Tag Archives: DHCPv6

IPv6 Part 15: To sum up

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While I’ve been working on this series on IPv6, my thinking has evolved. It’s clear from reading RFC 7421 that the fixed 64-bit length of the interface ID is now so baked into IPv6 that there’s no point in debating it. If sixteen bits (or rather four nibbles) isn’t enough to encompass a site’s topology, then I accept the assurances (for example in Tom Coffeen’s IPv6 Address Planning) that a global prefix that’s shorter than the standard /48 can be obtained (I still think that the size of the IPv6 address space is over-hyped).

IPv6 has two ways of automating addressing and configuration: DHCPv6 and SLAAC. In practice neither are complete solutions, and they have to be combined in various ways; the standards are still evolving. Something as straightforward as mapping an IP address to a MAC address becomes much harder with IPv6. Such a complex and messy environment is not conducive to good security.

However, the biggest problem I have with IPv6 is the assumption that it means a return to end-to-end addressing. The original purpose of Network Address Port Translation (NAPT) was to stretch the IPv4 address space, and the massively increased address space of IPv6 makes that no longer necessary. The crude one-way filter effect of NAPT needs to be replaced with a proper firewall. Nevertheless, NAPT has had other benefits too, such as:

  • Host identity hiding
  • Network identity hiding
  • Topology hiding
  • Simplification of provider change when using provider-assigned addresses
  • Simplification of multi-homing when using provider-assigned addresses

Various attempts have been made to try and replicate these benefits in IPv6: temporary addresses, “local network protection” (using host routes or Mobile IP), ULAs, NPTv6 and multi-homing via DHCPv6. In chasing the end-to-end addressing dream these have all added complexity to the endpoint device (difficult to manage on an enterprise scale); all, that is, apart from NPTv6, which is a form of NAT.

IPv6 enthusiasts often express a desire to return to the time in the early days of the Internet when end-to-end addressing was feasible. I think that the world has moved on since then: the Internet is no longer as simple or as safe, and we need to adapt our architectures accordingly. We should not assume that end-to-end addressing is always the end goal. When applying the end-to-end principle to the enterprise, it makes more sense to think of the endpoint as lying at the enterprise boundary, where complexity can be more easily handled.

IPv6 Part 6: SLAAC vs DHCPv6

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Address auto-configuration was important to IPv6 right from the start. The original proposal was for a 64-bit address space, but 128 bits was chosen to enable address auto-configuration based on MAC address, in the style of XNS/IPX. It was always intended that address assignment via DHCPv6 would also be supported for those sites that preferred it. [UPDATE: I would argue that there’s an architectural issue here. DNS is the key to accessing any network resources, especially local ones. If you are an enterprise running a highly available IPAM system for your DNS, then adding DHCPv6 to that has more pros than cons. After all, if the DHCP is down and hosts can’t get an address, then the DNS is probably down as well, and you’re not going anywhere anyway. If you’re a smaller enterprise, then decentralised addressing using SLAAC is a better fit, if only because of the limitations of Microsoft’s DHCP implementation.]

However I can see that there might be some hardliners who see SLAAC as the “real” address configuration method for IPv6, and DHCP as a hangover from IPv4. Google’s lead IPv6 developer for Android has set his face against DHCPv6, and so there’s no DHCPv6 support in Android. I don’t have much patience with such “religious” beliefs (in technical matters I’m an atheist): I think users should be given the choice as to which address configuration method they use.

It’s the needs of auto-configuration that impose the fixed 64-bit length of the interface ID. Ironically this fixed length network prefix represents a kind of return to the very earliest days of IPv4, when addresses were made up of an 8-bit network address and a 24-bit host address. The shift to classful addressing, which provided three (later four) classes of addresses with 8- 16- and 24-bit network parts, gave much more flexibility; the later move to classless addressing gave more flexibility still. Even though there is a massive address space (64 bits) in each half of an IPv6 address, it still doesn’t feel like a good idea to me to have a fixed structure like that.

Whether that fixed structure is sustainable in the long run is open to question. As we’ve seen the address space available to sites for subnetting is not that generous. If enterprises require more subnetting in the future, for example for security reasons or to accommodate the Internet of Things, then pressure may grow on the /64 network prefix boundary. My concern is that the /64 structure will have been coded so deeply into many IPv6 implementations that changing it in the future may be very painful.

In the next post, I’ll look at another big cultural shift that IPv6 introduces.

IPv6 Part 5: Stateful DHCPv6

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Using DHCP to manage IPv6 addresses is known as stateful DHCPv6, because the DHCP server maintains the state of IP address leases centrally. In the last post we looked at its limitations, as opposed to SLAAC. Why would you use DHCPv6 to manage IPv6 addresses? Well the main reason is accountability. One of the most important aspects of this is host DNS registration: having your hosts registered in DNS and available for reverse DNS lookup is the most basic way of tracking them. If you use DHCPv6 for address assignment then the DHCP server will register the host with DNS (typically the two services run on the same server). If you are using SLAAC then hosts themselves are responsible for registering themselves in DNS : this is inherently less reliable.

In fact DHCPv6 has made DNS registration more robust than under IPv4. With IPv4, it’s a simple model: each MAC address is a separate entity, and each interface makes its own independent registration in DHCP. This can cause problems with DNS registration when a host has multiple interfaces, as is increasingly the case where laptops are connected both to the office wired LAN and to a wireless network. If the host is registered in DNS under one interface, then depending on how things are configured the second interface may not be registered in DNS at all. Alternatively the second interface may supersede the DNS registration of the first interface.

DHCPv6 introduces the concept of the DHCP Unique Identifier (DUID). The DUID is a persistent identifier of the host rather than any specific interface. IP addresses are actually assigned to Interface Associations (IAs) which identify the individual interfaces. Thus DHCPv6 understands that hosts can have multiple interfaces and DNS hostname registrations can be done according to the DUID rather than the IA.

Another reason that you might want to use DHCP to manage addresses is to make it easier to find out an interface’s MAC address from its IP address. Take the example of a security alert you receive from your intrusion prevention system or DNS firewall: an IP address is trying to contact a known botnet command and control (C2) server. You want to disconnect that IP address from the network as quickly as possible, preferably automatically. Under DHCPv4 it’s possible by looking up the IP address in the DHCP server and extracting the MAC address, and then using whatever tools your network may provide to track down the specific network port that that MAC address is connected to. You can then disable the port.

Sadly, the way that DHCPv6 works makes this much harder. Although the DUID of a host usually contains a MAC address, it is typically generated at install time from the MAC address of any interface on the system. That interface can in fact be removed from the host after the DUID has been generated and the DUID will stay the same. The Interface Association Identifier (IAID) is generated in an arbitrary way by the host, and may have no relation to the physical MAC address of the interface. If the DHCP server is not on the same subnet as the client, and is receiving the DHCP request via a relay, then the server has no visibility of the client’s MAC address. Thus there can be no way of building a centralised mapping of IP addresses to MAC addresses. RFC 6939 does provide a mechanism for DHCP relays to forward the client’s MAC address to the DHCPv6 server, but this is quite a recent proposal. So, one step forward, one step back.

Next post I’ll summarise the SLAAC vs stateful DHCPv6 debate.