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

From Red Hat Enterprise Linux 6 Security Technical Implementation Guide

Associated with: CCI-001095

SV-50340r2_rule The system must be configured to use TCP syncookies when experiencing a TCP SYN flood.

Vulnerability discussion

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.

Check content

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

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

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