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The Network Information Service (NIS)
provides a simple network lookup service consisting of databases and processes. The databases are ndbm files in a directory tree rooted at /var/yp. See ndbm(3C). These files are described in ypfiles(4). The processes are /usr/lib/netsvc/yp/ypserv, the NIS database lookup server, and /usr/lib/netsvc/yp/ypbind, the NIS binder. The programmatic interface to the NIS service is described in ypclnt(3NSL). Administrative tools are described in yppoll(1M), yppush(1M), ypset(1M), ypxfr(1M),
and ypwhich(1). Tools to see the contents of NIS maps are described in ypcat(1), and ypmatch(1). Database generation and maintenance tools are described in ypinit(1M), ypmake(1M), and makedbm(1M).
The ypserv utility is a daemon process typically activated at system startup from /etc/init.d/rpc. Alternatively, you can, as the root user, startNIS services using ypstart(1M) from the command-liner. ypserv runs only on NIS server machines with a complete NIS database. You can halt all NIS
services using the ypstop(1M) command.
The ypxfrd utility transfers entire NIS maps in an efficient manner. For systems that use this daemon, map transfers are 10 to 100 times faster, depending on the map. To use this daemon, be sure ypxfrd is running on the master
server. See /usr/lib/netsvc/yp/ypstart. ypxfr attempts to use ypxfrd first. If that fails, it prints a warning, then uses the older transfer method.
The ypserv daemon's primary function is to look up information in its local database of NIS maps.
The operations performed by ypserv are defined for the implementor by the YP Protocol Specification, and for the programmer by the header file <rpcsvc/yp_prot.h>.
Communication to and from ypserv is by means of RPC calls. Lookup functions are described in ypclnt(3NSL),
and are supplied as C-callable functions in the libnsl(3LIB) library. There are four lookup functions, all of which are performed on a specified map within
some NIS domain: yp_match(3NSL), yp_first(3NSL), yp_next(3NSL), and yp_all(3NSL). The yp_match operation takes a key, and returns the associated value. The yp_first operation returns the first key-value pair from the map, and yp_next can
be used to enumerate the remainder. yp_all ships the entire map to the requester as the response to a single RPC request.
A number of special keys in the DBM files can alter the way in which ypserv operates. The keys of interest are:
-
YP_INTERDOMAIN
- The presence of this key causes ypserv to forward to a DNS server host lookups that cannot be satisfied by the DBM
files.
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YP_SECURE
- This key causes ypserv to answer only questions coming from clients on reserved ports.
-
YP_MULTI_hostname
- This is a special key in the form, YP_MULTI_hostname addr1,...,addrN. A client looking for hostname receives the closest address.
Two other functions supply information about the map, rather than map entries: yp_order(3NSL), and yp_master(3NSL). In fact, both order number and master name exist in the map as key-value pairs, but the server will not return either through the normal lookup functions. If you examine the map with makedbm(1M), however, they are visible. Other functions are used within the NIS service subsystem itself, and are not of general interest to NIS clients. These
functions include do_you_serve_this_domain?, transfer_map, and reinitialize_internal_state.
On start up, ypserv checks for the existence of the NIS to LDAP (N2L) configuration file /var/yp/NISLDAPmapping. If it is present then a master server starts in N2L mode. If the file is not present it starts in "traditional" (non N2L) mode. Slave
servers always start in traditional mode.
In N2L mode, a new set of map files, with an LDAP_ prefix, are generated, based on the contents of the LDAP DIT. The old map files, NIS source files and ypmake(1M)
are not used.
It is possible that ypmake(1M) can be accidentally run in N2L mode. If the occurs, the old style map files are overwritten. That the map files are overwritten is harmless. However, any
resulting yppush(1M) operation will push information based on the DIT rather than the source files. The user may not expect information based on the DIT. ypserv keeps track
of the last modification date of the old style map files. If the map files have been updated, a warning is logged that suggests that the user call yppush directly instead of ypmake.
If a server attempts to run in N2L mode and a LDAP server cannot be contacted, it behaves as follows:
- When ypserv is started, a warning will be logged.
- When a NIS read access is made and the TTL entry has expired, a warning is logged.Information that is returned from the cache has not been updated.
- When a NIS write access is made, a warning is logged. The cache will not be updated, and a NIS failure will be returned.
If ypxfrd is running in N2L mode and is asked to transfer a map, ypxfrd first checks whether the map is out of date. If the map is out of date,ypxfrd initiates an update from the DIT. ypxfrd cannot wait for the update
to complete. If ypxfrd waited, the client end ypxfr operation could time out. To prevent ypxfrd from timing out, the existing map is transferred from the cache. The most up to date map will be transferred on subsequent ypxfrd
operations.
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