*This document is written for a software developer audience. For information on using ZeroTier, see the: [Website](https://www.zerotier.com), [Documentation Site](https://docs.zerotier.com), and [Discussion Forum](https://discuss.zerotier.com).*
ZeroTier is a smart programmable Ethernet switch for planet Earth. It allows all networked devices, VMs, containers, and applications to communicate as if they all reside in the same physical data center or cloud region.
This is accomplished by combining a cryptographically addressed and secure peer to peer network (termed VL1) with an Ethernet emulation layer somewhat similar to VXLAN (termed VL2). Our VL2 Ethernet virtualization layer includes advanced enterprise SDN features like fine grained access control rules for network micro-segmentation and security monitoring.
All ZeroTier traffic is encrypted end-to-end using secret keys that only you control. Most traffic flows peer to peer, though we offer free (but slow) relaying for users who cannot establish peer to peer connections.
The goals and design principles of ZeroTier are inspired by among other things the original [Google BeyondCorp](https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/43231.pdf) paper and the [Jericho Forum](https://en.wikipedia.org/wiki/Jericho_Forum) with its notion of "deperimeterization."
Visit [ZeroTier's site](https://www.zerotier.com/) for more information and [pre-built binary packages](https://www.zerotier.com/download/). Apps for Android and iOS are available for free in the Google Play and Apple app stores.
ZeroTier is licensed under the [BSL version 1.1](https://mariadb.com/bsl11/). See [LICENSE.txt](LICENSE.txt) and the [ZeroTier pricing page](https://www.zerotier.com/pricing) for details. ZeroTier is free to use internally in businesses and academic institutions and for non-commercial purposes. Certain types of commercial use such as building closed-source apps and devices based on ZeroTier or offering ZeroTier network controllers and network management as a SaaS service require a commercial license.
A small amount of third party code is also included in ZeroTier and is not subject to our BSL license. See [AUTHORS.md](AUTHORS.md) for a list of third party code, where it is included, and the licenses that apply to it. All of the third party code in ZeroTier is liberally licensed (MIT, BSD, Apache, public domain, etc.).
Everything in the ZeroTier world is controlled by two types of identifier: 40-bit/10-digit *ZeroTier addresses* and 64-bit/16-digit *network IDs*. These identifiers are easily distinguished by their length. A ZeroTier address identifies a node or "device" (laptop, phone, server, VM, app, etc.) while a network ID identifies a virtual Ethernet network that can be joined by devices.
ZeroTier addresses can be thought of as port numbers on an enormous planet-wide enterprise Ethernet smart switch supporting VLANs. Network IDs are VLAN IDs to which these ports may be assigned. A single port can be assigned to more than one VLAN.
A ZeroTier address looks like `8056c2e21c` and a network ID looks like `8056c2e21c000001`. Network IDs are composed of the ZeroTier address of that network's primary controller and an arbitrary 24-bit ID that identifies the network on this controller. Network controllers are roughly analogous to SDN controllers in SDN protocols like [OpenFlow](https://en.wikipedia.org/wiki/OpenFlow), though as with the analogy between VXLAN and VL2 this should not be read to imply that the protocols or design are the same. You can use our convenient and inexpensive SaaS hosted controllers at [my.zerotier.com](https://my.zerotier.com/) or [run your own controller](controller/) if you don't mind messing around with JSON configuration files or writing scripts to do so.
-`attic/`: old stuff and experimental code that we want to keep around for reference.
-`controller/`: the reference network controller implementation, which is built and included by default on desktop and server build targets.
-`debian/`: files for building Debian packages on Linux.
-`doc/`: manual pages and other documentation.
-`ext/`: third party libraries, binaries that we ship for convenience on some platforms (Mac and Windows), and installation support files.
-`include/`: include files for the ZeroTier core.
-`java/`: a JNI wrapper used with our Android mobile app. (The whole Android app is not open source but may be made so in the future.)
-`macui/`: a Macintosh menu-bar app for controlling ZeroTier One, written in Objective C.
-`node/`: the ZeroTier virtual Ethernet switch core, which is designed to be entirely separate from the rest of the code and able to be built as a stand-alone OS-independent library. Note to developers: do not use C++11 features in here, since we want this to build on old embedded platforms that lack C++11 support. C++11 can be used elsewhere.
-`osdep/`: code to support and integrate with OSes, including platform-specific stuff only built for certain targets.
-`service/`: the ZeroTier One service, which wraps the ZeroTier core and provides VPN-like connectivity to virtual networks for desktops, laptops, servers, VMs, and containers.
-`zeroidc/`: OIDC implementation used by ZeroTier service to log into SSO-enabled networks. (This part is written in Rust, and more Rust will be appearing in this repository in the future.)
To build on Mac and Linux just type `make`. On FreeBSD and OpenBSD `gmake` (GNU make) is required and can be installed from packages or ports. For Windows there is a Visual Studio solution in `windows/`.
- Linux makefiles automatically detect and prefer clang/clang++ if present as it produces smaller and slightly faster binaries in most cases. You can override by supplying CC and CXX variables on the make command line.
Typing `make selftest` will build a *zerotier-selftest* binary which unit tests various internals and reports on a few aspects of the build environment. It's a good idea to try this on novel platforms or architectures.
The service is controlled via the JSON API, which by default is available at 127.0.0.1 port 9993. We include a *zerotier-cli* command line utility to make API calls for standard things like joining and leaving networks. The *authtoken.secret* file in the home folder contains the secret token for accessing this API. See [service/README.md](service/README.md) for API documentation.
* **Windows**: `\ProgramData\ZeroTier\One` (That's the default. The base 'shared app data' folder might be different if Windows is installed with a non-standard drive letter assignment or layout.)
If you are running a local system firewall, we recommend adding a rules permitting zerotier. If you installed binaries for Windows this should be done automatically. Other platforms might require manual editing of local firewall rules depending on your configuration.
On CentOS check `/etc/sysconfig/iptables` for IPTables rules. For other distributions consult your distribution's documentation. You'll also have to check the UIs or documentation for commercial third party firewall applications like Little Snitch (Mac), McAfee Firewall Enterprise (Windows), etc. if you are running any of those. Some corporate environments might have centrally managed firewall software, so you might also have to contact IT.
ZeroTier One peers will automatically locate each other and communicate directly over a local wired LAN *if UDP port 9993 inbound is open*. If that port is filtered, they won't be able to see each others' LAN announcement packets. If you're experiencing poor performance between devices on the same physical network, check their firewall settings. Without LAN auto-location peers must attempt "loopback" NAT traversal, which sometimes fails and in any case requires that every packet traverse your external router twice.
Users behind certain types of firewalls and "symmetric" NAT devices may not able able to connect to external peers directly at all. ZeroTier has limited support for port prediction and will *attempt* to traverse symmetric NATs, but this doesn't always work. If P2P connectivity fails you'll be bouncing UDP packets off our relay servers resulting in slower performance. Some NAT router(s) have a configurable NAT mode, and setting this to "full cone" will eliminate this problem. If you do this you may also see a magical improvement for things like VoIP phones, Skype, BitTorrent, WebRTC, certain games, etc., since all of these use NAT traversal techniques similar to ours.
If a firewall between you and the Internet blocks ZeroTier's UDP traffic, you will fall back to last-resort TCP tunneling to rootservers over port 443 (https impersonation). This will work almost anywhere but is *very slow* compared to UDP or direct peer to peer connectivity.
