Possible SYN flooding on port 53

My YunoHost server

Hardware: VPS bought online
YunoHost version: 3.6.5.3
OS: Debian 9.12 64bit (Linux 5.4.20)
I have access to my server : Through SSH | through the webadmin | direct access via keyboard / screen | …
Are you in a special context or did you perform some particular tweaking on your YunoHost instance ? : no

Description of my issue

When I login to my VPN, the terminal interface has the following message

Possible SYN flooding on port 53. Sending cookies. Check NMP counters

Being a beginner I have no idea what this means, how serious it is and how it can be checked/fixed.

If anyone has any idea, please share…

Merci

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