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


Finding ID Version Rule ID IA Controls Severity
V-217923 RHEL-06-000095 SV-217923r505923_rule Medium
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.
Red Hat Enterprise Linux 6 Security Technical Implementation Guide 2020-09-03


Check Text ( C-19404r376784_chk )
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 (F-19402r376785_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

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