Test if port is open, even if nothing there to respond?

sporkit · 08-15-2011, 04:40 PM

Hi Guys,

This might be a dumb question, but is it possible to test if a remote port is open, even if there is nothing there to respond to it?

For example a command like "telnet mydomain.com 80" will usually receive a response from an Apache server. What if I shut it down? Is there a way to validate it's a least open by connecting from a remote machine?

acid_kewpie · 08-15-2011, 04:45 PM

This doesn't make sense, the port is not open if there is nothing there. You'll see the same as if iptables was rejecting packets. Mind you, maybe that is enough for you if you would expect a firewall to drop silently rather than reject. The reciept of a TCP RESET packet does generally identify that the system exists.

theNbomr · 08-15-2011, 05:55 PM

The use of the term 'open' in this scenario seems to lead to all sorts of confusion. There isn't any 'gate' or other such obstacle that stops or enables traffic. A port that is 'open' is simply a port to which a process has bound, and is listening for a connection request. No listener = not open. Simple.

--- rod.

anomie · 08-15-2011, 09:36 PM

@sporkit:

The telnet(1) test is fine; there are a couple other, more refined utilities as well. nmap(1), nc(1), and hping(1) all do a great job.

Example with nmap:

Code:

$ nmap -PN foo.local -p 80

If a tcp handshake is established, nmap says "open". If a tcp RST is received, nmap says "closed". If no reply is received, nmap says "filtered".

sporkit · 08-16-2011, 12:03 PM

Quote:

Originally Posted by anomie

@sporkit:

The telnet(1) test is fine; there are a couple other, more refined utilities as well. nmap(1), nc(1), and hping(1) all do a great job.

Example with nmap:

Code:

$ nmap -PN foo.local -p 80

If a tcp handshake is established, nmap says "open". If a tcp RST is received, nmap says "closed". If no reply is received, nmap says "filtered".

THANK YOU... You hit the nail on the head Anomie.

And I don't think the term "open" is a misnomer here. I'm testing connectivity to a port lying beyond several complex hardware firewalls and subnets. I would have said "accepting connections" if I was concerned with the program running underneath it.

acid_kewpie · 08-17-2011, 02:21 AM

it definitely IS a misnomer. Open = accepting connections.

baldy3105 · 08-18-2011, 02:23 PM

Quote:

Originally Posted by acid_kewpie

it definitely IS a misnomer. Open = accepting connections.

Agree completely. An open port is one with a listening daemon that can accept connections. A closed port is one with no listening process. The connection will either be refused or if you have a decent firewall totally ignored.