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pam_fail_delay(3) [opendarwin man page]

PAM_FAIL_DELAY(3)						Programmers' Manual						 PAM_FAIL_DELAY(3)

NAME
pam_fail_delay - request a delay on failure SYNOPSIS
#include <security/pam_appl.h> or, #include <security/pam_modules.h> int pam_fail_delay(pam_handle_t *pamh, unsigned int usec); DESCRIPTION
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 short timeouts, it may prove possible to attempt a brute force 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 (indi- cating 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 covert channel of useful information. To minimize the effectiveness of such attacks, it is desirable to introduce a random delay in a failed authentication process. Linux-PAM provides such a facility. The delay occurs upon failure of the pam_authenticate(3) and pam_chauthtok(3) functions. It occurs after all authentication modules have been called, but before control is returned to the service application. The function, pam_fail_delay(3), is used to specify a required minimum for the length of the failure-delay; the usec argument. This func- tion can be called by the service application and/or the authentication modules, both may have an interest in delaying a reapplication for service by the user. The length of the delay is computed at the time it is required. Its length is pseudo-gausianly distributed about the maximum requested value; the resultant delay will differ by as much as 25% of this maximum requested value (both up and down). On return from pam_authenticate(3) or pam_chauthtok(3), independent of success or failure, the new requested delay is reset to its default value: zero. EXAMPLE
For example, a login application may require a failure delay of roughly 3 seconds. It will contain the following code: pam_fail_delay(pamh, 3000000 /* micro-seconds */ ); pam_authenticate(pamh, 0); if the modules do not request a delay, the failure delay will be between 2.25 and 3.75 seconds. However, the modules, invoked in the authentication process, may also request delays: (module #1) pam_fail_delay(pamh, 2000000); (module #2) pam_fail_delay(pamh, 4000000); in this case, it is the largest requested value that is used to compute the actual failed delay: here between 3 and 5 seconds. RETURN VALUE
Following a successful call to pam_fail_delay(3), PAM_SUCCESS is returned. All other returns should be considered serious failures. ERRORS
May be translated to text with pam_strerror(3). CONFORMING TO
Under consideration by the X/Open group for future inclusion in the PAM RFC. 1996/1/10 BUGS
none known. SEE ALSO
pam_start(3), pam_get_item(3) and pam_strerror(3). Also, see the three Linux-PAM Guides, for System administrators, module developers, and application developers. Linux-PAM 0.56 1997 Jan 12 PAM_FAIL_DELAY(3)

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PAM_FAIL_DELAY(3)						 Linux-PAM Manual						 PAM_FAIL_DELAY(3)

NAME
pam_fail_delay - request a delay on failure SYNOPSIS
#include <security/pam_appl.h> int pam_fail_delay(pam_handle_t *pamh, unsigned int usec); DESCRIPTION
The pam_fail_delay function provides a mechanism by which an application or module can suggest a minimum delay of usec micro-seconds. The function keeps a record of the longest time requested with this function. Should pam_authenticate(3) fail, the failing return to the application is delayed by an amount of time randomly distributed (by up to 25%) about this longest value. 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 after all authentication modules have been called, but before control is returned to the service application. When using this function the programmer should check if it is available with: #ifdef HAVE_PAM_FAIL_DELAY .... #endif /* HAVE_PAM_FAIL_DELAY */ 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 PAM_FAIL_DELAY item. It can be queried and set with pam_get_item(3) and pam_set_item (3) respectively. The value used to set it should be a function pointer of the following prototype: void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr); The arguments being the retval return code of the module stack, the usec_delay micro-second delay that libpam is requesting and the appdata_ptr that the application has associated with the current pamh. This last value was set by the application when it called pam_start(3) or explicitly with pam_set_item(3). Note, if PAM_FAIL_DELAY item is unset (or set to NULL), then no delay will be performed. RATIONALE
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 short timeouts, it may prove possible to attempt a brute force 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 covert channel of useful information. 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. EXAMPLE
For example, a login application may require a failure delay of roughly 3 seconds. It will contain the following code: pam_fail_delay (pamh, 3000000 /* micro-seconds */ ); pam_authenticate (pamh, 0); if the modules do not request a delay, the failure delay will be between 2.25 and 3.75 seconds. However, the modules, invoked in the authentication process, may also request delays: module #1: pam_fail_delay (pamh, 2000000); module #2: pam_fail_delay (pamh, 4000000); in this case, it is the largest requested value that is used to compute the actual failed delay: here between 3 and 5 seconds. RETURN VALUES
PAM_SUCCESS Delay was successful adjusted. PAM_SYSTEM_ERR A NULL pointer was submitted as PAM handle. SEE ALSO
pam_start(3), pam_get_item(3), pam_strerror(3) STANDARDS
The pam_fail_delay function is an Linux-PAM extension. Linux-PAM Manual 06/04/2011 PAM_FAIL_DELAY(3)

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