Trying to think out a hack for NSS and pw(8)

Mike Kelly pioto at
Fri Sep 9 21:07:21 UTC 2016

You may find that the best way to handle this is to disable enumeration of
your LDAP users in NSS.

For example, if you're using sssd for your LDAP NSS & PAM provider, it is,
in fact, disabled by default.

This means that calls to getpwent(3) will only end up enumerating the users
in your local files, and not those in LDAP. But, calls to getpwnam(3),
getpwuid(3), etc will return the details of a specific username or user id,
even if it's only present in LDAP.

On Fri, Sep 9, 2016 at 4:11 PM Garrett Wollman <wollman at>

> Presently, we have a bunch of machines under configuration management
> (using Puppet, but that's not really relevant here).  I'm hoping to
> implement LDAP via nsswitch on these machines, but I've run into an
> issue: the standard getpw*(3) mechanisms can't tell the difference
> between users or groups in the local databases and those in the remote
> LDAP database.  We need Puppet to manage entries for users and groups
> in the local database, without respect to what entries might be
> imported from LDAP (because they are supposed to override the data
> returned by LDAP).  Puppet invokes pw(8) to actually perform the
> modifications, but I suspect it also uses native code from the Ruby
> standard library to actually do pre-modification lookups.
> Looking at the code in both nss-pam-ldapd and libc, it seems like the
> only plausible way to fix this is to add functionality to nsswitch
> which would allow it to use different configurations depending on the
> identity of the process invoking getpwnam(3) or getgrnam(3).  Does
> anyone have opinions on how this ought to be implemented, or indeed
> how it could be implemented securely?
> (As a side issue, the net/nss-pam-ldapd port completely ignores
> account expiration dates.  This bug is due to the fact that Linux has
> this ships-in-the-night "shadow" mechanism, getspent(3), rather than
> having it integrated in getpwent(3) like it should be, but the
> ultimate upshot is that if you're using nss-pam-ldapd you can't rely
> on shadowExpire attributes in the directory actually have an effect on
> FreeBSD.  I'll open a bugzilla issue about this.)
> -GAWollman
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Mike Kelly

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