Various functions in the C Library need to be configured to work correctly in the local environment. Traditionally, this was done by using files (e.g., `/etc/passwd'), but other nameservices (like the Network Information Service (NIS) and the Domain Name Service (DNS)) became popular, and were hacked into the C library, usually with a fixed search order (see section `frobnicate' in The Jargon File).
The GNU C Library contains a cleaner solution of this problem. It is designed after a method used by Sun Microsystems in the C library of Solaris 2. GNU C Library follows their name and calls this scheme Name Service Switch (NSS).
Though the interface might be similar to Sun's version there is no common code. We never saw any source code of Sun's implementation and so the internal interface is incompatible. This also manifests in the file names we use as we will see later.
The basic idea is to put the implementation of the different services offered to access the databases in separate modules. This has some advantages:
To fulfill the first goal above the ABI of the modules will be described below. For getting the implementation of a new service right it is important to understand how the functions in the modules get called. They are in no way designed to be used by the programmer directly. Instead the programmer should only use the documented and standardized functions to access the databases.
The databases available in the NSS are
aliases
ethers
group
hosts
netgroup
networks
protocols
passwd
rpc
services
shadow
There will be some more added later (automount, bootparams,
netmasks, and publickey).
Somehow the NSS code must be told about the wishes of the user. For this reason there is the file `/etc/nsswitch.conf'. For each database this file contain a specification how the lookup process should work. The file could look like this:
# /etc/nsswitch.conf # # Name Service Switch configuration file. # passwd: db files nis shadow: files group: db files nis hosts: files nisplus nis dns networks: nisplus [NOTFOUND=return] files ethers: nisplus [NOTFOUND=return] db files protocols: nisplus [NOTFOUND=return] db files rpc: nisplus [NOTFOUND=return] db files services: nisplus [NOTFOUND=return] db files
The first column is the database as you can guess from the table above. The rest of the line specifies how the lookup process works. Please note that you specify the way it works for each database individually. This cannot be done with the old way of a monolithic implementation.
The configuration specification for each database can contain two different items:
files, db, or nis.
[NOTFOUND=return].
The above example file mentions four different services: files,
db, nis, and nisplus. This does not mean these
services are available on all sites and it does also not mean these are
all the services which will ever be available.
In fact, these names are simply strings which the NSS code uses to find the implicitly addressed functions. The internal interface will be described later. Visible to the user are the modules which implement an individual service.
Assume the service name shall be used for a lookup. The code for this service is implemented in a module called `libnss_name'. On a system supporting shared libraries this is in fact a shared library with the name (for example) `libnss_name.so.2'. The number at the end is the currently used version of the interface which will not change frequently. Normally the user should not have to be cognizant of these files since they should be placed in a directory where they are found automatically. Only the names of all available services are important.
The second item in the specification gives the user much finer control on the lookup process. Action items are placed between two service names and are written within brackets. The general form is
[(!? status=action )+]
where
status => success | notfound | unavail | tryagain action => return | continue
The case of the keywords is insignificant. The status values are the results of a call to a lookup function of a specific service. They mean
return.
continue.
continue.
continue.
If we have a line like
ethers: nisplus [NOTFOUND=return] db files
this is equivalent to
ethers: nisplus [SUCCESS=return NOTFOUND=return UNAVAIL=continue
TRYAGAIN=continue]
db [SUCCESS=return NOTFOUND=continue UNAVAIL=continue
TRYAGAIN=continue]
files
(except that it would have to be written on one line). The default value for the actions are normally what you want, and only need to be changed in exceptional cases.
If the optional ! is placed before the status this means
the following action is used for all statuses but status itself.
I.e., ! is negation as in the C language (and others).
Before we explain the exception which makes this action item necessary
one more remark: obviously it makes no sense to add another action
item after the files service. Since there is no other service
following the action always is return.
Now, why is this [NOTFOUND=return] action useful? To understand
this we should know that the nisplus service is often
complete; i.e., if an entry is not available in the NIS+ tables it is
not available anywhere else. This is what is expressed by this action
item: it is useless to examine further services since they will not give
us a result.
The situation would be different if the NIS+ service is not available
because the machine is booting. In this case the return value of the
lookup function is not notfound but instead unavail. And
as you can see in the complete form above: in this situation the
db and files services are used. Neat, isn't it? The
system administrator need not pay special care for the time the system
is not completely ready to work (while booting or shutdown or
network problems).
Finally a few more hints. The NSS implementation is not completely helpless if `/etc/nsswitch.conf' does not exist. For all supported databases there is a default value so it should normally be possible to get the system running even if the file is corrupted or missing.
For the hosts and networks databases the default value is
dns [!UNAVAIL=return] files. I.e., the system is prepared for
the DNS service not to be available but if it is available the answer it
returns is definitive.
The passwd, group, and shadow databases are
traditionally handled in a special way. The appropriate files in the
`/etc' directory are read but if an entry with a name starting
with a + character is found NIS is used. This kind of lookup
remains possible by using the special lookup service compat
and the default value for the three databases above is
compat [NOTFOUND=return] files.
For all other databases the default value is
nis [NOTFOUND=return] files. This solution give the best
chance to be correct since NIS and file based lookup is used.
A second point is that the user should try to optimize the lookup
process. The different service have different response times.
A simple file look up on a local file could be fast, but if the file
is long and the needed entry is near the end of the file this may take
quite some time. In this case it might be better to use the db
service which allows fast local access to large data sets.
Often the situation is that some global information like NIS must be
used. So it is unavoidable to use service entries like nis etc.
But one should avoid slow services like this if possible.
Now it is time to describe what the modules look like. The functions contained in a module are identified by their names. I.e., there is no jump table or the like. How this is done is of no interest here; those interested in this topic should read about Dynamic Linking.
The name of each function consist of various parts:
_nss_service_function
service of course corresponds to the name of the module this
function is found in.(3) The function part is derived
from the interface function in the C library itself. If the user calls
the function gethostbyname and the service used is files
the function
_nss_files_gethostbyname_r
in the module
libnss_files.so.2
is used. You see, what is explained above in not the whole truth. In
fact the NSS modules only contain reentrant versions of the lookup
functions. I.e., if the user would call the gethostbyname_r
function this also would end in the above function. For all user
interface functions the C library maps this call to a call to the
reentrant function. For reentrant functions this is trivial since the
interface is (nearly) the same. For the non-reentrant version The
library keeps internal buffers which are used to replace the user
supplied buffer.
I.e., the reentrant functions can have counterparts. No service
module is forced to have functions for all databases and all kinds to
access them. If a function is not available it is simply treated as if
the function would return unavail
(see section Actions in the NSS configuration).
The file name `libnss_files.so.2' would be on a Solaris 2 system `nss_files.so.2'. This is the difference mentioned above. Sun's NSS modules are usable as modules which get indirectly loaded only.
The NSS modules in the GNU C Library are prepared to be used as normal libraries themselves. This is not true at the moment, though. However, the organization of the name space in the modules does not make it impossible like it is for Solaris. Now you can see why the modules are still libraries.(4)
Now we know about the functions contained in the modules. It is now time to describe the types. When we mentioned the reentrant versions of the functions above, this means there are some additional arguments (compared with the standard, non-reentrant version). The prototypes for the non-reentrant and reentrant versions of our function above are:
struct hostent *gethostbyname (const char *name)
int gethostbyname_r (const char *name, struct hostent *result_buf,
char *buf, size_t buflen, struct hostent **result,
int *h_errnop)
The actual prototype of the function in the NSS modules in this case is
enum nss_status _nss_files_gethostbyname_r (const char *name,
struct hostent *result_buf,
char *buf, size_t buflen,
int *errnop, int *h_errnop)
I.e., the interface function is in fact the reentrant function with the
change of the return value and the omission of the result
parameter. While the user-level function returns a pointer to the
result the reentrant function return an enum nss_status value:
NSS_STATUS_TRYAGAIN
-2
NSS_STATUS_UNAVAIL
-1
NSS_STATUS_NOTFOUND
0
NSS_STATUS_SUCCESS
1
Now you see where the action items of the `/etc/nsswitch.conf' file are used.
If you study the source code you will find there is a fifth value:
NSS_STATUS_RETURN. This is an internal use only value, used by a
few functions in places where none of the above value can be used. If
necessary the source code should be examined to learn about the details.
In case the interface function has to return an error it is important
that the correct error code is stored in *errnop. Some
return status value have only one associated error code, others have
more.
@multitable @columnfractions .3 .2 .50
NSS_STATUS_TRYAGAIN @tab
EAGAIN @tab One of the functions used ran temporarily out of
resources or a service is currently not available.
ERANGE @tab The provided buffer is not large enough.
The function should be called again with a larger buffer.
NSS_STATUS_UNAVAIL @tab
ENOENT @tab A necessary input file cannot be found.
NSS_STATUS_NOTFOUND @tab
ENOENT @tab The requested entry is not available.
These are proposed values. There can be other error codes and the
described error codes can have different meaning. With one
exception: when returning NSS_STATUS_TRYAGAIN the error code
ERANGE must mean that the user provided buffer is too
small. Everything is non-critical.
The above function has something special which is missing for almost all
the other module functions. There is an argument h_errnop. This
points to a variable which will be filled with the error code in case
the execution of the function fails for some reason. The reentrant
function cannot use the global variable h_errno;
gethostbyname calls gethostbyname_r with the last argument
set to &h_errno.
The getXXXbyYYY functions are the most important
functions in the NSS modules. But there are others which implement
the other ways to access system databases (say for the
password database, there are setpwent, getpwent, and
endpwent). These will be described in more detail later.
Here we give a general way to determine the
signature of the module function:
int;
STRUCT_TYPE *result_buf
STRUCT_TYPE is
normally a struct which corresponds to the database.
char *buffer
size_t buflen
set...ent
and end...ent functions.
One of the advantages of NSS mentioned above is that it can be extended quite easily. There are two ways in which the extension can happen: adding another database or adding another service. The former is normally done only by the C library developers. It is here only important to remember that adding another database is independent from adding another service because a service need not support all databases or lookup functions.
A designer/implementor of a new service is therefore free to choose the databases s/he is interested in and leave the rest for later (or completely aside).
The sources for a new service need not (and should not) be part of the GNU C Library itself. The developer retains complete control over the sources and its development. The links between the C library and the new service module consists solely of the interface functions.
Each module is designed following a specific interface specification.
For now the version is 2 (the interface in version 1 was not adequate)
and this manifests in the version number of the shared library object of
the NSS modules: they have the extension .2. If the interface
changes again in an incompatible way, this number will be increased.
Modules using the old interface will still be usable.
Developers of a new service will have to make sure that their module is created using the correct interface number. This means the file itself must have the correct name and on ElF systems the soname (Shared Object Name) must also have this number. Building a module from a bunch of object files on an ELF system using GNU CC could be done like this:
gcc -shared -o libnss_NAME.so.2 -Wl,-soname,libnss_NAME.so.2 OBJECTS
section `Link Options' in GNU CC, to learn more about this command line.
To use the new module the library must be able to find it. This can be
achieved by using options for the dynamic linker so that it will search
the directory where the binary is placed. For an ELF system this could be
done by adding the wanted directory to the value of
LD_LIBRARY_PATH.
But this is not always possible since some programs (those which run
under IDs which do not belong to the user) ignore this variable.
Therefore the stable version of the module should be placed into a
directory which is searched by the dynamic linker. Normally this should
be the directory `$prefix/lib', where `$prefix' corresponds to
the value given to configure using the --prefix option. But be
careful: this should only be done if it is clear the module does not
cause any harm. System administrators should be careful.
Until now we only provided the syntactic interface for the functions in the NSS module. In fact there is not much more we can say since the implementation obviously is different for each function. But a few general rules must be followed by all functions.
In fact there are four kinds of different functions which may appear in
the interface. All derive from the traditional ones for system databases.
db in the following table is normally an abbreviation for the
database (e.g., it is pw for the password database).
enum nss_status _nss_database_setdbent (void)
int setdbent (int)). section Host Names, which describes the
sethostent function.
The return value should be NSS_STATUS_SUCCESS or according to the
table above in case of an error (see section The Interface of the Function in NSS Modules).
enum nss_status _nss_database_enddbent (void)
enum nss_status _nss_database_getdbent_r (STRUCTURE *result, char *buffer, size_t buflen, int *errnop)
host and networks.
The function shall return NSS_STATUS_SUCCESS as long as there are
more entries. When the last entry was read it should return
NSS_STATUS_NOTFOUND. When the buffer given as an argument is too
small for the data to be returned NSS_STATUS_TRYAGAIN should be
returned. When the service was not formerly initialized by a call to
_nss_DATABASE_setdbent all return value allowed for
this function can also be returned here.
enum nss_status _nss_DATABASE_getdbbyXX_r (PARAMS, STRUCTURE *result, char *buffer, size_t buflen, int *errnop)
setDBent function whenever this makes sense.
Before the function returns the implementation should store the value of
the local errno variable in the variable pointed to be
errnop. This is important to guarantee the module working in
statically linked programs.
Again, this function takes an additional last argument for the
host and networks database.
The return value should as always follow the rules given above
(see section The Interface of the Function in NSS Modules).
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