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pam_tacplus

This PAM module support the following functions:

  • authentication
  • authorization (account management)
  • accounting (session management)

All are performed using TACACS+ protocol [1], designed by Cisco Systems. This is remote AAA protocol, supported by most Cisco hardware. A free TACACS+ server is available [2], which I'm using without any major problems for about a year. Advantages of TACACS+ is that all (unlike RADIUS) packets exchanged with the authentication server are encrypted. This module is an attempt to provide most useful part of TACACS+ functionality to applications using the PAM interface on Linux.

Persistent connections are not supported, because libtac is single threaded. You must make a new connection via tac_connect_single() or equivalent for each new accounting, authorization, or authentication request.

Recognized options:

Option Management group Description
debug ALL output debugging information via syslog(3); note, that the debugging is heavy, including passwords!
secret=STRING ALL can be specified more than once; secret key used to encrypt/decrypt packets sent/received from the server
server=HOSTNAME server=IP_ADDR server=HOSTNAME:PORT server=IP_ADDR:PORT auth, session can be specified more than once; adds a TACACS+ server to the servers list
timeout=INT ALL connection timeout in seconds default is 5 seconds
login=STRING auth TACACS+ authentication service, this can be "shell", "pap", "chap" or "login" at the moment. Default is pap.
prompt=STRING auth Custom password prompt. If you want to use a space use '_' character instead.
acct_all session if multiple servers are supplied, pam_tacplus will send accounting start/stop packets to all servers on the list
service account, session TACACS+ service for authorization and accounting
protocol account, session TACACS+ protocol for authorization and accounting

The last two items are widely described in TACACS+ draft [1]. They are required by the server, but it will work if they don't match the real service authorized :) During PAM account the AV pairs returned by the TACACS+ servers are made available to the PAM environment, so you can use i.e. pam_exec.so to do something with these AV pairs. Not all service types return AV pairs. If you need privilege levels for accounting, e.g., at least some servers require the service to be "shell"

Basic installation:

This project is using autotools for building, so please run autoreconf first.

$ autoreconf -i
$ ./configure && make && sudo make install

Example configuration

(This will be different with systems such as Debian Wheezy and recent
 Ubuntu that use the common-* configfile method pam-auth-update).

Also see Pam.d.common-example for examples with user mapping,  and more
comments.

Common parameters can also be set in /etc/tacplus_servers, rather than
the commandline by using the include=/etc/tacplus_servers paramter.
For the secret parameter, this also improves security

#%PAM-1.0
auth       required     /lib/security/pam_tacplus.so debug server=1.1.1.1 secret=SECRET-1
account	   required	/lib/security/pam_tacplus.so debug secret=SECRET-1 service=ppp protocol=lcp
account    sufficient	/lib/security/pam_exec.so /usr/local/bin/showenv.sh
password   required	/lib/security/pam_cracklib.so
password   required	/lib/security/pam_pwdb.so shadow use_authtok
session    required	/lib/security/pam_tacplus.so debug server=1.1.1.1 server=2.2.2.2 secret=SECRET-1 secret=SECRET-2 service=ppp protocol=lcp

If you need AV attributes back, such as privilege level, then for some servers, you'll need to use service=shell for "account"

More on server lists:

  1. Having more that one TACACS+ server defined for given management group has following effects on authentication:

    • if the first server on the list is unreachable or failing pam_tacplus will try to authenticate the user against the other servers until it succeeds

    • the `first_hit' option has been deprecated

    • when the authentication function gets a positive reply from a server, it saves its address for future use by account management function (see below)

  2. The account management (authorization) function asks only one TACACS+ server and it ignores the whole server list passed from command line. It uses server saved by authentication function after successful authenticating user on that server. We assume that the server is authoriative for queries about that user.

  3. The session management (accounting) functions obtain their server lists independently from the other functions. This allows you to account user sessions on different servers than those used for authentication and authorization.

    • normally, without the `acct_all' modifier, the extra servers on the list will be considered as backup servers, mostly like in point 1. i.e. they will be used only if the first server on the list will fail to accept our accounting packets

    • with `acct_all' pam_tacplus will try to deliver the accounting packets to all servers on the list; failure of one of the servers will make it try another one

      this is useful when your have several accounting, billing or logging hosts and want to have the accounting information appear on all of them at the same time

Short introduction to PAM via TACACS+:

This diagram should show general idea of how the whole process looks:

                                              +-----+
          Authen -user/pass valid?----------> | T S |
        /                                     | A e |
     PAM- Author -service allowed?----------> | C r |
      ^ \                                     | A v |
      |   Acct ,-----start session----------> | C e |
      |         `----stop session-----------> | S r |
  Application                                 +-----+

  *Client Host*          *Network*           *Server Host*

Consider `login' application:

  1. Login accepts username and password from the user.
  2. Login calls PAM function pam_authenticate() to verify if the supplied username/password pair is valid.
  3. PAM loads pam_tacplus module (as defined in /etc/pam.d/login) and calls pam_sm_authenticate() function supplied by this module.
  4. This function sends an encrypted packet to the TACACS+ server. The packet contains username and password to verify. TACACS+ server replied with either positive or negative response. If the reponse is negative, the whole thing is over ;)
  5. PAM calls another function from pam_tacplus - pam_sm_acct_mgmt(). This function is expected to verify whether the user is allowed to get the service he's requesting (in this case: unix shell). The function again verifies the permission on TACACS+ server. Assume the server granted the user with requested service.
  6. Before user gets the shell, PAM calls one another function from pam_tacplus - pam_sm_open_session(). This results in sending an accounting START packet to the server. Among other things it contains the terminal user loggen in on and the time session started.
  7. When user logs out, pam_sm_close_session() sends STOP packet to the server. The whole session is closed.

TACACS+ client program

The library comes with a simple TACACS+ client program tacc which can be used for testing as well as simple scripting. Sample usage:

tacc --authenticate --authorize --account --username test1
    --password test1 --server localhost --remote 1.1.1.1
    --secret test1 --service ppp --protocol ip

This configuration runs full AAA round (authentication, authorization and accounting). The server and secret option specify server connection parameters and all remaining options supply data specific to TACACS+ protocol. The tac_plus daemon (found in tacacs+ package in Debian and Ubuntu) can be used for testing with the following example configuration:

key = test1
user = test1 {
    global = cleartext "test1"
    service = ppp protocol = ip {
            addr=8.8.8.8
    }
}

Limitations:

Many of them for now :)

  • only subset of TACACS+ protocol is supported; it's enough for most need, though
  • utilize PAM_SERVICE item obtained from PAM for TACACS+ services
  • clean options and configuration code

Authors:

Pawel Krawczyk [email protected] https://ipsec.pl/

Jeroen Nijhof [email protected]