Basic windows listening

A very basic implementation of listening UDP connections in Windows.

It is based on

#521 (comment)

It works good enough. There is probably something missing, but a windows
binary using it is capable at listening.

Signed-off-by: Gonzalo Fernandez-Victorio <[email protected]>

dhcpv4/server4/conn_windows.go

@@ -1,11 +1,21 @@
+//go:build windows
+
 package server4
 
 import (
-	"errors"
+	"fmt"
 	"net"
 )
 
-// NewIPv4UDPConn fails on Windows. Use WithConn() to pass the connection.
+// NewIPv4UDPConn returns an UDPv4 connection bound to the IP and port provider
 func NewIPv4UDPConn(iface string, addr *net.UDPAddr) (*net.UDPConn, error) {
-	return nil, errors.New("not implemented on Windows")
+	connection, err := net.ListenPacket("udp4", addr.String())
+	if err != nil {
+		return nil, fmt.Errorf("We cannot listen on %s and port %d: %v", addr.IP, addr.Port, err)
+	}
+	udpConn, ok := connection.(*net.UDPConn)
+	if !ok {
+		return nil, fmt.Errorf("The connection is not of the proper type")
+	}
+	return udpConn, nil
 }