LPI Linux Certification/LPIC2 Exam 202/Network Client Management

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Section Overview[edit]

  • DHCP configuration
  • NIS configuration
  • LDAP configuration
  • PAM authentication
  • DHCP configuration

DHCP configuration[edit]


Description: The candidate should be able to configure a DHCP server and set default options, create a subnet, and create a dynamically-allocated range. This objective includes adding a static host, setting options for a single host, and adding bootp hosts. Also included is to configure a DHCP relay agent, and reload the DHCP server after making changes.

Key files, terms, and utilities include:




Most people reading this will already know the DHCP protocol. Just as a quick reminder. DHCP stands for Dynamic Host Configuration Protocol and is commonly used to distribute specific network settings in networks. Settings such as the default gateway, nameservers, IP addresses and much more.

As for a small illustration of the protocol itself.

<insert schematic about DHCP Requests here>

Configuring dhcpd[edit]

After the installation of dhcpd the main configuration file can be found at /etc/dhcpd.conf. For Debian installations, one should edit /etc/default/dhcp as soon as the installation is finished and change the following line according to your setup.

INTERFACES="eth1" # or "eth1 eth2", whatever interfaces you wish to serve ip's.

The dhcpd.conf file is divided in global parameters and subnet specific parameters. Each subnet can override the global parameters. The most commonly used parameters are the following.

option domain-name "example.com"; 
option domain-name-servers ","
option subnet-mask; # global Subnet mask
default-lease-time 600; # Seconds each DHCP lease is granted and after which a request for the same ip is launched.
max-lease-time 7200; # If DHCP server does not respond, keep IP till 7200 seconds are passed.

subnet netmask { # Subnet for first 13 devices, 10 of which are servers, 3 printers
 range;  # Range of IP's for our printers
 option subnet-mask;
 option broadcast-address; # This is the subnets broadcast address
 option routers; # The gateway of this subnet
 option time-servers; # Gateway is running a timeserver
 option ntp-servers; # Gateway running a timeserver
subnet netmask { # Subnet for 29 computers
 option subnet-mask;
 option broadcast-address;
 option routers;
group {
 host server1 { # the first fixed server for subnet
  server-name server1;
  hardware ethernet 0f:45:d3:23:11:90;
 host server2 {
  server-name server2;
  hardware ethernet 0f:45:d3:23:11:91;

This example is just providing a hint about possible options and overrides.

More info can be found on dhcpd.conf and dhcp-options in man pages. Look in those pages too for information about using the DHCP server to serve BOOTP as well, useful for diskless clients..

NIS configuration[edit]


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 


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[edit]

The ypbind daemon

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


If the system cam resolv 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 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:


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[edit]

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[edit]

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[edit]

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[edit]

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


  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 ===.

LDAP configuration[edit]


Key terms, files and utilities : Slapd slapd.conf


== PAM authentication == test

PAM (Pluggable Authentication Modules) is a flexible mechanism for authenticating users. Since the beginnings of UNIX, authenticating a user has been accomplished via the user entering a password and the system checking if the entered password corresponds to the encrypted official password that is stored in /etc/passwd . The idea being that the user *is* really that user if and only if they can correctly enter their secret password. That was in the beginning. Since then, a number of new ways of authenticating users have become popular. Including more complicated replacements for the /etc/passwd file, and hardware devices Smart cards etc.. The problem is that each time a new authentication scheme is developed, it requires all the necessary programs (login, ftpd etc...) to be rewritten to support it. PAM provides a way to develop programs that are independent of authentication scheme. These programs need "authentication modules" to be attatched to them at run-time in order to work. Which authentication module is to be attatched is dependent upon the local system setup and is at the discretion of the local system administrator. PAM authentication Linux-PAM (Pluggable Authentication Modules for Linux) is a suite of shared libraries that enable the local system administrator to choose how applications authenticate users. In other words, without (rewriting and) recompiling a PAM-aware application, it is possible to switch between the authentication mechanism(s) it uses. Indeed, one may entirely upgrade the local authentication system without touching the applications themselves. Historically an application that has required a given user to be authenticated, has had to be compiled to use a specific authentication mechanism. For example, in the case of traditional UN*X systems, the identity of the user is verified by the user entering a correct password. This password, after being prefixed by a two character ``salt, is encrypted (with crypt(3)). The user is then authenticated if this encrypted password is identical to the second field of the user's entry in the system password database (the /etc/passwd file). On such systems, most if not all forms of privileges are granted based on this single authentication scheme. Privilege comes in the form of a personal user-identifier (uid) and membership of various groups. Services and applications are available based on the personal and group identity of the user. Traditionally, group membership has been assigned based on entries in the /etc/group file. PAM authentication Unfortunately, increases in the speed of computers and the widespread introduction of network based computing, have made once secure authentication mechanisms, such as this, vulnerable to attack. In the light of such realities, new methods of authentication are continuously being developed. It is the purpose of the Linux-PAM project to separate the development of privilege granting software from the development of secure and appropriate authentication schemes. This is accomplished by providing a library of functions that an application may use to request that a user be authenticated. This PAM library is configured locally with a system file, /etc/pam.conf (or a series of configuration files located in /etc/pam.d/) to authenticate a user request via the locally available authentication modules. The modules themselves will usually be located in the directory /lib/security and take the form of dynamically loadable object files (see dlopen(3)). PAM authentication Overview For the uninitiated, we begin by considering an example. We take an application that grants some service to users; login is one such program. Login does two things, it first establishes that the requesting user is whom they claim to be and second provides them with the requested service: in the case of login the service is a command shell (bash, tcsh, zsh, etc.) running with the identity of the user. Traditionally, the former step is achieved by the login application prompting the user for a password and then verifying that it agrees with that located on the system; hence verifying that as far as the system is concerned the user is who they claim to be. This is the task that is delegated to Linux-PAM. From the perspective of the application programmer (in this case the person that wrote the login application), Linux-PAM takes care of this authentication task -- verifying the identity of the user. PAM authentication The flexibility of Linux-PAM is that you, the system administrator, have the freedom to stipulate which authentication scheme is to be used. You have the freedom to set the scheme for any/all PAM-aware applications on your Linux system. That is, you can authenticate from anything as naive as simple trust (pam_permit) to something as paranoid as a combination of a retinal scan, a voice print and a one-time password! To illustrate the flexibility you face, consider the following situation: a system administrator (parent) wishes to improve the mathematical ability of her users (children). She can configure their favorite ``Shoot 'em up game (PAM-aware of course) to authenticate them with a request for the product of a couple of random numbers less than 12. It is clear that if the game is any good they will soon learn their multiplication tables. As they mature, the authentication can be upgraded to include (long) division! PAM authentication Linux-PAM deals with four separate types of (management) task. These are: authentication management; account management; session management; and password management. The association of the preferred management scheme with the behavior of an application is made with entries in the relevant Linux-PAM configuration file. The management functions are performed by modules specified in the configuration file. The Linux-PAM library consults the contents of the PAM configuration file and loads the modules that are appropriate for an application. These modules fall into one of four management groups and are stacked in the order they appear in the configuration file. These modules, when called by Linux-PAM, perform the various authentication tasks for the application. Textual information, required from/or offered to the user, can be exchanged through the use of the application-supplied conversation function. PAM authentication Linux-PAM is designed to provide the system administrator with a great deal of flexibility in configuring the privilege granting applications of their system. The local configuration of those aspects of system security controlled by Linux-PAM is contained in one of two places: either the single system file, /etc/pam.conf; or the /etc/pam.d/ directory. Linux-PAM specific tokens in this file are case insensitive. The module paths, however, are case sensitive since they indicate a file's name and reflect the case dependence of typical Linux file-systems. The case-sensitivity of the arguments to any given module is defined for each module in turn. In addition to the lines described below, there are two special characters provided for the convenience of the system administrator: comments are preceded by a `#' and extend to the next end-of-line; also, module specification lines may be extended with a `\' escaped newline. A general configuration line of the /etc/pam.conf file has the following form: : service-name module-type control-flag module-path args PAM authentication Below, we explain the meaning of each of these tokens. The second (and more recently adopted) way of configuring Linux-PAM is via the contents of the /etc/pam.d/ directory. Once we have explained the meaning of the above tokens, we will describe this method.

Service-name The name of the service associated with this entry. Frequently the service name is the conventional name of the given application. For example, `ftpd', `rlogind' and `su', etc. . There is a special service-name, reserved for defining a default authentication mechanism. It has the name `OTHER' and may be specified in either lower or upper case characters. Note, when there is a module specified for a named service, the `OTHER' entries are ignored.

PAM authentication Module-type One of (currently) four types of module. The four types are as follows:

auth; this module type provides two aspects of authenticating the user. Firstly, it establishes that the user is who they claim to be, by instructing the application to prompt the user for a password or other means of identification. Secondly, the module can grant group membership (independently of the /etc/groups file discussed above) or other privileges through its credential granting properties.

account; this module performs non-authentication based account management. It is typically used to restrict/permit access to a service based on the time of day, currently available system resources (maximum number of users) or perhaps the location of the applicant user---`root' login only on the console.

session; primarily, this module is associated with doing things that need to be done for the user before/after they can be given service. Such things include the logging of information concerning the opening/closing of some data exchange with a user, mounting directories, etc. .

password; this last module type is required for updating the authentication token associated with the user. Typically, there is one module for each `challenge/response' based authentication (auth) module-type. PAM authentication Control-flag The control-flag is used to indicate how the PAM library will react to the success or failure of the module it is associated with. Since modules can be stacked (modules of the same type execute in series, one after another), the control-flags determine the relative importance of each module. The application is not made aware of the individual success or failure of modules listed in the `/etc/pam.conf' file. Instead, it receives a summary success or fail response from the Linux-PAM library. The order of execution of these modules is that of the entries in the /etc/pam.conf file; earlier entries are executed before later ones. As of Linux-PAM v0.60, this control-flag can be defined with one of two syntaxes. The simpler (and historical) syntax for the control-flag is a single keyword defined to indicate the severity of concern associated with the success or failure of a specific module. There are four such keywords: required, requisite, sufficient and optional. PAM authentication Control-flag The Linux-PAM library interprets these keywords in the following manner: required; this indicates that the success of the module is required for the module-type facility to succeed. Failure of this module will not be apparent to the user until all of the remaining modules (of the same module-type) have been executed.

requisite; like required, however, in the case that such a module returns a failure, control is directly returned to the application. The return value is that associated with the first required or requisite module to fail. Note, this flag can be used to protect against the possibility of a user getting the opportunity to enter a password over an unsafe medium. It is conceivable that such behavior might inform an attacker of valid accounts on a system. This possibility should be weighed against the not insignificant concerns of exposing a sensitive password in a hostile environment.

sufficient; the success of this module is deemed `sufficient' to satisfy the Linux-PAM library that this module-type has succeeded in its purpose. In the event that no previous required module has failed, no more `stacked' modules of this type are invoked. (Note, in this case subsequent required modules are not invoked.). A failure of this module is not deemed as fatal to satisfying the application that this module-type has succeeded.

Optional; as its name suggests, this control-flag marks the module as not being critical to the success or failure of the user's application for service. In general, Linux-PAM ignores such a module when determining if the module stack will succeed or fail. However, in the absence of any definite successes or failures of previous or subsequent stacked modules this module will determine the nature of the response to the application. One example of this latter case, is when the other modules return something like PAM_IGNORE. PAM authentication Control-flag : The more elaborate (newer) syntax is much more specific and gives the administrator a great deal of control over how the user is authenticated. This form of the control flag is delimeted with square brackets and consists of a series of value=action tokens:

   [value1=action1 value2=action2 ...]

Here, valueI is one of the following return values: success; open_err; symbol_err; service_err; system_err; buf_err; perm_denied; auth_err; cred_insufficient; authinfo_unavail; user_unknown; maxtries; new_authtok_reqd; acct_expired; session_err; cred_unavail; cred_expired; cred_err; no_module_data; conv_err; authtok_err; authtok_recover_err; authtok_lock_busy; authtok_disable_aging; try_again; ignore; abort; authtok_expired; module_unknown; bad_item; and default. The last of these (default) can be used to set the action for those return values that are not explicitly defined. The actionI can be a positive integer or one of the following tokens: ignore; ok; done; bad; die; and reset. A positive integer, J, when specified as the action, can be used to indicate that the next J modules of the current module-type will be skipped. In this way, the administrator can develop a moderately sophisticated stack of modules with a number of different paths of execution. Which path is taken can be determined by the reactions of individual modules. PAM authentication

ignore - when used with a stack of modules, the module's return status will not contribute to the return code the application obtains.

bad - this action indicates that the return code should be thought of as indicative of the module failing. If this module is the first in the stack to fail, its status value will be used for that of the whole stack.

die - equivalent to bad with the side effect of terminating the module stack and PAM immediately returning to the application.

ok - this tells PAM that the administrator thinks this return code should contribute directly to the return code of the full stack of modules. In other words, if the former state of the stack would lead to a return of PAM_SUCCESS, the module's return code will override this value. Note, if the former state of the stack holds some value that is indicative of a modules failure, this 'ok' value will not be used to override that value.

done - equivalent to ok with the side effect of terminating the module stack and PAM immediately returning to the application.

reset - clear all memory of the state of the module stack and start again with the next stacked module. PAM authentication Each of the four keywords: required; requisite; sufficient; and optional, have an equivalent expression in terms of the [...] syntax. They are as follows:

required is equivalent to [success=ok new_authtok_reqd=ok ignore=ignore default=bad]

requisite is equivalent to [success=ok new_authtok_reqd=ok ignore=ignore default=die]

sufficient is equivalent to [success=done new_authtok_reqd=done default=ignore]

optional is equivalent to [success=ok new_authtok_reqd=ok default=ignore]

Just to get a feel for the power of this new syntax, here is a taste of what you can do with it. With Linux-PAM-0.63, the notion of client plug-in agents was introduced. This is something that makes it possible for PAM to support machine-machine authentication using the transport protocol inherent to the client/server application. With the ``[ ... value=action ... ] control syntax, it is possible for an application to be configured to support binary prompts with compliant clients, but to gracefully fall over into an alternative authentication mode for older, legacy, applications. PAM authentication


The path-name of the dynamically loadable object file; the pluggable module itself. If the first character of the module path is `/', it is assumed to be a complete path. If this is not the case, the given module path is appended to the default module path: /lib/security

Args The args are a list of tokens that are passed to the module when it is invoked. Much like arguments to a typical Linux shell command. Generally, valid arguments are optional and are specific to any given module. Invalid arguments are ignored by a module, however, when encountering an invalid argument, the module is required to write an error to syslog(3). For a list of generic options see the next section.

Any line in (one of) the configuration file(s), that is not formatted correctly, will generally tend (erring on the side of caution) to make the authentication process fail. A corresponding error is written to the system log files with a call to syslog(3). PAM authentication Directory based configuration More flexible than the single configuration file, as of version 0.56, it is possible to configure libpam via the contents of the /etc/pam.d/ directory. In this case the directory is filled with files each of which has a filename equal to a service-name (in lower-case): it is the personal configuration file for the named service. Linux-PAM can be compiled in one of two modes. The preferred mode uses either /etc/pam.d/ or /etc/pam.conf configuration but not both. That is to say, if there is a /etc/pam.d/ directory then libpam only uses the files contained in this directory. However, in the absence of the /etc/pam.d/ directory the /etc/pam.conf file is used (this is likely to be the mode your preferred distribution uses). The other mode is to use both /etc/pam.d/ and /etc/pam.conf in sequence. In this mode, entries in /etc/pam.d/ override those of /etc/pam.conf. The syntax of each file in /etc/pam.d/ is similar to that of the /etc/pam.conf file and is made up of lines of the following form: module-type control-flag module-path arguments The only difference being that the service-name is not present. The service-name is of course the name of the given configuration file. For example, /etc/pam.d/login contains the configuration for the login service. PAM authentication This method of configuration has a number of advantages over the single file approach. We list them here to assist the reader in deciding which scheme to adopt: A lower chance of misconfiguring an application. There is one less field to mis-type when editing the configuration files by hand. Easier to maintain. One application may be reconfigured without risk of interfering with other applications on the system. It is possible to symbolically link different services configuration files to a single file. This makes it easier to keep the system policy for access consistent across different applications. (It should be noted, to conserve space, it is equally possible to hard link a number of configuration files. However, care should be taken when administering this arrangement as editing a hard linked file is likely to break the link.) A potential for quicker configuration file parsing. Only the relevant entries are parsed when a service gets bound to its modules. It is possible to limit read access to individual Linux-PAM configuration files using the file protections of the filesystem. Package management becomes simpler. Every time a new application is installed, it can be accompanied by an /etc/pam.d/xxxxxx file. PAM authentication The following are optional arguments which are likely to be understood by any module. Arguments (including these) are in general optional. Debug : Use the syslog(3) call to log debugging information to the system log files.

no_warn : Instruct module to not give warning messages to the application. 
use_first_pass : The module should not prompt the user for a password. Instead, it should obtain the previously typed password (from the preceding auth module), and use that. If that doesn't work, then the user will not be authenticated. (This option is intended for auth and password modules only). 
try_first_pass : The module should attempt authentication with the previously typed password (from the preceding auth module). If that doesn't work, then the user is prompted for a password. (This option is intended for auth modules only). 
use_mapped_pass : This argument is not currently supported by any of the modules in the Linux-PAM distribution because of possible consequences associated with U.S. encryption exporting restrictions. Within the U.S., module developers are, of course, free to implement it (as are developers in other countries).

expose_account : In general the leakage of some information about user accounts is not a secure policy for modules to adopt. Sometimes information such as users names or home directories, or preferred shell, can be used to attack a user's account. In some circumstances, however, this sort of information is not deemed a threat: displaying a user's full name when asking them for a password in a secured environment could also be called being 'friendly'. The expose_account argument is a standard module argument to encourage a module to be less discrete about account information as it is deemed appropriate by the local administrator. PAM authentication Example configuration file entries Default policy : If a system is to be considered secure, it had better have a reasonably secure `OTHER' entry. The following is a paranoid setting (which is not a bad place to start!):

  1. default; deny access

OTHER auth required pam_deny.so OTHER account required pam_deny.so OTHER password required pam_deny.so OTHER session required pam_deny.so

Whilst fundamentally a secure default, this is not very sympathetic to a misconfigured system. For example, such a system is vulnerable to locking everyone out should the rest of the file become badly written. The module pam_deny is not very sophisticated. For example, it logs no information when it is invoked so unless the users of a system contact the administrator when failing to execute a service application, the administrator may go for a long while in ignorance of the fact that his system is misconfigured. PAM authentication The addition of the following line before those in the above example would provide a suitable warning to the administrator.

  1. default; wake up! This application is not configured

OTHER auth required pam_warn.so OTHER password required pam_warn.so

Having two ``OTHER auth lines is an example of stacking. On a system that uses the /etc/pam.d/ configuration, the corresponding default setup would be achieved with the following file:

  1. default configuration: /etc/pam.d/other

auth required pam_warn.so auth required pam_deny.so account required pam_deny.so password required pam_warn.so password required pam_deny.so session required pam_deny.so PAM authentication On a less sensitive computer, one on which the system administrator wishes to remain ignorant of much of the power of Linux-PAM, the following selection of lines (in /etc/pam.conf) is likely to mimic the historically familiar Linux setup.

  1. default; standard UN*X access

OTHER auth required pam_unix.so OTHER account required pam_unix.so OTHER password required pam_unix.so OTHER session required pam_unix.so

PAM authentication Key terms, files and utilities : /etc/pam.d /etc/pam.conf /lib/libpam.so.*