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yp - description of the YP/NIS system
The YP subsystem is started automatically in /etc/rc if it has been initialized in /etc/rc.conf and if the directory /var/yp exists (which it does in the default distribution). The default NIS domain must also be set with the domainname(1) command, which will happen automatically at system startup if it is specified in /etc/rc.conf
NIS is an RPC -based client/server system that allows a group of machines within an NIS domain to share a common set of configuration files. This permits a system administrator to set up NIS client systems with only minimal configuration data and add, remove or modify configuration data from a single location.
The canonical copies of all NIS information are stored on a single machine called the NIS master server The databases used to store the information are called NIS maps In Fx , these maps are stored in /var/yp/ Aq domainname where Aq Ar domainname is the name of the NIS domain being served. A single NIS server can support several domains at once, therefore it is possible to have several such directories, one for each supported domain. Each domain will have its own independent set of maps.
In Fx , the NIS maps are Berkeley DB hashed database files (the same format used for the passwd(5) database files). Other operating systems that support NIS use old-style ndbm databases instead (largely because Sun Microsystems originally based their NIS implementation on ndbm and other vendors have simply licensed Sun's code rather than design their own implementation with a different database format). On these systems, the databases are generally split into .dir and .pag files which the ndbm code uses to hold separate parts of the hash database. The Berkeley DB hash method instead uses a single file for both pieces of information. This means that while you may have passwd.byname.dir and passwd.byname.pag files on other operating systems (both of which are really parts of the same map), Fx will have only one file called passwd.byname The difference in format is not significant: only the NIS server, ypserv(8), and related tools need to know the database format of the NIS maps. Client NIS systems receive all NIS data in ASCII form.
There are three main types of NIS systems:
A NIS client establishes what is called a binding to a particular NIS server using the ypbind(8) daemon. The ypbind(8) utility checks the system's default domain (as set by the domainname(1) command) and begins broadcasting RPC requests on the local network. These requests specify the name of the domain for which ypbind(8) is attempting to establish a binding. If a server that has been configured to serve the requested domain receives one of the broadcasts, it will respond to ypbind(8), which will record the server's address. If there are several servers available (a master and several slaves, for example), ypbind(8) will use the address of the first one to respond. From that point on, the client system will direct all of its NIS requests to that server. The ypbind(8) utility will occasionally ``ping'' the server to make sure it is still up and running. If it fails to receive a reply to one of its pings within a reasonable amount of time, ypbind(8) will mark the domain as unbound and begin broadcasting again in the hopes of locating another server.
NIS master and slave servers handle all NIS requests with the ypserv(8) daemon. The ypserv(8) utility is responsible for receiving incoming requests from NIS clients, translating the requested domain and map name to a path to the corresponding database file and transmitting data from the database back to the client. There is a specific set of requests that ypserv(8) is designed to handle, most of which are implemented as functions within the standard C library:
There are a few other requests which ypserv(8) is capable of handling (i.e., acknowledge whether or not you can handle a particular domain (YPPROC_DOMAIN ) or acknowledge only if you can handle the domain and be silent otherwise (YPPROC_DOMAIN_NONACK ) but these requests are usually generated only by ypbind(8) and are not meant to be used by standard utilities.
On networks with a large number of hosts, it is often a good idea to use a master server and several slaves rather than just a single master server. A slave server provides the exact same information as a master server: whenever the maps on the master server are updated, the new data should be propagated to the slave systems using the yppush(8) command. The NIS Makefile (/var/yp/Makefile ) will do this automatically if the administrator comments out the line which says ``NOPUSH=true '' ( NOPUSH is set to true by default because the default configuration is for a small network with only one NIS server). The yppush(8) command will initiate a transaction between the master and slave during which the slave will transfer the specified maps from the master server using ypxfr(8). (The slave server calls ypxfr(8) automatically from within ypserv(8); therefore it is not usually necessary for the administrator to use it directly. It can be run manually if desired, however.) Maintaining slave servers helps improve NIS performance on large networks by:
The Fx ypserv(8) is specially designed to provide enhanced security (compared to other NIS implementations) when used exclusively with Fx client systems. The Fx password database system (which is derived directly from BSD 4.4 includes support for shadow passwords The standard password database does not contain users' encrypted passwords: these are instead stored (along with other information) in a separate database which is accessible only by the super-user. If the encrypted password database were made available as an NIS map, this security feature would be totally disabled, since any user is allowed to retrieve NIS data.
To help prevent this, Fx Ns 's NIS server handles the shadow password maps ( master.passwd.byname and master.passwd.byuid in a special way: the server will only provide access to these maps in response to requests that originate on privileged ports. Since only the super-user is allowed to bind to a privileged port, the server assumes that all such requests come from privileged users. All other requests are denied: requests from non-privileged ports will receive only an error code from the server. Additionally, Fx Ns 's ypserv(8) includes support for An Wietse Venema Ns 's tcp wrapper package; with tcp wrapper support enabled, the administrator can configure ypserv(8) to respond only to selected client machines.
While these enhancements provide better security than stock NIS they are by no means 100% effective. It is still possible for someone with access to your network to spoof the server into disclosing the shadow password maps.
On the client side, Fx Ns 's getpwent(3) functions will automatically search for the master.passwd maps and use them if they exist. If they do, they will be used, and all fields in these special maps (class, password age and account expiration) will be decoded. If they are not found, the standard passwd maps will be used instead.
Some systems, such as
SunOS
4.x, need
NIS
to be running in order
for their hostname resolution functions
Fn ( gethostbyname ,
gethostbyaddr (,);
etc.) to work properly.
On these systems,
ypserv(8)
performs
DNS
lookups when asked to return information about
a host that does not exist in its
hosts.byname
or
hosts.byaddr
maps.
Fx Ns 's
resolver uses
DNS
by default (it can be made to use
NIS
if desired), therefore its
NIS
server does not do
DNS
lookups
by default.
However,
ypserv(8)
can be made to perform
DNS
lookups if it is started with a special
flag.
It can also be made to register itself as an
NIS
v1 server
in order to placate certain systems that insist on the presence of
a v1 server
( Fx uses only
NIS
v2, but many other systems,
including
SunOS
4.x, search for both a v1 and v2 server when binding).
Fx Ns 's
ypserv(8)
does not actually handle
NIS
v1 requests, but this
``kludge mode''
is useful for silencing stubborn systems that search for both
a v1 and v2 server.
(Please see the ypserv(8) manual page for a detailed description of these special features and flags.)
The getservent(3) and getprotoent(3) functions do not yet have NIS support. Fortunately, these files do not need to be updated that often.
Many more manual pages should be written, especially ypclnt(3). For the time being, seek out a local Sun machine and read the manuals for there.
Neither Sun nor this author have found a clean way to handle the problems that occur when ypbind cannot find its server upon bootup.
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