BGP Session Teardown due to AS_CONFED_SEQUENCE in AS4_PATH

Tico tico-nanog at
Fri Jan 16 16:10:56 UTC 2009

Comments below.

Rob Shakir wrote:
> Strict RFC 4893 (4-byte ASN support) BGP4 implementations are vulnerable to a
> session reset by distant (not directly connected) ASes. This vulnerability is a
> feature of the standard, and unless immediate action is taken an increasingly
> significant number of networks will be open to attack. Accidental triggering of
> this vulnerability has already been seen in the wild, although the limited
> number of RFC 4893 deployments has limited its effect.  
> Summary:
> It is possible to cause BGP sessions to remotely reset by injecting invalid data
> into the AS4_PATH attribute provided to store 4-byte ASN paths. Since AS4_PATH
> is an optional transitive attribute, the invalid data will be transited through
> many intermediate ASes which will not examine the content. To be vulnerable, an
> operator does not have to be actively using 4-byte AS support. This problem was
> first reported by Andy Davidson on NANOG in December 2008 [0], furthermore we
> have been able to demonstrate that a device running Cisco IOS release
> 12.0(32)S12 behaves as per this description.
> Details:
> When a prefix is learnt from a BGP neighbour that does not support 4-byte ASNs,
> the AS4_PATH attribute is retained, and appended to UPDATE messages sent to
> other neighbours [1, 3]. RFC4893 specifies that AS_CONFED_SEQUENCE and
> AS_CONFED_SET are invalid in an AS4_PATH, the intention of which is to ensure
> that an AS with a mix of AS4-aware BGP speakers, and AS4-unaware BGP speakers
> does not propagate confederation AS paths outside of the confederation [1, 3].
> Upon receiving an invalid BGP UPDATE message, a BGP speaker must send a
> NOTIFICATION message [2, 6.3], after a NOTIFICATION message, the BGP connection
> is closed [2, 4.5].
> Analysis of the Reported Path:   
> On 10th December 2008, a BGP update was propagated with illegal/invalid
> confederation attributes in the AS4_PATH.  When this update was received by AS4
> aware BGP speakers, the RFCs described above were interpreted literally and the
> session was torn down. Because the illegal attributes were learned on a transit
> session, an affected network can have global reachability impaired.
> Please note that the analysis of this path describes what we expect to have
> happened in this case, it has not been confirmed by any of the ASNs involved.
> 	Path Attributes - Origin: Incomplete 
> 	Flags: 0x40 (Well-known, Transitive, Complete) 
> 	Origin: Incomplete (2) 
> 	AS_PATH: xx xx 35320 23456 (13 bytes) 
> 	AS4_PATH: (65044 65057) 196629 (7 bytes) 
> In this data, the AS_PATH indicates that a prefix is announced by an AS4 speaker
> (as indicated by AS23456) and propagated through by AS35320. The AS4_PATH data
> shows that the AS4 originator is AS196629, the rest of this path is an
> AS_CONFED_SEQUENCE [3, 5]. It would appear that in this case, AS196629 peers
> with AS35320, which is AS4-aware on this border. The prefix is then propagated
> through AS35320, with the AS4 aware routers appending their ASN to the
> AS_CONFED_SEQUENCE. This is in contravention of RFC 4893 [1, 3]. The border
> which announces this route to AS35320's upstream does not appear to be
> AS4-aware. During normal announcements, the BGP speaker on a border with an
> upstream ASN that is not part of the confederation will remove the left-most
> AS_CONFED_SETs or AS_CONFED_SEQUENCEs that exist in the AS_PATH [3, 6.1] and
> replace them with the confederation identifier. However, due to the fact that
> both AS_CONFED_SET and AS_CONFED_SEQUENCE are invalid in an AS4_PATH, then no
> such action is taken on the border between an AS4 aware AS, and a non-AS4 aware
> AS. In addition, since the AS35320 border is not AS4 aware, then it does not
> update the AS4_PATH.
> This malformed UPDATE is then sent to AS35320's upstream, if there are no
> AS4-aware routers in the path between the AS35320 border, and an AS receiving
> this update, the AS4_PATH will not have been analysed. The first AS4-aware
> router to receive this update will reset the session towards the neighbour from
> whom it receives the update. 
> The border which announces this route to AS35320's upstream does not appear to
> be AS4-aware; If it were a strict AS4 implementation it would reset the BGP
> session due to the malformed AS4_PATH, and a broken implementation that treats
> AS4_PATH as an equivalent of the AS_PATH would sanitise the AS4_PATH. This
> allows the AS4_PATH containing an AS_CONFED_SET to be passed to neighbouring
> networks.
> This escape of an AS_CONFED_SET from a network with only partial AS4 support is
> exactly the situation that RFC 4893 attempts to avoid by forbidding the presence
> of an AS_CONFED_SET in the AS4_PATH. In the ideal world the neighbouring network
> receiving an UPDATE containing this obviously malformed AS4_PATH would reset the
> session, preventing further propagation and isolating the broken network.
> Unfortunately the vast majority of networks do not support AS4 so pass on this
> malformed AS4_PATH to their neighbours. The first AS4-aware router to receive
> this update will reset the session towards the neighbour from whom it received
> the update.
> Cisco IOS Behaviour:
> In a lab environment, a Cisco 7200 running IOS 12.0(32)S12, which is able to
> support 4-byte ASNs, was peered with a Cisco 2811 running 12.4(19). When the BGP
> session to the upstream 2811 is established by the 7200, the following log
> messages are observed:
> *Jan 16 11:29:58.531: %BGP-5-ADJCHANGE: neighbor Up 
> *Jan 16 11:30:02.595: %BGP-6-ASPATH: Invalid AS path (65044 65048 65062) 3.21 23456 received from Confederation found in AS4_PATH
> *Jan 16 11:30:02.595: %BGP-5-ADJCHANGE: neighbor Down BGP Notification sent
> *Jan 16 11:30:02.595: %BGP-3-NOTIFICATION: sent to neighbor 3/1 (update malformed) 27 bytes E0111803 030000FE 140000FE 180000FE 26 FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 0050 0200 0000 3540 0101 0240 020C 0205 3D25 2114 89F8 5BA0 5BA0 4003 04C1 EF20 02E0 1118 0303 0000 FE14 0000 FE18 0000 FE26 0202 0003 0015 0000 5BA0 175B CFDA
> The configuration on the 7200 is as follows:
> router bgp 65123
>  no synchronization
>  bgp log-neighbor-changes
>  neighbor remote-as 15653
>  no auto-summary
> The BGP session will continue to be reset each time the invalid AS4_PATH is
> received.
> Possible Impact:
> During a BGP conversation, it is expected that a neighbour's UPDATE messages are
> sanitised by the immediate neighbour, during a 'normal' BGP conversation, if a
> BGP speaker receives an invalid UPDATE, it will teardown the session, and this
> invalid UPDATE will not propagate any further. In the case of optional
> transitive attributes such as AS4_PATH, this invalid update can be transited
> through many ASes, as the content of the invalid attribute in the UPDATE message
> is not examined.
> In a hypothetical scenario, an AS4 aware service provider (A) has a transit
> provider (T) that is not AS4 aware. BGP speaker B, a large distance from A has a
> bug affecting their equipment that introduces an AS_CONFED_SET in the AS4_PATH.
> Since B's updates are propagated through to A via T, A will tear down the
> session to T due to the malformed attribute. This is an out of proportion
> reaction as the update may affect only one prefix in a full BGP table. If this
> update is also propagated through A's other transit providers A may lose
> full-table visibility until one of their transit providers filters the route.
> Examining the UPDATE message to establish which route caused session teardown
> may be a non-trivial activity.
> Conclusion:
> Whilst this description may be applied to invalid data in any optional
> transitive element, it has a greater impact with AS4_PATH due to the large
> number of BGP speakers that currently do not examine any 4-byte ASN data in an
> UPDATE. There has been a discussion of this matter on the IETF IDR mailing list
> [4], however, due to availability of Cisco IOS containing AS4 support
> (12.0(32)S12), and an observation of this problem 'in the wild', we believe that
> it is of operational concern to those that are planning on deployment of
> AS4-aware platforms [5].
> Any input from the operational community relating to this problem is much
> appreciated, either publicly, or privately.
> Regards,
> 	Andy Davidson, NetSumo (andy.davidson at,
> 	Jonathan Oddy, Hostway UK (jonathan.oddy at,
> 	Rob Shakir, GX Networks (rjs at
> References:
> [0]: Andy Davidson - prefix in DFZ - AS3.21 / AS196629 -
>      announced with AS_CONFED_SEQUENCE in AS4_PATH - propagated by 35320,
> [1]: rfc4893: BGP Support for Four-octet AS Number Space
> [2]: rfc4271: A Border Gateway Protocol 4 (BGP-4)
> [3]: rfc3054: Autonomous System Confederations for BGP
> [4]: Kaliraj Vairavakkalai, Juniper Networks, [Idr] RFC-4893 handling malformed
>      AS4_PATH attributes,
> [5]:
> Thanks to Will Hargrave (LONAP) for assistance with this document.

I'm sure you (and others) already aware of this, but for the archives' 
sake, when Andy first brought this up on [email protected] there was a 
corresponding discussion on the misc at mailing list:

And patched (in OpenBGPD) here:

though I see it's now in CVS:


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