'\" t
.\"     Title: pam_fail_delay
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: 09/03/2021
.\"    Manual: Linux-PAM Manual
.\"    Source: Linux-PAM Manual
.\"  Language: English
.\"
.TH "PAM_FAIL_DELAY" "3" "09/03/2021" "Linux-PAM Manual" "Linux-PAM Manual"
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.SH "NAME"
pam_fail_delay \- request a delay on failure
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <security/pam_appl\&.h>
.fi
.ft
.HP \w'int\ pam_fail_delay('u
.BI "int pam_fail_delay(pam_handle_t\ *" "pamh" ", unsigned\ int\ " "usec" ");"
.SH "DESCRIPTION"
.PP
The
\fBpam_fail_delay\fR
function provides a mechanism by which an application or module can suggest a minimum delay of
\fIusec\fR
micro\-seconds\&. The function keeps a record of the longest time requested with this function\&. Should
\fBpam_authenticate\fR(3)
fail, the failing return to the application is delayed by an amount of time randomly distributed (by up to 50%) about this longest value\&.
.PP
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
\fIafter\fR
all authentication modules have been called, but
\fIbefore\fR
control is returned to the service application\&.
.PP
When using this function the programmer should check if it is available with:
.sp
.if n \{\
.RS 4
.\}
.nf
#ifdef HAVE_PAM_FAIL_DELAY
    \&.\&.\&.\&.
#endif /* HAVE_PAM_FAIL_DELAY */
      
.fi
.if n \{\
.RE
.\}
.PP
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
\fIPAM_FAIL_DELAY\fR
item\&. It can be queried and set with
\fBpam_get_item\fR(3)
and
\fBpam_set_item\fR(3)
respectively\&. The value used to set it should be a function pointer of the following prototype:
.sp
.if n \{\
.RS 4
.\}
.nf
void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);
      
.fi
.if n \{\
.RE
.\}
.sp
The arguments being the
\fIretval\fR
return code of the module stack, the
\fIusec_delay\fR
micro\-second delay that libpam is requesting and the
\fIappdata_ptr\fR
that the application has associated with the current
\fIpamh\fR\&. This last value was set by the application when it called
\fBpam_start\fR(3)
or explicitly with
\fBpam_set_item\fR(3)\&.
.PP
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\&.
.SH "RATIONALE"
.PP
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
\fIshort\fR
timeouts, it may prove possible to attempt a
\fIbrute force\fR
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
\fIcovert channel\fR
of useful information\&.
.PP
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\&.
.SH "EXAMPLE"
.PP
For example, a login application may require a failure delay of roughly 3 seconds\&. It will contain the following code:
.sp
.if n \{\
.RS 4
.\}
.nf
    pam_fail_delay (pamh, 3000000 /* micro\-seconds */ );
    pam_authenticate (pamh, 0);
    
.fi
.if n \{\
.RE
.\}
.PP
if the modules do not request a delay, the failure delay will be between 1\&.5 and 4\&.5 seconds\&.
.PP
However, the modules, invoked in the authentication process, may also request delays:
.sp
.if n \{\
.RS 4
.\}
.nf
module #1:    pam_fail_delay (pamh, 2000000);
module #2:    pam_fail_delay (pamh, 4000000);
    
.fi
.if n \{\
.RE
.\}
.PP
in this case, it is the largest requested value that is used to compute the actual failed delay: here between 2 and 6 seconds\&.
.SH "RETURN VALUES"
.PP
PAM_SUCCESS
.RS 4
Delay was successful adjusted\&.
.RE
.PP
PAM_SYSTEM_ERR
.RS 4
A NULL pointer was submitted as PAM handle\&.
.RE
.SH "SEE ALSO"
.PP
\fBpam_start\fR(3),
\fBpam_get_item\fR(3),
\fBpam_strerror\fR(3)
.SH "STANDARDS"
.PP
The
\fBpam_fail_delay\fR
function is an Linux\-PAM extension\&.