6.37. pam_unix - traditional password authentication
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## 6.37. pam_unix - traditional password authentication

`pam_unix.so` [ ... ]

### 6.37.1. DESCRIPTION

This is the standard Unix authentication module. It uses standard calls from the system's libraries to retrieve and set account information as well as authentication. Usually this is obtained from the /etc/passwd and the /etc/shadow file as well if shadow is enabled.

The account component performs the task of establishing the status of the user's account and password based on the following shadow elements: expire, last_change, max_change, min_change, warn_change. In the case of the latter, it may offer advice to the user on changing their password or, through the PAM_AUTHTOKEN_REQD return, delay giving service to the user until they have established a new password. The entries listed above are documented in the shadow(5) manual page. Should the user's record not contain one or more of these entries, the corresponding shadow check is not performed.

The authentication component performs the task of checking the users credentials (password). The default action of this module is to not permit the user access to a service if their official password is blank.

A helper binary, unix_chkpwd(8), is provided to check the user's password when it is stored in a read protected database. This binary is very simple and will only check the password of the user invoking it. It is called transparently on behalf of the user by the authenticating component of this module. In this way it is possible for applications like xlock(1) to work without being setuid-root. The module, by default, will temporarily turn off SIGCHLD handling for the duration of execution of the helper binary. This is generally the right thing to do, as many applications are not prepared to handle this signal from a child they didn't know was `fork()`d. The `noreap` module argument can be used to suppress this temporary shielding and may be needed for use with certain applications.

The maximum length of a password supported by the pam_unix module via the helper binary is PAM_MAX_RESP_SIZE - currently 512 bytes. The rest of the password provided by the conversation function to the module will be ignored.

The password component of this module performs the task of updating the user's password.

The session component of this module logs when a user logins or leave the system.

Remaining arguments, supported by others functions of this module, are silently ignored. Other arguments are logged as errors through syslog(3).

### 6.37.2. OPTIONS

`debug`

  Turns on debugging via syslog(3).

`audit`

  A little more extreme than debug.

`nullok`

  The default action of this module is to not permit the user access to a service if their official password is blank. The `nullok` argument overrides this default.

`try_first_pass`

  Before prompting the user for their password, the module first tries the previous stacked module's password in case that satisfies this module as well.

`use_first_pass`

  The argument `use_first_pass` forces the module to use a previous stacked modules password and will never prompt the user - if no password is available or the password is not appropriate, the user will be denied access.

`nodelay`

  This argument can be used to discourage the authentication component from requesting a delay should the authentication as a whole fail. The default action is for the module to request a delay-on-failure of the order of two second.

`use_authtok`

  When password changing enforce the module to set the new password to the one provided by a previously stacked `password` module (this is used in the example of the stacking of the pam_cracklib module documented below).

`not_set_pass`

  This argument is used to inform the module that it is not to pay attention to/make available the old or new passwords from/to other (stacked) password modules.

`nis`

  NIS RPC is used for setting new passwords.

`remember=`n``

  The last `n` passwords for each user are saved in `/etc/security/opasswd` in order to force password change history and keep the user from alternating between the same password too frequently.

`shadow`

  Try to maintain a shadow based system.

`md5`

  When a user changes their password next, encrypt it with the MD5 algorithm.

`bigcrypt`

  When a user changes their password next, encrypt it with the DEC C2 algorithm.

`sha256`

  When a user changes their password next, encrypt it with the SHA256 algorithm. If the SHA256 algorithm is not known to the crypt(3) function, fall back to MD5.

`sha512`

  When a user changes their password next, encrypt it with the SHA512 algorithm. If the SHA512 algorithm is not known to the crypt(3) function, fall back to MD5.

`blowfish`

  When a user changes their password next, encrypt it with the blowfish algorithm. If the blowfish algorithm is not known to the crypt(3) function, fall back to MD5.

`rounds=`n``

  Set the optional number of rounds of the SHA256, SHA512 and blowfish password hashing algorithms to `n`.

`broken_shadow`

  Ignore errors reading shadow information for users in the account management module.

`no_pass_expiry`

  When set ignore password expiration as defined by the shadow entry of the user. The option has an effect only in case pam_unix was not used for the authentication or it returned authentication failure meaning that other authentication source or method succeeded. The example can be public key authentication in sshd. The module will return PAM_SUCCESS instead of eventual PAM_NEW_AUTHTOK_REQD or PAM_AUTHTOK_EXPIRED.

Invalid arguments are logged with syslog(3).

### 6.37.3. MODULE TYPES PROVIDED

All module types (`account`, `auth`, `password` and `session`) are provided.

### 6.37.4. RETURN VALUES

PAM_IGNORE

  Ignore this module.

### 6.37.5. EXAMPLES

An example usage for `/etc/pam.d/login` would be:

    # Authenticate the user
    auth       required   pam_unix.so
    # Ensure users account and password are still active
    account    required   pam_unix.so
    # Change the users password, but at first check the strength
    # with pam_cracklib(8)
    password   required   pam_cracklib.so retry=3 minlen=6 difok=3
    password   required   pam_unix.so use_authtok nullok md5
    session    required   pam_unix.so

### 6.37.6. AUTHOR

pam_unix was written by various people.

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