[management,proxy,client] Add L4 capabilities (TLS/TCP/UDP) (#5530)

proxy/internal/tcp/proxyprotocol.go

@@ -0,0 +1,29 @@
+package tcp
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/pires/go-proxyproto"
+)
+
+// writeProxyProtoV2 sends a PROXY protocol v2 header to the backend connection,
+// conveying the real client address.
+func writeProxyProtoV2(client, backend net.Conn) error {
+	tp := proxyproto.TCPv4
+	if addr, ok := client.RemoteAddr().(*net.TCPAddr); ok && addr.IP.To4() == nil {
+		tp = proxyproto.TCPv6
+	}
+
+	header := &proxyproto.Header{
+		Version:           2,
+		Command:           proxyproto.PROXY,
+		TransportProtocol: tp,
+		SourceAddr:        client.RemoteAddr(),
+		DestinationAddr:   client.LocalAddr(),
+	}
+	if _, err := header.WriteTo(backend); err != nil {
+		return fmt.Errorf("write PROXY protocol v2 header: %w", err)
+	}
+	return nil
+}