add proxy protocol support for UDP proxies (#4810)

pkg/util/net/proxyprotocol.go

@@ -0,0 +1,45 @@
+// Copyright 2025 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package net
+
+import (
+	"bytes"
+	"fmt"
+	"net"
+
+	pp "github.com/pires/go-proxyproto"
+)
+
+func BuildProxyProtocolHeaderStruct(srcAddr, dstAddr net.Addr, version string) *pp.Header {
+	var versionByte byte
+	if version == "v1" {
+		versionByte = 1
+	} else {
+		versionByte = 2 // default to v2
+	}
+	return pp.HeaderProxyFromAddrs(versionByte, srcAddr, dstAddr)
+}
+
+func BuildProxyProtocolHeader(srcAddr, dstAddr net.Addr, version string) ([]byte, error) {
+	h := BuildProxyProtocolHeaderStruct(srcAddr, dstAddr, version)
+
+	// Convert header to bytes using a buffer
+	var buf bytes.Buffer
+	_, err := h.WriteTo(&buf)
+	if err != nil {
+		return nil, fmt.Errorf("failed to write proxy protocol header: %v", err)
+	}
+	return buf.Bytes(), nil
+}