Revert "Rollback new routing functionality (#1805)" (#1813)

This reverts commit 9f32ccd453.

util/net/net_linux.go

@@ -0,0 +1,35 @@
+//go:build !android
+
+package net
+
+import (
+	"fmt"
+	"syscall"
+)
+
+// SetSocketMark sets the SO_MARK option on the given socket connection
+func SetSocketMark(conn syscall.Conn) error {
+	sysconn, err := conn.SyscallConn()
+	if err != nil {
+		return fmt.Errorf("get raw conn: %w", err)
+	}
+
+	return SetRawSocketMark(sysconn)
+}
+
+func SetRawSocketMark(conn syscall.RawConn) error {
+	var setErr error
+
+	err := conn.Control(func(fd uintptr) {
+		setErr = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_MARK, NetbirdFwmark)
+	})
+	if err != nil {
+		return fmt.Errorf("control: %w", err)
+	}
+
+	if setErr != nil {
+		return fmt.Errorf("set SO_MARK: %w", setErr)
+	}
+
+	return nil
+}