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Re: user/group mapping for NFS

Hi Charles,

On Jan 10 15:24, Charles Hedrick wrote:
> We have a system that is doing a Kerberized NFS mount of directories
> on Linux. We have our LDAP system set up as a mapping server, with
> samaccountname added to users so that the Windows mount command does
> the mount as the right user.
> ls -l causes a query that looks like
> (&(objectClass=posixAccount)(uidNumber=26780))" attrs=“uid”. We sent
> back a response. If there’s a local account for the user, “ls -l”
> shows them. But we only have loal accounts for people who actually use
> the Windows box. I’d like “ls -l” to work for everyone.

Sorry, I don't understand the problem, maybe because I'm looking at it
from the other side.

ls -l calls stat(2).  Stat(2) on NFS returns the uids and gids returned
by a stat(2) call on the NFS server.  ls -l then calls (basically)
getpwuid(3)/getgrgid(3) which in turn ask the account server via LDAP;
differently, depending on the server being AD or non-AD LDAP.

If there's no mapping from a uidNumber/gidNumber to a SID in AD, nor a
mapping from uidNumber/gidNumber to an account name ("uid" for users,
"cn" for groups) in LDAP, Cygwin will create a mapping in memory on the
fly.  Example:

  $ ls -l foo
  -rwxr-xr-x 1 Unix_User+4 Unix_Group+7 47456 Oct 30  2015 foo

So the fake account names are "Unix_User+<uid>" and "Unix_Group+<gid>"
The Cygwin uids/gids follow a certain computation rule (details don't
matter here) which is used for Samba accounts as well.  Ultimately
the in-memory passwd and group entries look like this:

  $ getent passwd Unix_User+4
  $ getent group Unix_Group+7

> It appears that cygwin generates SIDs for entries in /etc/passwd.

No, it generates the info on the fly in memory.  In fact, if there's
info in /etc/passwd or /etc/group, and if /etc/nsswitch.conf is set up
to actually _use_ these files (*), you can override the user name to
something more readable:

  $ echo 'MyUser:*:4278190084:4278190084:U-Unix_User\4,S-1-22-1-4:/:/sbin/nologin' >> /etc/passwd
  $ echo 'MyGroup:S-1-22-2-7:4278190087:' >> /etc/group

  [exit from Cygwin terminal, start a new one]

  $ ls -l foo
  -rwxr-xr-x 1 MyUser MyGroup 47456 Oct 30  2015 foo

For the more exact details how to use /etc/nsswitch.conf and how the
mapping is performed, see

(*) https://cygwin.com/cygwin-ug-net/ntsec.html

> How can I make it do that for kid’s returned from LDAP? I’m willing to
> generate SIDs and put them in LDAP, but it doesn’t look like Cygwin is
> asking for a SID.

Well, it should.  What happens is this:  After asking the non-AD LDAP
server for the account name, it asks the account fetching algorithm for
that name from scratch.  This depends on the /etc/nsswitch.conf
settings, of course (*).  Assuming "passwd: files db", it first checks
the local /etc/passwd file for a matching entry for that account name,
then the OS, preferring AD on an AD member machine, then local SAM.

Does this help?


Corinna Vinschen
Cygwin Maintainer

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