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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 by launchd(8) if an NIS domain
     is specified in the /etc/defaultdomain configuration file, and if the
     directory /var/yp exists (which it does in the default distribution).

     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 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
     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 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
     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); there-
     fore 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
         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 provide 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
     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 stub-
     born 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 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.


     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.

BSD                              April 5, 1993                             BSD

Mac OS X 10.7 - Generated Wed Sep 7 20:06:57 CDT 2011
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