What is a Router Advertisement Guard? IPv6, SLAAC, DAD, and NDP

A Router Advertisement (RA) Guard is a network hardening technique that can be implemented to IPv6 networks. But why do you need an RA guard specifically for IPv6 networks? Let’s first take a look at how IPv6 works on a fundamental level.


All networks require a way to identify hosts to effectively route traffic to their intended destinations. Traditionally, networks leaned on DHCP to allocate local addresses in IPv4 ecosystems. This procedure involves a DHCP server, which is typically a service running on your router. The DHCP server possesses a reservoir of IP addresses at its disposal ready to be allocated to connecting hosts. The interaction begins with a client broadcasting a DHCP request across the network, this prompts the server to counter with an IP address for the client’s use. For a more comprehensive explanation on IPv4 routing, this article is a valuable resource.

IPv6, on the other hand, employs an entirely different mechanism for host configuration. Unlike in IPv4 networks, where the scarcity of addresses creates a need for local IP allocation and Network Address Translation (NAT), IPv6 presents a breakthrough with its vast address space.


Stateless Address Autoconfiguration (SLAAC) serves as the mechanism that allows IPv6 hosts to autonomously set up their distinctive IP addresses. Unlike protocols involving dedicated servers to oversee address assignments, SLAAC operates without the need for such servers. SLAAC generates unique IPv6 addresses by utilizing the device’s MAC address, segmenting it into parts, and combining it with the network prefix provided by the router.

Nevertheless, the issue of duplicate addresses still looms. To tackle this concern, IPv6 hosts employ a function called Duplicate Address Detection (DAD) that effectively resolves any issues arising from address overlaps. It works by the host sending a Neighbor Solicitation (NS) message to the IPv6 address it just created. If no response is received within a specified time frame, the host concludes that the address is unique within the network and begins using it.

This method, in which each device holds its own unique IP address, enhances wide area network communication and optimizes network management. This streamlined approach creates communication efficiency by eliminating the need for IP address translation.

The heightened security at the protocol level in IPv6 networks, with features like IPSec, renders the protective role of NAT less vital in most scenarios.

IPv6 Nodes and Neighbor Discovery

Duplicate Address Detection (DAD) is a part of the Neighbor Discovery Protocol (NDP) protocol in IPv6 networks. NDP, which replaces ARP in IPv4 networks, is a set of ICMPv6 (Internet Control Message Protocol) messages and mechanisms. NDP facilitates various functions in IPv6 networks, including address autoconfiguration, router discovery, and neighbor reachability detection.

NDP serves the purpose of enabling communication and interaction between devices on the same local network segment, also known as a link. Each device on the network, known as a “node”, uses Neighbor Discovery to broadcast its presence and existence to its neighboring nodes. This allows devices to become aware of each other’s presence on the same link.

Router Advertisements

Router Advertisements (RAs) are essential components of the Neighbor Discovery Protocol (NDP) in IPv6 networks. RAs are messages sent by routers to announce their presence and provide critical configuration information to devices on the same network segment. These messages include details like network prefixes, default gateway addresses, and other relevant parameters.

Connecting a new router to an IPv6 network becomes necessary when expanding or reconfiguring the network to accommodate more devices or different segments. During this process, router advertisements transmit essential network configuration details, such as prefixes and default gateway information, to devices on the network segment. This enables devices to autoconfigure their IPv6 addresses through SLAAC and facilitates proper routing.

Link-local addresses are IPv6 addresses that are automatically assigned to interfaces within the same network segment or link. They start with the prefix “fe80::” and serve the purpose of enabling direct communication and interaction between devices that reside on the same network segment without the need for routing.

Having explored the significance of router advertisements and their crucial role in IPv6 networks, an important concern emerges: these messages lack security measures and are vulnerable to forgery attacks. To prevent a malicious device from forging router advertisements and creating links directly with hosts on the network we can implement Router Advertisement Guards.

Router Advertisement Guards

Router Advertisement Guards (RAGs) are security mechanisms that are implemented onto network devices that filter incoming RA messages to prevent unauthorized or malicious routers from being accepted into the network. These guards analyze the attributes of RA messages, such as the source MAC address, source IPv6 address, IPv6 address prefix, hop-count limit, router preference priority, and configuration flags, to validate the authenticity of the messages.

RA Guards are commonly deployed on switches which link devices within a specific network segment known as the “first hop.” In this context, the “first hop” signifies the very first point where data from end-user devices enters the network. Imagine a scenario where several devices are connected to a switch, which acts as the first point of interaction with the broader network. RA Guard is put in place on this switch to protect against unauthorized or malicious Router Advertisement (RA) messages. These messages could potentially misconfigure devices on the network or create security vulnerabilities.

By setting up RA Guard, administrators can allocate roles for routers and hosts within an RA policy framework. Subsequently, this policy is enforced on designated interfaces or a range of interfaces, dictating how the network processes RA messages.

The process of implementing RA Guard involves several steps, beginning with the creation of a global RA Guard policy that specifies the type of device, such as a router. This policy is then associated with a particular interface, known as an “uplink interface,” which connects to higher-level networks or other switches. Essentially, an uplink interface serves as the gateway through which data travels to and from external networks. This connection point is crucial for preventing unauthorized router advertisements from entering the network. The uplink interface needs to have the RA Guard policy attached to it to ensure that only valid router advertisements are allowed into the network.

Rouge Router Infiltration

The infiltration of a rogue Router Advertisement (RA) into a network can trigger a cascade of security vulnerabilities that expose the network to severe risks. Once a malicious RA gains entry, it opens the gateway for a range of attacks, including RA spoofing, eavesdropping, and data manipulation. By initially planting a rogue RA, an attacker can effectively compromise the network’s routing infrastructure. This enables the attacker to forge and distribute counterfeit RAs, confusing legitimate devices into diverting their network traffic through the attacker-controlled nodes. Malicious redirection lays the groundwork for eavesdropping on sensitive communications or enabling data interception.

The implications of a rogue RA extend beyond eavesdropping and data manipulation, it also enables sophisticated man-in-the-middle attacks. By exploiting the trust that users place in authentic RAs, attackers can position themselves between users and the actual services they intend to access. This positioning grants the attacker the ability to intercept and monitor user interactions with a variety of online resources. As a result, login credentials, financial information, or any other sensitive data exchanged during these interactions can be captured by the attacker.


As technology advances, vulnerabilities evolve too. Our journey through IPv6’s complexities highlights the importance of RA Guards in securing networks. This tool helps shield our network from potential security breaches by protecting it from rouge RAs from entering the network.