The system must use a reverse-path filter for IPv4 network traffic when possible.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-22420	GEN003613	SV-38869r1_rule	ECSC-1	Medium

Description
Reverse-path filtering provides protection against spoofed source addresses by causing the system to discard packets with source addresses for which the system has no route or if the route does not point towards the interface on which the packet arrived. Reverse-path filtering should be used whenever possible. Depending on the role of the system, reverse-path filtering may cause legitimate traffic to be discarded and, therefore, should be used in a more permissive mode or not at all.

Description

Reverse-path filtering provides protection against spoofed source addresses by causing the system to discard packets with source addresses for which the system has no route or if the route does not point towards the interface on which the packet arrived. Reverse-path filtering should be used whenever possible. Depending on the role of the system, reverse-path filtering may cause legitimate traffic to be discarded and, therefore, should be used in a more permissive mode or not at all.

STIG	Date
Draft AIX Security Technical Implementation Guide	2011-08-17

Details

Check Text ( C-37863r1_chk )
Determine if the system is configured to use reverse-path filtering. Examine the IPSec rules on the system. # lsfilt -a All systems must block inbound traffic destined to the loopback address. Additionally, if the system is multihomed and the attached networks are isolated or perform symmetric routing, traffic with source addresses expected on one interface must be blocked when received on another interface. If this filtering is not configured on the system, this is a finding.

Check Text ( C-37863r1_chk )

Determine if the system is configured to use reverse-path filtering.
Examine the IPSec rules on the system.
# lsfilt -a

All systems must block inbound traffic destined to the loopback address.

Additionally, if the system is multihomed and the attached networks are isolated or perform symmetric routing, traffic with source addresses expected on one interface must be blocked when received on another interface.

If this filtering is not configured on the system, this is a finding.

Fix Text (F-33123r1_fix)
Configure the system to use reverse-path filtering. Add a rule to block traffic with loopback network source addresses from being received on interfaces other than the loopback. Use smitty or genfilt command to block loopback address from network interfaces. #smitty ipsec4 #genfilt –v4 –a D –s 127.0.0.0 –m 255.0.0 –d 0.0.0.0 –M 0.0.0.0 –c all –O any –w I –l y –a en0 If the system is multihomed and the attached networks are isolated or perform symmetric routing, add rules to block traffic with source addresses expected on one interface when received on another interface. For example, consider a system with two network interfaces, one attached to an isolated management network with address 10.0.0.55/24 and the other attached to a production network with address 192.168.1.2/24 and a default route. Traffic with a source address on the 10.0.0.0/24 network must be the only traffic accepted on the management interface and must not be accepted on the production interface. This can be accomplished with IPF rules such as: #smitty ipsec4 #genfilt –v4 –a D –s 10.0.0.0 –m 255.255.255.0 –d 0.0.0.0 –M 0.0.0.0 –c all –O any –w I –l y –a (prod en{x}) #genfilt –v4 –a D –s 192.168.1.0 –m 255.255.255.0 –d 0.0.0.0 –M 0.0.0.0 –c all –O any –w I –l y –a (mgmt en{x})

Fix Text (F-33123r1_fix)

Configure the system to use reverse-path filtering.

Add a rule to block traffic with loopback network source addresses from being received on interfaces other than the loopback.
Use smitty or genfilt command to block loopback address from network interfaces.
#smitty ipsec4

#genfilt –v4 –a D –s 127.0.0.0 –m 255.0.0 –d 0.0.0.0 –M 0.0.0.0 –c all –O any –w I –l y –a en0

If the system is multihomed and the attached networks are isolated or perform symmetric routing, add rules to block traffic with source addresses expected on one interface when received on another interface.

For example, consider a system with two network interfaces, one attached to an isolated management network with address 10.0.0.55/24 and the other attached to a production network with address 192.168.1.2/24 and a default route. Traffic with a source address on the 10.0.0.0/24 network must be the only traffic accepted on the management interface and must not be accepted on the production interface. This can be accomplished with IPF rules such as:

#smitty ipsec4
#genfilt –v4 –a D –s 10.0.0.0 –m 255.255.255.0 –d 0.0.0.0 –M 0.0.0.0 –c all –O any –w I –l y –a (prod en{x})
#genfilt –v4 –a D –s 192.168.1.0 –m 255.255.255.0 –d 0.0.0.0 –M 0.0.0.0 –c all –O any –w I –l y –a (mgmt en{x})