diff --git a/usr/lib/sysctl.d/990-security-misc.conf b/usr/lib/sysctl.d/990-security-misc.conf
index 1194489..800e626 100644
--- a/usr/lib/sysctl.d/990-security-misc.conf
+++ b/usr/lib/sysctl.d/990-security-misc.conf
@@ -275,6 +275,7 @@ net.ipv4.tcp_rfc1337=1
 ## Prevents IP spoofing and mitigates vulnerabilities such as CVE-2019-14899.
 ##
 ## https://en.wikipedia.org/wiki/IP_address_spoofing
+## https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/security_guide/sect-security_guide-server_security-reverse_path_forwarding#sect-Security_Guide-Server_Security-Reverse_Path_Forwarding
 ## https://forums.whonix.org/t/enable-reverse-path-filtering/8594
 ## https://seclists.org/oss-sec/2019/q4/122
 ##