cis-gcp-cos-3.2.8

DevOps & Security

Ensure TCP SYN Cookies is enabled

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3.2.8 Ensure TCP SYN Cookies is enabled (Automated)

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. Instead, the kernel simply replies to the SYN with a SYN|ACK, but will include a specially crafted TCP sequence number that encodes the source and destination IP address and port number and the time the packet was sent. A legitimate connection would send the ACK packet of the three way handshake with the specially crafted sequence number. This allows the system to verify that it has received a valid response to a SYN cookie and allow the connection, even though there is no corresponding SYN in the queue.

Rationale

Attackers use SYN flood attacks to perform a denial of service attacked 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.

Audit Procedure

Run the following commands and verify output matches:

# sysctl net.ipv4.tcp_syncookies
net.ipv4.tcp_syncookies = 1
# grep "net\.ipv4\.tcp_syncookies" /etc/sysctl.conf /etc/sysctl.d/*
net.ipv4.tcp_syncookies = 1

Expected Result

The sysctl command should return a value of 1 and the grep command should confirm the setting is persisted in the sysctl configuration files.

Remediation

Set the following parameters in /etc/sysctl.conf or a /etc/sysctl.d/* file:

net.ipv4.tcp_syncookies = 1

Run the following commands to set the active kernel parameters:

# sysctl -w net.ipv4.tcp_syncookies=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.

CIS Controls

Controls Version	Control	IG 1	IG 2	IG 3
v8	12.2 Establish and Maintain a Secure Network Architecture - Establish and maintain a secure network architecture. A secure network architecture must address segmentation, least privilege, and availability, at a minimum.		X	X
v7	5.1 Establish Secure Configurations - Maintain documented, standard security configuration standards for all authorized operating systems and software.	X	X	X

Profile

Level 1 - Server