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NAME |SYNOPSIS |DESCRIPTION |RATIONALE |EXAMPLE |RETURN VALUES |SEE ALSO |STANDARDS |COLOPHON | |
PAM_FAIL_DELAY(3) Linux-PAM ManualPAM_FAIL_DELAY(3)pam_fail_delay - request a delay on failure
#include <security/pam_appl.h>int pam_fail_delay(pam_handle_t *pamh, unsigned intusec);
Thepam_fail_delayfunction provides a mechanism by which an application or module can suggest a minimum delay ofusec micro-seconds. The function keeps a record of the longest time requested with this function. Shouldpam_authenticate(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. 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 occursafter all authentication modules have been called, butbefore 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 thePAM_FAIL_DELAY item. It can be queried and set withpam_get_item(3) andpam_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 theretval return code of the module stack, theusec_delay micro-second delay that libpam is requesting and theappdata_ptr that the application has associated with the currentpamh. This last value was set by the application when it calledpam_start(3) or explicitly withpam_set_item(3). 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.
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 ofshort timeouts, it may prove possible to attempt abrute 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 acovert 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.
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 1.5 and 4.5 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 2 and 6 seconds.
PAM_SUCCESS Delay was successful adjusted. PAM_SYSTEM_ERR A NULL pointer was submitted as PAM handle.
pam_start(3),pam_get_item(3),pam_strerror(3)
Thepam_fail_delayfunction is an Linux-PAM extension.
This page is part of thelinux-pam (Pluggable Authentication Modules for Linux) project. Information about the project can be found at ⟨http://www.linux-pam.org/⟩. If you have a bug report for this manual page, see ⟨//www.linux-pam.org/⟩. This page was obtained from the project's upstream Git repository ⟨https://github.com/linux-pam/linux-pam.git⟩ on 2023-12-22. (At that time, the date of the most recent commit that was found in the repository was 2023-12-18.) If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up-to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux-PAM Manual 12/22/2023PAM_FAIL_DELAY(3)Pages that refer to this page:pam_get_item(3), pam_set_item(3), pam_faildelay(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |