tac_plus - Man Page

tacacs plus daemon


tac_plus -C <configfile> [-GghiLPSstv] [-B <bind_address>] [-d <level>] [-l <logfile>] [-m <max_listen_queue>] [-p <tcp_port>] [-Q <setgid>] [-U <setuid>] [-u <wtmpfile>] [-w <wholog>]


By default, tac_plus listens on tcp port 49 and provides network devices (normally routers and access servers) with authentication, authorization and accounting services.

A configuration file controls the details of authentication, authorization and accounting.

Command-Line Options

-C <configfile>

Specify the configuration file name.  The -C option is required.

-B <bind address>

Specify the address on which the daemon should bind(2). Successive instances of -B override previous instances.  By default, the daemon listens on all addresses. Note: this changes the name of the pid file created by the daemon.


Remain in the foreground, but not single-threaded nor logging to the tty.

-d <level>

Switch on debugging.  By default the output will appear in the log file and syslog(3).

NOTE: The -g flag will cause these messages to also appear on stdout.  The -t flag will cause these messages to also be written to /dev/console.

The value of level is as described below.  These values represent bits that can be logically OR'd together.  The daemon logically ORs successive occurrences of the -d option.

Value   Meaning
2       configuration parsing debugging
4       fork(1) debugging
8       authorization debugging
16      authentication debugging
32      password file processing debugging
64      accounting debugging
128     config file parsing & lookup
256     packet transmission/reception
512     encryption/decryption
1024    MD5 hash algorithm debugging
2048    very low level encryption/decryption
32768   max session debugging
65536   lock debugging

Single threaded mode.  The daemon will only accept and service a single connection at a time without forking and without closing file descriptors.  All log messages appear on standard output.

This is intended only for debugging and not for normal service.

This option does not work with single-connection sessions.


Display help message.


tac_plus will be run from inetd(8).  In inetd mode, the configuration file is parsed every time tac_plus starts.

If the configuration is large or the frequency of connections is high, this negatively will affect the responsiveness of the daemon.

If the config file is small, connections are infrequent, and authentication is being done via passwd(5) files or SKEY (which are not cached), running in inetd mode should be tolerable, but still is not recommended.

This option does not work with single-connection sessions.

-l <logfile>

Specify an alternate log file location. This file is only used when the -d option is used. The logs are still posted to syslog.

-m <max_listen_queue>

Specify an alternative client listen queue limit. The default is SOMAXCONN or 64, if your O/S does not specify one.


Lookup DNS PTR (Domain Name System PoinTeR) record of client addresses. The resulting FQDN (Fully Qualified Domain Name), if it resolves, will be used in log messages, libwrap (tcp_wrappers) checks, and for matching host clauses of the configuration file.  Also see tac_plus.conf(5).


Parse the configuration file, echo it to standard output while parsing, and then exit. tac_plus will exit non-zero when a parser error occurs.

Useful for debugging configuration file syntax.

-p <port>

Listen on the specified port number instead of the default port 49 for incoming tcp connections.  Note: this changes the name of the pid file created by the daemon.

-Q <setgid groupname>

Specify the groupname or GID to setgid(2). If the daemon was compiled with a specific GID, this option overrides that value. By default, the daemon inherits the GID from its parent process.


Enables or allows client single-connection mode, where-by the client will create one connection and interleave queries.

Note: this is broken in IOS and IOS-XE.

Note: this is currently only partially supported in the daemon.


Causes the daemon to always reject authentication requests which contain a minor version number of zero (SENDPASS).  This enhances security in the event that someone discovers your encryption key.  SENDPASS requests permit requesters to obtain CHAP, PAP and ARAP passwords from the daemon, iff the encryption key is known.

Note: IOS versions preceding 11.2 will fail.


Log all informational, debugging or error messages to /dev/console in addition to logging to syslogd. Useful for debugging.

Specify the username or UID to

setuid(2). If the daemon was compiled with a specific UID, this option overrides that value. The daemon must be started by root to open the privileged port. By default, it does not change it's UID and therefore remains root.

-u <wtmpfile>

Write wtmp entries to the specified wtmp file.


Display version information and exit.

-w <wholog>

Specify the location of the max session file.


tac_plus is normally invoked by root, as follows:

   # tac_plus -C <configfile>

where <configfile> is a full path to the configuration file.  Tac_plus will background itself and start listening on port 49 for incoming tcp connections.

Tac_plus must be invoked as root to obtain privileged network socket 49 and to read the protected configuration file, which may contain confidential information such as encryption keys and cleartext passwords.

After the port is acquired and the config file is read, root privileges are no longer required.  You can arrange that tac_plus will change its user and group IDs to a more innocuous user and group via the configuration file.

NOTE: The new user and group still needs permission to read any passwd(5) (and shadow(5)) files and S/KEY database if these are being used.

TCP Wrappers

If tac_plus was compiled with libwrap (aka. tcp_wrappers) support, upon connection the daemon will consult with tcp_wrappers on whether the client has permission to connect.  The daemon name used in a daemon list of the access control file is the name of the executable, normally "tac_plus". See hosts_access(5).


The configuration file should be unreadable and unwriteable by anyone except root, as it contains passwords and keys.


If the daemon is receives a SIGHUP or SIGUSR1, it will reinitialize itself and re-read its configuration file.

Note: if an error is encountered in the configuration file or the file can not be opened for reading, such as due to insufficient permissions resulting from process ownership and file permissions, the daemon will exit.

Likewise, if the daemon is configured to send accounting records to a file and that file can not be opened for writing, such as due to insufficient permissions resulting from process ownership and file permissions, the daemon will exit.

Log Messages

tac_plus logs error and informational messages to syslog facility LOG_DAEMON.



Default accounting file.


Default log file used when the -d option is used.


Pid file. If the -B option is used, ".bind_address" is appended. If the -p option is used, ".port_number" is appended.

See Also

tac_plus.conf(5), tac_pwd(8)

Also see the   tac_plus User Guide (user_guide) that came with the distribution. The user guide does not cover all the modifications to the original  Cisco version.


There are at least 3 versions of the authentication protocol that people commonly refer to as "TACACS".

The first is ordinary tacacs, which was the first one offered on Cisco boxes and has been in use for many years.  The second is an extension to the first, commonly called Extended Tacacs or XTACACS, introduced in 1990.

The third one is TACACS+ (or T+ or tac_plus) which is what is documented here.  TACACS+ is NOT COMPATIBLE with any previous versions of tacacs.


The tac_plus (tacacs+) developer's kit is a product of Cisco Systems, written by Lol Grant.  Made available at no cost and with no warranty of any kind.  See the file COPYING and source files that came with the distribution for specifics.

Though heavily modified from the original Cisco manual pages, much of the modifications are derived from the tacacs IETF draft and the Cisco user guide.

Referenced By

tac_plus.conf(5), tac_pwd(8).

29 December 2014