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If I require a SEO link to my site (with that my software installed) from every installation of my software, does it make my license not FOSS? Can I use GPL components in so licensed software?
Can you recommend an existing license with such the requirement?
I'd formulate it like "You are obliged not to remove or in any way hinder (e.g. adding rel="nofollow" and likewise) the link to the author's site."
Let's break down this question in parts and start with the FOSS licenses common ground.
The common things about FOSS licenses are that
The GPL as a strong copyleft license goes a step further and additionally requires that
Now to the first question, including GPL - licensed components and adding additional restrictions: No, that does not fly. You cannot require active code or advertisement in any specific form. You can only require the proper attribution, and you must keep the conditions on distribution without making them stricter. The GPL FAQ has a few answers on various restriction scenarios.
This might be different, if you include a "library" under the LGPL; in that case you might still be allowed to choose whatever license you fancy provided you allow users to replace the library by whatever they want.
The more permissive licenses have no such restrictions either - they only require proper attribution and allow you to distribute your code even under proprietary licenses (including no distribution).
There cannot be any FOSS license with a restriction of required advertisement like you suggest as it fails the desert island test: such software would not work in an environment where links don't work.
All this said, I'd still recommend to use the GPL. Add an appropriate copyright notice to your files like
(c) 2022 Porton from http://my-fancy-software.example.com/
and if you feel like add these links where you consider them appropriate. Also add in the readme a kind request like
If you like this software and want to help its continued development, please support us by keeping the links to our software or linking us in this form: (add the code snippet)
The idea to force users of the software to advertise the original author is not new. It was first seen in the 4-clause BSD License. There was a lot of criticism about the advertising clause, and it was removed (leading to BSD-3 and BSD-2 licenses). This advertising clause is also incompatible with GPL. I would not recommend anyone to use BSD-4 as the license for any new code.
Obviously, what you can do is to put your URL into the copyright attribution language into a notice.txt file, which will then need to be kept and reproduced by everyone if that is required by the respective license. For example §4 of the Apache 2.0 License includes language enforcing that, and this is GPL compliant. But it might not show up as prominently as you wish, and while your URL in the notice.txt might show up there, it does not necessarily need to be included as a clickable link.
I think the GPL3 allows what you're asking for. Section 7(b) says
Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms:
b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it;
Section 5(d) says
d) If the work has interactive user interfaces, each must display Appropriate Legal Notices; however, if the Program has interactive interfaces that do not display Appropriate Legal Notices, your work need not make them do so.
"Appropriate Legal Notices" is defined as
An interactive user interface displays “Appropriate Legal Notices” to the extent that it includes a convenient and prominently visible feature that (1) displays an appropriate copyright notice, and (2) tells the user that there is no warranty for the work (except to the extent that warranties are provided), that licensees may convey the work under this License, and how to view a copy of this License. If the interface presents a list of user commands or options, such as a menu, a prominent item in the list meets this criterion.
Merriam-Webster defines "preservation" as
a: the activity or process of keeping something valued alive, intact, or free from damage or decay
So, if you display a copyright statement, license notice, and attribution to yourself as the author, along with a link to your website, on the "user interface" of your application (the HTML as shown to the end user), the GPL3 allows you to legally require that it be "preserved" on all modified versions, which I believe prohibits both removing and altering the requirement (so adding rel="nofollow" would be illegal modification).
If I require a SEO link to my site (with that my software installed) from every installation of my software, does it make my license not FOSS?
Yes. I'm going to disagree with the other answers and claim that this is outright a non-free license. If the requirement were just to ship an advertisement with the software, it would be "obnoxious" or "odious" like the legacy BSD advertising clause. But if it's actually governing the functionality of the software and requiring you to keep odious functionality that the user does not want, it lacks freedom 1 ("freedom to...change it so it does your computing as you wish") and arguably lacks part of freedom 0.
If the clause only applied to distributing modified versions, including the odious behavior by default in such distribution, it would probably still be FOSS since you could just also include instructions on how to remove it (see some legacy "nasty license" software that only allowed distribution of original source along with patch files, not modified sources) but would still be user-hostile.
