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In the following nmap output:

$ sudo nmap -sS -sV -vv -n -Pn -T5 x.x.x.1-255 -p80,23,21 -oG - | grep 'open'

Host: x.x.x.x ()  Ports: 21/closed/tcp//ftp///, 23/open/tcp//telnet?///, 80/filtered/tcp//http///
Host: x.x.x.x ()  Ports: 21/open/tcp//ftp//D-Link or USRobotics ADSL router firmware update ftpd/, 23/open/tcp//telnet//Broadcom BCM96338 ADSL router telnetd/, 80/open/tcp//http?///
Host: x.x.x.x ()  Ports: 21/open/tcp//ftp?///, 23/open/tcp//telnet?///, 80/open/tcp//http?///
Host: x.x.x.x ()  Ports: 21/closed/tcp//ftp///, 23/open/tcp//telnet?///, 80/open/tcp//http?///
Host: x.x.x.x ()  Ports: 21/closed/tcp//ftp///, 23/closed/tcp//telnet///, 80/open/tcp//http?///

there is a question mark after the service name (ftp?, http?).

What does the question mark mean exactly?

  • Why are people down-voting this? – Steve May 15 '13 at 18:36
6

If Nmap isn't able to identify the service based on the signature of its response to the probing it falls back to port identification. It checks the port against a list of known port-service pairs.

So the question mark indicates that the identification was based on the port number rather than response signature.

You can find the full list in nmap-services

2

It basically means that nmap is not certain what service is running on that port. The more in-depth probes that the -sV flag uses does not return enough information. nmap is simply making an assumption about what service is running on that particular port based on a list of common services and their ports, for example port 80 and http.

1

As the others already mentioned the return values for -sV do not provide enough information to be sure about the service running on a certain port (Remember: you can run any service on any port, e.g. ftp on port 80 although this is rarely used).

To get more information you can try nmap -A which in addition to -sV also activates the nmap scripting engine. This usually provides improved information about a service.

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