[Samba] Corruption of winbind cache after converting NT4 to AD domain

[Rowland Penny]

On Sat, 2022-02-12 at 01:48 +0300, Michael Tokarev wrote:
> 12.02.2022 01:05, Rowland Penny via samba пишет:
> > On Sat, 2022-02-12 at 00:34 +0300, Michael Tokarev via samba wrote:
> ...
> > > Now, the most "interesting" part, besides the obvious wrong
> > > behavour somewhere.
> > > 
> > > For a long time, we had unix users with their own regular home
> > > directories,
> > > shell access and lots of work in linux.  As far as I can see, in
> > > order to
> > > use AD domain, we should convert linux users to AD, so that a
> > > user is EITHER
> > > in linux OR in AD, but not both.  I found nothing conclusive
> > > about this,
> > 
> > The old way was to have a Unix user and a Samba user, this mapped
> > Windows users to Unix users. Now, with AD, you only have one user
> > and
> > that user is stored in AD. Winbind maps the AD user to a Unix ID
> > and
> > hence makes the user a Unix user. This all means that if you have a
> > user called 'fred' in AD and /etc/passwd , you should remove the
> > local
> > Unix user from /etc/passwd.
> 
> This is very important point, Rowland. And once I started to realize
> it,
> I started wondering why it is not written in ALL BOLD in all HOWTOs
> and
> wikis out there. Because when you live in NT4-domain world, the "AD
> world"
> is VERY different in this respect, and you just don't understand it.
> This is, in fact, the main reason why I asked for the wiki account -
> to draw a summary of some sort, stating how and especially WHY things
> should be done and how/why they're different between NT4 and AD.
> Because everything tells about converting your users from NT4 to AD,
> but this is just a very beginning, while it seems it is all what
> should be done.

This is one of the reasons that I keep saying that if you have run an
NT4-style domain, then you should forget a lot of what you know, it is
no longer relevant.

> 
> > >   it
> > > is just my gut feeling, - there's no direct requirement like this
> > > in
> > > the docs
> > 
> > This was explained in the Samba wiki, but someone has just removed
> > it.
> 
> It should be in ALL BOLD. Really, I'm not joking. This concept is so
> much foreign to anyone who's used to NT4 or unix.. I'm oldscool, but
> even for many modern sysadmins this is something foreign too.
> That's why, I think, a wiki about AD should start from its concepts
> and some WHYs.  I think most misunderstanding is due to this.

It helped, but needed slightly expanding, so it looks like I will have
to put it back again.

> 
> Again, I have much experience in this area, I understand how it works
> (whole picture), but for ones who has less knowledge it is even more
> difficult, - the HOWTOs describes steps which should be done but does
> not add any understanding...

The problem is if you start to add such information to a howto, it
quickly gets very large.

> 
> > > I found so far.  But I see that people do it like this, not
> > > mixing uids and
> > > usernames.  It is just my gut feeling maybe I'm wrong..
> > 
> > It is not so much that you are mixing uids and usernames, you seem
> > to
> > be possibly mixing users.
> 
> What do you mean "mixing users"? 

If you have users with the same name in /etc/passwd and AD, then you
are mixing users. The user in /etc/passwd is NOT the same user in AD,
even if the usernames are identical.

> What I want to achieve is to have one
> user with its home directory, files and processes, who can login to
> linux environment using natural linux way (either desktop or ssh or
> whatever), and to be able to access his home directory from windows,
> using windows ways to authenticate, with the help of samba AD. 

Very easy, create the user in AD and then use one of the winbind idmap
backends to map the user to a Unix user.

> Roughly
> speaking, we have local linux users with their passwords and ssh
> keys,
> and their windows passwords are stored within AD.

This is where you are going wrong, you do not local Unix users, you
make AD users into Unix users.

>  The uid numbers are
> the same, the names are the same.

But the SIDs will be different.

>   And I don't understand that it is
> bad and especially WHY it is bad. Besides bad (to my view: buggy)
> behavor of winbind (it should either give meaningful error message
> or should work, but not error out randomly with a very difficult to
> debug issues).
> 
> It is not mixed users, - I view it is the same single user whos
> windows-related attributes are stored in the AD. That's it.

No, it is two users with the same name, one in /etc/passwd, one in AD.

> 
> You see - this is why it's so difficult to grok this concept even
> when you have strong background.

As I keep saying, if you have upgraded from an NT4-style domain to AD,
forget most of what you know.

> 
> > > So there are two parts of the question:
> > > 
> > > First, how such setup should be done? We really used to linux
> > > auth
> > > and linux
> > > work, it's somewhat unnatural to rely on the AD when dealing with
> > > local linux
> > > accounts.  But at the same time, these account should have access
> > > from windows
> > > to their files.  And most important, _why_ this setup should be
> > > done?

Lets go back to my username:

rowland at devstation:~$ getent passwd rowland
rowland:*:10000:10000:Rowland Penny:/home/rowland:/bin/bash
rowland at devstation:~$ grep -c 'rowland' /etc/passwd 
0

As you can see, I have a Unix ID (and the second '10000' is Domain
Users), but I am NOT in /etc/passwd. This means that Unix knows who I
am.
The 'passwd' line in /etc/nsswitch.conf is this:
passwd:         compat winbind

Which means that /etc/passwd is checked first, then AD, can you see why
you cannot have users in /etc/passwd and AD ?

> > 
> > 
> > I can assure that 'rowland' isn't in /etc/passwd
> 
> The main question is why. And my inside is fighting with this idea
> too:
> why should we move our lovely local users to some remote location and
> make our main server dependant from some other machine(s) while it
> already
> self-contained?  We do have local accounts on all linux servers
> (sharing
> the same uids), - this may be difficult to administer (it's not with
> a
> bit of getting used to), but it is 100% reliable.

Because it gives you one place to administrate everything, as I keep
saying, forget most things you know.

> 
> > > And second, what to do with this cache corruption, how to prevent
> > > it?
> > 
> > Setup your system correctly.
> 
> Why it is "incorrect"?  I just don't understand the main concept, it
> seems...

You have users in /etc/passwd and in AD, this is incorrect.
> 
> > > smb.conf:
> > > [global]
> > >     server string = %h samba server %v
> > >     netbios name = TSRV
> > >     netbios aliases = LINUX FS
> > 
> > I do not recommend using 'netbios aliases' use a dns 'CNAME'
> > instead.
> 
> Hm.
> It's actually interesting.
> 
> I didn't plan to mention this but we faced an issue here too.
> 
> When I added a CNAME for a host, it didn't work, - neither from
> windows
> nor from smbclient, when logging in the server returned "wrong
> password"
> when connecting to //cname/foo -U foo, but worked fine when using
> //mainname/foo -U foo.  And it didn't work until I added the above
> netbios aliases line and re-joined this server to the domain
> (net ads leave | join). Only after that clients were able to connect.
> It took me lots of time to figure it out.  In the AD some AltName
> attributes appeared after the rejoin (I don't remember exactly).

AD lives on dns and netbios is going away.

> 
> I didn't experiment with this further, because nothing works on my
> side for over a week and I need to fix _that_ first :)
> 
> ..in another email, you wrote:
> 
>  > If you have AD, there is no point in using Samba as a standalone
>  > server, in fact, if you later decide to join the 'standalone
> server' to
>  > the domain, that is where your troubles start.
> 
> This is exactly what we're doing actually: converting a standalone
> server to a member of a domain.

The problem is usually because of different IDs, not just your users
and groups, but also the Well Known SIDs

Rowland