diff options
Diffstat (limited to 'doc/man/pam_fail_delay.3.xml')
| -rw-r--r-- | doc/man/pam_fail_delay.3.xml | 202 |
1 files changed, 202 insertions, 0 deletions
diff --git a/doc/man/pam_fail_delay.3.xml b/doc/man/pam_fail_delay.3.xml new file mode 100644 index 00000000..a101cf39 --- /dev/null +++ b/doc/man/pam_fail_delay.3.xml @@ -0,0 +1,202 @@ +<?xml version="1.0" encoding="UTF-8"?> +<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" + "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"> + +<refentry id="pam_fail_delay"> + + <refmeta> + <refentrytitle>pam_fail_delay</refentrytitle> + <manvolnum>3</manvolnum> + <refmiscinfo class='setdesc'>Linux-PAM Manual</refmiscinfo> + </refmeta> + + <refnamediv id="pam_fail_delay-name"> + <refname>pam_fail_delay</refname> + <refpurpose>request a delay on failure</refpurpose> + </refnamediv> + +<!-- body begins here --> + + <refsynopsisdiv> + <funcsynopsis id="pam_fail_delay-synopsis"> + <funcsynopsisinfo>#include <security/pam_appl.h></funcsynopsisinfo> + <funcprototype> + <funcdef>int <function>pam_fail_delay</function></funcdef> + <paramdef>pam_handle_t *<parameter>pamh</parameter></paramdef> + <paramdef>unsigned int <parameter>usec</parameter></paramdef> + </funcprototype> + </funcsynopsis> + </refsynopsisdiv> + + <refsect1 id='pam_fail_delay-description'> + <title>DESCRIPTION</title> + <para> + The <function>pam_fail_delay</function> function provides a + mechanism by which an application or module can suggest a minimum + delay of <emphasis>usec</emphasis> micro-seconds. The + function keeps a record of the longest time requested with this + function. Should + <citerefentry> + <refentrytitle>pam_authenticate</refentrytitle><manvolnum>3</manvolnum> + </citerefentry> fail, the failing return to the application is + delayed by an amount of time randomly distributed (by up to 25%) + about this longest value. + </para> + <para> + Independent of success, the delay time is reset to its zero + default value when the PAM service module returns control to + the application. The delay occurs <emphasis>after</emphasis> all + authentication modules have been called, but <emphasis>before</emphasis> + control is returned to the service application. + </para> + <para> + When using this function the programmer should check if it is + available with: + </para> + <programlisting> +#ifdef HAVE_PAM_FAIL_DELAY + .... +#endif /* HAVE_PAM_FAIL_DELAY */ + </programlisting> + + <para> + For applications written with a single thread that are event + driven in nature, generating this delay may be undesirable. + Instead, the application may want to register the delay in some + other way. For example, in a single threaded server that serves + multiple authentication requests from a single event loop, the + application might want to simply mark a given connection as + blocked until an application timer expires. For this reason + the delay function can be changed with the + <emphasis>PAM_FAIL_DELAY</emphasis> item. It can be queried and + set with + <citerefentry> + <refentrytitle>pam_get_item</refentrytitle><manvolnum>3</manvolnum> + </citerefentry> + and + <citerefentry> + <refentrytitle>pam_set_item </refentrytitle><manvolnum>3</manvolnum> + </citerefentry> respectively. The value used to set it should be + a function pointer of the following prototype: + <programlisting> +void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr); + </programlisting> + The arguments being the <emphasis>retval</emphasis> return code + of the module stack, the <emphasis>usec_delay</emphasis> + micro-second delay that libpam is requesting and the + <emphasis>appdata_ptr</emphasis> that the application has associated + with the current <emphasis>pamh</emphasis>. This last value was set + by the application when it called + <citerefentry> + <refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum> + </citerefentry> or explicitly with + <citerefentry> + <refentrytitle>pam_set_item</refentrytitle><manvolnum>3</manvolnum> + </citerefentry>. + Note, if PAM_FAIL_DELAY item is unset (or set to NULL), then no delay + will be performed. + </para> + </refsect1> + + <refsect1 id='pam_fail_delay-rationale'> + <title>RATIONALE</title> + <para> + It is often possible to attack an authentication scheme by exploiting + the time it takes the scheme to deny access to an applicant user. In + cases of <emphasis>short</emphasis> timeouts, it may prove possible + to attempt a <emphasis>brute force</emphasis> dictionary attack -- + with an automated process, the attacker tries all possible passwords + to gain access to the system. In other cases, where individual + failures can take measurable amounts of time (indicating the nature + of the failure), an attacker can obtain useful information about the + authentication process. These latter attacks make use of procedural + delays that constitute a <emphasis>covert channel</emphasis> + of useful information. + </para> + <para> + To minimize the effectiveness of such attacks, it is desirable to + introduce a random delay in a failed authentication process. + Preferable this value should be set by the application or a special + PAM module. Standard PAM modules should not modify the delay + unconditional. + </para> + </refsect1> + + <refsect1 id='pam_fail_delay-example'> + <title>EXAMPLE</title> + <para> + For example, a login application may require a failure delay of + roughly 3 seconds. It will contain the following code: + </para> + <programlisting> + pam_fail_delay (pamh, 3000000 /* micro-seconds */ ); + pam_authenticate (pamh, 0); + </programlisting> + + <para> + if the modules do not request a delay, the failure delay will be + between 2.25 and 3.75 seconds. + </para> + + <para> + However, the modules, invoked in the authentication process, may + also request delays: + </para> + + <programlisting> +module #1: pam_fail_delay (pamh, 2000000); +module #2: pam_fail_delay (pamh, 4000000); + </programlisting> + + <para> + in this case, it is the largest requested value that is used to + compute the actual failed delay: here between 3 and 5 seconds. + </para> + </refsect1> + + <refsect1 id='pam_fail_delay-return_values'> + <title>RETURN VALUES</title> + <variablelist> + <varlistentry> + <term>PAM_SUCCESS</term> + <listitem> + <para> + Delay was successful adjusted. + </para> + </listitem> + </varlistentry> + <varlistentry> + <term>PAM_SYSTEM_ERR</term> + <listitem> + <para> + A NULL pointer was submitted as PAM handle. + </para> + </listitem> + </varlistentry> + </variablelist> + </refsect1> + + <refsect1 id='pam_fail_delay-see_also'> + <title>SEE ALSO</title> + <para> + <citerefentry> + <refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum> + </citerefentry>, + <citerefentry> + <refentrytitle>pam_get_item</refentrytitle><manvolnum>3</manvolnum> + </citerefentry>, + <citerefentry> + <refentrytitle>pam_strerror</refentrytitle><manvolnum>3</manvolnum> + </citerefentry> + </para> + </refsect1> + + <refsect1 id='pam_fail_delay-standards'> + <title>STANDARDS</title> + <para> + The <function>pam_fail_delay</function> function is an + Linux-PAM extension. + </para> + </refsect1> + +</refentry> |