STIGQter: STIG Summary: Red Hat Enterprise Linux 9 Security Technical Implementation Guide Version: 2 Release: 9 Benchmark Date: 01 Jul 2026:

RHEL 9 must be configured to use TCP syncookies.

DISA Rule

SV-257957r1155618_rule

Vulnerability Number

V-257957

Group Title

SRG-OS-000480-GPOS-00227

Rule Version

RHEL-09-253010

Severity

CAT II

CCI(s)

• CCI-001095 - Manage capacity, bandwidth, or other redundancy to limit the effects of information flooding types of denial of service attacks.
• CCI-002385 - Protect against or limit the effects of organization-defined types of denial of service events.

Weight

10

Fix Recommendation

Configure RHEL 9 to use TCP syncookies.

Create a configuration file if it does not already exist:

$ sudo vi /etc/sysctl.d/99-ipv4_tcp_syncookies.conf

Add the following line to the file:
net.ipv4.tcp_syncookies = 1

Reload settings from all system configuration files with the following command:

$ sudo sysctl --system

Check Contents

Verify RHEL 9 is configured to use IPv4 TCP syncookies.

Check the value of all "tcp_syncookies" variables with the following command:

$ sudo sysctl net.ipv4.tcp_syncookies
net.ipv4.tcp_syncookies = 1

If the network parameter "ipv4.tcp_syncookies" is not equal to "1" or nothing is returned, this is a finding.

Vulnerability Number

V-257957

Documentable

False

Rule Version

RHEL-09-253010

Severity Override Guidance

Verify RHEL 9 is configured to use IPv4 TCP syncookies.

Check the value of all "tcp_syncookies" variables with the following command:

$ sudo sysctl net.ipv4.tcp_syncookies
net.ipv4.tcp_syncookies = 1

If the network parameter "ipv4.tcp_syncookies" is not equal to "1" or nothing is returned, this is a finding.

Check Content Reference

M

Target Key

5551