# Cis Ubuntu2004 V300 3 3 5

> Ensure icmp redirects are not accepted

- Skill: `cyberstrikeus/cis-ubuntu2004-v300-3-3-5` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu2004-v300-3-3-5`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu2004-v300-3-3-5/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cyberstrikeus/cis-ubuntu2004-v300-3-3-5

---


# 3.3.5 Ensure icmp redirects are not accepted (Automated)

## Profile

- Level 1 - Server
- Level 1 - Workstation

## Description

ICMP redirect messages are packets that convey routing information and tell your host (acting as a router) to send packets via an alternate path. It is a way of allowing an outside routing device to update your system routing tables.

## Rationale

ICMP redirect messages are packets that convey routing information and tell your host (acting as a router) to send packets via an alternate path. It is a way of allowing an outside routing device to update your system routing tables. By setting `net.ipv4.conf.all.accept_redirects`, `net.ipv4.conf.default.accept_redirects`, `net.ipv6.conf.all.accept_redirects`, and `net.ipv6.conf.default.accept_redirects` to 0, the system will not accept any ICMP redirect messages, and therefore, won't allow outsiders to update the system's routing tables.

## Impact

None expected.

## Audit Procedure

### Command Line

Run the following script to verify the following kernel parameters are set in the running configuration and correctly loaded from a kernel parameter configuration file:

- `net.ipv4.conf.all.accept_redirects` is set to 0
- `net.ipv4.conf.default.accept_redirects` is set to 0
- `net.ipv6.conf.all.accept_redirects` is set to 0
- `net.ipv6.conf.default.accept_redirects` is set to 0

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 If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the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.
- IPv6 kernel parameters only apply to systems where IPv6 is enabled

## Expected Result

Audit Result: ** PASS ** with all four accept_redirects parameters set to 0 (or IPv6 parameters not applicable if IPv6 is disabled).

## Remediation

### Command Line

Set the following parameters in `/etc/sysctl.conf` or a file in `/etc/sysctl.d/` ending in `.conf`:

- `net.ipv4.conf.all.accept_redirects = 0`
- `net.ipv4.conf.default.accept_redirects = 0`

Example:

```bash
# printf '%s\n' "net.ipv4.conf.all.accept_redirects = 0" "net.ipv4.conf.default.accept_redirects = 0" >>
/etc/sysctl.d/60-netipv4_sysctl.conf
```

Run the following script to set the active kernel parameters:

```bash
#!/usr/bin/env bash

{
  sysctl -w net.ipv4.conf.all.accept_redirects=0
  sysctl -w net.ipv4.conf.default.accept_redirects=0
  sysctl -w net.ipv4.route.flush=1
}
```

- IF - IPv6 is enabled on the system:
  Set the following parameters in `/etc/sysctl.conf` or a file in `/etc/sysctl.d/` ending in `.conf`:

- `net.ipv6.conf.all.accept_redirects = 0`
- `net.ipv6.conf.default.accept_redirects = 0`

Example:

```bash
# printf '%s\n' "net.ipv6.conf.all.accept_redirects = 0" "net.ipv6.conf.default.accept_redirects = 0" >>
/etc/sysctl.d/60-netipv6_sysctl.conf
```

Run the following script to set the active kernel parameters:

```bash
#!/usr/bin/env bash

{
  sysctl -w net.ipv6.conf.all.accept_redirects=0
  sysctl -w net.ipv6.conf.default.accept_redirects=0
  sysctl -w net.ipv6.route.flush=1
}
```

Note: If these settings appear in a canonically later file, or later in the same file, these settings will be overwritten.

## Default Value

net.ipv4.conf.all.accept_redirects = 1

net.ipv4.conf.default.accept_redirects = 1

net.ipv6.conf.all.accept_redirects = 1

net.ipv6.conf.default.accept_redirects = 1

## References

1. NIST SP 800-53 :: CM-6 b
2. NIST SP 800-53A :: CM-6.1 (iv)
3. RHEL 8 STIG GROUP ID: V-230535
4. RHEL 8 STIG RULE ID: SV-230535r858793
5. RHEL 8 STIG GROUP ID: V-230544
6. RHEL 8 STIG RULE ID: SV-230544r858820
7. RHEL 8 STIG GROUP ID: V-230550
8. RHEL 8 STIG RULE ID: SV-230550r627750
9. RHEL 8 STIG GROUP ID: V-230553
10. RHEL 8 STIG RULE ID: SV-230553r809324

## CIS Controls

| Controls Version | Control                                                                         | IG 1 | IG 2 | IG 3 |
| ---------------- | ------------------------------------------------------------------------------- | ---- | ---- | ---- |
| v8               | 4.8 Uninstall or Disable Unnecessary Services on Enterprise Assets and Software |      | x    | x    |
| v7               | 9.2 Ensure Only Approved Ports, Protocols and Services Are Running              |      | x    | x    |