This is the README for pam_unix in Linux-PAM-0.66.
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pam_unix comes as four separate modules:

pam_unix_auth:		authentication module providing
		pam_authenticate() and pam_setcred() hooks

       The following options are recognized:
               debug:          enables logging of debug information
               audit:          enables even more extensive logging
               try_first_pass: tries to acquire the authentication
                               token set by an earlier module
               use_first_pass: like try_first_pass, but causes the
                               module to fail if the authtok isn't
                               available
               nullok:         allows null passwords (default is to
                               reject)

       Credential facilities are trivial (function simply returns)

pam_unix_sess:		session module, providing session logging

	"debug" and "trace" arguments are accepted, which indicate the
	logging-level for syslog.

		"debug" ->	LOG_DEBUG    [ also default ]
		"trace" ->	LOG_AUTHPRIV

pam_unix_acct:		account management, providing shadow account
		managment features, password aging etc..

	NO options are recognized. Account managment trivial without
	shadow active.

pam_unix_passwd:	password updating facilities providing
		cracklib password strength checking facilities.

	if compiled, the default behavior is to check passwords
	strictly using CrackLib. This behavior can be turned off
	with the argument

		"strict=false"


	the "hash" argument is also accepted, allowing the administrator to
	specify the algorithm used to hash passwords, e.g.,

		"hash=md5"

	currently recognized values are "md5" and "crypt".  This is only
	supported on systems where crypt() accepts alternate salts.

	invalid arguments are logged to syslog.

