PAM authenticate a user in C

Question

Ok, so I'm a pretty awful coder, and I'm wondering (as the title says) how I can authenticate a Linux user with PAM using C. The only thing I really understand is including the pam_appl.h and pam_misc.h header files, that I need to place pam_start and pam_end somewhere in my code, and not much else. So how would I achieve what I'm looking for?

Answer 1

You need to use the pam_authenticate function.

Working with PAM requires answers that are a bit longer than the average StackOverflow answer; but, fortunately there is an entire online book on the subject http://www.linux-pam.org/Linux-PAM-html/Linux-PAM_ADG.html

If you want to just get a code snippet, you can start with this one

#include <security/pam_appl.h>
#include <security/pam_misc.h>
#include <stdio.h>

static struct pam_conv conv = {
    misc_conv,
    NULL
};

int main(int argc, char *argv[])
{
    pam_handle_t *pamh=NULL;
    int retval;
    const char *user="nobody";

    if(argc == 2) {
        user = argv[1];
    }

    if(argc > 2) {
        fprintf(stderr, "Usage: check_user [username]\n");
        exit(1);
    }

    retval = pam_start("check_user", user, &conv, &pamh);

    if (retval == PAM_SUCCESS)
        retval = pam_authenticate(pamh, 0);    /* is user really user? */

    if (retval == PAM_SUCCESS)
        retval = pam_acct_mgmt(pamh, 0);       /* permitted access? */

    /* This is where we have been authorized or not. */

    if (retval == PAM_SUCCESS) {
        fprintf(stdout, "Authenticated\n");
    } else {
        fprintf(stdout, "Not Authenticated\n");
    }

    if (pam_end(pamh,retval) != PAM_SUCCESS) {     /* close Linux-PAM */
        pamh = NULL;
        fprintf(stderr, "check_user: failed to release authenticator\n");
        exit(1);
    }

    return ( retval == PAM_SUCCESS ? 0:1 );       /* indicate success */
}

Which will work with a correct PAM setup including

  You need to add the following (or equivalent) to the
  /etc/pam.d/check_user file:
  # check authorization
  auth       required     pam_unix.so
  account    required     pam_unix.so
 */

as detailed within the mentioned book. The original source of this code snippet can be found at http://www.linux-pam.org/Linux-PAM-html/adg-example.html