/usr/src/xtables-addons-1.40/xt_ECHO.Kconfig is in xtables-addons-dkms 1.40-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 | config NETFILTER_XT_TARGET_ECHO
	tristate '"ECHO" sample target'
	depends on NETFILTER_XTABLES && NETFILTER_ADVANCED
	---help---
	The ECHO target provides a demonstrational implementation of an
	Xtables target implementing RFC 862 for UDP.
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