[CentOS] What is /etc/subuid ?

Wed Oct 9 20:33:42 UTC 2019
Valeri Galtsev <galtsev at kicp.uchicago.edu>

On 2019-10-09 14:58, Jonathan Billings wrote:
> On Wed, Oct 09, 2019 at 02:47:19PM -0500, Valeri Galtsev wrote:
>> 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
> I'm not sure what else it's used for, but /etc/subuid and /etc/subgid
> are used by podman for rootless containers (i.e. you can run a
> container without any root permissions).  subuid/subgid is used to map
> a range of UID/GIDs to the process namespace inside the kernel.
> Some details here:
> https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux_atomic_host/7/html-single/managing_containers/index#set_up_for_rootless_containers
> It's actually pretty cool.
> So, now when accounts are created with useradd, subuids are assigned
> to that new user.
> Unfortunately, this doesn't really work in an enterprise environment
> when users are defined via LDAP, since no subuid/subgid entries are
> created, but I've heard that there's an effort to make that happen in
> the NSS layer in the future.

Thank you, Michael and Jonathan for your answers.

I have one more question (which I probably will just answer myself by 
kickstart installing fresh new system...):

Did something changed and now by default useradd command adds user in 
that file (by default without me using extra flag etc)? In other words 
is it just me or indeed the command we used since forever suddenly 
changed its behavior?

Thanks again for your insights everybody.



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