CODEWITHSUNDEEP
socketsport
Imran Khan
Imran Khan

Reputation: 31

How do I check whether an open port is TCP or UDP?

I am working on making a port scanner . I can check whether ports are open or not by trying to make a connection with the port but, how I will check the open port is udp or tcp?

Upvotes: 2

Views: 5622

Answers (2)

Steffen Ullrich
Steffen Ullrich

Reputation: 123551

... how I will check the open port is udp or tcp?

What you seem to have in mind is a single street where the ports are house numbers and in each house TCP and/or UDP might live. And you knock on the door and then need to figure out if UDP or TCP opened the door. But this view is wrong.

It is more like two separate streets which just have the same house numbers. There is one street for TCP and another for UDP and you decide which street to walk by choosing the type of socket. No need to check who is living in there since in the TCP street only TCP can live and in the UDP street only UDP.

A TCP socket gets created with AF_INET (or AF_INET6) and SOCK_STREAM. A UDP socket uses SOCK_DGRAM instead of SOCK_STREAM. So you know by creating the socket what you expect, i.e. which street to walk.

Apart from that UDP does not have the concept of an actual connection: while one can connect a UDP socket this will set the destination address on the local socket. It does not actually send any data to the peer to establish a connection like done with TCP. Thus a connect on a UDP socket will basically always succeed, no matter if something is expecting traffic on the destination (i.e port is "open") or not.

Upvotes: 5

John C
John C

Reputation: 1981

A single port can have any number of services running on it, using different protocols. Connecting only means that you connected with a specific protocol, and you'll need to check the other(s).

So, while it's probably unsatisfying as an answer, if you've been probing with TCP, you've probably never found a UDP service, and vice versa. And if you've been probing with XTP or something similarly obscure you've probably never found an open port.

Of course, be careful. "Connection" has different meanings under different protocols, particularly the stream-oriented protocols like TCP. You might have "connected" in an informal sense (provoked the server to acknowledge you), but you may not be "connected" in the sense of establishing a connection where the you definitely have the server's attention.

Upvotes: 1

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