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YP(8)			  BSD System Manager's Manual			 YP(8)

     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	stan-
     dard C library for	reading	bootparams.  NIS support is enabled in

     The YP subsystem is started automatically in /etc/rc if it	has been ini-
     tialized 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	ma-
     chine called the NIS master server.  The databases	used to	store the in-
     formation are called NIS maps.  In	FreeBSD, these maps are	stored in
     /var/yp/<domainname> where	<domainname> is	the name of the	NIS domain be-
     ing 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 re-
     ally 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 sys-
     tem'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 do-
     main 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	ad-
     dress 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)	util-
     ity 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 re-
     quests 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 func-
     tions 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 map/do-

     yp_match()	  lookup the data corresponding	to a given in key in a partic-
		  ular map/domain

     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 func-
		  tions	yp_first() and yp_next() can be	used to	do a sequen-
		  tial 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 ad-
     ministrator comments out the line which says "NOPUSH=true"	(NOPUSH	is set
     to	true by	default	because	the default configuration is for a small net-
     work 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); there-
     fore it is	not usually necessary for the administrator to use it di-
     rectly.  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
	 becomes unreachable

     o	 Spreading the client load out over several machines instead of	caus-
	 ing 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	fea-
     ture 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, pass-
     word 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 set-
     ting 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	de-
     fault (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	sup-
     port.  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.

BSD				 April 5, 1993				   BSD


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