cis-ubuntu2004-v300-3-3-7

3.3.7 Ensure reverse path filtering is enabled (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

Setting net.ipv4.conf.all.rp_filter and net.ipv4.conf.default.rp_filter to 1 forces the Linux kernel to utilize reverse path filtering on a received packet to determine if the packet was valid. Essentially, with reverse path filtering, if the return packet does not go out the same interface that the corresponding source packet came from, the packet is dropped (and logged if log_martians is set).

Rationale

Setting net.ipv4.conf.all.rp_filter and net.ipv4.conf.default.rp_filter to 1 is a good way to deter attackers from sending your system bogus packets that cannot be responded to. One instance where this feature breaks down is if asymmetrical routing is employed. This would occur when using dynamic routing protocols (bgp, ospf, etc) on your system. If you are using asymmetrical routing on your system, you will not be able to enable this feature without breaking the routing.

Impact

If you are using asymmetrical routing on your system, you will not be able to enable this feature without breaking the routing.

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.rp_filter is set to 1
• net.ipv4.conf.default.rp_filter 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 If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the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 both net.ipv4.conf.all.rp_filter and net.ipv4.conf.default.rp_filter set to 1.

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.rp_filter = 1
• net.ipv4.conf.default.rp_filter = 1

Example:

# printf '%s\n' "net.ipv4.conf.all.rp_filter = 1" "net.ipv4.conf.default.rp_filter = 1" >> /etc/sysctl.d/60-
netipv4_sysctl.conf

Run the following script to set the active kernel parameters:

#!/usr/bin/env bash

{
  sysctl -w net.ipv4.conf.all.rp_filter=1
  sysctl -w net.ipv4.conf.default.rp_filter=1
  sysctl -w net.ipv4.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.rp_filter = 2

net.ipv4.conf.default.rp_filter = 1

References

1. NIST SP 800-53 Rev. 5: CM-1,CM-2, CM-6, CM-7, IA-5
2. NIST SP 800-53A :: CM-6.1 (iv)
3. RHEL 8 STIG Vul ID: V-230549
4. RHEL 8 STIG Rule ID: SV-230549r858830

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