| Internet Engineering Task Force | M.R. Smith |
| Internet-Draft | IMOT |
| Updates: 4861 (if approved) | October 13, 2012 |
| Intended status: Standards Track | |
| Expires: April 14, 2013 |
Mitigating IPv6 Router Neighbor Cache DoS Using Stateless Neighbor Discovery
draft-smith-6man-mitigate-nd-cache-dos-slnd-01
The IPv6 neighbor discovery cache is vulnerable to a Denial of Service attack that purposely exhausts the state used during the neighbor discovery address resolution process. This attack can be very disruptive when the target is a router. This memo proposes a stateless form of neighbor discovery to be used by routers to mitigate this attack. It does not require any changes to hosts.
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The IPv6 neighbor discovery cache [RFC4861] is vulnerable to a Denial of Service attack that purposely exhausts the state used during the neighbor discovery address resolution process [RFC3756].
When a router is the target of this attack, an off-link attacker sends traffic towards many non-existent addresses within a prefix attached to the router. This causes the router to create neighbor cache state for neighbor solicitations for these non-existent addresses. The denial of service occurs when the router's neighbor cache state capacity is exhausted due to too many outstanding neighor solicitations.
Sizing a prefix proportional to the number of attached hosts, rather than using the standard /64 prefix size [RFC4291], would mitigate this attack. However, operational conveniences and benefits such as universal fixed length prefixes and interface identifiers, Stateless Address Autoconfiguration (SLAAC) [RFC4862] and privacy addresses [RFC4941], and never having to resize the prefix or add secondary prefixes to attach more hosts to the link would be lost.
This memo proposes a stateless form of neighbor discovery to prevent this type of DoS attack on a router. It does not require any changes to the operation of neighbor discovery on hosts. It takes advantage of hosts' ability to recover from packet loss in the network, necessary due to IPv6's best effort nature. This method would be used for unknown or untrusted packet sources, when the router's neighbor cache's state capacity reaches a medium to high threshold of use, suggesting a neighbor cache DoS attack is occuring. Trusted packet sources would continue to be provided with traditional stateful neighbor discovery.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
Stateful Neighbor Discovery (SFND): Traditional neighbor discovery, as specified in [RFC4861]. This form of neighbor discovery maintains per packet destination state for all unresolved destinations during the neighbor discovery process. The neighbor cache's state capacity is intentionally exhausted to cause the neighbor cache Denial of Service attack.
Stateless Neighbor Discovery (SLND): The form of neighbor discovery described in this memo. This form of neighbor discovery does not maintain per packet destination state for unresolved destinations during the neighbor discovery process.
To perform stateless neighbor discovery, four variables are maintained:
SLND Flag - This flag indicates whether or not the interface will perform SLND if necessary.
SLDN Activate Threshold - This variable specifies the threshold when stateless neighbor discovery is activated. The threshold specifies a neighbor cache utilisation level. It is expressed as a percentage, with a default value of 80%. It may be either a per-interface or router global variable depending on whether the router implementation has per-interface neighbor caches or a global neighbor cache used by all interfaces.
SLND Active Flag - This flag indicates whether or not the interface is performing SLND for untrusted packet sources. It is maintained for each interface on the router.
Trusted/Untrusted Sources Prefix List ("TUSP List") - This variable specifies a list of trusted and/or untrusted packet source address prefixes. It is a per-interface variable, as different interfaces on the router may have different sets of trusted and/or untrusted packet sources. A router may also maintain a single global TUSP List, used by interfaces that don't have an interface specific TUSP List.
SLND Neighbor Solicitation Rate Limit ("SLND NS Rate Limit") - This variable specifies a threshold for multicast Neighbor Solictiations when the interface is performing SLND, specified in packets per second. It is a per-interface attribute, as different interfaces may have different thresholds. The rate value should be an appropriate portion of the multicast packet per second capabilities of the interface link technology, such as 10%. A router may maintain a global threshold that is applied to interfaces that do not have an interface specific rate limit.
The stateless neighbor discovery process may occur once a router has determined the outgoing interface for a packet, and that the packet's destination is on-link.
If the packet's destination address is present in the neighbor cache, and the link-layer address has been resolved, the packet is forwarded to it's destination.
If the packet's destination address is not present in the neighbor cache, and the SLND Flag is off, traditional stateful neighbor discovery is performed for the packet's destination.
If the packet's destination address is not present in the neighbor cache, and the SLND Flag is on, the packet's source address is compared to the TUSP List.
If the packet's source address is determined to be trusted, traditional stateful neighbor discovery is performed.
If the packet's source address is determined to be untrusted, stateless neighbor discovery is performed. The stateless neighbor discovery process is as follows:
The utilisation of the neighbor cache has to be measured to determine if it crosses the SLDN Activate Threshold. If the utilisation increases above the SLDN Activate Threshold, the SLND Active Flag is switched on, and if it decreases below the SLDN Activate Threshold, the SLND Active Flag is switched off. Neighbor cache utlisation should be measured and compared to the SLDN Activate Threshold when:
During traditional stateful neighbor discovery, state is used to perform the following:
Stateless neighbor discovery sacrifices these functions and the related state to mitigate the neighbor cache DoS attack.
Ensuring received neighbor advertisements correspond to previously sent neighbor solicitations prevents on-link nodes from sending unsolicited neighbor advertisements to the router, and then having them added to the router's neighbor cache without validation. This would allow on-link hosts to perform a neighbor cache DoS attack, as they could send many neighbor advertisements for non-existent addresses within the link assigned prefixes, exhausting the neighbor cache capacity.
If neighbor advertisement validation occurs, then the router is vulnerable to an off-link sourced neighbor cache DoS attack, but is not vulnerable to an on-link sourced neighbor cache DoS attack. If neighbor advertisement validation does not occur, then the router is vulnerable to an on-link sourced neighbore cache DoS attack, but is now not vulnerable to an off-link sourced neighbor cache DoS attack.
Considering that on-link nodes will usually have a vested interest in the router continuing to operate, that there will be a much smaller set of on-link sources, and that they'll be far better known and possibly access controlled, the likelihood of an on-link sourced neighbor cache DoS is much lower than an off-link sourced neighbor cache DoS. It is therefore beneficial to sacrifice on-link neighbor cache DoS protection to gain off-link neighbor cache DoS protection. Also note that during the stateless neighbor discovery process proposed in this memo, neighbor advertisement validation is only sacrificed when an off-link sourced neighbor cache DoS appears to be taking place. Under normal circumstances on-link sourced neighbor advertisement validation continues to occur.
The nature of IPv6 is best effort, meaning that there is a possibility that packets may be lost as they transit the network, and that IPv6 will not make any attempt to recover lost packets. If an application residing on an IPv6 node requires reliable packet delivery, it will need to utilise locally implemented reliable upper layer protocols such as TCP and SCTP, or implement it's own reliability mechanisms. These reliability mechanisms involve retransmitting packets. Alternatively, the application needs to accept the possibility of packet loss.
The remaining uses of stateful neighbor discovery state are not assured of success. The limited number of neighbor solicitation retransmissions may not be enough, causing neighbor discovery to fail even though the target node exists. There may be more packets sent that trigger neighbor discovery than are stored for transmission when neighbor discovery completes successfully, causing them to be dropped. The ICMPv6 destination unreachable message may be dropped on the way back to the traffic originating node, perhaps intentionally by a network located firewall.
This means that these functions are useful but not essential optimisations. If necessary, they do not need to be performed, as the packet source will retransmit it's packets, reinitiating the neighbor discovery process, or accept that packet loss has occured. This provides the opportunity to perform a stateless form of neighbor discovery if there is evidence that a neighbor cache DoS attack is occuring, mitigating the off-link sourced neighbor cache DoS attack.
As previously described, the Trusted/Untrused Source Prefix list (TUSP List) is used to determine whether a packet source is trusted or untrusted, with trusted sources continuing to receive traditional stateful neighbor discovery services, and untrusted hosts receiving stateless neighbor discovery services.
For routers where it may not be operationally convenient or possible to implement comprehensive trusted and untrusted packet source selection, such as on low-end or embedded devices, it would be acceptable to consider all packet sources untrusted when stateless neighbor discovery is active.
For routers that can support more comprehensive trusted and untrusted packet source selection, the following information sources can be used to construct the trusted/untrusted source prefix list (TUSP List).
The first TUSP List source is an operator configured list of prefixes and their lengths, each with a flag indicating whether traffic with source addresses that falls within the specified prefix is from a trusted or untrusted source.
How this list is evaluated would be implementation dependent, however it is likely to be either sequential from first to last entry, or using a longest match algorithm.
This list should have a default entry of the ULA prefix (fc00::/7) [RFC4193], flagged as a trusted source. An implementation must allow this entry to be removed.
The second TUSP List source is the network's routing information.
The network's routing information can be used to distinguish trusted and untrusted packet sources. An advantage of using routing information for this purpose is that it will typically be dynamically and automatically distributed to all routers within the network, when dynamic routing protocols are used. This avoids individual routers in the network having to be manually reconfigured wht trusted prefixes when subnets are added or removed from the network.
The contents of a stub network's route table is typically all the internal routes for the network, and then a default route used to reach the Internet. The list of internal routes can be used to distinguish between trusted and untrusted sources, with packet sources matching internal routes being trusted, and all other packet sources being untrusted.
In more complex routing environments, such as those using one or more IGPs and an EGP such as BGP, there may be other methods available to distinguish between trusted and untrusted sources. For example, routes carried in an IGP could be considered trusted, while routes carried in BGP are untrusted. For a network using BGP to carry all reachability information, except network transit and loopback interface routes, routes may be tagged with one or more BGP communities which indicate internal and therefore trusted prefixes.
A default route should never be used to define a trusted packet source prefix. If a router's operator wishes to trust all packet sources, they should specify ::/0 as a configured trusted prefix.
Implementations should provide convenient methods to use the network's routing information to distinguish between trusted and untrusted packet source prefixes.
Finally, should none of the previous trusted or untrusted source prefix information sources match the source address of traffic that would trigger neighbor discovery, the packet source should be considered untrusted.
Review and comments were provided by Ray Hunter and Matthew Moyle-Croft.
This memo was prepared using the xml2rfc tool.
This memo proposes a security mitigation for an off-link sourced neighbor cache Denial of Service attack, aimed at a router.
As discussed in Section 4.1, the method proposed creates an opportunity for an on-link sourced neighbor cache DoS attack, when mitigating the off-link sourced neighbor cache DoS. This is considered to be an acceptable security trade-off.
draft-smith-6man-mitigate-nd-cache-dos-slnd-00, initial version, 2012-09-04
draft-smith-6man-mitigate-nd-cache-dos-slnd-01, more clarity, 2012-10-13
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC4861] | Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. |
| [RFC3756] | Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor Discovery (ND) Trust Models and Threats", RFC 3756, May 2004. |
| [RFC4291] | Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. |
| [RFC4862] | Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. |
| [RFC4941] | Narten, T., Draves, R. and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, September 2007. |
| [RFC4193] | Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, October 2005. |