FAQ service-question

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Why do some services have a question mark (?) after them?

When you ask for service version detection (-sV), Nmap does its best to find out what service is listening by sending probes and matching responses. If it can't match any known service, it falls back to a lookup in the nmap-services file. Since this is only guessing the service based on port number, Nmap appends a ? to the end of the service name.