Syncookie (#372)

* Enabled SYN cookie sysctl.

Signed-off-by: Farid Joubbi <[email protected]>

* Removed SYN cookies from here since it's a default now.

Signed-off-by: Farid Joubbi <[email protected]>

roles/os_hardening/defaults/main.yml

@@ -171,6 +171,14 @@ sysctl_config:
   # RFC 1337 fix F1 | sysctl-10
   net.ipv4.tcp_rfc1337: 1
 
+  # Attackers use SYN flood attacks to perform a denial of service attack on a system
+  # by sending many SYN packets without completing the three way handshake.
+  # This will quickly use up slots in the kernel's half-open connection queue and
+  # prevent legitimate connections from succeeding.
+  # SYN cookies allow the system to keep accepting valid connections, even if
+  # under a denial of service attack. CIS Distro Independent 3.2.8.
+  net.ipv4.tcp_syncookies: 1
+
   # Send(router) or accept(host) RFC1620 shared media redirects | sysctl-12
   net.ipv4.conf.all.shared_media: 1
   net.ipv4.conf.default.shared_media: 1

roles/os_hardening/vars/Amazon.yml

@@ -40,7 +40,5 @@ os_always_ignore_users: ['root', 'sync', 'shutdown', 'halt', 'ec2-user']
 sysctl_rhel_config:
   # ExecShield protection against buffer overflows
   kernel.exec-shield: 1
-  # Syncookies is used to prevent SYN-flooding attacks.
-  net.ipv4.tcp_syncookies: 1
 
 hidepid_option: '2'  # allowed values: 0, 1, 2