freebsd/share/man/man4/yp.4

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.\"     from: @(#)yp.8	1.0 (deraadt) 4/26/93
.\"	$Id$
.\"
.Dd April 5, 1993
.Dt YP 4
.Os BSD 4.2
.Sh NAME
.Nm yp
.Nd description of the YP/NIS system
.Sh SYNOPSIS
.Nm yp
.Sh DESCRIPTION
The
.Nm YP
subsystem allows network management of passwd, group, netgroup, hosts,
services, rpc, bootparams and ethers file
entries through the functions
.Xr getpwent 3 ,
.Xr getgrent 3 ,
.Xr getnetgrent 3 ,
.Xr gethostent 3 ,
.Xr getnetent 3 ,
.Xr getrpcent 3 ,
and
.Xr ethers 3 .
The
.Xr bootparamd 8
daemon makes direct NIS library calls since there are no
functions in the standard C library for reading bootparams. NIS
support for the hosts, services and rpc databases is enabled by
uncommenting the
.Pa nis
line in
.Pa /etc/host.conf.
NIS support for the remaining services is
activated by adding a special '+' entry to the appropriate file.
.Pp
The
.Nm YP 
subsystem is started automatically in
.Pa /etc/rc
if it has been initialized in
.Pa /etc/sysconfig
and if the directory
.Pa /var/yp
exists (which it does in the default distribution). The default
NIS domain must also be set with the
.Xr domainname 1
command, which will happen automatically at system startup if it is
specified in
.Pa /etc/sysconfig.
.Pp
NIS is an RPC-based client/server system that allows a group of
machines within an NIS
.Em 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.
.Pp
The canonical copies of all NIS information are stored on a single machine
called the
.Pa NIS master server .
The databases used to store the information are called
.Pa NIS maps.
In FreeBSD, these maps are stored in
.Pa /var/yp/[domainname]
where
.Pa [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.
.Pp
In FreeBSD, the NIS maps are Berkeley DB hashed database files (the
same format used for the
.Xr 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
.Em .dir
and
.Em .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
.Pa passwd.byname.dir
and
.Pa 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
.Pa passwd.byname .
The difference in format is not significant: only the
NIS server,
.Xr 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.
.Pp
There are three main types of NIS systems:
.Bl -enum -offset indent
.It
.Pa NIS clients ,
which query NIS servers for information.
.It
.Pa NIS master servers ,
which maintain the canonical copies of all NIS maps.
.It
.Pa NIS slave servers ,
which maintain backup copies of NIS maps that are periodically
updated by the master.
.El
.Pp
An NIS client establishes what is called a
.Em binding
to a particular NIS server using the
.Xr ypbind 8
daemon.
.Xr Ypbind 8
checks the system's default domain (as set by the
.Xr domainname 1
command) and begins broadcasting RPC requests on the local network.
These requests specify the name of the domain for which
.Xr 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
.Xr ypbind 8 ,
which will record the server's address. If there are several servers
available (a master and several slaves, for example),
.Xr 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.
.Xr Ypbind 8
will occasionally ``ping'' the server to make sure it's still up
and running. If it fails to receive a reply to one of its pings
within a reasonable amount of time,
.Xr ypbind 8
will mark the domain as unbound and begin broadcasting again in the
hopes of locating another server.
.Pp
NIS master and slave servers handle all NIS requests with the
.Xr ypserv 8
daemon.
.Xr Ypserv 8
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
.Xr ypserv 8
is designed to handle, most of which are implemented as functions
within the standard C library:
.Bl -bullet -offset indent
.It
.Fn yp_order
-- check the creation date of a particular map
.It
.Fn yp_master
-- obtain the name of the NIS master server for a given
map/domain
.It
.Fn yp_match
-- lookup the data corresponding to a given in key in a particular
map/domain
.It
.Fn yp_first
-- obtain the first key/data pair in a particular map/domain
.It
.Fn yp_next
-- pass
.Xr ypserv 8
a key in a particular map/domain and have it return the
key/data pair immediately following it (the functions
.Fn yp_first
and
.Fn yp_next
can be used to do a sequential search of an NIS map)
.It
.Fn yp_all
-- retrieve the entire contents of a map
.El
.Pp
There are a few other requests which
.Xr 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
.Xr ypbind 8
and are not meant to be used by standard utilities.
.Pp
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
.Xr yppush 8
command. The NIS Makefile (
.Pa /var/yp/Makefile )
will do this automatically if the administrator comments out the
line which says
.Em NOPUSH=true
(NOPUSH is set to true by default because the default configuration is
for a small network with only one NIS server). The
.Xr 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
.Xr ypxfr 8 .
(The slave server calls
.Xr ypxfr 8
automatically from within
.Xr ypserv 8 ;
therefore 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:
.Pp
.Bl -bullet -offset indent
.It
Providing backup services in the event that the NIS master crashes
or becomes unreachable
.It
Spreading the client load out over several machines instead of
causing the master to become overloaded
.It
Allowing a single NIS domain to extend beyond
a local network (the
.Xr 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
.Xr ypbind 8
to bind to a particular server with
.Xr ypset 8
but this is sometimes inconvenient. This problem can be avoided simply by
placing a slave server on the local network.)
.El
.Pp
The FreeBSD
.Xr 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
.Bx 4.4 )
includes support for
.Em "shadow passwords" .
The standard password database does not contain users' encrypted
passwords: these are instead stored (along with other information)
is 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 feature would be totally disabled, since any user
is allowed to retrieve NIS data.
.Pp
To help prevent this, FreeBSD's NIS server
handles the shadow password maps (
.Pa master.passwd.byname
and
.Pa 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
.Xr ypserv 8
includes support for Wietse Venema's tcp wrapper package; with tcp
wrapper support enabled, the administrator can configure
.Xr ypserv 8
to respond only to selected client machines. 
.Pp
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 maps.
.Pp
On the client side, FreeBSD's
.Fn getpwent 3
functions will automatically search for the
.Pa master.passwd
maps and use them if they exist. If they do, they will be used, and
all fields in these special maps (class, password age and account
expiration) will be decoded. If they aren't found, the standard
.Pa passwd
maps will be used instead.
.Sh COMPATIBILITY
Some systems, such as SunOS 4.x, need NIS to be running in order
for their hostname resolution functions (
.Fn gethostbyname ,
.Fn gethostbyaddr ,
etc) to work properly. On these systems,
.Xr ypserv 8
performs DNS lookups when asked to return information about
a host that doesn't exist in its
.Pa hosts.byname
or
.Pa hosts.byaddr
maps. FreeBSD's resolver uses DNS by default (it can be made to use
NIS, if desired), therefore its NIS server doesn't do DNS lookups
by default. However,
.Xr 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
.Xr 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.
.Pp
(Please see the
.Xr ypserv 8
manual page for a detailed description of these special features
and flags.)
.Sh BUGS
While FreeBSD now has both NIS client and server capabilities,
it does not yet have support for
.Xr ypupdated 8
or the
.Fn yp_update
function. Both of these require secure RPC, which FreeBSD doesn't
support yet either.
.Pp
The
.Xr getservent 3
and
.Xr getprotoent 3
functions do not yet have NIS support. Fortunately, these files
don't need to be updated that often.
.Pp
Many more manual pages should be written, especially
.Xr ypclnt 3 .
For the time being, seek out a local Sun machine and read the
manuals for there.
.Pp
Neither Sun nor this author have found a clean way to handle
the problems that occur when ypbind cannot find its server
upon bootup.
.Sh HISTORY
The
.Nm 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.