[Samba] UID translation mystery or Festivus miracle?

[Rowland Penny]

On Wed, 2021-12-22 at 10:49 -0600, Patrick Goetz via samba wrote:
> I'm guessing this is an rsync thing I wasn't aware of, but I've
> gotten 
> useful tips from the rsync therapists on this list before.
> 
> I'm transferring data from an old Samba PDC system to a new Samba
> 4.15.3 
> file server bound to a samba-ad-dc using the RID id mapping.  All
> the 
> user ID's are changing as as result, so I was preparing to write a 
> script to translate the file/directory owner uids/gids when I
> noticed 
> something really odd.
> 
> On the old PDC/fileserver:
> 
> root at data:home# id pgoetz
> uid=1004(pgoetz) gid=1004(pgoetz) 
> groups=1004(pgoetz),27(sudo),100(users),5005(gco)
> 
> root at data:home# ls -ld pgoetz
> drwxr-xr-x 10 pgoetz pgoetz 4096 Dec 10 06:20 pgoetz
> 
> 
> Rsync the data over to the new file server:
> 
> root at data:/# cd home
> root at data:home# rsync -axW * data2:/data/home-old/
> 
> 
> And now on the new fileserver:
> 
> archives at data2:/data/home-old$ id pgoetz
> uid=11103(pgoetz) gid=10513(domain users) groups=10513(domain 
> users),11103(pgoetz),11112(ea-staff),11113(ea-
> admins),3001(BUILTIN\users)
> 
> archives at data2:/data/home-old$ ls -ld pgoetz
> drwxr-xr-x 10 pgoetz pgoetz 4096 Dec 10 06:20 pgoetz
> 
> ----
> 
> I'm only showing my home folder, but for every user account created
> in 
> the new domain, the transferred files already have the correct file 
> ownership on the new system.
> 
> My best guess is rsync is using the username to convert the UIDs?

Of course, rather than guessing, you could read 'man rsync', where you
would find this:

--numeric-ids
With this option rsync will transfer numeric group and user IDs rather
than
using user and group names and mapping them at both ends.

By  default  rsync will use the username and groupname to determine
what
ownership to give files. The special uid 0 and the special group 0 are
never mapped via user/group names even if the --numeric-ids option is
not specified.

> 
> A secondary mystery is that my group is still "pgoetz" even though
> there 
> is no such group defined in the domain.  How does that work?

It is Magic :-D

The magic of 'id_type_both', Samba creates a usergroup if one does not
exist.

Rowland