Will a web server get blacklisted for port scanning?

As I understand it, port scanning is an activity that can be a precursor to malicious activity, as one of the things it is used for is to scan for open ports that one can attack.

In that train of thought, why do sites like the following exist? Won't they get blacklisted by some web servers as a result?

• https://www.whatismyip.com/port-scanner/
• https://www.advanced-port-scanner.com/
• https://hidemy.name/en/port-scanner/

I am asking because I am building a port scanner on my own website, and I am wary that if I start generating traffic for it, when people use it, it may create some issues for me because it will get my server's traffic blacklisted from other websites.

Aside from 1) rate-limiting the number of queries, 2) preventing remote REST queries from other servers using CSRF blockers, and 3) blacklisting / whitelisting certain domains / IPs, how else can you host such a tool on your server without having it be abused?

why do sites offering portscans exist?
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I am asking because I am building a port scanner on my own website

Why do YOU want to offer that on your website?

There are several scenario's that can happen in response to a portscan:

• fairly often: nothing at all.

  Either it isn't detected at all, or simply deemed not sufficiently malicious and anomalous to warrant further (automated) action.

• the target of the port scan has an IDS that does recognise the portscan and it deems the act of a port scan malicious.
  Then it might take a follow up action:

  • the system blocks all future traffic from the IP-address of your webserver.

    That is probably not actually be a problem for you though, maybe only for whomever used your site to scan their system(s).

  • the system sends a complaint to the abuse contact for your IP-address.

    That can be problematic for you, when your ISP follows up on that (by cutting off your connectivity for example) because that ins't allowed by their terms of service.

• Your ISP detects that your system performs port scans and follows up on that (by cutting off your connectivity for example) because that ins't allowed by their terms of service.

Your website might be categorised as "hacking tools" or a similar BS category by one or more content control providers and thus access to your website will effectively be blocked for the users of such internet filtering.