| Operational Security Capabilities for IP Network Infrastructure | M. Behringer |
| Internet-Draft | E. Vyncke |
| Intended status: Informational | Cisco |
| Expires: March 23, 2013 | September 21, 2012 |
Using Only Link-Local Addressing Inside an IPv6 Network
draft-ietf-opsec-lla-only-01
In an IPv6 network it is possible to use only link-local addresses on infrastructure links between routers. This document discusses the advantages and disadvantages of this approach to help the decision process for a given network.
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An infrastructure link between a set of routers typically does not require global or even unique local addressing [RFC4193]. Using link-local addressing on such links has a number of advantages, for example that routing tables do not need to carry link addressing, and can therefore be significantly smaller. This helps to decrease failover times in certain routing convergence events. An interface of a router is also not reachable beyond the link boundaries, therefore reducing the attack horizon.
We propose to configure neither globally routable IPv6 addresses nor unique local addresses on infrastructure links of routers, wherever possible. We recommend to use exclusively link-local addresses on such links.
This document discusses the advantages and caveats of this approach.
Note: [I-D.ietf-ospf-prefix-hiding] describes another approach for OPSFv2 and OSPFv3 by modifying the existing protocols while this document does not modify any protocol but works only for IPv6.
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 RFC2119 [RFC2119] when they appear in ALL CAPS. These words may also appear in this document in lower case as plain English words, absent their normative meanings.
This document proposes to use only link-local addresses (LLA) on all router interfaces on infrastructure links. Routers typically do not need to be reached from nodes of the network, nor from outside the network. For an network operator there may be reasons to send packets to an infrastructure link for certain monitoring tasks; many of those tasks could also be handled differently, not requiring routable address space on infrastructure links.
Neither global IPv6 addresses nor unique local addresses are configured on infrastructure links. In the absence of specific global or unique local address definitions, the default behavior of routers is to use link-local addresses notably for routing protocols. These link-local addresses SHOULD be hard-coded to prevent the change of EUI-64 addresses when changing of MAC address (such as after changing a network interface card).
ICMPv6 [RFC4443] error messages (packet-too-big, time-exceeded...) are required for routers, therefore a loopback interface MUST be configured with an IPv6 address with a greater scope than link-local (this will usually be a global scope). This greater-than-link scope IPv6 address MUST be used as the source IPv6 address for all generated ICMPv6 messages sent to a non-link-local address and MUST belong to the operator to avoid being dropped by other routers implementing [RFC3704].
The effect on specific traffic types is as follows:
We therefore conclude that it is possible to construct a working network in this way.
Smaller routing tables: Since the routing protocol only needs to carry one loopback address per router, it is smaller than in the traditional approach where every infrastructure link addresses are carried in the routing protocol. This reduces memory consumption, and increases the convergence speed in some routing failover cases (notably because the Forwarding Information Base to be downloaded to line cards are smaller but also because there are less prefixes in the Routing Information Base hence accelerating the routing algorithm). Note: smaller routing tables can also be achieved by putting interfaces in passive mode for the IGP.
Reduced attack surface: Every routable address on a router constitutes a potential attack point: a remote attacker can send traffic to that address, for example a TCP SYN flood, or he can intent SSH brute force password attacks. If a network only uses loopback addresses for the routers, only those loopback addresses need to be protected from outside the network. This significantly eases protection measures, such as infrastructure access control lists. See also [I-D.ietf-grow-private-ip-sp-cores] for further discussion on this topic.
Lower configuration complexity: LLAs require no specific configuration, thereby lowering the complexity and size of router configurations. This also reduces the likelihood of configuration mistakes.
Simpler DNS: Less greater-than-link-local address space in use also means less DNS mappings to maintain because DNS is not really suitable to contain link-local addresses as DNS has no clue to the link scope.
Interface ping: If an interface doesn't have a routable address, it can only be pinged from a node on the same link. Therefore it is not possible to ping a specific link interface remotely. A possible workaround is to ping the loopback address of a router instead. In most cases today it is not possible to see which link the packet was received on; however, RFC5837 [RFC5837] suggests to include the interface identifier of the interface a packet was received on in the ICMP response; it must be noted that there are little implemention of this ICMP extension. With this approach it would be possible to ping a router on the loopback address, yet see which interface the packet was received on. To check liveliness of a specific interface it may be necessary to use other methods, for example to connect to the router via SSH and to check locally or use SNMP.
Traceroute: Similar to the ping case, a reply to a traceroute packet would come from a loopback address with a greater than link-local address. Today this does not display the specific interface the packets came in on. Also here, RFC5837 [RFC5837] provides a solution.
Hardware dependency: LLAs are usually EUI-64 based, hence, they change when the MAC address is changed. This could pose problem in a case where the routing neighbor must be configured explicitly (e.g. BGP) and a line card needs to be physically replaced hence changing the EUI-64 LLA and breaking the routing neighborship. But, LLAs can be statically configured such as fe80::1 and fe80::2 which can be used to configure any required static routing neighborship.
Network Management System (NMS) toolkits: If there is any NMS tool that makes use of interface IP address of a router to carry out any of NMS functions, then it would no longer work, if the interface is missing routable address. A possible workaround for such tools is to use the routable loopback address of the router instead.
MPLS and RSVP-TE [RFC3209] allows establishing MPLS LSP on a path that is explicitly identified by a strict sequence of IP prefixes or addresses (each pertaining to an interface or a router on the path). This is commonly used for FRR. However, if an interface uses only a link-local address, then such LSPs can not be established. A possible workaround is to use loose sequence of IP prefixes or addresses (each pertaining to a router) to identify an explicit path along with shared-risk-link-group (to not use a set of common interfaces).
Using link-local addressing only on infrastructure links has a number of advantages, such as a smaller routing table size and a reduced attack surface. It also simplifies router configurations. However, the way certain network management tasks are carried out today has to be adapted to provide the same level of detail, for example interface identifiers in traceroute.
Using LLAs only on infrastructure links reduces the attack surface of a router: loopback addresses with routed addresses are still reachable and must be secured, but infrastructure links can only be attacked from the local link. This simplifies security of control and management planes. The proposal does not impact the security of the data plane. This proposal does not address control plane [RFC6192] attacks generated by data plane packets (such as hop-limit expiration or packets containing a hop-by-hop extension header).
As in the traditional approach, this approach relies on the assumption that all routers can be trusted due to physical and operational security.
There are no IANA considerations or implications that arise from this document.
The authors would like to thank Salman Asadullah, Brian Carpenter, Benoit Claise, Simon Eng, Wes George, Janos Mohacsi, Alvaro Retana for their useful comments about this work.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |