I have a question related to the concept of port number and its associated service.

Given that a port number is a way to identify a specific process to which an Internet or other network message is to be forwarded when it arrives at a server (example: web server is supposed to be found on port 80 also 8080...etc)

My question in a nutshell:

if nmap scan shows:

host: 10.0.xxx.xx
port    state   service reason
20000   open    dnp     syn-ack ttl 56

am I right in thinking that I have actually found a SCADA system? (DNP3 as a Distributed Network Protocol is used by SCADA Master Stations)

Well, My own opinion is yes!...but unfortunately once I got in touch with the server's owner he told me that I am completely wrong since he installed a java based application which uses port 20000 (https://babelfish.arc.nasa.gov/trac/jpf/wiki/user/output)

Am I right in saying that the guy should have chosen another port, since port 20000 is considered to be the official port number for DNP protocol (https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers)?

Thanks a lot

  • 2
    Any service can be run on any port. If you don't use the default port, whatever client is connecting needs to know what port to connect to. Some services like HTTP, SSH, etc. that is trivial to do. Others like DHCP, it is impossible to do.... FWIW my /etc/services says that port 2000 is for "Cisco SCCP" .... – ivanivan Nov 20 '17 at 23:47

There is no hard requirement that a particular port can only be used by a specific service. I could run a web server on port 1234 instead of port 80, and aside from the difficulty in finding it, there would be nothing stopping me doing so. (On the public facing side of my home network I do run one or two services on non-standard ports simply to keep the script kiddies at bay.)

Ports can be reserved by IANA (for example tcp/22 is for ssh), and there's even a range for "user defined services", but these days on our nastier and less friendly Internet much of that is generally ignored. Aside from the fact that many Internet users simply don't know about it, of course.

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