<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="pam_fail_delay">

  <refmeta>
    <refentrytitle>pam_fail_delay</refentrytitle>
    <manvolnum>3</manvolnum>
    <refmiscinfo class="source">Linux-PAM</refmiscinfo>
    <refmiscinfo class="manual">Linux-PAM Manual</refmiscinfo>
  </refmeta>

  <refnamediv xml:id="pam_fail_delay-name">
    <refname>pam_fail_delay</refname>
    <refpurpose>request a delay on failure</refpurpose>
  </refnamediv>

<!-- body begins here -->

  <refsynopsisdiv>
    <funcsynopsis xml:id="pam_fail_delay-synopsis">
      <funcsynopsisinfo>#include &lt;security/pam_appl.h&gt;</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 xml: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 50%)
      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>.
    </para>
    <para>
      Note that the PAM_FAIL_DELAY item is set to NULL by default. This
      indicates that PAM should perform a random delay as described
      above when authentication fails and a delay has been suggested.
      If an application does not want the PAM library to perform any
      delay on authentication failure, then the application must define
      a custom delay function that executes no statements and set
      the PAM_FAIL_DELAY item to point to this function.
    </para>
  </refsect1>

  <refsect1 xml: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 xml: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 1.5 and 4.5 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 2 and 6 seconds.
    </para>
  </refsect1>

  <refsect1 xml: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 xml: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 xml:id="pam_fail_delay-standards">
    <title>STANDARDS</title>
    <para>
      The <function>pam_fail_delay</function> function is an
      Linux-PAM extension.
    </para>
  </refsect1>

</refentry>