nss_wrapper - Online in the Cloud

PROGRAM:

NAME

nss_wrapper - A wrapper for the user, group and hosts NSS API

SYNOPSIS

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

DESCRIPTION

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.

· Provides information for user and group accounts.

· Network name resolution using a hosts file.

· Loading and testing of NSS modules.

LIMITATIONS

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

NSS_WRAPPER_PASSWD, NSS_WRAPPER_GROUP
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.

NSS_WRAPPER_HOSTS
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

NSS_WRAPPER_HOSTNAME
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

NSS_WRAPPER_MODULE_SO_PATH, NSS_WRAPPER_MODULE_FN_PREFIX
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:

NSS_WRAPPER_MODULE_FN_PREFIX=winbind

NSS_WRAPPER_DEBUGLEVEL
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

EXAMPLE

$ 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
test.example.org

2015-09-12 NSS_WRAPPER(1)

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