TUN-5481: Create abstraction for Origin UDP Connection

Creates an abstraction over UDP Conn for origin "connection" which can
be useful for future support of complex protocols that may require
changing ports during protocol negotiation (eg. SIP, TFTP)
In addition, it removes a dependency from ingress on connection package.

ingress/origin_connection_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"net"
 	"net/http"
@@ -19,7 +20,6 @@ import (
 	"golang.org/x/net/proxy"
 	"golang.org/x/sync/errgroup"
 
-	"github.com/cloudflare/cloudflared/connection"
 	"github.com/cloudflare/cloudflared/logger"
 	"github.com/cloudflare/cloudflared/socks"
 	"github.com/cloudflare/cloudflared/websocket"
@@ -192,8 +192,10 @@ func TestSocksStreamWSOverTCPConnection(t *testing.T) {
 
 func TestWsConnReturnsBeforeStreamReturns(t *testing.T) {
 	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		eyeballConn, err := connection.NewHTTP2RespWriter(r, w, connection.TypeWebsocket)
-		assert.NoError(t, err)
+		eyeballConn := &readWriter{
+			w: w,
+			r: r.Body,
+		}
 
 		cfdConn, originConn := net.Pipe()
 		tcpOverWSConn := tcpOverWSConnection{
@@ -319,3 +321,16 @@ func echoTCPOrigin(t *testing.T, conn net.Conn) {
 	_, err = conn.Write(testResponse)
 	assert.NoError(t, err)
 }
+
+type readWriter struct {
+	w io.Writer
+	r io.Reader
+}
+
+func (r *readWriter) Read(p []byte) (n int, err error) {
+	return r.r.Read(p)
+}
+
+func (r *readWriter) Write(p []byte) (n int, err error) {
+	return r.w.Write(p)
+}

ingress/origin_udp_proxy.go

@@ -0,0 +1,27 @@
+package ingress
+
+import (
+	"fmt"
+	"io"
+	"net"
+)
+
+type UDPProxy struct {
+	io.ReadWriteCloser
+}
+
+func DialUDP(dstIP net.IP, dstPort uint16) (*UDPProxy, error) {
+	dstAddr := &net.UDPAddr{
+		IP:   dstIP,
+		Port: int(dstPort),
+	}
+
+	// We use nil as local addr to force runtime to find the best suitable local address IP given the destination
+	// address as context.
+	udpConn, err := net.DialUDP("udp", nil, dstAddr)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create UDP proxy to origin (%v:%v): %w", dstIP, dstPort, err)
+	}
+
+	return &UDPProxy{udpConn}, nil
+}