Title

The system must be configured to use TCP syncookies when experiencing a TCP SYN flood. (Cat II impact)

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 net.ipv4.tcp_syncookies = 1 $ grep net.ipv4.tcp_syncookies /etc/sysctl.conf /etc/sysctl.d/* net.ipv4.tcp_syncookies = 1 If "net.ipv4.tcp_syncookies" is not configured in the /etc/sysctl.conf file or in the /etc/sysctl.d/ directory, is commented out or does not have a value of "1", 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 Set the system to the required kernel parameter by adding the following line to "/etc/sysctl.conf" or a config file in the /etc/sysctl.d/ directory (or modify the line to have the required value): net.ipv4.tcp_syncookies = 1 Issue the following command to make the changes take effect: # sysctl --system

Additional Identifiers

Rule ID: SV-217923r603264_rule

Vulnerability ID: V-217923

Group Title: SRG-OS-000142

Expert Comments

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