Network Working Group S. Roy Request for Comments: 4943 Sun Microsystems, Inc. Category: Informational A. Durand Comcast J. Paugh Nominum, Inc. September 2007 IPv6 Neighbor Discovery On-Link Assumption Considered Harmful Status of This Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
AbstractThis document describes the historical and background information behind the removal of the "on-link assumption" from the conceptual host sending algorithm defined in Neighbor Discovery for IP Version 6 (IPv6). According to the algorithm as originally described, when a host's default router list is empty, the host assumes that all destinations are on-link. This is particularly problematic with IPv6-capable nodes that do not have off-link IPv6 connectivity (e.g., no default router). This document describes how making this assumption causes problems and how these problems outweigh the benefits of this part of the conceptual sending algorithm. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Background on the On-link Assumption . . . . . . . . . . . . . 2 3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. First Rule of Destination Address Selection . . . . . . . . 3 3.2. Delays Associated with Address Resolution . . . . . . . . . 3 3.3. Multi-interface Ambiguity . . . . . . . . . . . . . . . . . 4 3.4. Security-Related Issues . . . . . . . . . . . . . . . . . . 4 4. Changes to RFC 2461 . . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 6. Normative References . . . . . . . . . . . . . . . . . . . . . 6 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 7
RFC4861] defines a conceptual sending algorithm for hosts. The version of the algorithm described in [RFC2461] states that if a host's default router list is empty, then the host assumes that all destinations are on-link. This memo documents the removal of this assumption in the updated Neighbor Discovery specification [RFC4861] and describes the reasons why this assumption was removed. This assumption is problematic with IPv6-capable nodes that do not have off-link IPv6 connectivity. This is typical when systems that have IPv6 enabled on their network interfaces (either on by default or administratively configured that way) are attached to networks that have no IPv6 services such as off-link routing. Such systems will resolve DNS names to AAAA and A records, and they may attempt to connect to unreachable IPv6 off-link nodes. The on-link assumption creates problems for destination address selection as defined in [RFC3484], and it adds connection delays associated with unnecessary address resolution and neighbor unreachability detection. The behavior associated with the assumption is undefined on multi-interface nodes and has some subtle security implications. All of these issues are discussed in this document. RFC2461] conceptual sending algorithm was created to facilitate communication on a single link between systems configured with different global prefixes in the absence of an IPv6 router. For example, consider the case where two systems on separate links are manually configured with global addresses and are then plugged in back-to-back. They can still communicate with each other via their global addresses because they'll correctly assume that each is on-link. Without the on-link assumption, the above scenario wouldn't work, and the systems would need to be configured to share a common prefix such as the link-local prefix. On the other hand, the on-link assumption introduces several problems to various parts of the networking stack described in Section 3. As such, this document points out that the problems introduced by the on-link assumption outweigh the benefit that the assumption lends to this scenario. It is more beneficial to the end user to remove the on-link assumption from Neighbor Discovery and declare this scenario illegitimate (or a misconfiguration).
RFC3484] defines a destination address selection algorithm that takes an unordered list of destination addresses as input and produces a sorted list of destination addresses as output. The algorithm consists of destination address sorting rules, the first of which is "Avoid unusable destinations". The idea behind this rule is to place unreachable destinations at the end of the sorted list so that applications will be least likely to try to communicate with those addresses first. The on-link assumption could potentially cause false positives when attempting unreachability determination for this rule. On a network where there is no IPv6 router (all off-link IPv6 destinations are unreachable), the on-link assumption states that destinations are assumed to be on-link. An implementation could interpret that as, if the default router list is empty, then all destinations are reachable on-link. This may cause the rule to prefer an unreachable IPv6 destination over a reachable IPv4 destination. RFC1122] Section 126.96.36.199 requires that TCP retransmit for at least 3 minutes before aborting the connection attempt.
When the application has a large list of off-link unreachable IPv6 addresses followed by at least one reachable IPv4 address, the delay associated with Neighbor Unreachability Detection (NUD) of each IPv6 address before successful communication with the IPv4 address is unacceptable. Section 4.2.2 of IPv6 Neighbor Discovery Trust Models and Threats [RFC3756] titled "Default router is 'killed'". There is a threat that a host's router can be maliciously killed in order to cause the host to start
sending all packets on-link. The attacker can then spoof off-link nodes by sending packets on the same link as the host. The vulnerability is discussed in detail in [RFC3756]. Another security-related side-effect of the on-link assumption has to do with virtual private networks (VPNs). It has been observed that some commercially available VPN software solutions that don't support IPv6 send IPv6 packets to the local media in the clear (their security policy doesn't simply drop IPv6 packets). Consider a scenario where a system has a single Ethernet interface with VPN software that encrypts and encapsulates certain packets. The system attempts to send a packet to an IPv6 destination that it obtained by doing a DNS lookup, and the packet ends up going in the clear to the local media. A malicious third party could then spoof the destination on-link. RFC2461] and [RFC4861]: The last sentence of the second paragraph of Section 5.2 ("Conceptual Sending Algorithm") was removed. This sentence was, "If the Default Router List is empty, the sender assumes that the destination is on-link." Bullet item 3) in Section 6.3.6 ("Default Router Selection") was removed. The item read, "If the Default Router List is empty, assume that all destinations are on-link as specified in Section 5.2." APPENDIX A was modified to remove on-link assumption related text in bullet item 1) under the discussion on what happens when a multihomed host fails to receive Router Advertisements. The result of these changes is that destinations are considered unreachable when there is no routing information for that destination (through a default router or otherwise). Instead of attempting link- layer address resolution when sending to such a destination, a node should send an ICMPv6 Destination Unreachable message (code 0 - no route to destination) message up the stack. Section 3.4.
[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [RFC3484] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, February 2003. [RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor Discovery (ND) Trust Models and Threats", RFC 3756, May 2004. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.
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