TMA-COS-3.2.5

Ensure broadcast ICMP requests are ignored

MediumLevel 1Automatedcos

Description

Setting net.ipv4.icmp_echo_ignore_broadcasts to 1 will cause the system to ignore all ICMP echo and timestamp requests to broadcast and multicast addresses. Accepting ICMP echo and timestamp requests with broadcast or multicast destinations for your network could be used to trick your host into starting (or participating) in a Smurf attack. A Smurf attack relies on an attacker sending large amounts of ICMP broadcast messages with a spoofed source address. All hosts receiving this message and responding would send echo-reply messages back to the spoofed address, which is probably not routable. If many hosts respond to the packets, the amount of traffic on the network could be significantly multiplied.

How it is checked

Audit method: Shell command

shell

chk() { p=$1; allowed=" $2 "; case $p in net.ipv6.*) [ -d /proc/sys/net/ipv6 ] || return 0;; esac; v=$(sysctl -n "$p" 2>/dev/null) || { echo "$p: not available"; return 1; }; case "$allowed" in *" $v "*) ;; *) echo "$p is $v (running)"; return 1;; esac; sd=$(readlink -e /lib/systemd/systemd-sysctl || readlink -e /usr/lib/systemd/systemd-sysctl); f=$("$sd" --cat-config 2>/dev/null | grep -Po "^\h*${p//./\.}\h*=\h*\K\H+" | tail -n1); [ -n "$f" ] || { echo "$p is not set in any sysctl configuration file"; return 1; }; case "$allowed" in *" $f "*) ;; *) echo "$p is $f in configuration"; return 1;; esac; }; if chk net.ipv4.icmp_echo_ignore_broadcasts 1; then echo compliant; fi

Expected output: compliant

How it is fixed

Fix method: Guidance or infrastructure patch

Set the following parameters in /etc/sysctl.conf or a /etc/sysctl.d/* file: net.ipv4.icmp_echo_ignore_broadcasts = 1 Run the following commands to set the active kernel parameters: # sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1 # sysctl -w net.ipv4.route.flush=1 /etc is stateless on Container-Optimized OS. Therefore, /etc cannot be used to make these changes persistent across reboots. The steps mentioned above needs to be performed after every boot.

Properties

Operating systems: Any
Tags: container-optimized-oscos
Reference: CIS Google Container-Optimized OS Benchmark v1.2.0 - 3.2.5 Ensure broadcast ICMP requests are ignored

Check this host

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

shell

tmaegis scan --target cos