nss_wrapper - Man Page

A wrapper for the user, group and hosts NSS API


LD_PRELOAD=libnss_wrapper.so NSS_WRAPPER_PASSWD=/path/to/passwd NSS_WRAPPER_GROUP=/path/to/group NSS_WRAPPER_HOSTS=/path/to/host ./myapplication


There are projects which provide daemons needing to be able to create, modify and delete Unix users. Or just switch user ids to interact with the system e.g. a user space file server. To be able to test that you need the privilege to modify the passwd and groups file. With nss_wrapper it is possible to define your own passwd and groups file which will be used by software to act correctly while under test.

If you have a client and server under test they normally use functions to resolve network names to addresses (dns) or vice versa. The nss_wrappers allow you to create a hosts file to setup name resolution for the addresses you use with socket_wrapper.


Some calls in nss_wrapper will only work if uid_wrapper is loaded and active. One of this functions is initgroups() which needs to run setgroups() to set the groups for the user. setgroups() is wrapped by uid_wrapper.

Environment Variables


For user and group accounts you need to create two files: passwd and group. The format of the passwd file is described in man 5 passwd and the group file in man 5 group. So you can fill these files with made up accounts. You point nss_wrapper to them using the two variables NSS_WRAPPER_PASSWD=/path/to/your/passwd and NSS_WRAPPER_GROUP=/path/to/your/group.


If you also need to emulate network name resolution in your enviornment, especially with socket_wrapper, you can write a hosts file. The format is described in man 5 hosts. Then you can point nss_wrapper to your hosts file using: NSS_WRAPPER_HOSTS=/path/to/your/hosts


If you need to return a hostname which is different from the one of your machine is using you can use: NSS_WRAPPER_HOSTNAME=test.example.org


If you have a project which also provides user and group information out of a database, you normally write your own nss modules. nss_wrapper is able to load nss modules and ask them first before looking into the faked passwd and group file. To point nss_wrapper to the module you can do that using NSS_WRAPPER_MODULE_SO_PATH=/path/to/libnss_yourmodule.so. As each nss module has a special prefix like _nss_winbind_getpwnam() you need to set the prefix too so nss_wrapper can load the functions with NSS_WRAPPER_MODULE_FN_PREFIX=<prefix>.

For _nss_winbind_getpwnam() this would be:


If you need to see what is going on in nss_wrapper itself or try to find a bug, you can enable logging support in nss_wrapper if you built it with debug symbols.

  • 0 = ERROR
  • 1 = WARNING
  • 2 = DEBUG
  • 3 = TRACE

This allows you to disable deep binding in nss_wrapper. This is useful for running valgrind tools or sanitizers like (address, undefined, thread).


$ echo "bob:x:1000:1000:bob gecos:/home/test/bob:/bin/false" > passwd
$ echo "root:x:65534:65532:root gecos:/home/test/root:/bin/false" >> passwd
$ echo "users:x:1000:" > group
$ echo "root:x:65532:" >> group
$ LD_PRELOAD=libnss_wrapper.so NSS_WRAPPER_PASSWD=passwd \
  NSS_WRAPPER_GROUP=group getent passwd bob
bob:x:1000:1000:bob gecos:/home/test/bob:/bin/false
$ LD_PRELOAD=libnss_wrapper.so NSS_WRAPPER_HOSTNAME=test.example.org hostname


Samba Team