Operational Recommendations on Handling Invalid AS4_PATH Attributes

[Rob Shakir]

As discussed on the IETF IDR list last month, there are concerns relating to the
treatment of AS_CONFED_SET/SEQUENCE in AS4_PATH as described in RFC4893 [0].
Since the last post to that thread the situation has been made more urgent with
the release of Cisco IOS 12.0(32)S12, which responds to malformed AS4_PATH
attributes by sending a NOTIFICATION to the neighbour, and tearing down the BGP
adjacency. This behaviour seems to be required by RFC4721 section 6.3, as there
is no alternative error handling defined in RFC4893. As posted last Friday [1],
and discussed on the IDR list, this strict implementation introduces a new
attack vector by which a BGP session can be torn down due to a an attribute
populated by a distant BGP neighbour. These malformed attributes have already
been seen in the wild as a result of a error in Juniper's implementation of
RFC4893.

Following discussions with a number of operators, we have attempted to generate
some recommendations relating to the behaviour that would be operationally most
useful when treating the invalid data in the AS4_PATH optional transitive
attribute.

There are two cases to consider when an invalid AS4_PATH is received:
   (1) A path to the prefix is not already known from that neighbour.
   (2) A path to the prefix has already been learnt from that neighbour;
   
In case (1) we recommend that the BGP speaker should discard the UPDATE and log
the fact. The log entry should include the received AS_PATH and
AS4_PATH to aid in debugging.

In case (2) we recommend that the BGP speaker should treat the UPDATE as a
withdrawal of existing path to the prefix. As per case (1) a log entry should be
raised to indicate that this has occurred.

It is quite possible that in both cases this behaviour may result in the BGP
speaker no longer having a valid path to the destination. We foresee that this
lack of a prefix in a BGP speaker's routing table may cause some operational
load initially, however, we feel that this is acceptable, considering the
alternate behaviours.

Should a prefix be injected into the global table with an invalid AS4_PATH, and
should the newly advertised (invalid) path be selected by all upstreams
available to a given ASN then this ASN will lose reachability to the prefix.
Whilst this can be abused we do not see this as more serious than the existing
possibility of malicious injection and blackholing of a prefix by a 3rd party.
As long as the rejection of paths due to invalid AS4_PATHs is clearly reported
to the administrator the source of the problem can be clearly identified.

We consider that attempting to extract a valid AS4 or AS_PATH from the invalid
UPDATE is a mistake since this allows the propagation of invalid BGP data. In
addition, incorrect implementation of this comparatively complex mechanism by a
vendor may result in loops. By explicitly not installing prefixes with invalid
AS_PATH or AS4_PATH into the routing table, the possibility of loops caused by
these invalid paths is avoided.

The defined behaviour in RFC4893 and RFC4271 has significantly harmful effects
and it seems only by virtue of the fact that the implementations of many vendors
do not strictly comply with the RFCs that this problem has not had the same
impact for every vendor. At the current time, however, one cannot deploy a
4-byte capable Cisco IOS device, or an OpenBGP (current stable release) router
into the global table, without risking teardown of a every session via which a
global table is learnt.

Further discussion of this issue would be much appreciated, as a common and
consistent approach to rectifying the problem will benefit network operators far
more than individual vendor implementing their own solution. Should a consensus
be reached an update to the RFC is required in order to ensure that future
implementations do not exhibit this harmful behaviour.

Kind regards,
        Andy Davidson (NetSumo), andy.davidson@...
        Jonathan Oddy (HostWay), jonathan.oddy@...
        Rob Shakir (GX Networks), rjs@...

[0]: http://www.ietf.org/mail-archive/web/idr/current/msg03368.html
[1]: http://www.merit.edu/mail.archives/nanog/msg14345.html

Many thanks to David Freedman (Claranet) for assistance in developing the
recommendations in this document.
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[John Leslie]

Rob Shakir <rjs@...> wrote:
>
> ... This behaviour seems to be required by RFC4721 section 6.3, as there
> is no alternative error handling defined in RFC4893.
>...
> Following discussions with a number of operators, we have attempted to
> generate some recommendations relating to the behaviour that would be
> operationally most useful when treating the invalid data in the
> AS4_PATH optional transitive attribute.

   I have no desire to criticize, but I think it may be helpful to step
back for a moment.

   The _useful_ information in AS4_PATH is the 4-byte ASNs where OLD
speakers must place AS_TRANS. We might consider whether that information
is useful in cases where RFC4893's algorithm produces invalid data. (I
frankly doubt we have found all possible cases yet.)

   I can imagine a treatment where we abandon the 4893 algorithm and
simply substitute 4-byte ASNs for AS_TRANS in the AS_PATH received from
an OLD speaker. This would preserve loop-detection for 4-byte ASNs
while matching other characteristics of the NLRI received from our OLD
peer.

   Obviously this would be better than tearing down the session with our
OLD peer; the question is, would it be better than discarding the new
NLRI?

--
John Leslie <john@...>
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