yp -- description of the YP/NIS system
The YP subsystem allows network management of passwd, group, netgroup,
hosts, services, rpc, bootparams and ethers file entries through the
functions getpwent(3), getgrent(3), getnetgrent(3), gethostent(3),
getnetent(3), getrpcent(3), and ethers(3). The bootparamd(8) daemon
makes direct NIS library calls since there are no functions in the standard
C library for reading bootparams. NIS support is enabled in
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 FreeBSD, these maps are stored in
/var/yp/<domainname> where <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 FreeBSD, 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), FreeBSD 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:
1. NIS clients, which query NIS servers for information.
2. NIS master servers, which maintain the canonical copies of all NIS
3. NIS slave servers, which maintain backup copies of NIS maps that are
periodically updated by the master.
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:
yp_order() check the creation date of a particular map
yp_master() obtain the name of the NIS master server for a given
yp_match() lookup the data corresponding to a given in key in a particular
yp_first() obtain the first key/data pair in a particular map/domain
yp_next() pass ypserv(8) a key in a particular map/domain and have it
return the key/data pair immediately following it (the functions
yp_first() and yp_next() can be used to do a sequential
search of an NIS map)
yp_all() retrieve the entire contents of a map
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
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:
+o Providing backup services in the event that the NIS master crashes or
+o Spreading the client load out over several machines instead of causing
the master to become overloaded
+o Allowing a single NIS domain to extend beyond a local network (the
ypbind(8) daemon might not be able to locate a server automatically
if it resides on a network outside the reach of its broadcasts. It
is possible to force ypbind(8) to bind to a particular server with
ypset(8) but this is sometimes inconvenient. This problem can be
avoided simply by placing a slave server on the local network.)
The FreeBSD ypserv(8) is specially designed to provided enhanced security
(compared to other NIS implementations) when used exclusively with
FreeBSD client systems. The FreeBSD password database system (which is
derived directly from 4.4BSD) 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
To help prevent this, FreeBSD'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, FreeBSD's ypserv(8) includes support for Wietse Venema'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
On the client side, FreeBSD'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.
When using a non-FreeBSD NIS server for passwd(5) files, it is unlikely
that the default MD5-based format that FreeBSD uses for passwords will be
accepted by it. If this is the case, the value of the passwd_format setting
in login.conf(5) should be changed to "des" for compatibility.
Some systems, such as SunOS 4.x, need NIS to be running in order for
their hostname resolution functions (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. FreeBSD'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 (FreeBSD uses only NIS
v2, but many other systems, including SunOS 4.x, search for both a v1 and
v2 server when binding). FreeBSD'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.)
While FreeBSD now has both NIS client and server capabilities, it does
not yet have support for ypupdated(8) or the yp_update() function. Both
of these require secure RPC, which FreeBSD does not support yet either.
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.
The YP subsystem was written from the ground up by Theo de Raadt to be
compatible to Sun's implementation. Bug fixes, improvements and NIS
server support were later added by Bill Paul. The server-side code was
originally written by Peter Eriksson and Tobias Reber and is subject to
the GNU Public License. No Sun code was referenced.
FreeBSD 5.2.1 April 5, 1993 FreeBSD 5.2.1 [ Back ]