Whoever it is, it's not us... as @pitaj suggests it could be bots issuing HEAD requests to determine site availability before crawling? Who knows... I am looking at some of them coming in our logging system now, but they have no user agent either, so 🤷
@pawan-patil You are able to modify the source code and use it on your own site. You are only limited if you are intending to redistribute the code. If you intend to, then you must also open-source your changed code.
If in doubt, you'll want to contact a lawyer to review the GPLv3 license that comes with NodeBB.