How do I find the port number assigned to a UDP client (in .net/C#)?

Question

If I create a socket using

var socket = new UdpClient(0,AddressFamily.InterNetwork);

How do I then find the port of the socket?

I'm probably being daft, but I'm not having luck in MSDN/Google (probably because it is 4:42 on a Friday and the sun is shining).

Background:

What I want to do is find an open port, and then report to another process to forward messages to me on that port. There may be multiple clients, so I don't want to used a fixed port.

Thanks.

score 17 · Accepted Answer · answered Aug 21 '09 at 23:57

17

UdpClient is a wrapper around the Socket class which exposes the endpoint it's bound to through the LocalEndPoint property. Since you're using an UDP/IP client it's an IPEndPoint which has the desired Port property:

int port = ((IPEndPoint)socket.Client.LocalEndPoint).Port;

answered Aug 21 '09 at 23:57

dtb

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thanks. I'll implement this first thing Monday morning. What I was missing was the cast to IPEndPoint, so then intellisense didn't help me any. :( – Charley Rathkopf Aug 22 '09 at 00:04
Damn... never thought of that... thanx. :D – Cipi Jun 23 '11 at 12:05
Sadly, at least in .NET 4.0, the system never sets the Port field of a UDP socket. And since it is a get-only property, you can't manually set it either. – Jesse Chisholm Sep 25 '14 at 18:57
OOPS. I should have said: The system does not set the Port on RAW UDP Sockets when you bind. it does on DGRAM Sockets. – Jesse Chisholm Sep 25 '14 at 19:42

score 0 · Answer 2 · answered Sep 25 '14 at 20:42

For those (like me) who need to use a RAW socket, here is the workaround.

goal:

to create a RAW UDP socket on an arbitrary Port
learn what port the system chose.

expected: (socket.LocalEndPoint as IPEndPoint).Port

problems

whereas a DGRAM UDP Socket knows its (socket.LocalEndPoint as IPEndPoint).Port
a RAW UDP Socket always returns zero

solution:

create a normal DGRAM UDP socket
bind that socket
find out its Port
close that normal socket
create the RAW UDP socket

CAVEAT:

Use the modified local IPEndPoint variable to know the port, as the socket will always report zero.

code:

public Socket CreateBoundRawUdpSocket(ref IPEndPoint local)
{
    if (0 == local.port)
    {
        Socket wasted = new Socket(local.AddressFamily,
                                   SocketType.Dgram,
                                   ProtocolType.Udp);
        wasted.Bind(local);
        local.Port = (wasted.LocalEndPoint as IPEndPoint).Port;
        wasted.Close();
    }
    Socket goal = new Socket(local.AddressFamily,
                             SocketType.Raw,
                             ProtocolType.Udp);
    goal.Bind(local);
    return goal;
}

How do I find the port number assigned to a UDP client (in .net/C#)?

2 Answers2

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