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YPSERV(8)               FreeBSD 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
     machines to share a common set of configuration files.  Rather than
     requiring 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 does not hurt and it is consid-
     ered 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
     allowed 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
     information from a file called /var/yp/securenets.  (Note that this path
     varies depending on the path specified with the -p option, which is
     explained 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 are not 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
           cannot find an entry for a given host in its hosts maps, it will
           return an error and perform no further processing.  With the -n
           flag, ypserv will go one step further: rather than giving up imme-
           diately, it will try to resolve the hostname or address using a DNS
           nameserver query.  If the query is successful, ypserv will con-
           struct 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
           resolver 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 fail-
           ures) 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 nor-
           mally 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 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)

     This version of ypserv first appeared in FreeBSD 2.2.

     Bill Paul <>

FreeBSD 6.2                    February 4, 1995                    FreeBSD 6.2


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