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https://github.com/google/libpam-policycache

Password caching module with advanced policies for PAM.
https://github.com/google/libpam-policycache

pam

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Password caching module with advanced policies for PAM.

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README 

        
# Overview



Caches passwords from other PAM modules and bypasses those modules based on a

configurable policy.



Why use libpam-policycache?



* Allows users to login when network is down.

* Reduces latency and load on remote login services.

* Restricts cache usage to specific users and groups.

* Enforces policies like cached password lifetime and rotation.



# Module Configuration



Arguments:



* `action=check`: Prompt the user for a password and check it against the cache.

* `action=update`: Set the user's cached password to the last successful

  password in the stack.

* `try_first_pass`: Try to use the password from a previous module (check only).

* `use_first_pass`: Only use the password from a previous module (check only).

* `policy=`: Default path-glob is

  `/etc/libpam-policycache.d/*.policy`.

* `storage=`: Default path is `/var/cache/libpam-policycache`.



## Example



```

auth [success=2 new_authtok_reqd=ok default=ignore] pam_policycache.so action=check

auth [success=ok new_authtok_reqd=ok default=die] pam_krb5.so use_first_pass

auth [default=ignore] pam_policycache.so action=update

```



# Policy Configuration



Each policy file is INI-style with one section for each potential policy. The

first of the most specific matching sections is chosen.



The section's name describes who the policy applies to:



* `user:`

* `group:`

* `netgroup:`



Each section has a combination of attributes:



* `tries`: Maximum number of failed attempts before entry is invalid.

* `refresh`: Duration (1h, 5d, 3w) an entry stays valid between successful uses

  (action=check).

* `renew`: Duration after the last update (action=update) when a successful use

  (action=check) will return `new_authtok_req` instead of `success`. Used when

  a password should be tried against another module opportunistically before the

  entry expires.

* `expire`: Duration after the last update (action=update) when the cache entry

  is marked invalid.



## TODO



* Add a `backoff` timer to rate-limit renew attempts.

* Add a `reset-tries` timer to reset the try count on an entry.

* Choose between "best-match" and "first-match" for the preferred policy.



## Example



In the following example policy, user "janedoe" may be the owner of the machine

and her login experience is similar to having a password in /etc/shadow. Her

password only needs to be checked by another module once a year.



Others in the "users" group may also use the cache, but only for a short time.

Their entries are evicted from the cache after an hour without use or two days

without being verified again using another module.



```

# /etc/libpam-policycache.d/foo.policy



[user:janedoe]

renew=1w

expire=52w



[group:users]

tries=3

refresh=1h

renew=1d

expire=2d

```



# Cache Storage



Policy entries are stored in /var/cache/libpam-policycache by default. Each

entry is stored in a file for the user it belongs to.



The cache entry format is human-readable for easier debugging:

```

{'version': <1>, 'tries': <0>, 'algorithm': <'SHA256'>, 'salt': <'0B8BAA809CDCA339910EE8F6F9FE22A5'>, 'hash': <'14BABCBC943B302EFDCC137419F7D3FB736602D77CF42975A6778A5B7F2D63CD'>, 'last_verified': <'2014-03-28T23:14:21Z'>, 'last_used': <'2014-03-28T23:14:21Z'>, 'last_tried': <'2014-03-28T23:14:21Z'>}

```



# User Applications



Applications like screensavers that try to use the cache directly will be

denied, since the cache directory is only usable by root. Those applications

should use `pam_escalate.so` instead, which proxies the authentication prompt(s)

through a pipe to a setuid-root helper called `pam-escalate-helper`. The helper

calls `pam_start()` and `pam_authenticate()` just like an application would, and

it always uses `/etc/pam.d/escalate` for the service configuration. It checks in

advance that the user calling it is the same user as the one who should be

authenticated so it doesn't work for cases like web oder mail servers who check

system users.



The escalate module clears all environment variables, ignores some PAM items,

and only implements `pam_sm_authenticate()` so it may not cover all use cases.



At some point before or during the 1.0 release, the escalate module will be

moved to its own project.



## Example



```

# /etc/pam.d/gnome-screensaver

auth requisite pam_escalate.so

```



```

# /etc/pam.d/escalate

# Include common-auth where cache check/update and other auth modules live.

@include common-auth

```



# Building



Built with autotools and includes an autogen.sh script:



1. ./autogen.sh (not needed when using source tarball)

2. ./configure

3. make

4. make check

5. sudo make install



Dependencies:



* GLib

* Linux-PAM

* libscrypt

* Autoconf, Automake, Libtool