Enhanced AS-Loop Detection for BGP

The Border Gateway Protocol (BGP), as an inter-Autonomous (AS) routing protocol, is used to exchange network reachability information between BGP systems. BGP is widely used by Internet Service Providers (ISPs) and large organizations. BGP is used to exchange reachable inter-AS routes, establish inter-AS paths, avoid routing loops, and apply routing policies between ASs. BGP loop detection mechanism is defined in section 9.1.2. of RFC4271: ... If the AS_PATH attribute of a BGP route contains an AS loop, the BGP route should be excluded from the Phase 2 decision function. AS loop detection is done by scanning the full AS path (as specified in the AS_PATH attribute), and checking that the autonomous system number of the local system does not appear in the AS path. Operations of a BGP speaker that is configured to accept routes with its own autonomous system number in the AS path are outside the scope of this document. ... In ordinary BGP, every AS announces its route information with different prefixes. However, its neighboring ASes cannot validate this route information, but rather directly propagate it across the Internet or simply discard AS-Loop routes directly. Obviously, this weak trust model allows forged route announcement propagations and rarely been found, which is a fundamental security weakness of BGP. Forged routes, which can be generated by configuration errors or malicious attacks, can cause large-scale network connectivity problems. Some cases can be worse, hackers exploit this property of BGP to achieve their ulterior motives. They can add some providers' AS number into the forged AS-Path and attempt to make it look like the route had passed through these ASNs, or perhaps they are there to prevent those providers from carrying the route. For example, the cases shown in Figure 1. Forged Case 1: One upstream ISP of AS64596 forged a route with the ASN 64596 as the origin ASN in the AS-Path. Forged Case 2: One upstream ISP of AS64596 forged a route with the ASN 64596 as the transit ASN in the AS-Path. After receiving the above routes, AS64596 treats them as normal loop routes during the loop detecting phase and discards them directly. In most NOSes (Network Operation Systems), such rejected routes are not logged and only visible by putting the router into debugging mode. If the AS64596 is slightly enhanced, it can find that someone has faked himself, which may cause unnecessary trouble for himself.

Split-Horizon for EBGP is an optional function that a BGP sender will not advertise any routes that were previously received from that same AS. In some current implementation, the BGP outbound route processing step will simply discard the route if AS-Loop being detected. For example, the cases shown in Figure 1. Forged Case 1: One upstream ISP of AS64597 forged a route with the ASN 64596 as the origin ASN in the AS-Path. Forged Case 2: One upstream ISP of AS64597 forged a route with the ASN 64596 as the transit ASN in the AS-Path. When sending the above routes, AS64597 treats them as normal loop routes and discards them directly. If AS64597 is slightly enhanced, it can find that someone has faked AS64596, which may cause large-scale network connectivity problems.

Above cases are also being known As-Path Poisoning Attacks. ASPA [I-D.ietf-sidrops-aspa-verification] can be used to verify the AS_PATH attribute of routes advertised in the Border Gateway Protocol, and it is a systematic deployment based on RPKI system. This mechanism requires a series of infrastructure implementations. This document proposes to enhance AS-Loop Detection for BGP Inbound/Outbound Route Processing. It is an enhancement to the current BGP's Inbound/Outbound processing and can be implemented directly on the device. This could empower networks to quickly and accurately figure out they're being victimized.

The following terminology is used in this document. AS: Autonomous System BGP: Border Gateway Protocol BGP hijacking : is the illegitimate takeover of groups of IP addresses by corrupting Internet routing tables maintained using the Border Gateway Protocol (BGP). (Sometimes referred to as prefix hijacking, route hijacking or IP hijacking) EBGP: External BGP ISP: Internet Service Provider

This section proposes to enhance AS Loop Detection for BGP Inbound Route Processing. As shown in Figure 3, when receiving the routes from AS64597, AS64596 should check whether its own AS number is already in the AS-Path, If yes, it further analyzes the location of the AS Number 64596 in the received AS_Path: Case 1: AS 64596 is listed as Origin AS in the AS-Path Lookup the local resource database (Such as ROA Cache) and determine whether the route is originated from the AS 64596. Result 1: AS 64596 has no corresponding prefix; it is identified as a purely forged AS_Path prefix hijacking event, which is recorded as incident type 1. Result 2: The corresponding prefix is a sub-prefix of a certain prefix of the AS 64596 and the AS 64596 has not advertise it. For example, the prefix being hold by the AS 64596 is 10.10.128.0/17, and the receiving route prefix is 10.10.192.0/24, the latter is a sub-prefix of the former, which indicates that this is a forged AS_Path sub-prefix hijacking event, which is recorded as incident type 2. Result 3: The corresponding prefix is a sub-prefix of a certain prefix of the AS 64596 and the AS 64596 has only advertised to some special ASNs, and only wants it to be used internally by those ASNs. The AS 64596 recognizes that At least one special AS violates the route policy. Which is recorded as incident type 3. Result 4: The corresponding prefix is originated by the AS 64596, this is the normal case. Case 2: AS 64596 is listed as transit AS in the AS-Path For example, AS-Path looks like the following form AS64596's perspective: (possible other ASes), left AS, local AS(64596), right AS, (possible other ASes) At this point, AS 64596 can lookup the local resource database and check whether there is a real AS relationship between the local AS and the left AS and the right AS. (From the perspective of the local AS, it can manage/hold the AS-relationship database between the local AS and each of its neighboring ASs (such as C2P, P2P, P2C, etc.).) Result 1: At least one of the AS ( the left AS or the right AS) has no actual AS relationship with the local AS (i.e. A never before seen AS-AS adjacency). It is a purely forged AS_Path prefix hijacking event. Which is recorded as incident type 4. Result 2: The AS relationships between the local AS and the left AS and the right AS are correct, but the local AS has not previously process this prefix , so it can be recognized that this is a forged route. We classify this incident type as type 5. Result 3: The AS relationships between the AS and the left AS and the right AS are correct, and the local AS 64596 has previously processed the prefix, this is the normal case.

The local AS 64596 inputs the detected result to the route hijacking management module, or/and records the log or/and the alarm information, and the maintenance team of the local AS 64596 can notify the maintenance team of the relevant AS to correct the error in their networks . After the above steps are added, the stability and security of the network can be improved.

This section proposes to enhance AS Loop Detection for BGP Outbound Route Processing.

As shown in Figure 4, when sending the routes from AS64597 to AS64596, AS64597 will check whether the AS number 64596 is already in the AS-Path, If yes, it can further analyzes the location of the AS64596 in the received AS_Path: The remaining processing steps are the same as the previous section.

After the enhancements of the AS Loop Detection for BGP Inbound/Outbound Route Processing are added, the stability and security of the network can be improved.

The authors would like to acknowledge the review and inputs from Gang Yan, Zhenbin Li, Aijun Wang, Jeff Haas, Robert Raszuk, Alexander Asimov, Ruediger Volk and the working group.

This document includes no request to IANA.

This document does not change the underlying security issues in the BGP protocol. It however, does provide an additional mechanism to protect against attacks based on the forged AS-Path in the BGP routes.