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<br>
<IMG src="../../common/images/Frederic_Raynal.png" alt=
"[Photo of the Author]" height="147" width="126">
<BR>by <A href="mailto:pappy@users.sourceforge.net">Frédéric Raynal</A>
<BR><BR>
<I>About the author:</I><BR>
<A href="mailto:pappy@users.sourceforge.net">Frédéric
Raynal</A> is writing his thesis in informatics at the <A href=
"http://www.inria.fr">INRIA</A>. He likes to read (Tolkien as well
as Balzac) and to listen to music (from Mozart to Philip Glass and
from Led Zeppelin to Massive Attack over Björk and Boris Vian,
but carefully avoiding rap, techno and some other kinds of noise
;-)
<BR><i>Content</i>:
<UL>
<LI><A HREF="#148lfindex0">Structure</A></LI>
<LI><A HREF="#148lfindex1">The maps</A></LI>
<LI><A HREF="http://cgi.linuxfocus.org/cgi-bin/lftalkback?anum=148&lang=en">Talkback form for this article</A></LI>
</UL>
</TD></TR></TABLE>
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<br>
<H2>Yellow Pages, part 1</H2>
<IMG src="../../common/images/illustration148.gif" alt=
"[Illustration]" hspace="10" height="90" width="100">
<!-- ABSTRACT OF THE ARTICLE -->
<P><i>Abstract</i>:
<P>
The <I>Network Information Service</I> (NIS) keeps a database on a
server. Every machine on the network on which an NIS client is
running can query the server to obtain information like (login
name, password, info about user groups, ... ). This allows for a
centralized management of a large number of machines (especially
when it is used together with a distributed network file system
like NFS) since the changes to this information will be passed on
through the server to all its clients.
</P>
<HR size="2" noshade align="right"><BR>
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<H1>Introduction</H1>
<P>The <I>Network Information Service</I> (NIS) was initially
created by Sun and known as <I>Sun Yellow Pages</I> (commonly known
simply as the <I>Yellow Pages</I> or YP). This, however, is a
trademark of British Telecom and consequently can not be used
without the proper permissions. These <I>Yellow Pages</I> refer to the
ones where we look up telephone numbers.</P>
<P>The NIS servers maintain copies of common configuration files on
several networked machines in a database. The NIS clients address
their requests to the servers instead of using their own
configuration files.</P>
<P>Let's pretend to be a user on the network who wants to change
his password. Let's first imagine YP is not installed. This user
will have to logon to all the machines on the network to change his
password. If YP were installed it would be possible for him/her to
change his password on one of the machines where a NIS client is
running, The new password will then be tranferred to the server
where it will be changed in the server database. After this when a user
wants to connect to a networked machine (on which a NIS client is
running of course ;-), the password will be compared to the one
registered in the database of the server.</P>
<P>There are different versions of YP but since this article is an
introduction, we will only look at the principles of how it
works without going into the details. We will come to the details
later on.</P>
glibc 2.x (libc6) supports the use of NSS (Name Switch Service)
which determines the order in which the information has to be
searched (see the file <FONT face=
"Courier New,Courier">/etc/nsswitch.conf</FONT>). It maintains the
aliases, subnets, groups, hosts, netgroups, networks, protocols,
publickey, passwd, rpc, services and shadow maps.
<H1>How does it work</H1>
<A NAME="148lfindex0"> </A>
<H2>Structure</H2>
<P>There will be machine on the network serving as an NIS server
for a domain. This <I>domain</I> corresponds, more or less, to the
name of the database the server will administer. This is the key
NIS clients use to locate the needed information on the NIS
server. This domain name has <B>absolutely nothing</B> to do with
the DNS domain name. There can be more than one NIS server on the
same domain. They can each administer a different domain (on the
NIS level), or they could administer the same domain (in this case
there will be a <I>master</I> server and <I>slave</I> servers).</P>
<P>The slave servers only have a copy of the master servers
database. These servers supplement the master server when it is
taking too long to answer the client's requests or when the master
server goes down.</P>
<P>The slave servers are notified of every change in the database
by the program <FONT face="Courier New,Courier">yppush</FONT> and
they will update there own databases to reflect exactly the state
of the database on the server.</P>
<P>The clients, on their side, don't need any "maintenance" since
they are continually contacting the NIS server to lookup the
information in its database.</P>
<A NAME="148lfindex1"> </A>
<H2>The maps</H2>
<P>The YP database are in the GDBM format, taken from the ASCII
format. They are set up during the installation of the server by
the <FONT face="Courier New,Courier">makedbm</FONT> program.</P>
<P>These <I>maps</I> establish correspondences between a <B>key</B>
and its <B>value</B>. All the YP maps are based on this model. From
the server's point of view, the contents have no meaning (well,
besides some exceptions concerning data about the main server or
dates). This means that, to the server, a map with passwords or
groups etc. is nothing more than a set of key/value pairs.
Only the YP client knows how to search these maps to find the
information it needs.</P>
<P>This representation of the date can be problematic. As the
server cannot "read" the value of a key to find a second key inside
it, it will be necessary to duplicate the data. For instance in the
case of passwords, one might want to be able to look them up by
using the login name or by the <I>user ID</I> or UID (a unique
identifier for each user on the network). This will lead to an
information redundancy, as can be seen by the presence of the <FONT
face="Courier New,Courier">passwd.byname</FONT> and <FONT face=
"Courier New,Courier">passwd.byuid</FONT> files. Consequently there
will be a new map created for every key, meaning that the data has
to be transmitted twice in case of a change.</P>
<P> Three parameters are needed for a client to find the
information it needs from the database :</P>
<OL>
<LI>the domain name : this is the name of the database on the YP
server</LI>
<LI>the map name</LI>
<LI>the key name</LI>
</OL>
So, for a client to find the password of a user named Toto in the
domain <FONT face="Courier New,Courier">titi</FONT>, it will read
the file <FONT face=
"Courier New,Courier">/var/yp/titi/passwd.byname</FONT> on the YP
server looking for the user Toto. <BR>
<BR>
<P>This leads to a very flexible system, since setting up a new
domain is reduced to the creation of the directory <FONT face=
"Courier New,Courier">/var/yp/new_domain</FONT>, copying the <FONT
face="Courier New,Courier">Makefile</FONT> and executing it with
the correct options.</P>
<H1>Remote Procedure Calls (RPC)</H1>
<P>YP's functionality is essentially based on Remote Procedure
Calls (RPCs) accepting requests between the server and its
clients.</P>
<P>The RPC portmapper (portmap) is a program that converts the RPC
program numbers into port numbers. When an RPC is started, it will
tell portmap which port it will use and the RPC program numbers it
is administering. When a client wants to make an RPC request to a
certain program number, it will first contact the portmap server to
obtain the port number on which the program is running. After
obtaining this port number it addresses the RPC packets to the
corresponding port. The client/server model of YPs is nothing more
than a particular case of client/server RPC.</P>
<P>The file <FONT face="Courier New,Courier">yp_prot.h</FONT>
contains the structures and the prototypes of 11 functions defining
the RPC protocol between the clients and the YP server.</P>
<UL>
<LI>YPPROC_DOMAIN and YPPROC_DOMAIN_NOACK allow the clients to
find a server for a given domain.</LI>
<LI>The functions YPPROC_MATCH, YPPROC_FIRST, YPPROC_NEXT and
YPPROC_ALL allow access to the maps data.</LI>
<LI>YPPROC_XFR is called by <FONT face=
"Courier New,Courier">yppush</FONT> to signal the slave servers
that the map on the master server has changed and they need to
update their copies.</LI>
<LI>YPPROC_CLEAR cleans out the cache and the file handles. This
function is called after updating a map, for instance after the
<FONT face="Courier New,Courier">makedbm -c</FONT> command.</LI>
<LI>YPPROC_MASTER, YPPROC_ORDER and YPPROC_MAPLIST allow to
obtain special information about the maps. For instance, to
change the password, the <FONT face=
"Courier New,Courier">yppasswd</FONT> program makes a call to
YPPROC_MASTER to determine the server before changing the
password.</LI>
</UL>
<H1>Conclusion</H1>
Now that we know the general principle, the next article will
address the client side of yellow pages : how it works, how to
configure it, the tools it uses, etc... We will also have a look at
the tools necessary to configure our client correctly, both for the
RPCs and for the YPs.
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<font size=2>Translation information:</font><TABLE>
<tr><td><font size=2>fr</font></td>
<td><font size=2>-></font></td>
<td><font size=2>--</font></td>
<td><font size=2><a href="mailto:pappy@users.sourceforge.net"><FONT COLOR="#FFFFFF">Frédéric Raynal</FONT></a></font></td>
</tr>
<tr><td><font size=2>fr</font></td>
<td><font size=2>-></font></td>
<td><font size=2>en</font></td>
<td><font size=2><a href="mailto:rainbow@linuxfocus.org"><FONT COLOR="#FFFFFF">Jo Simons</FONT></a></font></td>
</tr>
<tr><td><font size=2>en</font></td>
<td><font size=2>-></font></td>
<td><font size=2>en</font></td>
<td><font size=2><a href="mailto:sherm_pbody@yahoo.com"><FONT COLOR="#FFFFFF">Lorne Bailey</FONT></a></font></td>
</tr>
</TABLE></TD>
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<p><font size=1>2001-06-29, generated by lfparser version 2.16</font></p>
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