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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	utility	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	utility	is started by /etc/rc.d/ypserv if it has been enabled
     in	/etc/rc.conf.

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

     To	make up	for this, the FreeBSD version of ypserv	handles	the
     master.passwd.byname and master.passwd.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 the FreeBSD 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 maintained through NIS.

   Setting Up Master and Slave Servers
     ypinit(8) is a convenient script that will	help setup master and slave
     NIS servers.

     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	utility	also has support for Wietse Venema's tcpwrapper	pack-
     age.  This	allows the administrator 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.  The
     FreeBSD 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 cannot
     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	compatibility 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.  The FreeBSD 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	re-
	   ports only unusual errors (access violations, file access failures)
	   using the syslog(3) facility.  In debug mode, the server does not
	   background 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	there-
	   fore	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 file system.

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

     ypcat(1), db(3), hosts_access(5), rpc.yppasswdd(8), yp(8),	ypbind(8),
     ypinit(8),	yppush(8), ypxfr(8)

     Bill Paul <>

     This version of ypserv first appeared in FreeBSD 2.2.

BSD			       February	4, 1995				   BSD


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