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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:

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:

Semantics of these options only make sense in the context of the RFC 8907 (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 module is available on most Linux distibutions:

$ sudo apt install libpam-tacplus

To compile from source, the code uses GNU autotools and gnulib:

$ sudo apt install libpam-tacplus autoconf build-essential libtool automake libpam-dev libssl-dev gnulib
$ git clone https://github.com/kravietz/pam_tacplus.git
$ cd pam-tacplus
$ gnulib-tool --makefile-name=Makefile.gnulib --libtool --import \
                  fcntl crypto/md5 array-list list xlist getrandom realloc-posix \
                  explicit_bzero xalloc getopt-gnu
$ autoreconf -f -v -i
$ ./configure
$ make
$ sudo make install

You can use ./configure --libdir=/lib option to ensure pam_tacplus.so is installed into /lib/security along with rather than in /usr/local. In such case you need to adjust the lines in /etc/pam.d file accordingly.

Outdated gnulib

If you get errors like the one below during ./configure, you most likely have an outdated gnulib (notably, FreeBSD uses 2014 version):

error: GL_GENERATE_ALLOCA_H does not appear in AM_CONDITIONAL

This is fixed by installing the latest gnulib:

$ git clone https://git.savannah.gnu.org/git/gnulib.git $HOME/gnulib
$ $HOME/gnulib/gnulib-tool --makefile-name=Makefile.gnulib --libtool --import \
                  fcntl crypto/md5 array-list list xlist getrandom realloc-posix \
                  explicit_bzero xalloc
$ autoreconf -f -v -i
$ ./configure
...                  

Quick start

TACACS+ server

To do anything with TACACS+ protocol we need a TACACS+ server. Here's where fun begins as of 2021. There are two TACACS+ servers currently available - they have the same name and temptingly similar but different configuration syntax:

• pro-bono-publico tac_plus, available in FreeBSD net/tacacs
• shrubbery tac_plus, available on Debian as tacacs+

Unfortunately, as of Ubuntu 20.04 the tacacs+ package is no longer available. Unless on Ubuntu 18.04 or FreeBSD, you'll need to install from source.

FreeBSD

$ sudo pkg install net/tacacs

Ubuntu 18.04 or older

$ sudo apt install tacacs+

From source

$ git clone https://github.com/facebook/tac_plus.git
$ cd tac_plus
$ ./configure
$ make
$ sudo make install

pamtester

I recommend PAM testing utility pamtester:

apt install pamtester

PAM configuration

Create /etc/pam.d/test with the following contents:

#%PAM-1.0
auth       required /usr/local/lib/security/pam_tacplus.so server=127.0.0.1 secret=testkey123
account	   required /usr/local/lib/security/pam_tacplus.so server=127.0.0.1 secret=testkey123 service=ppp protocol=ip
session    required /usr/local/lib/security/pam_tacplus.so server=127.0.0.1 secret=testkey123 server=127.0.0.2 secret=testkey123 service=ppp protocol=ip

Ensure /etc/tacacs+/tac_plus.conf contains the following options (shrubbery syntax):

key = testkey123
user = testuser1 {
        global = cleartext "testpass123"
        service = ppp protocol = ip {
                addr=1.2.3.4
        }
}
user = testuser2 {
        global = cleartext "testpass123"
        service = ppp protocol = ip {
                addr=2.3.4.5
        }
}

As explained above, the service and protocol parameters could be really anything, they however need to match whatever is set in TACACS+ server configuration. The TACACS+ protocol transport encryptio key must be however the same on both TACACS+ (key=) and PAM (secret=) sides.

Note the above example uses absolutely trivial "authentication database" in the form of statically configured usernames and plaintext passwords. TACACS+ servers can do much more than that in real life.

Next, ensure the TACACS+ server is not running system-wide (systemctl stop tacacs_plus.service) and run it as root in foreground in debugging mode:

# tac_plus -C tac_plus.conf -G -g -d 512
Reading config
Version F4.0.4.27a Initialized 1
tac_plus server F4.0.4.27a starting
socket FD 4 AF 2
socket FD 5 AF 10
uid=0 euid=0 gid=0 egid=0 s=911824672

In another terminal run the pamtester command:

# pamtester -v -I rhost=localhost test testuser1 authenticate acct_mgmt open_session close_session
pamtester: invoking pam_start(test, testuser1, ...)
pamtester: performing operation - authenticate
Password: (enter testpass123)
pamtester: successfully authenticated
pamtester: performing operation - open_session
pamtester: successfully opened a session
pamtester: performing operation - close_session
pamtester: session has successfully been closed.

System logs (journalctl -e) will show debugging output from the pam_tacplus module:

Oct 04 18:24:05 sweet-jaguarundi PAM-tacplus[4530]: 2 servers defined
Oct 04 18:24:05 sweet-jaguarundi PAM-tacplus[4530]: server[0] { addr=127.0.0.1:49, key='testke
Oct 04 18:24:05 sweet-jaguarundi PAM-tacplus[4530]: server[1] { addr=127.0.0.2:49, key='testke
Oct 04 18:24:05 sweet-jaguarundi PAM-tacplus[4530]: tac_service='ppp'
Oct 04 18:24:05 sweet-jaguarundi PAM-tacplus[4530]: tac_protocol='ip'
Oct 04 18:24:05 sweet-jaguarundi PAM-tacplus[4530]: tac_prompt=''
Oct 04 18:24:05 sweet-jaguarundi PAM-tacplus[4530]: tac_login=''
Oct 04 18:24:05 sweet-jaguarundi pamtester[4530]: _pam_account: [stop] called (pam_tacplus v1.
Oct 04 18:24:05 sweet-jaguarundi pamtester[4530]: _pam_account: tac_srv_no=2
Oct 04 18:24:05 sweet-jaguarundi pamtester[4530]: _pam_account: username [testuser2] obtained
Oct 04 18:24:05 sweet-jaguarundi pamtester[4530]: _pam_account: tty [pts/0] obtained
Oct 04 18:24:05 sweet-jaguarundi pamtester[4530]: _pam_account: rhost [localhost] obtained
Oct 04 18:24:05 sweet-jaguarundi tac_plus[4526]: connect from 127.0.0.1 [127.0.0.1]
Oct 04 18:24:05 sweet-jaguarundi pamtester[4530]: _pam_account: connected with fd=4 (srv 0)
Oct 04 18:24:05 sweet-jaguarundi pamtester[4530]: _pam_account: [stop] for [testuser2] sent

TACACS+ server confirms successful authentication:

connect from 127.0.0.1 [127.0.0.1]
pap-login query for 'testuser1' port pts/0 from 127.0.0.1 accepted

This confirms correct operations of the core functions of TACACS+ — authentication and accounting.

Now, if you change the secret set in /etc/pam.d/test to say testkey999, you will get the following errors from pam_tacplus module:

Oct 04 18:31:48 sweet-jaguarundi tac_plus[4526]: Error 127.0.0.1 pts/0: Invalid AUTHEN/START packet (check keys)
Oct 04 18:31:48 sweet-jaguarundi pamtester[4896]: tac_authen_read: inconsistent reply body, incorrect key?

And accordingly from tac_plus server:

connect from 127.0.0.1 [127.0.0.1]
127.0.0.1 pts/0: Invalid AUTHEN/START packet (check keys)

Note that semantics of these operation is outside of the scope of this quick guide and is very much service dependent.

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 authoritative 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 like:

Consider a 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 authentication request packet to the TACACS+ server. The packet contains username and password to verify. TACACS+ server replies with either positive or negative response. If the reponse is negative, the failure is escalated back to the login program, and no further action is taken.
5. If authentication is successful, PAM calls another function from pam_tacplus — pam_sm_acct_mgmt(). This function is expected to verify whether the users are allowed to get the service they are requesting (in this case: Unix shell). The function again verifies the permission on TACACS+ server.
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 logged 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 1.1.1.1 --tty ttyS0
    --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 {
            addr=8.8.8.8
    }
}

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

Limitations:

• 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

References:

• RFC 8907: The Terminal Access Controller Access-Control System Plus (TACACS+) Protocol
• RFC 1994: PPP Challenge Handshake Authentication Protocol (CHAP)
• The Linux-PAM Application Developers' Guide

Authors:

• Pawel Krawczyk [email protected] https://krvtz.net/
• Jeroen Nijhof [email protected]