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8.6: Neighbor Discovery

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  • IPv6 Neighbor Discovery, or ND, is a set of related protocols that replaces several IPv4 tools, most notably ARP, ICMP redirects and most non-address-assignment parts of DHCP. The messages exchanged in ND are part of the ICMPv6 framework, 8.9   ICMPv6. The original specification for ND is in RFC 2461 [], later updated by RFC 4861 []. ND provides the following services:

    8.6.1 Router Discovery

    IPv6 routers periodically send Router Advertisement (RA) packets to the all-nodes multicast group. Ordinary hosts wanting to know what router to use can wait for one of these periodic multicasts, or can request an RA packet immediately by sending a Router Solicitation request to the all-routers multicast group. Router Advertisement packets serve to identify the routers; this process is sometimes called Router Discovery. In IPv4, by comparison, the address of the default router is usually piggybacked onto the DHCP response message (7.10   Dynamic Host Configuration Protocol (DHCP)).

    These RA packets, in addition to identifying the routers, also contain a list of all network address prefixes in use on the LAN. This is “prefix discovery”, described in the following section. To a first approximation on a simple network, prefix discovery supplies the network portion of the IPv6 address; on IPv4 networks, DHCP usually supplies the entire IPv4 address.

    RA packets may contain other important information about the LAN as well, such as an agreed-on MTU.

    These IPv6 router messages represent a change from IPv4, in which routers need not send anything besides forwarded packets. To become an IPv4 router, a node need only have IPv4 forwarding enabled in its kernel; it is then up to DHCP (or the equivalent) to inform neighboring nodes of the router. IPv6 puts the responsibility for this notification on the router itself: for a node to become an IPv6 router, in addition to forwarding packets, it “MUST” (RFC 4294 []) also run software to support Router Advertisement. Despite this mandate, however, the RA mechanism does not play a role in the forwarding process itself; an IPv6 network can run without Router Advertisements if every node is, for example, manually configured to know where the routers are and to know which neighbors are on-link. (We emphasize that manual configuration like this scales very poorly.)

    On Linux systems, the Router Advertisement agent is most often the radvd [] daemon. See 8.13   IPv6 Connectivity via Tunneling below.

    8.6.2 Prefix Discovery

    Closely related to Router Discovery is the Prefix Discovery process by which hosts learn what IPv6 network-address prefixes, above, are valid on the network. It is also where hosts learn which prefixes are considered to be local to the host’s LAN, and thus reachable at the LAN layer instead of requiring router assistance for delivery. IPv6, in other words, does not limit determination of whether delivery is local to the IPv4 mechanism of having a node check a destination address against each of the network-address prefixes assigned to the node’s interfaces.

    Even IPv4 allows two IPv4 network prefixes to share the same LAN (eg a private one and a public one, but a consequence of IPv4 routing is that two such LAN-sharing subnets can only reach one another via a router on the LAN, even though they should in principle be able to communicate directly. IPv6 drops this restriction.

    The Router Advertisement packets sent by the router should contain a complete list of valid network-address prefixes, as the Prefix Information option. In simple cases this list may contain a single globally routable 64-bit prefix corresponding to the LAN subnet. If a particular LAN is part of multiple (overlapping) physical subnets, the prefix list will contain an entry for each subnet; these 64-bit prefixes will themselves likely share a common site-wide prefix of length N<64. For multihomed sites the prefix list may contain multiple unrelated prefixes corresponding to the different address blocks. Finally, site-specific “unique local” IPv6 address prefixes may also be included.

    Each prefix will have an associated lifetime; nodes receiving a prefix from an RA packet are to use it only for the duration of this lifetime. On expiration (and likely much sooner) a node must obtain a newer RA packet with a newer prefix list. The rationale for inclusion of the prefix lifetime is ultimately to allow sites to easily renumber; that is, to change providers and switch to a new network-address prefix provided by a new router. Each prefix is also tagged with a bit indicating whether it can be used for autoconfiguration, as in 8.7.2   Stateless Autoconfiguration (SLAAC) below.

    Each prefix also comes with a flag indicating whether the prefix is on-link. If set, then every node receiving that prefix is supposed to be on the same LAN. Nodes assume that to reach a neighbor sharing the same on-link address prefix, Neighbor Solicitation is to be used to find the neighbor’s LAN address. If a neighbor shares an off-link prefix, a router must be used. The IPv4 equivalent of two nodes sharing the same on-link prefix is sharing the same subnet prefix. For an example of subnets with prefix-discovery information, see 8.10   IPv6 Subnets.

    Routers advertise off-link prefixes only in special cases; this would mean that a node is part of a subnet but cannot reach other members of the subnet directly. This may apply in some wireless settings, eg MANETs (3.7.8   MANETs) where some nodes on the same subnet are out of range of one another. It may also apply when using IPv6 Mobility (7.13   Mobile IP, RFC 3775 []).

    8.6.3 Neighbor Solicitation

    Neighbor Solicitation messages are the IPv6 analogues of IPv4 ARP requests. These are essentially queries of the form “who has IPv6 address X?” While ARP requests were broadcast, IPv6 Neighbor Solicitation messages are sent to the solicited-node multicast address, which at the LAN layer usually represents a rather small multicast group. This address is ff02::0001:x.y.z.w, where x, y, z and w are the low-order 32 bits of the IPv6 address the sender is trying to look up. Each IPv6 host on the LAN will need to subscribe to all the solicited-node multicast addresses corresponding to its own IPv6 addresses (normally this is not too many).

    Neighbor Solicitation messages are repeated regularly, but followup verifications are initially sent to the unicast LAN address on file (this is common practice with ARP implementations, but is optional). Unlike with ARP, other hosts on the LAN are not expected to eavesdrop on the initial Neighbor Solicitation message. The target host’s response to a Neighbor Solicitation message is called Neighbor Advertisement; a host may also send these unsolicited if it believes its LAN address may have changed.

    The analogue of Proxy ARP is still permitted, in that a node may send Neighbor Advertisements on behalf of another. The most likely reason for this is that the node receiving proxy services is a “mobile” host temporarily remote from the home LAN. Neighbor Advertisements sent as proxies have a flag to indicate that, if the real target does speak up, the proxy advertisement should be ignored.

    Once a node (host or router) has discovered a neighbor’s LAN address through Neighbor Solicitation, it continues to monitor the neighbor’s continued reachability.

    Neighbor Solicitation also includes Neighbor Unreachability Detection. Each node (host or router) continues to monitor its known neighbors; reachability can be inferred either from ongoing IPv6 traffic exchanges or from Neighbor Advertisement responses. If a node detects that a neighboring host has become unreachable, the original node may retry the multicast Neighbor Solicitation process, in case the neighbor’s LAN address has simply changed. If a node detects that a neighboring router has become unreachable, it attempts to find an alternative path.

    Finally, IPv4 ICMP Redirect messages have also been moved in IPv6 to the Neighbor Discovery protocol. These allow a router to tell a host that another router is better positioned to handle traffic to a given destination.

    8.6.4 Security and Neighbor Discovery

    In the protocols outlined above, received ND messages are trusted; this can lead to problems with nodes pretending to be things they are not. Here are two examples:

    • A host can pretend to be a router simply by sending out Router Advertisements; such a host can thus capture traffic from its neighbors, and even send it on – perhaps selectively – to the real router.

    • A host can pretend to be another host, in the IPv6 analog of ARP spoofing (7.9.2   ARP Security). If host A sends out a Neighbor Solicitation for host B, nothing prevents host C from sending out a Neighbor Advertisement claiming to be B (after previously joining the appropriate multicast group).

    These two attacks can have the goal either of eavesdropping or of denial of service; there are also purely denial-of-service attacks. For example, host C can answer host B’s DAD queries (below at 8.7.1   Duplicate Address Detection) by claiming that the IPv6 address in question is indeed in use, preventing B from ever acquiring an IPv6 address. A good summary of these and other attacks can be found in RFC 3756 [].

    These attacks, it is worth noting, can only be launched by nodes on the same LAN; they cannot be launched remotely. While this reduces the risk, though, it does not eliminate it. Sites that allow anyone to connect, such as Internet cafés, run the highest risk, but even in a setting in which all workstations are “locked down”, a node compromised by a virus may be able to disrupt the network.

    RFC 4861 [] suggested that, at sites concerned about these kinds of attacks, hosts might use the IPv6 Authentication Header or the Encapsulated Security Payload Header to supply digital signatures for ND packets (see 22.11   IPsec). If a node is configured to require such checks, then most ND-based attacks can be prevented. Unfortunately, RFC 4861 [] offered no suggestions beyond static configuration, which scales poorly and also rather completely undermines the goal of autoconfiguration.

    A more flexible alternative is Secure Neighbor Discovery, or SEND, specified in RFC 3971 []. This uses public-key encryption (22.9   Public-Key Encryption) to validate ND messages; for the remainder of this section, some familiarity with the material at 22.9   Public-Key Encryption may be necessary. Each message is digitally signed by the sender, using the sender’s private key; the recipient can validate the message using the sender’s corresponding public key. In principle this makes it impossible for one message sender to pretend to be another sender.

    In practice, the problem is that public keys by themselves guarantee (if not compromised) only that the sender of a message is the same entity that previously sent messages using that key. In the second bulleted example above, in which C sends an ND message falsely claiming to be B, straightforward applications of public keys would prevent this if the original host A had previously heard from B, and trusted that sender to be the real B. But in general A would not know which of B or C was the real B. A cannot trust whichever host it heard from first, as it is indeed possible that C started its deception with A’s very first query for B, beating B to the punch.

    A common solution to this identity-guarantee problem is to create some form of “public-key infrastructure” such as certificate authorities, as in   Certificate Authorities. In this setting, every node is configured to trust messages signed by the certificate authority; that authority is then configured to vouch for the identities of other nodes whenever this is necessary for secure operation. SEND implements its own version of certificate authorities; these are known as trust anchors. These would be configured to guarantee the identities of all routers, and perhaps hosts. The details are somewhat simpler than the mechanism outlined in   Certificate Authorities, as the anchors and routers are under common authority. When trust anchors are used, each host needs to be configured with a list of their addresses.

    SEND also supports a simpler public-key validation mechanism known as cryptographically generated addresses, or CGAs (RFC 3972 []). These are IPv6 interface identifiers that are secure hashes (22.6   Secure Hashes) of the host’s public key (and a few other non-secret parameters). CGAs are an alternative to the interface-identifier mechanisms discussed in 8.2.1   Interface identifiers. DNS names in the .onion domain used by TOR also use CGAs.

    The use of CGAs makes it impossible for host C to successfully claim to be host B: only B will have the public key that hashes to B’s address and the matching private key. If C attempts to send to A a neighbor advertisement claiming to be B, then C can sign the message with its own private key, but the hash of the corresponding public key will not match the interface-identifier portion of B’s address. Similarly, in the DAD scenario, if C attempts to tell B that B’s newly selected CGA address is already in use, then again C won’t have a key matching that address, and B will ignore the report.

    In general, CGI addresses allow recipients of a message to verify that the source address is the “owner” of the associated public key, without any need for a public-key infrastructure (22.9.3   Trust and the Man in the Middle). C can still pretend to be a router, using its own CGA address, because router addresses are not known by the requester beforehand. However, it is easier to protect routers using trust anchors as there are fewer of them.

    SEND relies on the fact that finding two inputs hashing to the same 64-bit CGA is infeasible, as in general this would take about 264 tries. An IPv4 analog would be impossible as the address host portion won’t have enough bits to prevent finding hash collisions via brute force. For example, if the host portion of the address has ten bits, it would take C about 210 tries (by tweaking the supplemental hash parameters) until it found a match for B’s CGA.

    SEND has seen very little use in the IPv6 world, partly because IPv6 itself has seen such slow adoption, but also because of the perception that the vulnerabilities SEND protects against are difficult to exploit.

    RA-guard is a simpler mechanism to achieve ND security, but one that requires considerable support from the LAN layer. Outlined in RFC 6105 [], it requires that each host connects directly to a switch; that is, there must be no shared-media Ethernet. The switches must also be fairly smart; it must be possible to configure them to know which ports connect to routers rather than hosts, and, in addition, it must be possible to configure them to block Router Advertisements from host ports that are not router ports. This is quite effective at preventing a host from pretending to be a router, and, while it assumes that the switches can do a significant amount of packet inspection, that is in fact a fairly common Ethernet switch feature. If Wi-Fi is involved, it does require that access points (which are a kind of switch) be able to block Router Advertisements; this isn’t quite as commonly available. In determining which switch ports are connected to routers, RFC 6105 [] suggests that there might be a brief initial learning period, during which all switch ports connecting to a device that claims to be a router are considered, permanently, to be router ports.