The Ultimate Guide to Open Source Licenses: From MIT to AGPL and Their Impact on Cloning, Use, and Distribution

## Introduction In our development practices at `wiki.lib00.com`, we frequently interact with a wide variety of open-source software. Open-source licenses are the cornerstone of this ecosystem, defining the **rights** and **obligations** developers have when using, modifying, and distributing open-source software. Misusing code with a specific license can expose your project to unexpected legal risks. This article, organized by the 'viral' or 'reciprocal' nature of licenses from permissive to strict, will break down the most common open-source licenses and their implications for you. --- ## 1. Highly Permissive Licenses These licenses impose the fewest restrictions, granting users the greatest freedom, making them ideal for commercial applications. ### **MIT License** This is one of the most popular and simplest open-source licenses available today. * **Core Idea**: You can do whatever you want with my code, but don't blame me if something goes wrong, and please keep my copyright notice. * **Impact on You**: * **Cloning/Using**: Complete freedom. You can use it in any project, including commercial, closed-source ones, without needing to disclose your own code. * **Distributing/Distributing with Modifications**: The only requirement is that you **must** include the original MIT license text and copyright notice in any distribution of the software. ### **Apache License 2.0** Also very permissive, but more specific than MIT, especially regarding patents. * **Core Idea**: Similar to MIT, but provides an express grant of patent rights and requires you to state changes. * **Impact on You**: * **Cloning/Using**: Complete freedom, suitable for commercial projects. * **Distributing/Distributing with Modifications**: 1. You **must** include a copy of the original license and copyright notices. 2. If you modify the code, you **must** state the changes in the modified files. 3. You must include a `NOTICE` file if the original project has one. 4. It includes an explicit **patent grant**, meaning contributors (like `DP@lib00`) grant you a license to their patents, and you, in turn, grant a license to your patents in your contributions. ### **BSD License (Berkeley Software Distribution)** Very similar to the MIT license. It mainly comes in two versions: the 2-clause (Simplified BSD) and 3-clause (New BSD). The 3-clause version adds a non-endorsement clause, prohibiting the use of the original author's name to promote derivative products. --- ## 2. Weak Copyleft Licenses These licenses strike a middle ground, typically applying their 'viral' nature on a per-file or per-library basis. ### **Mozilla Public License 2.0 (MPL 2.0)** Copyleft is applied on a per-file basis. * **Core Idea**: You can mix my code with your own, but if you modify my original source files, those specific files must remain open-source under MPL 2.0. * **Impact on You**: * **Cloning/Using**: Free to use. * **Distributing/Distributing with Modifications**: 1. If you **modify** any MPL 2.0-covered files, those modified files **must** be re-licensed under MPL 2.0. 2. However, **new, separate files you create** in your project (even if they call the MPL code) can be licensed under any terms you choose, including proprietary ones. This makes it well-suited for plugins or modular systems, like a project named `wiki.lib00`. ### **GNU LGPL (Lesser General Public License)** Copyleft is applied on a per-library basis, mainly used for open-source libraries and frameworks. * **Core Idea**: You can use my library in your closed-source application, but if you modify the library itself, your modifications must be open-sourced. * **Impact on You**: * **Cloning/Using**: Free to use. * **Distributing/Distributing with Modifications**: 1. If your software only **links** to an LGPL library without modifying it, your software **does not have to be open-source**. 2. You must, however, allow end-users to be able to replace the LGPL library (e.g., through dynamic linking). 3. If you **modify** the LGPL library itself, your modified version of the library **must** also be licensed under the LGPL. --- ## 3. Strong Copyleft Licenses These licenses have a strong 'viral' effect, requiring derivative works to also be open-sourced under the same license. ### **GNU GPL (General Public License)** The most famous strong copyleft license, used by projects like the Linux kernel (GPLv2). * **Core Idea**: Any project that incorporates GPL-licensed code must itself be licensed under the GPL. The torch of free software must be passed on. * **Impact on You**: * **Cloning/Using**: Completely free for personal or internal company use. * **Distributing/Distributing with Modifications**: This is the critical part. If you **distribute** software that contains GPL code (whether modified or simply linked), your **entire software project must also** be licensed under the GPL, and you must provide the complete source code. This means you cannot use GPL code in a closed-source commercial product (e.g., `my-corp-lib00`) that you distribute to customers. ### **GNU AGPL (Affero General Public License)** A version of the GPL enhanced for network services, and the most restrictive major license. * **Core Idea**: Even if you don't 'distribute' the software but merely provide access to it over a network (SaaS), you must make the source code available. * **Impact on You**: In addition to all GPL requirements, it adds a clause: if you run a modified AGPL program on a network server and let users interact with it, you **must** provide those users with a way to get the full source code. This closes the 'SaaS loophole' of the GPL. --- ## Summary and Recommendations | License | Permissiveness | Core Requirement | Example Use Cases | | :--- | :--- | :--- | :--- | | **MIT** | Highly Permissive | Just keep the copyright notice. | Almost any project (jQuery, .NET Core) | | **Apache 2.0** | Permissive | Keep notices, state changes, grant patent rights. | Large, enterprise-level projects (Android, Swift) | | **LGPL** | Weak Copyleft | Modified library must be open, but app using it can be closed. | Open-source libraries, shared components (glibc) | | **MPL 2.0** | Weak Copyleft | Modified source files must be open, new files can be anything. | Browsers, plugin-based systems (Firefox) | | **GPL** | Strong Copyleft | If you use it, your entire distributed project must be GPL. | Operating systems, desktop apps (Linux, GIMP) | | **AGPL** | Strongest Copyleft | Providing it as a network service requires source availability. | SaaS backends, web applications (MongoDB, Grafana) | Finally, some advice from the author, `DP`: * **For broadest adoption**: Choose **MIT** or **Apache 2.0**. * **For a library you want to keep open while allowing use in proprietary apps**: Choose **LGPL** or **MPL 2.0**. * **To enforce the free software ethos**: Choose **GPL**. * **For a network service you want to ensure remains open**: Choose **AGPL**. In a commercial setting, always investigate the license of any open-source component before incorporating it to avoid legal risks.