[CentOS] What is /etc/subuid ?

Wed Oct 9 19:47:19 UTC 2019
Valeri Galtsev <galtsev at kicp.uchicago.edu>

Dear Experts,

Could someone enlighten me about the following file:

/etc/subuid

? This file appears to be owned by "setup" package. This is CentOS 7 
system, and until now these files if existed were never changed. Today I 
have added user quite routine way, by doing

/usr/sbin/groupadd -g 4500 [username]
/usr/sbin/useradd -g [username] -u 4500 -c "User Name, email at domain" 
[username]

And the file /etc/subuid changed and user was added into it:

[username]:100000:65536

Nothing like that was happening before. This is first time I create 
account after update done on Oct 3, 2019. I checked several CentOS 7 
machines, basically doing this:

  # grep subuid /usr/sbin/useradd
Binary file /usr/sbin/useradd matches

And CentOS 7 machines indeed may have that file name in the useradd 
binary. None of CentOS 6 machines has that.

I tried to do FreeBSD-ism:

man /etc/subuid

came empty, and realized that I'm doing FreeBSD-ism.

I tried to do search on the web (did not "google", I use duckduckgo... 
so I "did search"), and came pretty much empty.

Is it just me, or indeed something in CentOS 7 indeed changed? And what 
is it?

Another question on the same note: how do we find out what the file is 
about and is used for in Linux, apart from searching on the web. (When 
there are surprises like the one I had today, one does like to know what 
this particular file is used for).


Thanks in advance for your answers.


Valeri


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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
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