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The system must be configured to use TCP syncookies when experiencing a TCP SYN flood.


Overview

Finding ID Version Rule ID IA Controls Severity
V-50683 OL6-00-000095 SV-64889r2_rule Medium
Description
A TCP SYN flood attack can cause a denial of service by filling a system's TCP connection table with connections in the SYN_RCVD state. Syncookies can be used to track a connection when a subsequent ACK is received, verifying the initiator is attempting a valid connection and is not a flood source. This feature is activated when a flood condition is detected, and enables the system to continue servicing valid connection requests.
STIG Date
Oracle Linux 6 Security Technical Implementation Guide 2018-09-19

Details

Check Text ( C-53177r2_chk )
The status of the "net.ipv4.tcp_syncookies" kernel parameter can be queried by running the following command:

$ sysctl net.ipv4.tcp_syncookies

The output of the command should indicate a value of "1". If this value is not the default value, investigate how it could have been adjusted at runtime, and verify it is not set improperly in "/etc/sysctl.conf".

$ grep net.ipv4.tcp_syncookies /etc/sysctl.conf

If the correct value is not returned, this is a finding.
Fix Text (F-55477r1_fix)
To set the runtime status of the "net.ipv4.tcp_syncookies" kernel parameter, run the following command:

# sysctl -w net.ipv4.tcp_syncookies=1

If this is not the system's default value, add the following line to "/etc/sysctl.conf":

net.ipv4.tcp_syncookies = 1