When is a port considered to be closed?

True, that would be the second part of my post where the user recieves no response or gets the icmp admin prohibited from the nearest local router responsible for that network segment.

Exactly the point, Juridian.

What is meant by the posters use of "Closed".

Is it rejecting, dropping, connecting, or connected? If there is a service listening (or as Roswell finehaired it, a superdaemon listening), the port is considered 'Open', agreed? If no service is listening, is it 'Closed' as TheSpecialist indicated? What about "Stealthed" as many port scanners will call unresponsive ports on obivously online hosts?

Semantics: not just an overbloated AntiVirus company anymore.

Stealthed is the same as filtered..

No responce at all..

A closed port should send a reset back..
The clients sends a SYN and the server sends a RST back.

And if a host shuts down right when it receives a "SYN" packet to a specific port?

Quote:

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Originally posted here by ThePastorGang
And if a host shuts down right when it receives a "SYN" packet to a specific port?

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The client precieves that as a broken request or a request to a filtered port . .

for me:

blocked = filtered = sthealthed --> a firewall is dropping the response packs or a firewall is blocking the input packets. its detected by timeout, since normal behavior is send a RST back

closed = no deamon, service is available on that port. It can be showed by a RST response (Juridian is correct about FIN flag)

The Port is considered to be closed if it is explicitly blocked by the Operating System .

thank you for the response.

So all of you suggest that - If explicitly Blocked by the OS or firewall in the system then the port is consifered to be closed.

If you have a different opinion pl. put it across.

Closed, blocked.... These are all just definitions and people tend to disagree with those definitions. Basically there are three situations with a port:
1) not a single process is using it.
2) A single process is listening to it.
3) Two processes are listening to it and thus communicate with each other.

In situation 2, it is possible that a process is listening and discarding each and every message that is sent to the port. This way, the process is actually blocking the port. In situation 1, I have the feeling that it is just closed since any process can decide to open the port to listen to it. But that's my personal feelings about it. Others have different opinions about this.

I love it when people over-analyze a question! :)

Think of any connection via TCP as a telephone conversation:

You dial the number of the other person (IP and Port) This immediatly initiates the SYN.

If the number is not in service (no service listening to port) you get that annoying mesage from the phone company (the RST).

IMHO, that is a closed port. There is no service or daemon running, so no process is listening on the port, preventing a connection from being established.

Any other event such as dropping the SYN and sending no response is simply stealthed, blocked, hidden, masked, etc. (which I personally collectively refer to as "blocked".

Also, While it is true that inetd will spawn telnet if it is requested, on most linux distros and MS OSes, telnet has its own service daemon that can be disabled, or simply not installed when setting up the OS, which effectively closes port 23 for telnet on these systems, and also prevents telnet access to SMTP (because there is no telnet service).