Restoring from backup, preserving uids

[Neil Gunton]

Matt McCutchen wrote:
> On Sat, 2008-08-02 at 08:26 -0500, Neil Gunton wrote:
>> I think rsync could match up the "old" uids from 
>> the backup with the "current" uids in the system if it had access to the 
>> old /etc/passwd file, which maps the uids to usernames. So would it be a 
>> good option to add to rsync to give it a path to the "old" passwd file, 
>> in order to match up the uids in the source dir to the ones in the 
>> destination? Destination would be assumed to use the current passwd, of 
>> course. This would help map the old uids to the new ones.
> 
> That's the right idea, but rsync relies on the getpw* functions of the C
> library rather than reading /etc/passwd directly, so the fix wouldn't be
> as simple as adding an option to use a different path to /etc/passwd.
> What you can do that works right now is pull from an rsync daemon
> configured to chroot into the backup so that the daemon's calls to
> getpw* access the /etc/passwd file inside the chroot.
> 
> Matt

Thanks for the tip. I just took at look at the man page for rsyncd.conf 
and it seems to warn you away from combining chroot:

"... disadvantages of ... complicating the preservation of users and 
groups by name (see below)."

I can see in principle how to use this, but my first reaction is that it 
seems like an overly complex solution to a very common problem. Surely a 
tool should aim to make the most commonly used tasks the simplest to 
accomplish? I would posit that most people use rsync for system backups, 
  I have for years, it's only now that I have bothered to talk about the 
uid issue.

I'm suggesting that it might be a nice addition to rsync if we could add 
an option to use a passwd file for the mapping of the src uids to the 
current (dest) uid/username system. This would then mean you don't have 
to set up a special chrooted rsync daemon simply in order to do restore 
from your local backup drive. Instead you could just do something like:

rsync -av --src-passwd-file=/backup/etc/passwd /backup/etc /

... or some other option name that makes sense, that's just off the top 
of my head. This would make restores much simpler. I accept that there's 
a workaround via the chroot daemon, and that this would likely require 
some refactoring of the rsync code. I'm just making an appeal to 
simplicity for something that's done quite commonly. Of course someone 
else will have to do the work, so it's just a humble suggestion.

Thanks again,

Neil