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LPI Linux Certification/NIS Configuration

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Detailed Objective

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Weight: 1

Description:
Candidates should be able to configure an NIS server. This objective includes configuring a system as an NIS client.

  • Key knowledge area(s):
    • NIS configuration files, terms and utilities
    • Create NIS maps for major configuration files
    • Manipulate nsswitch.conf to configure the ability to search local files, DNS, NIS, etc.
  • The following is a partial list of the used files, terms and utilities:
    • ypbind
    • ypcat
    • ypmatch
    • ypserv
    • yppasswd
    • yppoll
    • yppush
    • ypwhich
    • rpcinfo
    • nsswitch.conf
    • ypserv.conf
    • contents of /var/yp/*
    • netgroup
    • nicknames
    • securenets
    • Makefile

NIS configuration

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Overview

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Description: The candidate should be able to configure an NIS server and create NIS maps for major configuration files. This objective includes configuring a system as a NIS client, setting up an NIS slave server, and configuring ability to search local files, DNS, NIS, etc. in nsswitch.conf.

Key files, terms, and utilities include: 

nisupdate, ypbind, ypcat, ypmatch, ypserv, ypswitch, yppasswd, yppoll, yppush, ypwhich, rpcinfo
nis.conf, nsswitch.conf, ypserv.conf 
/etc/nis/netgroup
/etc/nis/nicknames
/etc/nis/securenets

NIS

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NIS stands for Network Information Service. Its purpose is to provide information, that has to be known throughout the network, to all machines on the network. Information likely to be distributed by NIS is login names/passwords/home directories (/etc/passwd) and group information (/etc/group)

If, for example, your password entry is recorded in the NIS passwd database, you will be able to login on all machines on the network which have the NIS client programs running.

Within a network there must be at least one machine acting as a NIS server. You can have multiple NIS servers, each serving different NIS "domains" - or you can have cooperating NIS servers, where one is the master NIS server, and all the other are so-called slave NIS servers (for a certain NIS "domain", that is!) - or you can have a mix of them...

Slave servers only have copies of the NIS databases and receive these copies from the master NIS server whenever changes are made to the master's databases. Depending on the number of machines in your network and the reliability of your network, you might decide to install one or more slave servers. Whenever a NIS server goes down or is too slow in responding to requests, a NIS client connected to that server will try to find one that is up or faster.

NIS databases are in so-called DBM format, derived from ASCII databases. For example, the files /etc/passwd and /etc/group can be directly converted to DBM format using ASCII-to-DBM translation software ("makedbm", included with the server software). The master NIS server should have both, the ASCII databases and the DBM databases.

Slave servers will be notified of any change to the NIS maps, (via the "yppush" program), and automatically retrieve the necessary changes in order to synchronize their databases. NIS clients do not need to do this since they always talk to the NIS server to read the information stored in it's DBM databases.

To run any of the software mentioned below you will need to run the program /usr/sbin/portmap. The RPC portmapper (portmap(8)) is a server that converts RPC program numbers into TCP/IP (or UDP/IP) protocol port numbers. It must be running in order to make RPC calls (which is what the NIS/NIS+ client software does) to RPC servers (like a NIS or NIS+ server) on that machine. When an RPC server is started, it will tell portmap what port number it is listening to, and what RPC program numbers it is prepared to serve. When a client wishes to make an RPC call to a given program number, it will first contact portmap on the server machine to determine the port number where RPC packets should be sent.

Since RPC servers could be started by inetd(8), portmap should be running before inetd is started. For secure RPC, the portmapper needs the Time service. Make sure, that the Time service is enabled in /etc/inetd.conf on all hosts: 

# Time service is used for clock synchronization.
#
time    stream  tcp     nowait  root    internal
time    dgram   udp     wait    root    internal

IMPORTANT: Don't forget to restart inetd after changes on its configuration file !

What do you need to set up NIS?

Determine whether you are a Server, Slave or Client : Your machine is going to be part of a network with existing NIS servers You do not have any NIS servers in the network yet

In the first case, you only need the client programs (ypbind, ypwhich, ypcat, yppoll, ypmatch). The most important program is ypbind. This program must be running at all times, which means, it should always appear in the list of processes. It is a daemon process and needs to be started from the system's startup file (eg. /etc/init.d/nis, /sbin/init.d/ypclient, /etc/rc.d/init.d/ypbind, /etc/rc.local). As soon as ypbind is running your system has become a NIS client.

In the second case, if you don't have NIS servers, then you will also need a NIS server program (usually called ypserv). Section 9 describes how to set up a NIS server on your Linux machine using the "ypserv" daemon.

Setting Up the NIS Client

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The ypbind daemon

Newer ypbind versions have a configuration file called /etc/yp.conf. You can hardcode a NIS server there - for more info see the manual page for ypbind(8). You also need this file for NYS. An example: 

 ypserver 10.10.0.1
 ypserver 10.0.100.8
 ypserver 10.3.1.1

If the system can resolve the hostnames without NIS, you may use the name, otherwise you have to use the IP address. ypbind 3.3 has a bug and will only use the last entry (ypserver 10.3.1.1 in the example). All other entries are ignored. ypbind-mt handle this correct and uses that one, which answered at first.

It might be a good idea to test ypbind before incorporating it in the startup files. To test ypbind do the following:

Make sure you have your YP-domain name set. If it is not set then issue the command: /bin/domainname nis.domain

where nis.domain should be some string _NOT_ normally associated with the DNS-domain name of your machine! The reason for this is that it makes it a little harder for external crackers to retrieve the password database from your NIS servers. If you don't know what the NIS domain name is on your network, ask your system/network administrator.

Start up "/usr/sbin/portmap" if it is not already running. Create the directory "/var/yp" if it does not exist. Start up "/usr/sbin/ypbind"

Use the command "rpcinfo -p localhost" to check if ypbind was able to register its service with the portmapper. The output should look like: 

      program vers proto   port
       100000    2   tcp    111  portmapper
       100000    2   udp    111  portmapper
       100007    2   udp    637  ypbind
       100007    2   tcp    639  ypbind

Or like this (depending on the version of ypbind you are using) : 

      program vers proto   port
       100000    2   tcp    111  portmapper
       100000    2   udp    111  portmapper
       100007    2   udp    758  ypbind
       100007    1   udp    758  ypbind
       100007    2   tcp    761  ypbind
       100007    1   tcp    761  ypbind

You may also run "rpcinfo -u localhost ypbind". This command should produce something like: 

       program 100007 version 1 ready and waiting
       program 100007 version 2 ready and waiting

The output depends on the ypbind version you have installed. Important is only the "version 2" message. At this point you should be able to use NIS client programs like ypcat, etc... For example, "ypcat passwd.byname" will give you the entire NIS password database.

IMPORTANT: If you skipped the test procedure then make sure you have set the domain name, and created the directory /var/yp. This directory MUST exist for ypbind to start up successfully. To check if the domainname is set correct, use the /bin/ypdomainname from yp-tools 2.2. It uses the yp_get_default_domain() function which is more restrict. It doesn't allow for example the "(none)" domainname, which is the default under Linux and makes a lot of problems.

If the test worked you may now want to change your startupd files so that ypbind will be started at boot time and your system will act as a NIS client. Make sure that the domainname will be set before you start ypbind. Well, that's it. Reboot the machine and watch the boot messages to see if ypbind is actually started. For host lookups you must set (or add) "nis" to the lookup order line in your /etc/host.conf file. Please read the manpage "resolv+.8" for more details. Add the following line to /etc/passwd on your NIS clients: 

+::::::

You can also use the + and - characters to include/exclude or change users. If you want to exclude the user guest just add -guest to your /etc/passwd file. You want to use a different shell (e.g. ksh) for the user "linux"? No problem, just add "+linux::::::/bin/ksh" (without the quotes) to your /etc/passwd. Fields that you don't want to change have to be left empty. You could also use Netgroups for user control.

For example, to allow login-access only to miquels, dth and ed, and all members of the sysadmin netgroup, but to have the account data of all other users available use: 

     +miquels:::::::
     +ed:::::::
     +dth:::::::
     +@sysadmins:::::::
     -ftp
     +:*::::::/etc/NoShell

Note that in Linux you can also override the password field, as we did in this example. We also remove the login "ftp", so it isn't known any longer, and anonymous ftp will not work. The netgroup would look like :

sysadmins (-,software,) (-,kukuk,)

The nsswitch.conf File

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The Network Services switch file /etc/nsswitch.conf determines the order of lookups performed when a certain piece of information is requested, just like the /etc/host.conf file which determines the way host lookups are performed. For example, the line : 

   hosts: files nis dns

specifies that host lookup functions should first look in the local /etc/hosts file, followed by a NIS lookup and finally through the domain name service (/etc/resolv.conf and named), at which point if no match is found an error is returned. This file must be readable for every user! You can find more information in the man-page nsswitch.5 or nsswitch.conf.5.

A good /etc/nsswitch.conf file for NIS is: 

# /etc/nsswitch.conf
passwd:     compat
group:      compat

# For libc5, you must use shadow: files nis
shadow:     compat
passwd_compat: nis
group_compat: nis
shadow_compat: nis
hosts:      nis files dns
services:   nis [NOTFOUND=return] files
networks:   nis [NOTFOUND=return] files
protocols:  nis [NOTFOUND=return] files
rpc:        nis [NOTFOUND=return] files
ethers:     nis [NOTFOUND=return] files
netmasks:   nis [NOTFOUND=return] files
netgroup:   nis
bootparams: nis [NOTFOUND=return] files
publickey:  nis [NOTFOUND=return] files
automount:  files
aliases:    nis [NOTFOUND=return] files

Setting up a NIS Server

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The Server Program ypserv

If you run your server as master, determine what files you require to be available via NIS and then add or remove the appropriate entries to the "all" rule in /var/yp/Makefile. You always should look at the Makefile and edit the Options at the beginning of the file.

There was one big change between ypserv 1.1 and ypserv 1.2. Since version 1.2, the file handles are cached. This means you have to call makedbm always with the -c option if you create new maps. Make sure, you are using the new /var/yp/Makefile from ypserv 1.2 or later, or add the -c flag to makedbm in the Makefile. If you don't do that, ypserv will continue to use the old maps, and not the updated one.

Now edit /var/yp/securenets and /etc/ypserv.conf. For more information, read the ypserv(8) and ypserv.conf(5) manual pages.

Make sure the portmapper (portmap(8)) is running, and start the server ypserv. The command « rpcinfo -u localhost ypserv » should output something like : 

   program 100004 version 1 ready and waiting
   program 100004 version 2 ready and waiting

The "version 1" line could be missing, depending on the ypserv version and configuration you are using. It is only necessary if you have old SunOS 4.x clients.

Now generate the NIS (YP) database. On the master, run : 

   % /usr/lib/yp/ypinit -m

On a slave make sure that ypwhich -m works. This means, that your slave must be configured as NIS client before you could run « /usr/lib/yp/ypinit -s masterhost » to install the host as NIS slave. That's it, your server is up and running.

If you have bigger problems, you could start ypserv and ypbind in debug mode on different xterms. The debug output should show you what goes wrong.

If you need to update a map, run make in the /var/yp directory on the NIS master. This will update a map if the source file is newer, and push the files to the slave servers. Please don't use ypinit for updating a map. You might want to edit root's crontab *on the slave* server and add the following lines: 

     20 *    * * *    /usr/lib/yp/ypxfr_1perhour
     40 6    * * *    /usr/lib/yp/ypxfr_1perday
     55 6,18 * * *    /usr/lib/yp/ypxfr_2perday

This will ensure that most NIS maps are kept up-to-date, even if an update is missed because the slave was down at the time the update was done on the master.

You can add a slave at every time later. At first, make sure that the new slave server has permissions to contact the NIS master. Then run : 

   % /usr/lib/yp/ypinit -s masterhost

on the new slave. On the master server, add the new slave server name to /var/yp/ypservers and run make in /var/yp to update the map.

The Program rpc.ypxfrd

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rpc.ypxfrd is used for speed up the transfer of very large NIS maps from a NIS master to NIS slave servers. If a NIS slave server receives a message that there is a new map, it will start ypxfr for transfering the new map. ypxfr will read the contents of a map from the master server using the yp_all() function. This process can take several minutes when there are very large maps which have to store by the database library.

The rpc.ypxfrd server speeds up the transfer process by allowing NIS slave servers to simply copy the master server's map files rather than building their own from scratch. rpc.ypxfrd uses an RPC-based file transfer protocol, so that there is no need for building a new map.

rpc.ypxfrd can be started by inetd. But since it starts very slow, it should be started with ypserv. You need to start rpc.ypxfrd only on the NIS master server.

The Program rpc.yppasswdd

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Whenever users change their passwords, the NIS password database and probably other NIS databases, which depend on the NIS password database, should be updated. The program "rpc.yppasswdd" is a server that handles password changes and makes sure that the NIS information will be updated accordingly. rpc.yppasswdd is now integrated in ypserv. You don't need the older, separate yppasswd-0.9.tar.gz or yppasswd-0.10.tar.gz, and you shouldn't use them any longer. The rpc.yppasswdd in ypserv 1.3.2 has full shadow support. yppasswd is now part of yp-tools-2.2.tar.gz.

You need to start rpc.yppasswdd only on the NIS master server. By default, users are not allowed to change their full name or the login shell. You can allow this with the -e chfn or -e chsh option. If your passwd and shadow files are not in another directory then /etc, you need to add the -D option. For example, if you have put all source files in /etc/yp and wish to allow the user to change his shell, you need to start rpc.yppasswdd with the following parameters: 

  rpc.yppasswdd -D /etc/yp -e chsh

or 

  rpc.yppasswdd -s /etc/yp/shadow -p /etc/yp/passwd -e chsh

There is nothing more to do. You just need to make sure, that rpc.yppasswdd uses the same files as /var/yp/Makefile. Errors will be logged using syslog.

If everything is fine (as it should be), you should be able to verify your installation with a few simple commands. Assuming, for example, your passwd file is being supplied by NIS, the command : 

   % ypcat passwd

should give you the contents of your NIS passwd file. The command : 

   % ypmatch userid passwd

(where userid is the login name of an arbitrary user) should give you the user's entry in the NIS passwd file. The "ypcat" and "ypmatch" programs should be included with your distribution of traditional NIS or NYS. Once you have NIS correctly configured on the server and client, you do need to be sure that the configuration will survive a reboot. On RedHat, create or modify the variable NISDOMAIN in the file /etc/sysconfig/network.

Exercises

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