Golang : UDP client server read write example

In this tutorial, we will learn how to build a bare bone client-server UDP program with Golang. This UDP example is adapted from previous tutorial on how to create TCP client-server example in Golang.

Here is an example of how a UDP server program looks like in Golang. The server will first setup a listener and then starts listening for incoming UDP client on localhost. You can change the host name and port number to suit your configuration.

udpserver.go

 package main

 import (
 "fmt"
 "log"
 "net"
 )

 func handleUDPConnection(conn *net.UDPConn) {

 // here is where you want to do stuff like read or write to client

 buffer := make([]byte, 1024)

 n, addr, err := conn.ReadFromUDP(buffer)

 fmt.Println("UDP client : ", addr)
 fmt.Println("Received from UDP client :  ", string(buffer[:n]))

 if err != nil {
 log.Fatal(err)
 }

 // NOTE : Need to specify client address in WriteToUDP() function
 // otherwise, you will get this error message
 // write udp : write: destination address required if you use Write() function instead of WriteToUDP()

 // write message back to client
 message := []byte("Hello UDP client!")
 _, err = conn.WriteToUDP(message, addr)

 if err != nil {
 log.Println(err)
 }

 }

 func main() {
 hostName := "localhost"
 portNum := "6000"
 service := hostName + ":" + portNum

 udpAddr, err := net.ResolveUDPAddr("udp4", service)

 if err != nil {
 log.Fatal(err)
 }

 // setup listener for incoming UDP connection
 ln, err := net.ListenUDP("udp", udpAddr)

 if err != nil {
 log.Fatal(err)
 }

 fmt.Println("UDP server up and listening on port 6000")

 defer ln.Close()

 for {
 // wait for UDP client to connect
 handleUDPConnection(ln)
 }

 }

Build this udpserver.go and run it on the background :

./udpserver &

then run the client on a separate machine(if possible) and connect to the server

udpclient.go

 package main

 import (
 "log"
 "net"
 "fmt"
 )

 func main() {
 hostName := "localhost"
 portNum := "6000"

 service := hostName + ":" + portNum

 RemoteAddr, err := net.ResolveUDPAddr("udp", service)

 //LocalAddr := nil
 // see https://golang.org/pkg/net/#DialUDP

 conn, err := net.DialUDP("udp", nil, RemoteAddr)

 // note : you can use net.ResolveUDPAddr for LocalAddr as well
 // for this tutorial simplicity sake, we will just use nil

 if err != nil {
 log.Fatal(err)
 }

 log.Printf("Established connection to %s \n", service)
 log.Printf("Remote UDP address : %s \n", conn.RemoteAddr().String())
 log.Printf("Local UDP client address : %s \n", conn.LocalAddr().String())

 defer conn.Close()

 // write a message to server
 message := []byte("Hello UDP server!")

 _, err = conn.Write(message)

 if err != nil {
 log.Println(err)
 }

 // receive message from server
 buffer := make([]byte, 1024)
 n, addr, err := conn.ReadFromUDP(buffer)

 fmt.Println("UDP Server : ", addr)
 fmt.Println("Received from UDP server : ", string(buffer[:n]))

 }

Sample output for udpserver.go :

UDP server up and listening on port 6000

UDP client : 127.0.0.1:63937

Received from UDP client : Hello UDP server!

Sample output for udpclient.go :

2015/11/24 11:14:56 Established connection to localhost:6000

2015/11/24 11:14:56 Remote UDP address : 127.0.0.1:6000

2015/11/24 11:14:56 Local UDP client address : 127.0.0.1:63937

UDP Server : 127.0.0.1:6000

Received from UDP server : Hello UDP client!

References :

https://www.socketloop.com/tutorials/golang-simple-client-server-example

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