| name | cis-ubuntu2004-v300-3-3-10 |
| description | Ensure tcp syn cookies is enabled |
| category | cis-networking |
| version | 3.0.0 |
| author | cyberstrike-official |
| tags | ["cis","ubuntu","linux","ubuntu-20.04","kernel-parameter"] |
| cis_id | 3.3.10 |
| cis_benchmark | CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 |
| tech_stack | ["ubuntu","linux"] |
| cwe_ids | [] |
| chains_with | [] |
| prerequisites | [] |
| severity_boost | {} |
3.3.10 Ensure tcp syn cookies is enabled (Automated)
Profile
- Level 1 - Server
- Level 1 - Workstation
Description
When tcp_syncookies is set, the kernel will handle TCP SYN packets normally until the half-open connection queue is full, at which time, the SYN cookie functionality kicks in. SYN cookies work by not using the SYN queue at all. Instead, the kernel simply replies to the SYN with a SYN/ACK, but will include a specially crafted TCP sequence number that encodes the source and destination IP address and port number and the time the packet was sent. A legitimate connection would send the ACK packet of the three way handshake with the specially crafted sequence number. This allows the system to verify that it has received a valid response to a SYN cookie and allow the connection, even though there is no corresponding SYN in the queue.
Rationale
Attackers use SYN flood attacks to perform a denial of service attacked on a system by sending many SYN packets without completing the three way handshake. This will quickly use up slots in the kernel's half-open connection queue and prevent legitimate connections from succeeding. Setting net.ipv4.tcp_syncookies to 1 enables SYN cookies, allowing the system to keep accepting valid connections, even if under a denial of service attack.
Impact
None expected.
Audit Procedure
Command Line
Run the following script to verify the following kernel parameter is set in the running configuration and correctly loaded from a kernel parameter configuration file:
net.ipv4.tcp_syncookies is set to 1
Note: kernel parameters are loaded by file and parameter order precedence. The following script observes this precedence as part of the auditing procedure. The parameters being checked may be set correctly in a file. If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the script and by the system during a normal kernel parameter load sequence.
Expected Result
Audit Result: ** PASS ** with net.ipv4.tcp_syncookies set to 1.
Remediation
Command Line
Set the following parameter in /etc/sysctl.conf or a file in /etc/sysctl.d/ ending in .conf:
net.ipv4.tcp_syncookies = 1
Example: