For media-based net art the Creative Commons Attribution Share-Alike licence is the best copyleft licence. For software based net art a different licence is required (and Creative Commons explicitly state that their licences should not be used for software).
The GNU GPL is the best copyleft licence for software that people use on their own computers, where it is “propagated” to them from elsewhere by downloading it or installing it from DVD. Software delivered to galleries or collections, or to other artists, counts as being propagated under the GPL, so the GPL is the best copyleft licence for software that will actually be delivered to its users.
Software accessed remotely on a server online does not count as being propagated, even if it is used as it would be locally but through a web interface. To handle this a variant of the GPL called the Affero GPL (AGPL) was created. When you use software over a network, for example through a web browser, the AGPL requires that you be able to acquire the source code of that software just as if you were using it locally under the GPL. The AGPL is therefore the best copyleft licence for software used over a network. This includes software-based net art.
The average piece of software-based net art will use a free operating system, and a free software scripting language, web server and web browser. It may use a free software database and many additional free software libraries of code as well. The difficulty of the artwork’s conception or production does not provide an excuse for making it non-free any more than the difficulties of creating the far greater body of work that it build on did.
It is much easier to install and maintain software that is not restricted by its licence and that provides its source code. Art that takes the form of software must be installed and maintained to curate and preserve it. Critics, artists, students and audience can benefit from studying the source code of net art. Even if they don’t fix bugs they can learn from it and maybe even appreciate it. And if the server goes down and you don’t have a backup, someone else may and will be able to give you a copy back. These freedoms are all protected by the AGPL, giving a strong practical benefit to using it. This fact should be borne in mind when discussing the curation, archiving and preservation of net art as well as when discussing its production.
The support of people’s freedom and the practical benefits to artists
from supporting the curation, preservation and scholarship of their
work provide strong reasons for making net art free software. Net artists can and should protect the freedom of the users of their software using the AGPL. See here for details of how to apply the AGPL to your work.