Ensure TCP SYN Cookies is enabled

MediumLevel 1Automatedbottlerocket

Description

When tcp_syncookies is set, the kernel will handle TCP SYN packets normally until the half-open connection queue is full, at which time, the SYN cookie functionality kicks in. SYN cookies work by not using the SYN queue at all. The audit reads the running value from /proc/sys. Run tmaegis on the host through the admin container root shell (sudo sheltie).

How it is checked

Audit method: Shell command

shell

bad=0
chk() { f=/proc/sys/$(printf %s "$1" | tr . /); v=$(cat "$f" 2>/dev/null) || { echo "$1: not readable"; bad=1; return; }; [ "$v" = "$2" ] || { echo "$1 = $v (expected $2)"; bad=1; }; }
chk net.ipv4.tcp_syncookies 1
[ "$bad" -eq 0 ] && echo compliant

Expected output: compliant

How it is fixed

Fix method: Guidance or infrastructure patch

Run the following command to set the active kernel parameter and persist the setting: # apiclient apply <<EOF [settings.kernel.sysctl] "net.ipv4.tcp_syncookies" = "1" EOF Run the following command to flush the routing cache: # sysctl -w net.ipv4.route.flush=1

Properties

Operating systems: Any
Tags: bottlerocketnetwork
Reference: CIS Bottlerocket Benchmark v1.0.0 - 3.2.7 Ensure TCP SYN Cookies is enabled

Check this host

Run this on a machine you are allowed to audit. The scan is read-only.

shell

tmaegis scan --target bottlerocket