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.
| 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 "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.
This project is using autotools for building, so please run autoreconf first.
$ autoreconf -i
$ ./configure && make && sudo make install
#%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
-
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)
-
-
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.
-
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
-
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:
- Login accepts username and password from the user.
- Login calls PAM function pam_authenticate() to verify if the supplied username/password pair is valid.
- PAM loads pam_tacplus module (as defined in /etc/pam.d/login) and calls pam_sm_authenticate() function supplied by this module.
- 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 ;)
- 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.
- 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.
- When user logs out, pam_sm_close_session() sends STOP packet to the server. The whole session is closed.
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
Pawel Krawczyk [email protected] http://ipsec.pl
Jeroen Nijhof [email protected]