The Linux-PAM System Administrators' Guide <author>Andrew G. Morgan, <tt>morgan@@linux.kernel.org</tt> <date>DRAFT v0.59 1998/1/7 <abstract> This manual documents what a system-administrator needs to know about the <bf>Linux-PAM</bf> library. It covers the correct syntax of the PAM configuration file and discusses strategies for maintaining a secure system. </abstract>  <toc>  <sect>Introduction <bf/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 <tt>/etc/passwd</tt> 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 (<tt/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 <tt>/etc/group</tt> file. 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 <bf/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, <tt>/etc/pam.conf</tt> (or a series of configuration files located in <tt>/etc/pam.d/</tt>) to authenticate a user request via the locally available authentication modules. The modules themselves will usually be located in the directory <tt>/usr/lib/security</tt> and take the form of dynamically loadable object files (see <tt/dlopen(3)/). <sect>Some comments on the text<label id="text-conventions"> Before proceeding to read the rest of this document, it should be noted that the text assumes that certain files are placed in certain directories. Where they have been specified, the conventions we adopt here for locating these files are those of the relevant RFC (RFC-86.0, see <ref id="see-also-sec" name="bibliography">). If you are using a distribution of Linux (or some other operating system) that supports PAM but chooses to distribute these files in a diferent way (Red Hat is one such distribution), you should be careful when copying examples directly from the text. As an example of the above, where it is explicit, the text assumes that PAM loadable object files (the <em/modules/) are to be located in the following directory: <tt>/usr/lib/security/</tt>. However, Red Hat Linux, in agreement with the Linux File System Standard (the FSSTND), places these files in <tt>/lib/security</tt>. Please be careful to perform the necessary transcription when using the examples from the text. <sect>Overview<label id="overview-section"> For the uninitiated, we begin by considering an example. We take an application that grants some service to users; <em/login/ is one such program. <em/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 <em/login/ the service is a command shell (bash, tcsh, zsh, etc.) running with the identity of the user. Traditinally, the former step is achieved by the <em/login/ application prompting the user for a password and then verifying that it agrees with that located on the system; hence verifying that the so far as the system is concerned the user is who they claim to be. This is the task that is delegated to <bf/Linux-PAM/. From the perspective of the application programmer (in this case the person that wrote the <em/login/ application), <bf/Linux-PAM/ takes care of this authentication task -- verifying the identity of the user. The flexibility of <bf/Linux-PAM/ is that <em/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 <em/simple trust/ (<tt/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 <em/multiplication tables/. As they mature, the authentication can be upgraded to include (long) division! <bf/Linux-PAM/ deals with four separate types of (management) task. These are: <em/authentication management/; <em/account management/; <em/session management/; and <em/password management/. The association of the preferred management scheme with the behavior of an application is made with entries in the relevant <bf/Linux-PAM/ configuration file. The management functions are performed by <em/modules/ specified in the configuration file. The syntax for this file is discussed in the section <ref id="configuration" name="below">. Here is a figure that describes the overall organization of <bf/Linux-PAM/. <tscreen> <verb> +----------------+ | application: X | +----------------+ / +----------+ +================+ | authentication-[---->--\--] Linux- |--<--| PAM config file| | + [----<--/--] PAM | |================| |[conversation()][--+ \ | | | X auth .. a.so | +----------------+ | / +-n--n-----+ | X auth .. b.so | | | | __| | | _____/ | service user | A | | |____,-----' | | | V A +----------------+ +------|-----|---------+ -----+------+ +---u-----u----+ | | | | auth.... |--[ a ]--[ b ]--[ c ] +--------------+ | acct.... |--[ b ]--[ d ] +--------------+ | password |--[ b ]--[ c ] +--------------+ | session |--[ e ]--[ c ] +--------------+ </verb> </tscreen> By way of explanation, the left of the figure represents the application; application X. Such an application interfaces with the <bf/Linux-PAM/ library and knows none of the specifics of its configured authentication method. The <bf/Linux-PAM/ library (in the center) consults the contents of the PAM configuration file and loads the modules that are appropriate for application-X. These modules fall into one of four management groups (lower-center) and are stacked in the order they appear in the configuaration file. These modules, when called by <bf/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 <em/conversation/ function. <sect>The Linux-PAM configuration file <label id="configuration"> <bf/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 <tt/Linux-PAM/ is contained in one of two places: either the single system file, <tt>/etc/pam.conf</tt>; or the <tt>/etc/pam.d/</tt> directory. In this section we discuss the correct syntax of and generic options respected by entries to these files. <sect1>Configuration file syntax The reader should note that the <bf/Linux-PAM/ specific tokens in this file are case <em/insensitive/. The module paths, however, are case sensitive since they indicate a file's <em/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 <em/special/ characters provided for the convenience of the system administrator: comments are preceded by a `<tt/#/' and extend to the next end-of-line; also, module specification lines may be extended with a `<tt/\/' escaped newline. A general configuration line of the <tt>/etc/pam.conf</tt> file has the following form: <tscreen> <verb> service-name module-type control-flag module-path arguments </verb> </tscreen> Below, we explain the meaning of each of these tokens. The second (and more recently adopted) way of configuring <bf/Linux-PAM/ is via the contents of the <tt>/etc/pam.d/</tt> directory. Once we have explained the meaning of the above tokens, we will describe this method. <descrip> <tag><tt/service-name/</tag> The name of the service associated with this entry. Frequently the service name is the conventional name of the given application. For example, `<tt/ftpd/', `<tt/rlogind/' and `<tt/su/', <em/etc./ . There is a special <tt/service-name/, reserved for defining a default authentication mechanism. It has the name `<tt/OTHER/' and may be specified in either lower or upper case characters. Note, when there is a module specified for a named service, the `<tt/OTHER/' entries are ignored. <tag><tt/module-type/</tag> One of (currently) four types of module. The four types are as follows: <itemize> <item> <tt/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 <tt/group/ membership (independently of the <tt>/etc/groups</tt> file discussed above) or other privileges through its <em/credential/ granting properties. <item> <tt/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---`<tt/root/' login only on the console. <item> <tt/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. . <item> <tt/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 (<tt/auth/) module-type. </itemize> <tag><tt/control-flag/</tag> 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 <em/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 `<tt>/etc/pam.conf</tt>' file. Instead, it receives a summary <em/success/ or <em/fail/ response from the <bf/Linux-PAM/ library. The order of execution of these modules is that of the entries in the <tt>/etc/pam.conf</tt> file; earlier entries are executed before later ones. As of Linux-PAM v0.60, this <em/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: <tt/required/, <tt/requisite/, <tt/sufficient/ and <tt/optional/. The Linux-PAM library interprets these keywords in the following manner: <itemize> <item> <tt/required/; this indicates that the success of the module is required for the <tt/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 <tt/module-type/) have been executed. <item> <tt/requisite/; like <tt/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 <em/first/ <tt/required/ or <tt/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. <item> <tt/sufficient/; the success of this module is deemed `<em/sufficient/' to satisfy the <bf/Linux-PAM/ library that this module-type has succeeded in its purpose. In the event that no previous <tt/required/ module has failed, no more `<em/stacked/' modules of this type are invoked. (Note, in this case subsequent <tt/required/ modules are <bf/not/ invoked.). A failure of this module is not deemed as fatal to satisfying the application that this <tt/module-type/ has succeeded. <item> <tt/optional/; as its name suggests, this <tt/control-flag/ marks the module as not being critical to the success or failure of the user's application for service. However, in the absence of any successes of previous or subsequent stacked modules this module will determine the nature of the response to the application. </itemize> 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 <tt/value=action/ tokens: <tscreen> <verb> [value1=action1 value2=action2 ...] </verb> </tscreen> Here, <tt/valueI/ is one of the following <em/return values/: <tt/success/; <tt/open_err/; <tt/symbol_err/; <tt/service_err/; <tt/system_err/; <tt/buf_err/; <tt/perm_denied/; <tt/auth_err/; <tt/cred_insufficient/; <tt/authinfo_unavail/; <tt/user_unknown/; <tt/maxtries/; <tt/new_authtok_reqd/; <tt/acct_expired/; <tt/session_err/; <tt/cred_unavail/; <tt/cred_expired/; <tt/cred_err/; <tt/no_module_data/; <tt/conv_err/; <tt/authtok_err/; <tt/authtok_recover_err/; <tt/authtok_lock_busy/; <tt/authtok_disable_aging/; <tt/try_again/; <tt/ignore/; <tt/abort/; <tt/authtok_expired/; <tt/module_unknown/; <tt/bad_item/; and <tt/default/. The last of these (<tt/default/) can be used to set the action for those return values that are not set explicitly. The <tt/actionI/ can be a positive integer or one of the following tokens: <tt/ignore/; <tt/ok/; <tt/done/; <tt/bad/; <tt/die/; and <tt/reset/. A positive integer, <tt/J/, when specified as the action can be used to indicate that the next <em/J/ modules of the current 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. <bf>Note, at time of writing, this newer syntax is so new that I don't want to write too much about it. Please play with this. Report all the bugs and make suggestions for new actions (etc.).</bf> <tag> <tt/module-path/</tag> The path-name of the dynamically loadable object file; <em/the pluggable module/ itself. If the first character of the module path is `<tt>/</tt>', 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: <tt>/usr/lib/security</tt> (but see the notes <ref id="text-conventions" name="above">). <tag> <tt/args/</tag> The <tt/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 <tt/syslog(3)/. For a list of <em/generic/ options see the next section. </descrip> Any line in (one of) the confiuration 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 <tt/syslog(3)/. <sect1>Directory based configuration More flexible than the single configuration file, as of version 0.56, it is possible to configure <tt>libpam</tt> via the contents of the <tt>/etc/pam.d/</tt> 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. <bf/Linux-PAM/ can be compiled in one of two modes. The preferred mode uses either <tt>/etc/pam.d/</tt> or <tt>/etc/pam.conf</tt> configuration but not both. That is to say, if there is a <tt>/etc/pam.d/</tt> directory then libpam only uses the files contained in this directory. However, in the absence of the <tt>/etc/pam.d/</tt> directory the <tt>/etc/pam.conf</tt> file is used. The other mode (and the one currently supported by Red Hat 4.2) is to use both <tt>/etc/pam.d/</tt> and <tt>/etc/pam.conf</tt> in sequence. In this mode, entries in <tt>/etc/pam.d/</tt> override those of <tt>/etc/pam.conf</tt>. The syntax of each file in <tt>/etc/pam.d/</tt> is similar to that of the <tt>/etc/pam.conf</tt> file and is made up of lines of the following form: <tscreen> <verb> module-type control-flag module-path arguments </verb> </tscreen> The only difference being that the <tt>service-name</tt> is not present. The service-name is of course the name of the given configuration file. For example, <tt>/etc/pam.d/login</tt> contains the configuration for the login service. 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: <itemize> <item>A lower chance of misconfiguring an application. There is one less field to mis-type when editing the configuration files by hand. <item>Easier to maintain. One application may be reconfigured without risk of interfering with other applications on the system. <item>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.) <item>A potential for quicker configuration file parsing. Only the relevant entries are parsed when a service gets bound to its modules. <item>It is possible to limit read access to individual <bf/Linux-PAM/ configuration files using the file protections of the filesystem. <item>Package management becomes simpler. Every time a new application is installed, it can be accompanied by an <tt>/etc/pam.d/</tt>xxxxxx file. </itemize> <sect1>Generic optional arguments The following are optional arguments which are likely to be understood by any module. Arguments (including these) are in general <em/optional/. <descrip> <tag><tt/debug/</tag> Use the <tt/syslog(3)/ call to log debugging information to the system log files. <tag> <tt/no_warn/</tag> Instruct module to not give warning messages to the application. <tag> <tt/use_first_pass/</tag> The module should not prompt the user for a password. Instead, it should obtain the previously typed password (from the preceding <tt/auth/ module), and use that. If that doesn't work, then the user will not be authenticated. (This option is intended for <tt/auth/ and <tt/password/ modules only). <tag> <tt/try_first_pass/</tag> The module should attempt authentication with the previously typed password (from the preceding <tt/auth/ module). If that doesn't work, then the user is prompted for a password. (This option is intended for <tt/auth/ modules only). <tag> <tt/use_mapped_pass/</tag> This argument is not currently supported by any of the modules in the <bf/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). For compatibility reasons we describe its use as suggested in the <bf/DCE-RFC 86.0/, see section <ref id="see-also-sec" name="bibliography"> for a pointer to this document. The <tt/use_mapped_pass/ argument instructs the module to take the clear text authentication token entered by a previous module (that requests such a token) and use it to generate an encryption/decryption key with which to safely store/retrieve the authentication token required for this module. In this way the user can enter a single authentication token and be quietly authenticated by a number of stacked modules. Obviously a convenient feature that necessarily requires some reliably strong encryption to make it secure. This argument is intended for the <tt/auth/ and <tt/password/ module types only. </descrip> <sect1>Example configuration file entries In this section, we give some examples of entries that can be present in the <bf/Linux-PAM/ configuration file. As a first attempt at configuring your system you could do worse than to implement these. <sect2>Default policy If a system is to be considered secure, it had better have a reasonably secure `<tt/OTHER/' entry. The following is a paranoid setting (which is not a bad place to start!): <tscreen> <verb> # # default; deny access # OTHER auth required /usr/lib/security/pam_deny.so OTHER account required /usr/lib/security/pam_deny.so OTHER password required /usr/lib/security/pam_deny.so OTHER session required /usr/lib/security/pam_deny.so </verb> </tscreen> 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 <tt/pam_deny/ (documented in a later section) 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. The addition of the following line before those in the above example would provide a suitable warning to the administrator. <tscreen> <verb> # # default; wake up! This application is not configured # OTHER auth required /usr/lib/security/pam_warn.so OTHER password required /usr/lib/security/pam_warn.so </verb> </tscreen> Having two ``<tt/OTHER auth/'' lines is an example of stacking. On a system that uses the <tt>/etc/pam.d/</tt> configuration, the corresponding default setup would be achieved with the following file: <tscreen> <verb> # # default configuration: /etc/pam.d/other # auth required /usr/lib/security/pam_warn.so auth required /usr/lib/security/pam_deny.so account required /usr/lib/security/pam_deny.so password required /usr/lib/security/pam_warn.so password required /usr/lib/security/pam_deny.so session required /usr/lib/security/pam_deny.so </verb> </tscreen> This is the only explicit example we give for an <tt>/etc/pam.d/</tt> file. In general, it should be clear how to transpose the remaining examples to this configuration scheme. On a less sensitive computer, one on which the system administrator wishes to remain ignorant of much of the power of <tt/Linux-PAM/, the following selection of lines (in <tt>/etc/pam.conf</tt>) is likely to mimic the historically familiar Linux setup. <tscreen> <verb> # # default; standard UNIX access # OTHER auth required /usr/lib/security/pam_unix_auth.so OTHER account required /usr/lib/security/pam_unix_acct.so OTHER password required /usr/lib/security/pam_unix_passwd.so OTHER session required /usr/lib/security/pam_unix_session.so </verb> </tscreen> In general this will provide a starting place for most applications. Unfortunately, most is not all. One application that might require additional lines is <em/ftpd/ if you wish to enable <em/anonymous-ftp/. To enable anonymous-ftp, the following lines might be used to replace the default (<tt/OTHER/) ones. (<bf/*WARNING*/ as of 1996/12/28 this does not work correctly with any ftpd. Consequently, this description may be subject to change or the application will be fixed.) <tscreen> <verb> # # ftpd; add ftp-specifics. These lines enable anonymous ftp over # standard UNIX access (the listfile entry blocks access to # users listed in /etc/ftpusers) # ftpd auth sufficient /usr/lib/security/pam_ftp.so ftpd auth required /usr/lib/security/pam_unix_auth.so use_first_pass ftpd auth required /usr/lib/security/pam_listfile.so \ onerr=succeed item=user sense=deny file=/etc/ftpusers </verb> </tscreen> Note, the second line is necessary since the default entries are ignored by a service application (here <em/ftpd/) if there are <em/any/ entries in <tt>/etc/pam.conf</tt> for that specified service. Again, this is an example of authentication module stacking. Note the use of the <tt/sufficient/ control-flag. It says that ``if this module authenticates the user, ignore the subsequent <tt/auth/ modules''. Also note the use of the ``<tt/use_first_pass/'' module-argument, this instructs the UNIX authentication module that it is not to prompt for a password but rely one already having been obtained by the ftp module. The standard UNIX modules, used above, are strongly tied to using the default `<tt/libc/' user database functions (see for example, <tt/man getpwent/). It is the opinion of the author that these functions are not sufficently flexible to make full use of the power of <bf/Linux-PAM/. For this reason, and as a small plug, I mention in passing that there is a pluggable replacement for the <tt/pam_unix_../ modules; <tt/pam_pwdb/. See the section below for a more complete description. <sect>Security issues of Linux-PAM This section will discuss good practices for using Linux-PAM in a secure manner. It is currently sadly lacking...suggestions are welcome! <sect1>If something goes wrong <bf/Linux-PAM/ has the potential to seriously change the security of your system. You can choose to have no security or absolute security (no access permitted). In general, <bf/Linux-PAM/ errs towards the latter. Any number of configuration errors can dissable access to your system partially, or completely. The most dramatic problem that is likely to be encountered when configuring <bf/Linux-PAM/ is that of deleting the configuration file(s): <tt>/etc/pam.d/*</tt> and/or <tt>/etc/pam.conf</tt>. This will lock you out of your own system! To recover, your best bet is to reboot the system in single user mode and set about correcting things from there. The following has been adapted from a life-saving email on the subject from David Wood: <verb> > What the hell do I do now? OK, don't panic. The first thing you have to realize is that this happens to 50% of users who ever do anything with PAM. It happened here, not once, not twice, but three times, all different, and in the end, the solution was the same every time. First, I hope you installed LILO with a delay. If you can, reboot, hit shift or tab or something and type: LILO boot: linux single (Replace 'linux' with 'name-of-your-normal-linux-image'). This will let you in without logging in. Ever wondered how easy it is to break into a linux machine from the console? Now you know. If you can't do that, then get yourself a bootkernel floppy and a root disk a-la slackware's rescue.gz. (Red Hat's installation disks can be used in this mode too.) In either case, the point is to get back your root prompt. Second, I'm going to assume that you haven't completely nuked your pam installation - just your configuration files. Here's how you make your configs nice again: cd /etc mv pam.conf pam.conf.orig mv pam.d pam.d.orig mkdir pam.d cd pam.d and then use vi to create a file called "other" in this directory. It should contain the following four lines: auth required pam_unix_auth.so account required pam_unix_acct.so password required pam_unix_passwd.so session required pam_unix_session.so Now you have the simplest possible PAM configuration that will work the way you're used to. Everything should magically start to work again. Try it out by hitting ALT-F2 and logging in on another virtual console. If it doesn't work, you have bigger problems, or you've mistyped something. One of the wonders of this system (seriously, perhaps) is that if you mistype anything in the conf files, you usually get no error reporting of any kind on the console - just some entries in the log file. So look there! (Try 'tail /var/log/messages'.) From here you can go back and get a real configuration going, hopefully after you've tested it first on a machine you don't care about screwing up. :/ Some pointers (to make everything "right" with Red Hat...): Install the newest pam, pamconfig, and pwdb from the redhat current directory, and do it all on the same command line with rpm... rpm -Uvh [maybe --force too] pam-* pamconfig-* pwdb-* Then make sure you install (or reinstall) the newest version of libc, util-linux, wuftp, and NetKit. For kicks you might try installing the newest versions of the affected x apps, like xlock, but I haven't gotten those to work at all yet. </verb> <sect1>Avoid having a weak `other' configuration It is not a good thing to have a weak default (<tt/OTHER/) entry. This service is the default configuration for all PAM aware applications and if it is weak, your system is likely to be vulnerable to attack. <sect>A reference guide for available modules Here, we collect together some descriptions of the various modules available for <bf/Linux-PAM/. In general these modules should be freely available. Where this is not the case, it will be indicated. Also please note the comments contained in the section <ref id="text-conventions" name="on text conventions above"> when copying the examples listed below.  <sect>Files <descrip> <tag><tt>/usr/lib/libpam.so.*</tt></tag> the shared library providing applications with access to <bf/Linux-PAM/. <tag><tt>/etc/pam.conf</tt></tag> the <bf/Linux-PAM/ configuration file. <tag><tt>/usr/lib/security/pam_*.so</tt></tag> the primary location for <bf/Linux-PAM/ dynamically loadable object files; the modules. </descrip> <sect>See also<label id="see-also-sec"> <itemize> <item>The <bf/Linux-PAM/ Application Writers' Guide. <item>The <bf/Linux-PAM/ Module Writers' Guide. <item>The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH PLUGGABLE AUTHENTICATION MODULES'', Open Software Foundation Request For Comments 86.0, October 1995. See this url: <tt><htmlurl url="http://www.pilgrim.umass.edu/pub/osf_dce/RFC/rfc86.0.txt" name="http://www.pilgrim.umass.edu/pub/osf_dce/RFC/rfc86.0.txt"></tt> </itemize> <sect>Notes I intend to put development comments here... like ``at the moment this isn't actually supported''. At release time what ever is in this section will be placed in the Bugs section below! :) Are we going to be able to support the <tt/use_mapped_pass/ module argument? Anyone know a cheap (free) good lawyer?! <itemize> <item> This issue may go away, as Sun have investigated adding a new management group for mappings. In this way, libpam would have mapping modules that could securely store passwords using strong cryptography and in such a way that they need not be distributed with Linux-PAM. </itemize> <sect>Author/acknowledgments This document was written by Andrew G. Morgan (morgan@@parc.power.net) with many contributions from   Craig S. Bell, Derrick J. Brashear, Ben Buxton, Oliver Crow, Marc Ewing, Cristian Gafton, Eric Hester, Eric Jacksch, Michael K. Johnson, David Kinchlea, Elliot Lee, Al Longyear, Marek Michalkiewicz, Aleph One, Sean Reifschneider, Eric Troan, Theodore Ts'o, Jeff Uphoff, Ronald Wahl, John Wilmes, Joseph S. D. Yao and Alex O. Yuriev. Thanks are also due to Sun Microsystems, especially to Vipin Samar and Charlie Lai for their advice. At an early stage in the development of <bf/Linux-PAM/, Sun graciously made the documentation for their implementation of PAM available. This act greatly accelerated the development of <bf/Linux-PAM/. <sect>Bugs/omissions More PAM modules are being developed all the time. It is unlikely that this document will ever be truely up to date! Currently there is no documentation for PAM-aware applications. This manual is unfinished. Only a partial list of people is credited for all the good work they have done. <sect>Copyright information for this document Copyright (c) Andrew G. Morgan 1996. All rights reserved. <newline> Email: <tt><morgan@@parc.power.net></tt> Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: <itemize> <item> 1. Redistributions of source code must retain the above copyright notice, and the entire permission notice in its entirety, including the disclaimer of warranties. <item> 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. <item> 3. The name of the author may not be used to endorse or promote products derived from this software without specific prior written permission. </itemize> <bf/Alternatively/, this product may be distributed under the terms of the GNU General Public License (GPL), in which case the provisions of the GNU GPL are required <bf/instead of/ the above restrictions. (This clause is necessary due to a potential bad interaction between the GNU GPL and the restrictions contained in a BSD-style copyright.) THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. <tt>$Id: pam_source.sgml,v 1.5 1997/04/05 06:49:14 morgan Exp morgan $</tt> </article> @ 1.1 log @Initial revision @ text @@ 1.1.1.1 log @Initial import of virgin Linux-PAM 0.65, slightly stripped down. @ text @@ 1.1.1.1.6.1 log @MFC: Latest vendor PAM code + local fixes. @ text @d5 1 a5 1 $Id: pam_source.sgml,v 1.5 2001/03/19 01:46:41 agmorgan Exp $ d7 1 a7 1 Copyright (c) Andrew G. Morgan 1996-2001. All rights reserved. d48 2 a49 2 <author>Andrew G. Morgan, <tt>morgan@@kernel.org</tt> <date>DRAFT v0.75 2001/03/18 d143 1 a143 1 Traditionally, the former step is achieved by the <em/login/ d145 2 a146 2 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. d218 1 a218 1 the order they appear in the configuration file. These modules, when a223 110 <sect1>Getting started The following text was contributed by Seth Chaiklin: <tscreen> <verb> To this point, we have described how PAM should work in an ideal world, in which all applications are coded properly. However, at the present time (October 1998), this is far from the case. Therefore, here are some practical considerations in trying to use PAM in your system. Why bother, is it really worth all the trouble? If you running Linux as a single user system, or in an environment where all the users are trusted, then there is no real advantage for using PAM. </verb> </tscreen> <BF>Ed:</BF> there is actually an advantage since you can <em/dummy down/ the authentication to the point where you don't have any... Almost like Win95. In a networked environment, it is clear that you need to think a little more about how users etc., are authenticated:] <tscreen> <verb> If you are running Linux as a server, where several different services are being provided (e.g., WWW with areas restricted by password control, PPP), then there can be some real and interesting value for PAM. In particular, through the use of modules, PAM can enable a program to search through several different password databases, even if that program is not explicitly coded for that particular database. Here are some examples of the possibilities that this enables. o Apache has a module that provides PAM services. Now authentication to use particular directories can be conducted by PAM, which means that the range of modules that are available to PAM can be used, including RADIUS, NIS, NCP (which means that Novell password databases can be used). o pppd has a PAMified version (available from RedHat) Now it is possible to use a series of databases to authenticate ppp users. In addition to the normal Linux-based password databases (such as /etc/passwd and /etc/shadow), you can use PAM modules to authenticate against Novell password databases or NT-based password databases. o The preceding two examples can be combined. Imagaine that the persons in your office/department are already registered with a username and password in a Novell or NT LAN. If you wanted to use this database on your Linux server (for PPP access, for web access, or even for normal shell access), you can use PAM to authenticate against this existing database, rather than maintain a separate database on both Linux and the LAN server. Can I use PAM for any program that requires authentication? Yes and no. Yes, if you have access to the source code, and can add the appropriate PAM functions. No, if you do not have access to the source code, and the binary does not have the PAM functions included. In other words, if a program is going to use PAM, then it has to have PAM functions explicitly coded into the program. If they are not, then it is not possible to use PAM. How can I tell whether a program has PAM coded into it or not? A quick-and-dirty (but not always reliable) method is to ldd <programname> If libpam and libpam_misc are not among the libraries that the program uses, then it is not going to work with PAM. However, it is possible that the libraries are included, but there are still problems, because the PAM coding in the program does not work as it should. So a more reliable method is to make the follow tests. In the /etc/pam.d directory, one needs to make a configuration file for the program that one wants to run. The exact name of the configuration file is hard-coded into the program. Usually, it is the same name as the program, but not always. For sake of illustration, let's assume that the program is named "pamprog" and the name of the configuration file is /etc/pam.d/pamprog. In the /etc/pam.d/pamprog but the following two lines: auth required pam_permit.so auth required pam_warn.so Now try to use pamprog. The first line in the configuration file says that all users are permitted. The second line will write a warning to your syslog file (or whether you syslog is writing messages). If this test succeeds, then you know that you have a program that can understand pam, and you can start the more interesting work of deciding how to stack modules in your /etc/pam.d/pamprog file. </verb> </tscreen> d366 3 a368 7 the user's application for service. In general, <bf/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 <tt/PAM_IGNORE/. d395 1 a395 1 action for those return values that are not explicitly defined. d400 1 a400 1 <tt/reset/. A positive integer, <tt/J/, when specified as the action, d408 3 a410 35 <itemize> <item><tt/ignore/ - when used with a stack of modules, the module's return status will not contribute to the return code the application obtains. <item><tt/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. <item><tt/die/ - equivalent to <tt/bad/ with the side effect of terminating the module stack and PAM immediately returning to the application. <item><tt/ok/ - this tells <bf/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 <tt/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. <item><tt/done/ - equivalent to <tt/ok/ with the side effect of terminating the module stack and PAM immediately returning to the application. <item><tt/reset/ - clear all memory of the state of the module stack and start again with the next stacked module. </itemize> Just to get a feel for the power of this new syntax, here is a taste of what you can do with it. With <bf/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 ``<tt/[ ... 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. Flexible eh? d434 1 a434 1 Any line in (one of) the configuration file(s), that is not formatted d456 4 a459 4 used. The other mode (and the one currently supported by Red Hat 4.2 and higher) is to use both <tt>/etc/pam.d/</tt> and <tt>/etc/pam.conf</tt> in sequence. In this mode, entries in <tt>/etc/pam.d/</tt> override those of <tt>/etc/pam.conf</tt>. a562 14 <tag><tt/expose_account/</tag> 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 <tt/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. d684 11 a803 22 Here is a sample "other" configuration file. The <em/pam_deny/ module will deny access and the <em/pam_warn/ module will send a syslog message to <tt/auth.notice/: <tscreen> <verb> # # The PAM configuration file for the `other' service # auth required pam_deny.so auth required pam_warn.so account required pam_deny.so account required pam_warn.so password required pam_deny.so password required pam_warn.so session required pam_deny.so session required pam_warn.so </verb> </tscreen> d850 2 a851 2 url="http://www.kernel.org/pub/linux/libs/pam/pre/doc/rfc86.0.txt.gz" name="http://www.kernel.org/pub/linux/libs/pam/pre/doc/rfc86.0.txt.gz"></tt> d878 1 a878 1 This document was written by Andrew G. Morgan (morgan@@kernel.org) d880 29 a908 1  d924 3 d933 1 a933 1 Copyright (c) Andrew G. Morgan 1996-9. All rights reserved. d935 1 a935 1 Email: <tt><morgan@@linux.kernel.org></tt> d983 1 a983 1 <tt>$Id: pam_source.sgml,v 1.5 2001/03/19 01:46:41 agmorgan Exp $</tt> @ 1.1.1.1.6.2 log @Back out the extremely unwise PAM MFC that I did about 4 days ago. My apologies to all. Please pass the pointy hat. @ text @d5 1 a5 2 $Id: pam_source.sgml,v 1.5 1997/04/05 06:49:14 morgan Exp morgan $ $FreeBSD$ d7 1 a7 1 Copyright (c) Andrew G. Morgan 1996,1997. All rights reserved. d48 2 a49 2 <author>Andrew G. Morgan, <tt>morgan@@linux.kernel.org</tt> <date>DRAFT v0.59 1998/1/7 d143 1 a143 1 Traditinally, the former step is achieved by the <em/login/ d145 2 a146 2 it agrees with that located on the system; hence verifying that the so far as the system is concerned the user is who they claim to be. d218 1 a218 1 the order they appear in the configuaration file. These modules, when d224 110 d476 7 a482 3 the user's application for service. However, in the absence of any successes of previous or subsequent stacked modules this module will determine the nature of the response to the application. d509 1 a509 1 action for those return values that are not set explicitly. d514 1 a514 1 <tt/reset/. A positive integer, <tt/J/, when specified as the action d522 35 a556 3 <bf>Note, at time of writing, this newer syntax is so new that I don't want to write too much about it. Please play with this. Report all the bugs and make suggestions for new actions (etc.).</bf> d580 1 a580 1 Any line in (one of) the confiuration file(s), that is not formatted d602 4 a605 4 used. The other mode (and the one currently supported by Red Hat 4.2) is to use both <tt>/etc/pam.d/</tt> and <tt>/etc/pam.conf</tt> in sequence. In this mode, entries in <tt>/etc/pam.d/</tt> override those of <tt>/etc/pam.conf</tt>. d709 14 a843 11 The standard UNIX modules, used above, are strongly tied to using the default `<tt/libc/' user database functions (see for example, <tt/man getpwent/). It is the opinion of the author that these functions are not sufficently flexible to make full use of the power of <bf/Linux-PAM/. For this reason, and as a small plug, I mention in passing that there is a pluggable replacement for the <tt/pam_unix_../ modules; <tt/pam_pwdb/. See the section below for a more complete description. d953 22 d1021 2 a1022 2 url="http://www.pilgrim.umass.edu/pub/osf_dce/RFC/rfc86.0.txt" name="http://www.pilgrim.umass.edu/pub/osf_dce/RFC/rfc86.0.txt"></tt> d1049 1 a1049 1 This document was written by Andrew G. Morgan (morgan@@parc.power.net) d1051 1 a1051 29   Craig S. Bell, Derrick J. Brashear, Ben Buxton, Oliver Crow, Marc Ewing, Cristian Gafton, Eric Hester, Eric Jacksch, Michael K. Johnson, David Kinchlea, Elliot Lee, Al Longyear, Marek Michalkiewicz, Aleph One, Sean Reifschneider, Eric Troan, Theodore Ts'o, Jeff Uphoff, Ronald Wahl, John Wilmes, Joseph S. D. Yao and Alex O. Yuriev. a1066 3 Currently there is no documentation for PAM-aware applications. d1073 1 a1073 1 Copyright (c) Andrew G. Morgan 1996. All rights reserved. d1075 1 a1075 1 Email: <tt><morgan@@parc.power.net></tt> d1123 1 a1123 1 <tt>$Id: pam_source.sgml,v 1.5 1997/04/05 06:49:14 morgan Exp morgan $</tt> @ 1.1.1.1.6.3 log @Switch importer @ text @d6 1 a6 1 $FreeBSD: stable/4/contrib/libpam/doc/pam_source.sgml 78074 2001-06-11 15:28:52Z markm $ @ 1.1.1.2 log @Vendor import Linux PAM 0.75 @ text @d5 1 a5 1 $Id: pam_source.sgml,v 1.5 2001/03/19 01:46:41 agmorgan Exp $ d7 1 a7 1 Copyright (c) Andrew G. Morgan 1996-2001. All rights reserved. d48 2 a49 2 <author>Andrew G. Morgan, <tt>morgan@@kernel.org</tt> <date>DRAFT v0.75 2001/03/18 d143 1 a143 1 Traditionally, the former step is achieved by the <em/login/ d145 2 a146 2 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. d218 1 a218 1 the order they appear in the configuration file. These modules, when a223 110 <sect1>Getting started The following text was contributed by Seth Chaiklin: <tscreen> <verb> To this point, we have described how PAM should work in an ideal world, in which all applications are coded properly. However, at the present time (October 1998), this is far from the case. Therefore, here are some practical considerations in trying to use PAM in your system. Why bother, is it really worth all the trouble? If you running Linux as a single user system, or in an environment where all the users are trusted, then there is no real advantage for using PAM. </verb> </tscreen> <BF>Ed:</BF> there is actually an advantage since you can <em/dummy down/ the authentication to the point where you don't have any... Almost like Win95. In a networked environment, it is clear that you need to think a little more about how users etc., are authenticated:] <tscreen> <verb> If you are running Linux as a server, where several different services are being provided (e.g., WWW with areas restricted by password control, PPP), then there can be some real and interesting value for PAM. In particular, through the use of modules, PAM can enable a program to search through several different password databases, even if that program is not explicitly coded for that particular database. Here are some examples of the possibilities that this enables. o Apache has a module that provides PAM services. Now authentication to use particular directories can be conducted by PAM, which means that the range of modules that are available to PAM can be used, including RADIUS, NIS, NCP (which means that Novell password databases can be used). o pppd has a PAMified version (available from RedHat) Now it is possible to use a series of databases to authenticate ppp users. In addition to the normal Linux-based password databases (such as /etc/passwd and /etc/shadow), you can use PAM modules to authenticate against Novell password databases or NT-based password databases. o The preceding two examples can be combined. Imagaine that the persons in your office/department are already registered with a username and password in a Novell or NT LAN. If you wanted to use this database on your Linux server (for PPP access, for web access, or even for normal shell access), you can use PAM to authenticate against this existing database, rather than maintain a separate database on both Linux and the LAN server. Can I use PAM for any program that requires authentication? Yes and no. Yes, if you have access to the source code, and can add the appropriate PAM functions. No, if you do not have access to the source code, and the binary does not have the PAM functions included. In other words, if a program is going to use PAM, then it has to have PAM functions explicitly coded into the program. If they are not, then it is not possible to use PAM. How can I tell whether a program has PAM coded into it or not? A quick-and-dirty (but not always reliable) method is to ldd <programname> If libpam and libpam_misc are not among the libraries that the program uses, then it is not going to work with PAM. However, it is possible that the libraries are included, but there are still problems, because the PAM coding in the program does not work as it should. So a more reliable method is to make the follow tests. In the /etc/pam.d directory, one needs to make a configuration file for the program that one wants to run. The exact name of the configuration file is hard-coded into the program. Usually, it is the same name as the program, but not always. For sake of illustration, let's assume that the program is named "pamprog" and the name of the configuration file is /etc/pam.d/pamprog. In the /etc/pam.d/pamprog but the following two lines: auth required pam_permit.so auth required pam_warn.so Now try to use pamprog. The first line in the configuration file says that all users are permitted. The second line will write a warning to your syslog file (or whether you syslog is writing messages). If this test succeeds, then you know that you have a program that can understand pam, and you can start the more interesting work of deciding how to stack modules in your /etc/pam.d/pamprog file. </verb> </tscreen> d366 3 a368 7 the user's application for service. In general, <bf/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 <tt/PAM_IGNORE/. d395 1 a395 1 action for those return values that are not explicitly defined. d400 1 a400 1 <tt/reset/. A positive integer, <tt/J/, when specified as the action, d408 3 a410 35 <itemize> <item><tt/ignore/ - when used with a stack of modules, the module's return status will not contribute to the return code the application obtains. <item><tt/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. <item><tt/die/ - equivalent to <tt/bad/ with the side effect of terminating the module stack and PAM immediately returning to the application. <item><tt/ok/ - this tells <bf/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 <tt/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. <item><tt/done/ - equivalent to <tt/ok/ with the side effect of terminating the module stack and PAM immediately returning to the application. <item><tt/reset/ - clear all memory of the state of the module stack and start again with the next stacked module. </itemize> Just to get a feel for the power of this new syntax, here is a taste of what you can do with it. With <bf/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 ``<tt/[ ... 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. Flexible eh? d434 1 a434 1 Any line in (one of) the configuration file(s), that is not formatted d456 4 a459 4 used. The other mode (and the one currently supported by Red Hat 4.2 and higher) is to use both <tt>/etc/pam.d/</tt> and <tt>/etc/pam.conf</tt> in sequence. In this mode, entries in <tt>/etc/pam.d/</tt> override those of <tt>/etc/pam.conf</tt>. a562 14 <tag><tt/expose_account/</tag> 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 <tt/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. d684 11 a803 22 Here is a sample "other" configuration file. The <em/pam_deny/ module will deny access and the <em/pam_warn/ module will send a syslog message to <tt/auth.notice/: <tscreen> <verb> # # The PAM configuration file for the `other' service # auth required pam_deny.so auth required pam_warn.so account required pam_deny.so account required pam_warn.so password required pam_deny.so password required pam_warn.so session required pam_deny.so session required pam_warn.so </verb> </tscreen> d850 2 a851 2 url="http://www.kernel.org/pub/linux/libs/pam/pre/doc/rfc86.0.txt.gz" name="http://www.kernel.org/pub/linux/libs/pam/pre/doc/rfc86.0.txt.gz"></tt> d878 1 a878 1 This document was written by Andrew G. Morgan (morgan@@kernel.org) d880 29 a908 1  d924 3 d933 1 a933 1 Copyright (c) Andrew G. Morgan 1996-9. All rights reserved. d935 1 a935 1 Email: <tt><morgan@@linux.kernel.org></tt> d983 1 a983 1 <tt>$Id: pam_source.sgml,v 1.5 2001/03/19 01:46:41 agmorgan Exp $</tt> @