/var/lib/rpcbind:/sbin/nologin
amandabackup:x:33:6:Amanda user:/var/lib/amanda:/bin/bash
pcp:x:388:388:Performance Co-Pilot:/var/lib/pcp:/sbin/nologin
geoclue:x:387:386:User for geoclue:/var/lib/geoclue:/sbin/nologin
setroubleshoot:x:386:385::/var/lib/setroubleshoot:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
memcached:x:385:384:Memcached daemon:/run/memcached:/sbin/nologin
rtkit:x:172:172:RealtimeKit:/proc:/sbin/nologin
mysql:x:27:27:MariaDB Server:/var/lib/mysql:/sbin/nologin
qemu:x:107:107:qemu user:/:/sbin/nologin
radvd:x:75:75:radvd user:/:/sbin/nologin
chrony:x:384:383::/var/lib/chrony:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin
sssd:x:383:382:User for sssd:/:/sbin/nologin
rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin
nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin
pulse:x:171:171:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
hsqldb:x:96:96::/var/lib/hsqldb:/sbin/nologin
tomcat:x:91:91:Apache Tomcat:/usr/share/tomcat:/sbin/nologin
pkiuser:x:17:17:Certificate System:/usr/share/pki:/sbin/nologin
gdm:x:42:42::/var/lib/gdm:/sbin/nologin
gnome-initial-setup:x:382:379::/run/gnome-initial-setup/:/sbin/nologin
avahi:x:70:70:Avahi mDNS/DNS-SD Stack:/var/run/avahi-daemon:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin
oprofile:x:16:16:Special user account to be used by OProfile:/var/lib/oprofile:/sbin/nologin
From the uid shown above, it seems to me that new services use uid from 999(ods) -> 998 (systemd-bus-proxy) -> 997(polkitd) -> ... -> 993(saslauth). So, if this scheme is followed by other new services as well (in future), there may be little risk for using 555 (for me). FYI, other users (except me) already have uid >= 1000, so no problem for them. But I've been using 555 for other machines (including Mac), so still wondering if it's better to use uid >= 1000 from now on.