Thursday, August 28, 2014

Architectural Security aspects of BGP/MPLS

The inherent benefits of the MPLS (Multi Protocol Label Switching), is gaining widespread use for providing IP VPN services. With the emerging trend of connected systems, a global enterprise today is well connected with their partners, with MPLS being the preferred choice. Border Gateway Routing Protocol (BGP) is used to interconnect such autonomous systems by exchanging the routing informaiton across such systems. The emergence of Multiprotocol Extension, and other variations of BGP Protocol, has furthered the choice of MPLS VPNs. On the same lines, the security concerns on using such a network is also on the rise. The specific demands of customers in terms of security is also emerging as they experience issues of data breaches and security incidents.

The objective of this blog is not to explain about the BGP / MPLS as such and instead let us examine how the BGP / MPLS addresses the typical security requirements in this blog. The following sections of this blog have been extracted from the RFC 4381 published by Internet Engineering Task Force (IETF) in 2006.

Address Space, Routing, and Traffic Separation

BGP/MPLS allows distinct IP VPNs to use the same address space, which can also be private address space. This is achieved by adding a 64-bit Route Distinguisher (RD) to each IPv4 route, making VPN-unique addresses also unique in the MPLS core. This "extended" address is also called a "VPN-IPv4 address". Thus, customers of a BGP/MPLS IP VPN service do not need to change their current addressing plan. The address space on the CE-PE link (including the peering PE address) is considered part of the VPN address space. Since address space can overlap between VPNs, the CE-PE link addresses can overlap between VPNs. For practical management considerations, SPs typically address CE-PE links from a global pool, maintaining uniqueness across the core.

On the data plane, traffic separation is achieved by the ingress PE pre-pending a VPN-specific label to the packets. The packets with the VPN labels are sent through the core to the egress PE, where the VPN label is used to select the egress VRF. Given the addressing, routing, and traffic separation across an BGP/ MPLS IP VPN core network, it can be assumed that this architecture offers in this respect the same security as a layer-2 VPN. It is not possible to intrude from a VPN or the core into another VPN unless this has been explicitly configured. If and when confidentiality is required, it can be achieved in BGP/ MPLS IP VPNs by overlaying encryption services over the network. However, encryption is not a standard service on BGP/MPLS IP VPNs.

Hiding of the BGP/MPLS IP VPN Core Infrastructure

Service providers and end-customers do not normally want their network topology revealed to the outside. This makes attacks more difficult to execute: If an attacker doesn't know the address of a victim, he can only guess the IP addresses to attack. Since most DoS attacks don't provide direct feedback to the attacker it would be difficult to attack the network. It has to be mentioned specifically that information hiding as such does not provide security. However, in the market this is a perceived requirement. 

With a known IP address, a potential attacker can launch a DoS attack more easily against that device. Therefore, the ideal is to not reveal any information about the internal network to the outside world. This applies to the customer network and the core. A number of additional security measures also have to be taken: most of all, extensive packet filtering. For security reasons, it is recommended for any core network to filter packets from the "outside" (Internet or connected VPNs) destined to the core infrastructure. This makes it very hard to attack the core, although some functionality such as pinging core routers will be lost. Traceroute across the core will still work, since it addresses a destination outside the core.

Being reachable from the Internet automatically exposes a customer network to additional security threats. Appropriate security mechanisms have to be deployed such as firewalls and intrusion detection systems. This is true for any Internet access, over MPLS or direct. A BGP/MPLS IP VPN network with no interconnections to the Internet has security equal to that of FR or ATM VPN networks. With an Internet access from the MPLS cloud, the service provider has to reveal at least one IP address (of the peering PE router) to the next provider, and thus to the outside world.

Resistance to Attacks

To attack an element of a BGP/MPLS IP VPN network, it is first necessary to know the address of the element. The addressing structure of the BGP/MPLS IP VPN core is hidden from the outside world. Thus, an attacker cannot know the IP address of any router in the core to attack. The attacker could guess addresses and send packets to these addresses. However, due to the address separation of MPLS each incoming packet will be treated as belonging to the address space of the customer. Thus, it is impossible to reach an internal router, even by guessing IP addresses.

In the case of a static route that points to an interface, the CE router doesn't need to know any IP addresses of the core network or even of the PE router. This has the disadvantage of needing a more extensive (static) configuration, but is the most secure option. In this case, it is also possible to configure packet filters on the PE interface to deny any packet to the PE interface. This protects the router and the whole core from attack. In all other cases, each CE router needs to know at least the router ID (RID, i.e., peer IP address) of the PE router in the core, and thus has a potential destination for an attack.

A potential attack could be to send an extensive number of routes, or to flood the PE router with routing updates. Both could lead to a DoS, however, not to unauthorised access. To reduce this risk, it is necessary to configure the routing protocol on the PE router to operate as securely as possible. This can be done in various ways: 

  • By accepting only routing protocol packets, and only from the CE router. The inbound ACL on each CE interface of the PE router should allow only routing protocol packets from the CE to the PE. 
  • By configuring MD5 authentication for routing protocols. This is available for BGP (RFC 2385 [6]), OSPF (RFC 2154 [4]), and RIP2 (RFC 2082 [3]), for example. 

This avoids packets being spoofed from other parts of the customer network than the CE router. It requires the service provider and customer to agree on a shared secret between all CE and PE routers. It is necessary to do this for all VPN customers. It is not sufficient to do this only for the customer with the highest security requirements.

It is theoretically possible to attack the routing protocol port to execute a DoS attack against the PE router. This in turn might have a negative impact on other VPNs on this PE router. For this reason, PE routers must be extremely well secured, especially on their interfaces to CE routers. ACLs must be configured to limit access only to the port(s) of the routing protocol, and only from the CE router.

Label Spoofing

Similar to IP spoofing attacks, where an attacker fakes the source IP address of a packet, it is also theoretically possible to spoof the label of an MPLS packet. For security reasons, a PE router should never accept a packet with a label from a CE router. RFC 3031 [9] specifies: "Therefore, when a labeled packet is received with an invalid incoming label, it MUST be discarded, UNLESS it is determined by some means that forwarding it unlabeled cannot cause any harm."

There remains the possibility to spoof the IP address of a packet being sent to the MPLS core. Since there is strict address separation within the PE router, and each VPN has its own VRF, this can only harm the VPN the spoofed packet originated from; that is, a VPN customer can attack only himself. MPLS doesn't add any security risk here. The Inter-AS and Carrier's Carrier cases are special cases, since on the interfaces between providers typically packets with labels are exchanged. See section 4 for an analysis of these architectures.

There are a number of precautionary measures outlined above that a service provider can use to tighten security of the core, but the security of the BGP/MPLS IP VPN architecture depends on the security of the service provider. If the service provider is not trusted, the only way to fully secure a VPN against attacks from the "inside" of the VPN service is to run IPsec on top, from the CE devices or beyond. This document discussed many aspects of BGP/MPLS IP VPN security. It has to be noted that the overall security of this architecture depends on all components and is determined by the security of the weakest part of the solution.

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