CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.2.7

---
name: cis-ubuntu1804-v220-3-4-2-7-b9c70965a9dc
description: "Ensure nftables outbound and established connections are configured"
category: cis-networking
version: "2.2.0"
author: cyberstrike-official
tags: [cis, ubuntu, linux, ubuntu-18.04, networking, firewall, nftables]
cis_id: "3.4.2.7"
cis_benchmark: "CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0"
tech_stack: [ubuntu, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.2.7

## Description

Configure the firewall rules for new outbound, and established connections.

## Rationale

If rules are not in place for new outbound, and established connections all packets will be dropped by the default policy preventing network usage.

## Impact

None.

## Audit Procedure

### Command Line

Run the following commands and verify all rules for established incoming connections match site policy:

```bash
nft list ruleset | awk '/hook input/,/}/' | grep -E 'ip protocol (tcp|udp|icmp) ct state'
```

Output should be similar to:

```
ip protocol tcp ct state established accept
ip protocol udp ct state established accept
ip protocol icmp ct state established accept
```

Run the following command and verify all rules for new and established outbound connections match site policy:

```bash
nft list ruleset | awk '/hook output/,/}/' | grep -E 'ip protocol (tcp|udp|icmp) ct state'
```

Output should be similar to:

```
ip protocol tcp ct state established,related,new accept
ip protocol udp ct state established,related,new accept
ip protocol icmp ct state established,related,new accept
```

## Expected Result

Established incoming connections and new/established outbound connections should be accepted as shown above.

## Remediation

### Command Line

Configure nftables in accordance with site policy. The following commands will implement a policy to allow all outbound connections and all established connections:

```bash
nft add rule inet filter input ip protocol tcp ct state established accept
nft add rule inet filter input ip protocol udp ct state established accept
nft add rule inet filter input ip protocol icmp ct state established accept
nft add rule inet filter output ip protocol tcp ct state new,related,established accept
nft add rule inet filter output ip protocol udp ct state new,related,established accept
nft add rule inet filter output ip protocol icmp ct state new,related,established accept
```

## Default Value

No connection tracking rules exist by default.

## References

1. NIST SP 800-53 Rev. 5: CA-9, SC-7
2. CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

## CIS Controls

Version 8

4.4 Implement and Manage a Firewall on Servers - Implement and manage a firewall on servers, where supported.

4.5 Implement and Manage a Firewall on End-User Devices - Implement and manage a host-based firewall or port-filtering tool on end-user devices.

Version 7

9.4 Apply Host-based Firewalls or Port Filtering - Apply host-based firewalls or port filtering tools on end systems, with a default-deny rule that drops all traffic except those services and ports that are explicitly allowed.

## Profile Applicability

- Level 1 - Server
- Level 1 - Workstation

## Assessment Status

Manual