| Internet Engineering Task Force | E. Chen, Ed. |
| Internet-Draft | Cisco Systems, Inc. |
| Updates: 1997, 4271, 4360, 4456, 4760, | J. Scudder, Ed. |
| 5543, 5701, 6368 (if approved) | Juniper Networks |
| Intended status: Standards Track | P. Mohapatra |
| Expires: June 15, 2015 | Sproute Networks |
| K. Patel | |
| Cisco Systems, Inc. | |
| December 12, 2014 |
Revised Error Handling for BGP UPDATE Messages
draft-ietf-idr-error-handling-17
According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable, because a session reset would impact not only routes with the offending attribute, but also other, valid routes exchanged over the session. This document partially revises the error handling for UPDATE messages and provides guidelines for the authors of documents defining new attributes. Finally, it revises the error handling procedures for a number of existing attributes.
This document updates error handling for RFCs 1997, 4271, 4360, 4456, 4760, 5543, 5701 and 6368.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 15, 2015.
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According to the base BGP specification [RFC4271], a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable, because a session reset impacts not only routes with the offending attribute, but also other, valid routes exchanged over the session. In the case of optional transitive attributes, the behavior is especially troublesome and may present a potential security vulnerability. This is because attributes may have been propagated without being checked by intermediate routers that don't recognize the attributes. In effect, the attributes may have been tunneled; and when they reach a router that recognizes and checks the attributes, the session that is reset may not be associated with the router that is at fault. To make matters worse, in such cases although the problematic attributes may have originated with a single update transmitted by a single BGP speaker, by the time they encounter a router that checks them they may have been replicated many times, and thus may cause the reset of many peering sessions. Thus the damage inflicted may be multiplied manyfold.
The goal for revising the error handling for UPDATE messages is to minimize the impact on routing by a malformed UPDATE message, while maintaining protocol correctness to the extent possible. This can be achieved largely by maintaining the established session and keeping the valid routes exchanged, but removing the routes carried in the malformed UPDATE from the routing system.
This document partially revises the error handling for UPDATE messages, and provides guidelines for the authors of documents defining new attributes. Finally, it revises the error handling procedures for a number of existing attributes. Specifically, the error handling procedures of [RFC1997], [RFC4271], [RFC4360], [RFC4456], [RFC4760], [RFC5543], [RFC5701], and [RFC6368] are revised.
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 RFC 2119 [RFC2119].
In this document we refer to four different approaches to handle errors found in BGP path attributes. They are as follows (listed in order, from the one with the "strongest" action to the one with the "weakest" action):
This specification amends [RFC4271] Section 6.3 in a number of ways. See also Section 7 for treatment of specific path attributes.
There are two error cases in which the Total Attribute Length value can be in conflict with the enclosed path attributes, which themselves carry length values. In the "overrun" case, as the enclosed path attributes are parsed, the length of the last encountered path attribute would cause the Total Attribute Length to be exceeded. In the "underrun" case, as the enclosed path attributes are parsed, after the last successfully-parsed attribute, fewer than three octets remain, or fewer than four octets, if the Attribute Flags field has the Extended Length bit set -- that is, there remains unconsumed data in the path attributes but yet insufficient data to encode a single minimum-sized path attribute. In either of these cases, an error condition exists and the treat-as-withdraw approach MUST be used (unless some other, more severe error is encountered dictating a stronger approach), and the Total Attribute Length MUST be relied upon to enable the beginning of the NLRI field to be located.
For all path attributes other than those specified as having an attribute length that may be zero, it SHALL be considered a syntax error for the attribute to have a length of zero. (Of the path attributes considered in this specification, only AS_PATH and ATOMIC_AGGREGATE may validly have an attribute length of zero.)
To facilitate the determination of the NLRI field in an UPDATE with a malformed attribute:
Since older BGP speakers may not implement these restrictions, an implementation MUST still be prepared to receive these fields in any position or combination.
If the encoding of [RFC4271] is used, the NLRI field for the IPv4 unicast address family is carried immediately following all the attributes in an UPDATE. When such an UPDATE is received, we observe that the NLRI field can be determined using the "Message Length", "Withdrawn Route Length" and "Total Attribute Length" (when they are consistent) carried in the message instead of relying on the length of individual attributes in the message.
[RFC4724] specifies an End-of-RIB message ("EoR") that can be encoded as an UPDATE message that contains only a MP_UNREACH_NLRI attribute that encodes no NLRI (it can also be a completely empty UPDATE message in the case of the "legacy" encoding). In all other well-specified cases, an UPDATE either carries only withdrawn routes (either in the Withdrawn Routes field, or the MP_UNREACH_NLRI attribute), or it advertises reachable routes (either in the Network Layer Reachability Information field, or the MP_REACH_NLRI attribute).
Thus, if an UPDATE message is encountered that does contain path attributes other than MP_UNREACH_NLRI and doesn't encode any reachable NLRI, we cannot be confident that the NLRI have been successfully parsed as Section 3 (j) requires. For this reason, if any path attribute errors are encountered in such an UPDATE message, and if any encountered error specifies an error-handling approach other than "attribute discard", then the "session reset" approach MUST be used.
The NLRI field or Withdrawn Routes field SHALL be considered "syntactically incorrect" if either of the following are true:
Similarly, the MP_REACH_NLRI or MP_UNREACH_NLRI attribute of an update SHALL be considered to be incorrect if any of the following are true:
Certain address families, for example MCAST-VPN [RFC6514], MCAST-VPLS [RFC7117] and EVPN [I-D.ietf-l2vpn-evpn] have NLRI that are typed. Since supported type values within the address family are not expressed in the MP-BGP capability [RFC4760], it is possible for a BGP speaker to advertise support for the given address family and sub-address family while still not supporting a particular type of NLRI within that AFI/SAFI.
A BGP speaker advertising support for such a typed address family MUST handle routes with unrecognized NLRI types within that address family by discarding them, unless the relevant specification for that address family specifies otherwise.
Although the "treat-as-withdraw" error-handling behavior defined in Section 2 makes every effort to preserve BGP's correctness, we note that if an UPDATE received on an IBGP session is subjected to this treatment, inconsistent routing within the affected Autonomous System may result. The consequences of inconsistent routing can include long-lived forwarding loops and black holes. While lamentable, this issue is expected to be rare in practice, and, more importantly, is seen as less problematic than the session-reset behavior it replaces.
When a malformed attribute is indeed detected over an IBGP session, we RECOMMEND that routes with the malformed attribute be identified and traced back to the ingress router in the network where the routes were sourced or received externally, and then a filter be applied on the ingress router to prevent the routes from being sourced or received. This will help maintain routing consistency in the network.
Even if inconsistent routing does not arise, the "treat-as-withdraw" behavior can cause either complete unreachability or sub-optimal routing for the destinations whose routes are carried in the affected UPDATE message.
Note that "treat-as-withdraw" is different from discarding an UPDATE message. The latter violates the basic BGP principle of incremental update, and could cause invalid routes to be kept.
Because of these potential issues, a BGP speaker MUST provide debugging facilities to permit issues caused by a malformed attribute to be diagnosed. At a minimum, such facilities MUST include logging an error listing the NLRI involved, and containing the entire malformed UPDATE message when such an attribute is detected. The malformed UPDATE message SHOULD be analyzed, and the root cause SHOULD be investigated.
Section 8 mentions that attribute discard should not be used in cases where "the attribute in question has or may have an effect on route selection." Although all cases that specify attribute discard in this document do not affect route selection by default, in principle routing policies could be written that affect selection based on such an attribute. Operators should take care when writing such policies to consider the possible consequences of an attribute discard. (In general, as long as such policies are only applied to external BGP sessions, correctness issues are not expected to arise.)
In the following subsections, we elaborate on the conditions for error-checking various path attributes, and specify what approach(es) should be used to handle malformations. It is possible that implementations may apply other error checks not contemplated here. If so, the error handling approach given here should generally be applied.
This section addresses all path attributes that are defined at the time of this writing, that were not defined with error-handling consistent with Section 8, and that are not marked as "deprecated" in [IANA-BGP-ATTRS]. Attributes 17 (AS4_PATH), 18 (AS4_AGGREGATOR), 22 (PMSI_TUNNEL), 23 (Tunnel Encapsulation Attribute), 26 (AIGP), 27 (PE Distinguisher Labels) and 29 (BGP-LS Attribute) do have error-handling consistent with Section 8 and thus are not further discussed herein. Attributes 11 (DPA), 12 (ADVERTISER), 13 (RCID_PATH / CLUSTER_ID), 19 (SAFI Specific Attribute), 20 (Connector Attribute), 21 (AS_PATHLIMIT) and 28 (BGP Entropy Label Capability Attribute) are deprecated and thus are not further discussed herein.
The attribute is considered malformed if its length is not 1, or it has an undefined value [RFC4271].
An UPDATE message with a malformed ORIGIN attribute SHALL be handled using the approach of "treat-as-withdraw".
An AS_PATH is considered malformed if an unrecognized segment type is encountered, or if it contains a malformed segment. A segment is considered malformed if any of the following obtains:
An UPDATE message with a malformed AS_PATH attribute SHALL be handled using the approach of "treat-as-withdraw".
[RFC4271] also says that an implementation optionally "MAY check whether the leftmost ... AS in the AS_PATH attribute is equal to the autonomous system number of the peer that sent the message". A BGP implementation SHOULD also handle routes that violate this check using "treat-as-withdraw", but MAY follow the session reset behavior if configured to do so.
The attribute is considered malformed if its length is not 4 [RFC4271]. A length of 16 is also permissible if [RFC5549] is in use and all conditions given in that specification have been satisfied.
An UPDATE message with a malformed NEXT_HOP attribute SHALL be handled using the approach of "treat-as-withdraw".
The attribute is considered malformed if its length is not 4 [RFC4271].
An UPDATE message with a malformed MULTI_EXIT_DISC attribute SHALL be handled using the approach of "treat-as-withdraw".
The error handling of [RFC4271] is revised as follows.
The attribute SHALL be considered malformed if its length is not 0 [RFC4271].
An UPDATE message with a malformed ATOMIC_AGGREGATE attribute SHALL be handled using the approach of "attribute discard".
The error conditions specified in [RFC4271] for the attribute are revised as follows:
The AGGREGATOR attribute SHALL be considered malformed if any of the following applies:
An UPDATE message with a malformed AGGREGATOR attribute SHALL be handled using the approach of "attribute discard".
The error handling of [RFC1997] is revised as follows:
The Community attribute SHALL be considered malformed if its length is not a nonzero multiple of 4.
An UPDATE message with a malformed Community attribute SHALL be handled using the approach of "treat-as-withdraw".
The error handling of [RFC4456] is revised as follows.
The error handling of [RFC4456] is revised as follows.
If the Length of Next Hop Network Address field of the MP_REACH attribute is inconsistent with that which was expected, the attribute is considered malformed. Since the next hop precedes the NLRI field in the attribute, in this case it will not be possible to reliably locate the NLRI, and thus the "session reset" or "AFI/SAFI disable" approach MUST be used.
"That which was expected", while somewhat vague, is intended to encompass the next hop specified for the Address Family Identifier and Subsequent Address Family Identifier fields and potentially modified by any extensions in use. For example, if [RFC5549] is in use then the next hop would have to have a length of 4 or 16.
Section 3 and Section 5 provide further discussion of the handling of this attribute.
Section 3 and Section 5 discuss the handling of this attribute.
We note that [RFC5543] does not detail what constitutes "malformation" for the Traffic Engineering path attribute. A future update to that specification may provide more guidance. In the interim, an implementation that determines (for whatever reason) that an UPDATE message contains a malformed Traffic Engineering path attribute MUST handle it using the approach of "treat-as-withdraw".
The error handling of [RFC4360] is revised as follows:
The Extended Community attribute SHALL be considered malformed if its length is not a nonzero multiple of 8.
An UPDATE message with a malformed Extended Community attribute SHALL be handled using the approach of "treat-as-withdraw".
Note that a BGP speaker MUST NOT treat an unrecognized Extended Community Type or Sub-Type as an error.
The error handling of [RFC5701] is revised as follows:
The IPv6 Address Specific Extended Community attribute SHALL be considered malformed if its length is not a nonzero multiple of 20.
An UPDATE message with a malformed IPv6 Address Specific Extended Community attribute SHALL be handled using the approach of "treat-as- withdraw".
Note that a BGP speaker MUST NOT treat an unrecognized IPv6 Address Specific Extended Community Type or Sub-Type as an error.
The final paragraph of Section 5 of [RFC6368] is revised as follows:
Furthermore, the normative reference to [OPT-TRANS-BGP] in [RFC6368] is removed.
A document that specifies a new BGP attribute MUST provide specifics regarding what constitutes an error for that attribute and how that error is to be handled. Allowable error-handling approaches are detailed in Section 2. The treat-as-withdraw approach is generally preferred. The document SHOULD also provide consideration of what debugging facilities may be required to permit issues caused by a malformed attribute to be diagnosed.
For any malformed attribute that is handled by the "attribute discard" instead of the "treat-as-withdraw" approach, it is critical to consider the potential impact of doing so. In particular, if the attribute in question has or may have an effect on route selection or installation, the presumption is that discarding it is unsafe, unless careful analysis proves otherwise. The analysis should take into account the tradeoff between preserving connectivity and potential side effects.
Authors can refer to Section 7 for examples.
This document makes no request of IANA.
This specification addresses the vulnerability of a BGP speaker to a potential attack whereby a distant attacker can generate a malformed optional transitive attribute that is not recognized by intervening routers (which thus propagate the attribute unchecked) but that causes session resets when it reaches routers that do recognize the given attribute type.
In other respects, this specification does not change BGP's security characteristics.
The authors wish to thank Juan Alcaide, Deniz Bahadir, Ron Bonica, Mach Chen, Andy Davidson, Bruno Decraene, Rex Fernando, Jeff Haas, Chris Hall, Joel Halpern, Dong Jie, Akira Kato, Miya Kohno, Warren Kumari, Tony Li, Alton Lo, Shin Miyakawa, Tamas Mondal, Jonathan Oddy, Tony Przygienda, Robert Raszuk, Yakov Rekhter, Eric Rosen, Shyam Sethuram, Rob Shakir, Naiming Shen, Adam Simpson, Ananth Suryanarayana, Kaliraj Vairavakkalai, Lili Wang and Ondrej Zajicek for their observations and discussion of this topic, and review of this document.
| [I-D.ietf-l2vpn-evpn] | Sajassi, A., Aggarwal, R., Henderickx, W., Balus, F., Isaac, A. and J. Uttaro, "BGP MPLS Based Ethernet VPN", Internet-Draft draft-ietf-l2vpn-evpn-03, February 2013. |
| [RFC6514] | Aggarwal, R., Rosen, E., Morin, T. and Y. Rekhter, "BGP Encodings and Procedures for Multicast in MPLS/BGP IP VPNs", RFC 6514, February 2012. |
| [RFC7117] | Aggarwal, R., Kamite, Y., Fang, L., Rekhter, Y. and C. Kodeboniya, "Multicast in Virtual Private LAN Service (VPLS)", RFC 7117, February 2014. |