D-Link NetDefend DFL-260E, Руководство пользователя

Страница: 243 / 552

6.1.2. IP Spoofing
Traffic that pretends it comes from a trusted host can be sent by an attacker to try and get past a
firewall's security mechanisms. Such an attack is commonly known as Spoofing.
IP spoofing is one of the most common spoofing attacks. Trusted IP addresses are used to bypass
filtering. The header of an IP packet indicating the source address of the packet is modified by the
attacker to be a local host address. The firewall will believe the packet came from a trusted source.
Although the packet source cannot be responded to correctly, there is the potential for unnecessary
network congestion to be created and potentially a Denial of Service (DoS) condition could occur.
Even if the firewall is able to detect a DoS condition, it is hard to trace or stop because of its nature.
VPNs provide one means of avoiding spoofing but where a VPN is not an appropriate solution then
Access Rules can provide an anti-spoofing capability by providing an extra filter for source address
verification. An Access Rule can verify that packets arriving at a given interface do not have a
source address which is associated with a network of another interface. In other words:
• Any incoming traffic with a source IP address belonging to a local trusted host is NOT allowed.
• Any outgoing traffic with a source IP address belonging to an outside untrusted network is NOT
allowed.
The first point prevents an outsider from using a local host's address as its source address. The
second point prevents any local host from launching the spoof.
6.1.3. Access Rule Settings
The configuration of an access rule is similar to other types of rules. It contains Filtering Fields as
well as the Action to take. If there is a match, the rule is triggered, and NetDefendOS will carry out
the specified Action.
Access Rule Filtering Fields
The Access Rule filtering fields used to trigger a rule are:
• Interface: The interface that the packet arrives on.
• Network: The IP span that the sender address should belong to.
Access Rule Actions
The Access Rule actions that can be specified are:
• Drop: Discard the packets that match the defined fields.
• Accept: Accept the packets that match the defined fields for further inspection in the rule set.
• Expect: If the sender address of the packet matches the Network specified by this rule, the
receiving interface is compared to the specified interface. If the interface matches, the packet is
accepted in the same way as an Accept action. If the interfaces do not match, the packet is
dropped in the same way as a Drop action.
Note: Enabling logging
Logging can be enabled as required for these actions.
Turning Off Default Access Rule Messages
6.1.3. Access Rule Settings Chapter 6. Security Mechanisms
243