Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
YPSERV(8)		  BSD System Manager's Manual		     YPSERV(8)

     ypserv -- NIS database server

     ypserv [-n] [-d] [-p path]

     NIS is an RPC-based service designed to allow a number of UNIX-based ma-
     chines to share a common set of configuration files. Rather than requir-
     ing a system administrator	to update several copies of files such as
     /etc/hosts, /etc/passwd and /etc/group, which tend	to require frequent
     changes in	most environments, NIS allows groups of	computers to share one
     set of data which can be updated from a single location.

     The ypserv	program	is the server that distributes NIS databases to	client
     systems within an NIS domain.  Each client	in an NIS domain must have its
     domainname	set to one of the domains served by ypserv using the
     domainname(1) command. The	clients	must also run ypbind(8)	in order to
     attach to a particular server, since it is	possible to have several
     servers within a single NIS domain.

     The databases distributed by ypserv are stored in /var/yp/[domainname]
     where domainname is the name of the domain	being served. There can	be
     several such directories with different domainnames, and you need only
     one ypserv	daemon to handle them all.

     The databases, or maps as they are	often called, are created by
     /var/yp/Makefile using several system files as source. The	database files
     are in db(3) format to help speed retrieval when there are	many records
     involved.	In FreeBSD, the	maps are always	readable and writable only by
     root for security reasons.	Technically this is only necessary for the
     password maps, but	since the data in the other maps can be	found in other
     world-readable files anyway, it doesn't hurt and it's considered good
     general practice.

     The ypserv	program	is started by /etc/ if it has	been enabled
     in	/etc/rc.conf.

     There are some problems associated	with distributing FreeBSD's password
     database via NIS: FreeBSD normally	only stores encrypted passwords	in
     /etc/master.passwd, which is readable and writable	only by	root. By turn-
     ing this file into	an NIS map, this security feature would	be completely

     To	make up	for this, the FreeBSD version of ypserv	handles	the
     master.passwd.byname and master.basswd.byuid maps in a special way. When
     the server	receives a request to access either of these two maps, it will
     check the TCP port	from which the request originated and return an	error
     if	the port number	is greater than	1023. Since only the superuser is al-
     lowed to bind to TCP ports	with values less than 1024, the	server can use
     this test to determine whether or not the access request came from	a
     privileged	user.  Any requests made by non-privileged users are therefore

     Furthermore, the getpwent(3) routines in FreeBSD's	standard C library
     will only attempt to retrieve data	from the master.passwd.byname and
     master.passwd.byuid maps for the superuser: if a normal user calls	any of
     these functions, the standard passwd.byname and passwd.byuid maps will be
     accessed instead. The latter two maps are constructed by /var/yp/Makefile
     by	parsing	the master.passwd file and stripping out the password fields,
     and are therefore safe to pass on to unprivileged users. In this way, the
     shadow password aspect of the protected master.passwd database is main-
     tained through NIS.

     There are two problems inherent with password shadowing in	NIS that users
     should be aware of:

	   1.	The `TCP port less than	1024' test is trivial to defeat	for
		users with unrestricted	access to machines on your network
		(even those machines which do not run UNIX-based operating

	   2.	If you plan to use a FreeBSD system to serve non-FreeBSD
		clients	that have no support for password shadowing (which is
		most of	them), you will	have to	disable	the password shadowing
		entirely by uncommenting the UNSECURE=True entry in
		/var/yp/Makefile.  This	will cause the standard	passwd.byname
		and passwd.byuid maps to be generated with valid encrypted
		password fields, which is necessary in order for non-FreeBSD
		clients	to perform user	authentication through NIS.

     In	general, any remote user can issue an RPC to ypserv and	retrieve the
     contents of your NIS maps,	provided the remote user knows your domain
     name. To prevent such unauthorized	transactions, ypserv supports a	fea-
     ture called securenets which can be used to restrict access to a given
     set of hosts.  At startup,	ypserv will attempt to load the	securenets in-
     formation from a file called /var/yp/securenets.  (Note that this path
     varies depending on the path specified with the -p	option,	which is ex-
     plained below.) This file contains	entries	that consist of	a network
     specification and a network mask separated	by white space.	 Lines start-
     ing with "#" are considered to be comments. A sample securenets file
     might look	like this:

	   # allow connections from local host -- mandatory
	   # allow connections from any	host
	   # on	the network
	   # allow connections from any	host
	   # between to

     If	ypserv receives	a request from an address that matches one of these
     rules, it will process the	request	normally. If the address fails to
     match a rule, the request will be ignored and a warning message will be
     logged. If	the /var/yp/securenets file does not exist, ypserv will	allow
     connections from any host.

     The ypserv	program	also has support for Wietse Venema's tcpwrapper	pack-
     age, though it is not compiled in by default since	the tcpwrapper package
     is	not distributed	with FreeBSD.  However,	if you have libwrap.a and
     tcpd.h, you can easily recompile ypserv with them.	This allows the	admin-
     istrator to use the tcpwrapper configuration files	( /etc/hosts.allow and
     /etc/hosts.deny) for access control instead of /var/yp/securenets.

     Note: while both of these access control mechanisms provide some secu-
     rity, they, like the privileged port test,	are both vulnerable to "IP
     spoofing" attacks.

   NIS v1 compatibility
     This version of ypserv has	some support for serving NIS v1	clients.
     FreeBSD's NIS implementation only uses the	NIS v2 protocol, however other
     implementations include support for the v1	protocol for backwards compat-
     ibility with older	systems. The ypbind(8) daemons supplied	with these
     systems will try to establish a binding to	an NIS v1 server even though
     they may never actually need it (and they may persist in broadcasting in
     search of one even	after they receive a response from a v2	server). Note
     that while	support	for normal client calls	is provided, this version of
     ypserv does not handle v1 map transfer requests; consequently, it can not
     be	used as	a master or slave in conjunction with older NIS	servers	that
     only support the v1 protocol. Fortunately,	there probably aren't any such
     servers still in use today.

   NIS servers that are	also NIS clients
     Care must be taken	when running ypserv in a multi-server domain where the
     server machines are also NIS clients. It is generally a good idea to
     force the servers to bind to themselves rather than allowing them to
     broadcast bind requests and possibly become bound to each other: strange
     failure modes can result if one server goes down and others are dependent
     upon on it. (Eventually all the clients will time out and attempt to bind
     to	other servers, but the delay involved can be considerable and the
     failure mode is still present since the servers might bind	to each	other
     all over again).

     Refer to the ypbind(8) man	page for details on how	to force it to bind to
     a particular server.

     The following options are supported by ypserv:

     -n	   This	option affects the way ypserv handles yp_match requests	for
	   the hosts.byname and	hosts.byaddress	maps. By default, if ypserv
	   can't find an entry for a given host	in its hosts maps, it will re-
	   turn	an error and perform no	further	processing. With the -n	flag,
	   ypserv will go one step further: rather than	giving up immediately,
	   it will try to resolve the hostname or address using	a DNS name-
	   server query. If the	query is successful, ypserv will construct a
	   fake	database record	and return it to the client, thereby making it
	   seem	as though the client's yp_match	request	succeeded.

	   This	feature	is provided for	compatiblity with SunOS	4.1.x, which
	   has brain-damaged resolver functions	in its standard	C library that
	   depend on NIS for hostname and address resolution.  FreeBSD's re-
	   solver can be configured to do DNS queries directly,	therefore it
	   is not necessary to enable this option when serving only FreeBSD
	   NIS clients.

     -d	   Cause the server to run in debugging	mode. Normally,	ypserv reports
	   only	unusual	errors (access violations, file	access failures) using
	   the syslog(3) facility. In debug mode, the server does not back-
	   ground itself and prints extra status messages to stderr for	each
	   request that	it receives. Also, while running in debug mode,	ypserv
	   will	not spawn any additional subprocesses as it normally does when
	   handling yp_all requests or doing DNS lookups. (These actions often
	   take	a fair amount of time to complete and are therefore handled in
	   subprocesses, allowing the parent server process to go on handling
	   other requests.) This makes it easier to trace the server with a
	   debugging tool.

     -p	path
	   Normally, ypserv assumes that all NIS maps are stored under
	   /var/yp.  The -p flag may be	used to	specify	an alternate NIS root
	   path, allowing the system administrator to move the map files to a
	   different place within the filesystem.

     /var/yp/[domainname]/[maps]       the NIS maps
     /etc/host.conf		       resolver	configuration file
     /var/yp/securenets		       host access control file

     ypcat(1), db(3), yp(4), ypbind(8),	yppasswdd(8), yppush(8), ypxfr(8)

     Bill Paul <>

     This version of ypserv first appeared in FreeBSD 2.2.

BSD			       February	4, 1995				   BSD


Want to link to this manual page? Use this URL:

home | help