TACACS+ protocol client library and PAM module in C. This PAM module support authentication, authorization (account management) and accounting (session management)performed using TACACS+ protocol designed by Cisco.
Clone or download


Build Status Analysis Status GitHub forks GitHub license Total alerts Coverity

TACACS+ client toolkit

This repository contains three modules that are typically used to perform requests to a TACACS+ server:

  • libtac - core TACACS+ client library
  • pam_tacplus - PAM module for authenticating users against TACACS+
  • tacc - a simple command-line TACACS+ client

The following core TACACS+ functions are supported:

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

The TACACS+ protocol was designed by Cisco Systems back in 90's and was intended to provide simple means of validating users connecting to simple network routers (e.g. over PPP) against a central authentication server. The router can send queries about authentication (validate user credentials), authorization (entitlement for requested service) and accounting (marking the start and end of user's session). The server can respond with either simple yes/no response, or send back attributes, such as text of a password prompt, effectively instructing the router to present it to the user and send back the obtained password.

Unlike RADIUS, which was designed for similar purposes, the TACACS+ protocol offers basic packet encryption but, as with most crypto designed back then, it's not secure and definitely should not be used over untrusted networks.

This package has been successfully used with free tac_plus TACACS+ server on variety of operating systems.

Recognized options:

Option Management group Description
debug ALL output debugging information via syslog(3); note, that the debugging is heavy, including passwords!
secret ALL string can be specified more than once; secret key used to encrypt/decrypt packets sent/received from the server
server auth, session string hostname, IP or hostname:port, can be specified more than once
timeout ALL integer connection timeout in seconds; default is 5 seconds
login auth TACACS+ authentication service, this can be pap, chap or login; default is pap
prompt auth string custom password prompt; use _ instead of spaces
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 string TACACS+ service for authorization and accounting
protocol account, session string TACACS+ protocol for authorization and accounting

Semantics of these options only makes sense in the context of the TACACS+ specification - for example, a dial-up router might request ppp service with protocol ip for their users, authenticating them with pap protocol which reflects the typical usage of TACACS+ back in 90's. These values however do not really need to match the actual service offered by your server as the TACACS+ server only cares about the service and protocol fields matching what it has in its configuration.

Basic installation:

The code uses standard GNU autotools:

$ ./auto.sh
$ ./configure && make && sudo make install

Example configuration:

auth       required     /lib/security/pam_tacplus.so debug server= 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= server= secret=SECRET-1 secret=SECRET-2 service=ppp protocol=lcp

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 user1
    --password pass1 --server localhost --remote
    --secret enckey1 --service ppp --protocol ip --login pap

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 = enckey1
user = user1 {
    global = cleartext "pass1"
    service = ppp protocol = ip {

For debugging run the tac_plus server with the following options - the -d 512 will debug encryption, for other values see man 8 tac_plus:

tac_plus -C /etc/tacacs+/tac_plus.conf -G -g -d 512


  • only subset of TACACS+ protocol is supported; it's enough for most need, though
  • tacc does not support password prompts and other interactive protocol features


Pawel Krawczyk pawel.krawczyk@hush.com https://ipsec.pl/

Jeroen Nijhof jeroen@jeroennijhof.nl