LIBTACPLUS(3)

NAME
libtacplus - TACACS+ client library

SYNOPSIS
#include <taclib.h>
int
tac_add_server(struct tac_handle *h, const char *host, int port, const char *secret, int timeout, int flags);
void
tac_close(struct tac_handle *h);
int
tac_config(struct tac_handle *h, const char *path);
int
tac_create_authen(struct tac_handle *h, int action, int type, int service);
void *
tac_get_data(struct tac_handle *h, size_t *len);
char *
tac_get_msg(struct tac_handle *h);
struct tac_handle *
tac_open(void);
int
tac_send_authen(struct tac_handle *h);
int
tac_set_data(struct tac_handle *h, const void *data, size_t data_len);
int
tac_set_msg(struct tac_handle *h, const char *msg);
int
tac_set_port(struct tac_handle *h, const char *port);
int
tac_set_priv(struct tac_handle *h, int priv);
int
tac_set_rem_addr(struct tac_handle *h, const char *addr);
int
tac_set_user(struct tac_handle *h, const char *user);
const char *
tac_strerror(struct tac_handle *h);

DESCRIPTION
The libtacplus library implements the client side of the TACACS+ network access control protocol. TACACS+ allows clients to perform authentication, authorization, and accounting by means of network requests to remote servers. This library currently supports only the authentication portion of the protocol.

INITIALIZATION
To use the library, an application must first call tac_open() to obtain a struct tac_handle *, which provides context for subsequent operations. Calls to tac_open() always succeed unless insufficient virtual memory is available. If the necessary memory cannot be allocated, tac_open() returns NULL.

Before issuing any TACACS+ requests, the library must be made aware of the servers it can contact. The easiest way to configure the library is to call tac_config(). tac_config() causes the library to read a configuration file whose format is described in tacplus.conf(5). The pathname of the configuration file is passed as the file argument to tac_config(). This argument may also be given as NULL, in which case the standard configuration file /etc/tacplus.conf is used. tac_config() returns 0 on success, or -1 if an error occurs.

The library can also be configured programmatically by calls to tac_add_server(). The host parameter specifies the server host, either as a fully qualified domain name or as a dotted-quad IP address in text form. The port parameter specifies the TCP port to contact on the server. If port is given as 0, the library uses port 49, the standard TACACS+ port. The shared secret for the server host is passed to the secret parameter. It may be any null-terminated string of bytes. The timeout for receiving replies from the server is passed to the timeout parameter, in units of seconds. The flags parameter is a bit mask of flags to specify various characteristics of the server. It may contain:

TAC_SRVR_SINGLE_CONNECT
    Causes the library to attempt to negotiate single connection mode when communicating with the server. In single connection mode, the original TCP connection is held open for multiple TACACS+ sessions. Older servers do not support this mode, and some of them become confused if the client attempts to negotiate it.

tac_add_server() returns 0 on success, or -1 if an error occurs.

tac_add_server() may be called multiple times, and it may be used together with tac_config(). At most 10 servers may be specified. When multiple servers are given, they are tried in round-robin fashion until a working, accessible server is found. Once the library finds such a server, it continues to use it as long as it works.

CREATING A TACACS+ AUTHENTICATION REQUEST
To begin constructing a new authentication request, call tac_create_authen(). The action, type, and service arguments must be be set to appropriate values as defined in the TACACS+ protocol specification. The <taclib.h> header file contains symbolic constants for these values.

After creating a request with tac_create_authen(), various optional parameters may be attached to it through calls to tac_set_data(), tac_set_port(), tac_set_priv(), tac_set_rem_addr(), and tac_set_user(). The library creates its own copies of any strings provided to these functions, so that it is not necessary for the caller to preserve them. By default, each of these parameters is empty except for the privilege level, which defaults to ‘USER’ privilege.

SENDING THE AUTHENTICATION REQUEST AND RECEIVING THE RESPONSE
After the TACACS+ request has been constructed, it is sent by means of tac_send_authen(). This function connects to a server if not already connected, sends the request, and waits for a reply. On failure, tac_send_authen() returns -1. Otherwise, it returns the TACACS+ status code and flags, packed into an integer value. The status can be extracted using the macro TAC_AUTHEN_STATUS(). Possible status codes, defined in <taclib.h>, include:

TAC_AUTHEN_STATUS_PASS
TAC_AUTHEN_STATUS_FAIL
TAC_AUTHEN_STATUS_GETDATA
TAC_AUTHEN_STATUS_GETUSER
TAC_AUTHEN_STATUS_GETPASS
TAC_AUTHEN_STATUS_RESTART
TAC_AUTHEN_STATUS_ERROR
TAC_AUTHEN_STATUS_FOLLOW

The only flag is the no-echo flag, which can be tested using the macro TAC_AUTHEN_NOECHO().

EXTRACTING INFORMATION FROM THE SERVER'S RESPONSE
An authentication response packet from the server may contain a server message, a data string, or both. After a successful call to tac_send_authen(), this information may be retrieved from the response by calling tac_get_msg() and tac_get_data(). These functions return dynamically-allocated copies of the information from the packet. The caller is responsible for freeing the copies when it no longer needs them. The data returned from these functions is guaranteed to be terminated by a null byte.

In the case of tac_get_data(), the len argument points to a location into which the library will store the actual length of the received data, not including the null terminator. This argument may be given as NULL if the caller is not interested in the length.

SENDING AUTHENTICATION CONTINUE PACKETS
If tac_send_authen() returns a value containing one of the status codes TAC_AUTHEN_STATUS_GETDATA, TAC_AUTHEN_STATUS_GETUSER, or TAC_AUTHEN_STATUS_GETPASS, then the client must provide additional information to the server by means of a TACACS+ CONTINUE packet. To do so, the application must first set the packet's user message and/or data fields using tac_set_msg() and tac_set_data(). The client then sends the CONTINUE packet with tac_send_authen(). N.B., tac_create_authen() should not be called to construct a CONTINUE packet; it is used only for the initial authentication request.

When it receives the CONTINUE packet, the server may again request more information by returning TAC_AUTHEN_STATUS_GETDATA, TAC_AUTHEN_STATUS_GETUSER, or TAC_AUTHEN_STATUS_GETPASS. The application should send further CONTINUEs until some other status is received from the server.

OBTAINING ERROR MESSAGES
Those functions which accept a struct tac_handle * argument record an error message if they fail. The error message can be retrieved by calling tac_strerror(). The message text is overwritten on each new error for the given struct tac_handle *. Thus the message must be copied if it is to be preserved through subsequent library calls using the same handle.

CLEANUP
To free the resources used by the TACACS+ library, call tac_close().

RETURN VALUES
The following functions return a non-negative value on success. If they detect an error, they return -1 and record an error message which can be retrieved using tac_strerror().

tac_add_server()
tac_config()
tac_create_authen()
tac_send_authen()
tac_set_data()
tac_set_msg()
tac_set_port()
tac_set_priv()
tac_set_rem_addr()
tac_set_user()

The following functions return a non-NULL pointer on success. If they are unable to allocate sufficient virtual memory, they return NULL and record an error message which can be retrieved using tac_strerror().

tac_get_data()
tac_get_msg()

The following functions return a non-NULL pointer on success. If they are unable to allocate sufficient virtual memory, they return NULL, without recording an error message.

tac_open()

FILES
/etc/tacplus.conf

SEE ALSO
tacplus.conf(5) D. Carrel, Lol Grant, The TACACS+ Protocol, Version 1.78, draft-grant-tacacs-02.txt (Internet Draft).

AUTHORS
This software was written by John Polstra, and donated to the FreeBSD project by Juniper Networks, Inc.