How to find local/epheremal port number?

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I have a UDP client program that uses Berkley sockets and Winsock (depending on the platform).

Basically it uses getaddrinfo(), then socket(), then sendto(). sendto() takes the address info returned by getaddrinfo(). My code looks like this:

struct addrinfo hint;
memset(&hint, 0, sizeof(hint));
hint.ai_socktype = SOCK_DGRAM;
struct addrinfo *address;
getaddrinfo("127.0.0.1", "9999", &hint, &address);

SOCKET s = socket(address->ai_family, address->ai_socktype, address->ai_protocol);
sendto(s, "test", 4, 0, address->ai_addr, address->ai_addrlen);

My question is, when is the local/ephemeral port number set? Is it set with the call to sendto()? If I send more data to a different server, does sendto() reuse the same ephemeral port number? How can I get the ephemeral port number (in a protocol independent way)? I know that knowing this may not be useful, and NAT can change it anyway, but I'm just trying to understand how it all works better.

I also know I can use bind() to set the local port, but my question is about what happens when the OS chooses the local port for me.

You want the getsockname function:

struct sockaddr_storage ss;
socklen_t len;

len = sizeof(ss);
if (getsockname(s, (struct sockaddr *)&ss, &len) == 0) {
    // print contents of ss
}

It populates the given sockaddr with the address and port that the socket is bound to.

This function is available in both winsock and Berkely sockets.

How to find local/epheremal port number?, How can I get the ephemeral port number (in a protocol independent way)? I know that knowing this may not be useful, and NAT can change it  Default Port Number Where Configurable; Oracle Notification Server Local Port: 6711 : Open the opmn.xml file and modify the “local” parameter. Then save the file. Oracle Notification Server Remote Port: 6712 : Open the opmn.xml file and modify the “remote” parameter. Then save the file.

MSDN's documentation for sendto() states:

Note If a socket is opened, a setsockopt call is made, and then a sendto call is made, Windows Sockets performs an implicit bind function call.

If the socket is unbound, unique values are assigned to the local association by the system, and the socket is then marked as bound. If the socket is connected, the getsockname function can be used to determine the local IP address and port associated with the socket.

If the socket is not connected, the getsockname function can be used to determine the local port number associated with the socket but the IP address returned is set to the wildcard address for the given protocol (for example, INADDR_ANY or "0.0.0.0" for IPv4 and IN6ADDR_ANY_INIT or "::" for IPv6).

Check the number of ephermal ports in use, This article describes how to check the number of TCP ports (that is, ephermal ports) netstat -nao | find /i "time_wait" #Gives a count of all lines/ports with a see this article for details on how to address this: Ephemeral Port  In the CurrPorts window, sort by the “Local Port” column, find the port you’re investigating, and you can see everything—the process name, PID, port, the full path to the process, and so on. To make it even easier, double-click on any process to see every single detail in one window.

Troubleshoot port exhaustion issues, TCP and UDP protocols work based on port numbers used for A browser on a client machine will use an ephemeral port to connect to Since outbound connections start to fail, you will see a lot of the below behaviors:. To find or change a port number, use the methods described in this appendix. E.3 Port Numbers and Protocols of Oracle Components Table E-1 lists the port numbers and protocols used by components that are configured during the installation.

43946 - Ephemeral port range, For details about how to change a range under Windows, see the Microsoft knowledge-base article 929851, "The default dynamic port range for  If the socket is not connected, the getsockname function can be used to determine the local port number associated with the socket but the IP address returned is set to the wildcard address for the given protocol (for example, INADDR_ANY or "0.0.0.0" for IPv4 and IN6ADDR_ANY_INIT or "::" for IPv6).

Ephemeral Port Exhaustion, Ephemeral, or dynamic ports are a port range used for short lived communications. When we connect to a server, we typically know the port  The Internet Assigned Numbers Authority (IANA) suggests the range 49152 to 65535 (2 15 +2 14 to 2 16 −1) for dynamic or private ports. Many Linux kernels use the port range 32768 to 61000. FreeBSD has used the IANA port range since release 4.6.

Best practices for managing TCP ephemeral port ranges and TCP , Determine the current lower and upper bound for the ephemeral port range. Platform, Commands, Default Value/Range. AIX, /usr/sbin/no -a |  To find or change a port number, use the methods described in this appendix. E.3 Port Numbers and Protocols of Oracle Components The following table lists the port numbers and protocols used by components that are configured during the installation.

Comments
  • My question is, when is the local/ephemeral port number set? Is it set with the call to sendto()? If I send more data to a different server, does sendto() reuse the same ephemeral port number?
  • Also, your solution uses sockaddr_in, which will not work with IPv6. How can I make it work with addrinfo instead?
  • @mark Because a UDP socket is identified only by the source IP/port, the port is set on the first send. As for sockaddr_in, I think you can use a sockaddr_storage instead to hold either an IPv4 or an IPv6 address, although I haven't tried it.