Golang : Get UDP client IP address and differentiate clients by port number

Problem :

Your UDP server program needs to find out the UDP client IP address before writing to client. However, you are not able to find out the client IP address from net.UDPConn.RemoteAddr()

For instance:

 conn *net.UDPConn
 clientIP := conn.RemoteAddr() 

 fmt.Println(clientIP) // <---- returns NIL

Solution :

Your UDP server must first receive data from the UDP client before the server is able to find out the client IP address. There are ReadFrom(), ReadFromUDP() and ReadMsgUDP() functions that your server will probably use to read the UDP data from client. These functions will return the source Addr ( https://golang.org/pkg/net/#Addr) that you can use to extract client IP address.

For example :

 buffer := make([]byte, 1024)

 n, addr, err := conn.ReadFromUDP(buffer)
 fmt.Println("UDP client : ", addr)

IF you have more than one client connection connecting from the same client machine and you need to distinguish them, use the port numbers to differentiate the client programs.

For example :

Client 1 program :

UDP client : 127.0.0.1:63937

Client 2 program :

UDP client : 127.0.0.1:52132

NOTE : See UDP client server example at https://socketloop.com/tutorials/golang-udp-client-server-read-write-example

References :

https://golang.org/pkg/net/#UDPConn.ReadFrom

https://golang.org/pkg/net/#UDPConn.ReadFromUDP

https://golang.org/pkg/net/#UDPConn.ReadMsgUDP