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I am making an NSS module answers depending on the name of the caller. For example, if sshd calls getpwnam_r(...), the pw_shell will be /bin/bash; if telnetd calls getpwnam_r(...), the pw_shell will be /bin/ksh.

A prototype is made and it works. However, when nscd is running and the cache is hot, the module's function will not be called. nscd's cached result is returned to every caller. nscd assumes the only variable to the result is time; it never think of process name will affect the result.

Suppose we can make some daemon or module to override nscd, the code should check the process name is on my list or not. If it is on the list, skip nscd; otherwise, let nscd answer getpwnam_r(...).

Is it possible?

Edit: Less preferable, but OK alternative is to bypass nscd when call getpwnam_r(...).

Cha Cha
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  • I think that if you really require this behavior your best bet is to simply disable `nscd` and see if you notice a performance hit. Many people run happily with nscd disabled; it all depends on the size of your environment. – larsks Oct 13 '11 at 13:56
  • I have little control on this. The environment varies. Most likely the users want *nscd*. – Cha Cha Oct 13 '11 at 14:31

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The calls to nscd are hardwired into the standard library such that any call to a map-related function (getpwnam(), gethostbyname() etc...) will query nscd first. The only solution is to turn nscd off or to write your own.

You can confirm this by using getent and strace:

strace -ttt getent passwd

Others have written nscd replacements - gnscd by Google, unscd for BusyBox. So if you cannot disable nscd then you must rewrite it....

Major Eccles
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Yes. It is possible to bypass nscd on a per-process basis, although it's a bit of a hack.

If you check out the glibc source code you'll observe there's a function called __nss_disable_nscd. This is used by nscd (or unscd) to ensure that it doesn't go recursive.

Probably easier to read the example in unscd. See http://busybox.net/~vda/unscd/nscd-0.51.c

mostlyb
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