GNU Gatekeeper (GnuGk) is a full featured H.323 gatekeeper, available freely under GPL license. It forms the basis for a free IP telephony (VOIP) or video conferencing system.

The GNU Gatekeeper is very stable. It is being used commercially by many organizations to provide VOIP or conferencing services.
1. we provide executables for Linux, Windows, MacOS X, Solaris, FreeBSD, OpenBSD and NetBSD
2. can be run as a Windows service
3. accounting and call authorization via SQL database, Radius, file or external application
4. database drivers for ODBC, MySQL, PostgreSQL, SQLite and Firebird
5. flexible call routing
6. number rewriting (calling and called)
7. support for NAT traversal (H.460.18/H.460.19, H.460.23/H.460.24 or own method)
8. full H.323 proxy
9. TCP interface to applications
10. CTI functions (eg. VOIP call-center, call transfers)
11. gatekeeper clustering support (neighbors, parent/child, alternates)
12. H.235 security
13. graphical user interface
14. it's free, including source code

The GNU Gatekeeper is an open-source project that implements a H.323 gatekeeper. A gatekeeper provides call control services to H.323 endpoints and is an integral part of most useful Internet telephony installations that are based on the H.323 standard.

According to Recommendation H.323, a gatekeeper shall provide the following services:
* Address Translation
* Admissions Control
* Bandwidth Control
* Zone Management
* Call Control Signaling
* Call Authorization
* Bandwidth Management
* Call Management

The GNU Gatekeeper implements most of these functions based on the OpenH323 protocol stack.

Recommendation H.323 is an international standard published by the ITU. It is a communications standard for audio, video, and data over the Internet.

Some protocols implemented in the GNU Gatekeeper are covered by patents (especially the firewall / NAT traversal protocols). To the best of our knowledge, the GNU Gatekeeper Project has a valid license for all its releases, but users making derived versions of this code must ensure that they have a valid license before enabling those features.

Generally speaking, the GNU GPL allows you to copy, distribute, resell or modify the software, but it requires that all derived works must also be published under the GNU GPL. This means that you must publish full source for all extensions to the gatekeeper and for all programs where you incorporate code from the GNU Gatekeeper. See the file COPYING for details.

If that's not what you want, you must interface to the gatekeeper through the status port and communicate with it via TCP. This allows you to integrate basic functionality into the gatekeeper (and provide source for that) but keep other parts of your application private.

The license of this software is Freeware, you can free download and free use this internet phone software.