Internet Draft L. Donnerhacke
Category: Proposed Standard IKS GmbH
Expires: September 2008 March 11, 2008
DNSSEC protected routing announcements for BGP
draft-donnerhacke-sidr-bgp-verification-dnssec-01
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Abstract
This document describes an infrastructure for real time verification
of routes reveived via BGP4. Some DNS query types are introduced to
check the origin of a prefix and validity of the AS path. The crypto
part can be offloaded from the routing engine by sending a DNS query
and checking the AD bit in the DNS response. The proposal depends on
the DNS scalability and caching mechanisms as well as PKI introduced
by DNSSEC.
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Table of Contents
1. Introduction .................................................. 3
2. DNS Mapping ................................................... 3
2.1. Prefix origin ............................................ 4
2.2. AS Peering ............................................... 5
2.3. Delegation hierarchy ..................................... 6
3. Verification .................................................. 6
3.1. Offloading crypto ........................................ 8
3.2. Zone slaving ............................................. 8
4. Related work .................................................. 8
5. Test environment .............................................. 9
6. Security Considerations ....................................... 10
7. IANA Considerations ........................................... 10
8. References .................................................... 10
8.1. Normative References ..................................... 10
8.2. Informal References ...................................... 11
9. Changes history ............................................... 12
10. Acknowledgements ............................................. 12
Nomenclature
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 [RFC2119].
The process of checking an DNS record set to match the DNSSEC key
hierarchy is called "validation" in this document.
The process of checking an BGP route for origin and path consistency
is called "verification" in this document.
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1. Introduction
BGP hijacking is a serious problem in the current internet. In an
ideal world those cases can't happen at all, because honest operators
apply filters on their BGP4 [RFC4271] peerings in order to catch fat-
fingered misconfigurations. The filters can automatically derived
from existing, well maintained routing databases. A look to actual
routing tables suffice for a reality check.
This document aims to real time verification of received BGP
announcements on the routers: An efficient, automatic, and external
filter. The described infrastructure does allow to filter bogus
announcements even after some steps of transit.
All the routing ressource meta information is simplified and mapped
into a DNS hierarchy. The allocation and assignment chains for AS
and IP numbers from the IANA via RIR and LIRs to the routing enities
are reflected by the approbriate DNS delegation chain [iananum].
At the routing entity level (i.e. the ISP or customer) the delegated
prefix is mapped to the AS(-Set), which inject the route into the
DFZ. Futhermore the peering state is modeled as a two way announce-
ment at this level.
Due to DNSSEC [RFC4033] all those delegations and announcements can
be validated. When querying the router can do the DNSSEC validation
itself or delegate it to the next validating resolver. A validated
response contains a special bit (Authenticated Data) assuming the
trustworthness of the link between the resolver and the router. So
the router can work with validated data without performing expensive
cryptographic operations and difficult lookup algorithms.
Some special issues arise from the interaction of bulding the routing
table while requiring a working interconnection for verification, and
from verification and other operational errors.
2. DNS Mapping
The mapping is designed to ease the route verification process. All
verification steps should be performed in a building a simple DNS
query and looking for a single value in the validated DNS response
set. Futhermore the whole process should be easy to debug.
A new zone BGP.ARPA is introduced to hold the routing ressouces. For
AS number mapping, the zone AS.BGP.ARPA is used. IPv4 prefixes are
mapped into IPV4.BGP.ARPA and IPv6 prefixes are mapped into
IPV6.BGP.ARPA.
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2.1. Prefix origin
To query the origin AS from a prefix, the prefix is transformed simi-
lar to reverse lookups and the DNS is queried for IN/TXT records.
The DNS response contains a (possibly empty) set of answer records.
Each answer record holds the human readable number of a origin AS in
asdot format [as4byte].
IPv4 prefixes are queried as a classless IN-ADDR.ARPA reverse delega-
tion [RFC2317], but in the zone IPV4.BGP.ARPA instead of IN-
ADDR.ARPA.
IPv6 prefixes are queried as a IP6.ARPA reverse delegation [RFC3596],
but in the zone IPV6.BGP.ARPA instead of IP6.ARPA. If prefix miss
the nibble boundary, the same technique MUST be used as for IPv4.
This is necessary, because the LIR needs contol over the whole prefix
to announce it.
For reserved ranges, the special keyword "reserved" is used as the
origin. Prefixes with origin "reserved" SHOULD NOT be accepted or
announced. Reserved ranges SHOULD use wildcards to mark all more
specific routes as "reserved", too.
Prefixes which MUST NOT appear in routing tables, so not get an entry
in the delegation hierarchy. DNSSEC provide authenticated denial of
existence. I.e. IPV6.BGP.ARPA should not contain an entry for F.
During rollout of this proposal, a transition period is necessary to
allow the AS operators to set up the necessary zones and get the del-
egations. During the transition, the RIRs MAY derive the AS data
from the [irdb] or insert the special keyword "transition" for the
allocated prefixes.
Please note, that for multicast routing the destination adresses are
not distributed via BGP4, but only the source addresses. That's why
the multicast group addresses from 224.0.0.0/3 or FF00::/8 are never
looked up and will not be delegated in BGP.ARPA.
Example:
$ORIGIN 192/20.17.217.IPV4.BGP.ARPA.
@ IN TXT "15725"
$ORIGIN 8.D.B.4.1.0.0.2.IPV6.BGP.ARPA.
@ IN TXT "15725"
$ORIGIN 10.IPV4.BGP.ARPA.
* IN TXT "reserved"
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2.2. AS Peering
Peering between two AS is fourfold: Sending and accepting on each
site. Futhermore peering policies depend on the address family of
the prefix [RFC2622].
To query the peering policy of AS A in regard to AS B, the both AS
numbers are put together and the DNS is queried for IN/TXT records.
The DNS response contains a (possibly empty) set of answer records.
Each answer record data consists of the peering direction, the
address family, and a space seperated (nonempty) set of AS numbers in
asdot format or the special term ANY. The data section is case
insensitive.
During rollout of this proposal, a transition period is necessary to
allow the AS operators to set up the necessary zones and get the del-
egations. During the transition, the RIRs MAY derive the AS data
from the [irdb] or insert the special keyword "transition" for the
assigned AS.
asrdata = direction SPACE family as-set
direction = "import" / "export"
family = familyversion "." familycast
familyversion = "ipv4" / "ipv6"
familycast = "unicast" / "multicast"
as-set = SPACE "any" / 1*as
as = SPACE [1*DIGIT "."] 1*DIGIT
To ease the delegation of AS numbers ranges to a RIR and in order to
keep the zone size small for efficent DNSSEC operation, the combining
of the two AS numbers is done in the following way: The 32-bit AS
number of A is written as <high order 16-bit value in decimal>.<low
order 16-bit value in decimal as five dot seperated digits>, then
reversed, and AS.BGP.ARPA appended. The asdot format of the AS B
number is used as a terminal in this zone.
Example:
$ORIGIN 5.2.7.5.1.0.AS.BGP.ARPA.
3.3 IN TXT "export ipv4.multicast 15725"
IN TXT "import ipv4.multicast ANY"
15725 IN TXT "export ipv4.multicast 15725"
IN TXT "import ipv4.multicast ANY"
$ORIGIN 3.0.0.0.0.3.AS.BGP.ARPA.
15725 IN TXT "export ipv4.multicast 3.3"
IN TXT "import ipv4.multicast 15725"
3.3 IN TXT "export ipv4.multicast ANY"
IN TXT "import ipv4.multicast ANY"
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2.3. Delegation hierarchy
IPv4 addresses are allocated to the RIRs as /8. The delegation at
IPV4.BGP.ARPA follows this and delegate the zones to the RIRs name
servers. This mimics the delegation vom IANA to the RIRs in IN-
ADDR.ARPA.
IPv4 addresses are allocated to the LIRs in various sizes. Delega-
tion of the allocate is done by the RIR in classless manner. Futher-
more the classful subdelegations has to be delegated to the LIR, too.
The use of DNAME to a subzone of the already delegated classless pre-
fix is RECOMMENDED.
Example:
$ORIGIN 17.217.IPV4.BGP.ARPA.
192/20 IN NS avalon.iks-jena.de.
$GENERATE 192-207 $ DNAME $.192/20
If the LIR announce the allocate itself, it can add the TXT record
set at the delegated zone apex. If the LIR deaggregate the allocate
and/or permit assignments to be seperatly announced, it adds further
TXT record sets or delegations to the zone.
IPv6 addresses delegation mimics the delegation in IP6.ARPA. Please
note the similarity to IPv4 if an allocate or assignment miss the
nibble boundary.
Example:
$ORIGIN 0.0.2.0.1.0.0.2.IPV6.BGP.ARPA.
C/35 IN NS ns.jp.example.
C DNAME C.C/35
D DNAME D.C/35
E DNAME E.C/35
F DNAME F.C/35
AS number allocation from IANA to the RIRs is done in large blocks.
IANA has to delegate every zone for with the RIR might be responsi-
ble, but not more. Additional zones MAY introduced to delegate sin-
gle AS numbers via RIRs, if the RIR can't maintain the end customer
data directly in the IANA zone.
RIRs assign single AS numbers to the LIRs and delegate the approbri-
ate zone.
3. Verification
A router receives routes in a given address family consisting of a
prefix and a AS path via BGP4. The router has to verify, if the
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incoming route is allowed or not.
The router has to check the following criteria:
- is the originating AS allowed to inject the route?
- does all the AS in the path peer as claimed?
- does the recorded path fullfill the peering policies?
To check the origin, the router queries for the prefix as described
in 2.1. If the last AS in the path is in the answer set, the origin
is verified. If the lookup for the prefix results in "reserved",
routers SHOULD accept the route only, if local configuration permits
the prefix. If the prefix can't be found, the route MUST be dropped
and MUST NOT advertised to other peers.
To check the peering policies, for each pair of sequenced AS in the
path a query as described in 2.2. is performed. The policy of the
sending AS MUST contain all AS numbers of the path tail including the
sending AS number or ANY for the address family and for the direction
"export". The policy of the receiving AS MUST contain all AS numbers
of the path tail including the sending AS number or ANY for the
address family and for the direction "import". AS path checks are
also performed for "reserved" prefixes. If an AS can't be found, the
check fails.
The router SHOULD NOT check the recursive peering policy for dupli-
cate AS numbers, which are the result of prepending. AS operators
SHOULD add a self peering entry, if they use prepending.
If all checks succeed, the route is accepted.
If the check fails, the processing for this route MUST be delayed and
retried. This is necessary, because BGP4 does announce a route only
once during a peering session. If the temporary (may be remote con-
figuration) problem with the DNS disappears, the route will not be
reannounced in the BGP4 session, but MUST be accepted now.
Routers SHOULD postphone all the checkings but accept all the routes
as long as the routing table stays below to a configurable value.
This behaviour allows a cold start after disasterous problems. The
verification is postphoned until DNS becomes useable.
Routers MAY record the TTL of the responses and assign the route the
minimum of all TTLs to regularly reverify the route. Routers MUST
NOT drop the route soley because the TTL times out.
If an AS or a prefix lookup return the special keyword "transition",
the verification step is assumed to be correct.
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3.1. Offloading crypto
Routers are not designed for DNS processing and should not do it.
DNSSEC offers a validating resolver and a Authenticated Data bit in
the response header. Routers SHOULD ask a validating resolver and
rely on the AD bit [RFC4033].
This way, the PKI processing, caching, and debugging is hand over to
specialized software and admins.
3.2. Zone slaving
Normally name servers of new AS can't be reached, because the new
route to the prefix of the AS can't be verified until the route to
the nameserver is active.
Thats why all zones in BGP.ARPA MUST have secondaries in other AS.
The RIRs are urged to provide public secondaries for their LIRs and
their routing customers.
To avoid a net split after a hypothetical major outage, running sec-
ondaries of other zones, especially of those of the peering AS, is
RECOMMENDED. Name server operators in BGP.ARPA SHOULD allow zone
transfers to everyone [RFC1034].
4. Related work
The idea is not new. Directly after the specification of DNSSEC, the
provided infrastructure was applied for verifying BGP announcements.
Prefix originating verification was proposed by [bates] and discussed
by [liauth]. AS mapping to DNS was proposed by [eastlake].
[bates] prefered to define the new record type AS in order to keep
the current semantics of TXT. This proposal prefers TXT. The
restrictions in TXT syntax are limited to the BGP.ARPA zone. In
IPV4.BGP.ARPA and IPV6.BGP.ARPA the TXT records only contain a space
seperated list of asdot formatted AS numbers. In AS.BGP.ARPA the
requried syntax is given in 2.2.
So there is the tradeoff beween defining two new record types and
deploy them to existing name server and resolver software, and modi-
fying the semantics of TXT records in special area. This proposal
prefers to break the pure semantics of TXT records without explicitly
updating [RFC1034].
[eastlake] define the AS mapping to DNS using the asplain notation
combined with a length indicator of the significat digits. With the
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introduction of four-byte AS numbers [RFC4893], IANA chooses to allo-
cate a whole <high 16bit> to the a single RIR only. Futhermore fixed
sized formats are easier to handle in embedded formats.
That's why this proposal chooses to expand the <low 16bit> to five
decimal digits and and append the whole <high 16bit> as a single dec-
imal number. This decision does only scale, as long as the number of
allocated <high 16bit> keeps small.
The alternate approach of coding the AS number in hex as in IP6.ARPA
offers the possibility to follow the IANA allocation policy more
closely (allocation step is 0x100). Tests show, that currently 3455
delegations based on decimal number vom IANA to RIRs are necessary,
but only 3440 based on hexadecimal numbers. Only if lookup would be
done on binary numbers, the number of delegations would drop to 70.
In order to ease debugging, this proposal chooses to stick on decimal
numbers.
The actual work of the SIDR WG focuses on automatic generation and
validation of filters [sidrwg]. AS Path checks are not yet devel-
oped.
There are other proposals, i.e. a redesign of the BGP4 protocol to
include cryptographic authentication of the path and origin [bar-
tels].
5. Test environment
A testbed was build to test implementations and verify assumptions
based on this recommendation. The data in the testbed is derived on
snapshots of the Internet Routing Registy [irdb].
The primary NS for the testbed of BGP.ARPA is IANA.BGP.IKS-JENA.DE.
If you run secondaries, I'm happy to add them as name servers for the
test zones. If you like to get an delegation to maintain your own
part in the testbed, please contact me.
IANA and RIRs are especially encouraged to maintain their own area of
responsibility. This way the testbed would be more accurate and the
communication channels between the participating parties could be
covered.
To gain experience with DNSSEC signed domains up to the root, I run a
signed root [iksroot], which is expanded to cover the BGP.ARPA
testbed.
You MUST NOT consider this environment as a permanent ressource. It
will vanish as soon as the root gets signed [rootsign].
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6. Security Considerations
All zones in BGP.ARPA MUST be signed. Local infrastructure between
the routers and the validating recursive resolvers SHOULD be secured
against data modification or spoofing attacks.
Operational errors in DNSSEC or DNS handling will cause routing prob-
lems. Operational errors at RIR or IANA will cause larger shutdowns
of global routing.
7. IANA Considerations
IANA should gracefully add the BGP.ARPA zone and maintain the delega-
tions to the RIRs.
IANA should sign the all the zones from the RIR delegation point down
to the root. IANA should maintain the resigning and key rollover
procedures for those zones.
IANA should set up a Delegate Signer (manual) update protocol for the
delegation points to allow the RIRs to change their keys.
8. References
8.1. Normative References
[RFC1034] Mockapetris, P, "Domain Names - Concepts and Facilities",
RfC 1034, November 1987
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, March 1997
[RFC2317] Eidnes, H. and de Groot, G. and Vixie, P., "Classless IN-
ADDR.ARPA delegation", RFC2317, March 1998
[RFC2622] Alaettinoglu, C. and Villamizar, C. and Gerich, E. and
Kessens, D. and Meyer, D. and Bates, T. and Karrenberg,
D. and Terpstra, M., "Routing Policy Specification Lan-
guage", RFC2622, June 1999
[RFC3596] Thomson, S. and Huitema, C. and Ksinant, V. and Souissi,
M., "DNS Extensions to support IP version 6", RFC 3596,
October 2003
[RFC4033] Arends, R. and Austein, R. and Larson, M. and Massey, D.
and Rose, S., "DNS Security Introduction and Require-
ments", RFC4033, March 2005
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[RFC4271] Rekhter, Y. and Li, T. and Hares, S., "A Border Gateway
Protocol 4", RFC 4271, January 2006
[RFC4893] Vohra, Q. and Chen, E., "BGP Support for Four-octet AS
Number Space", RFC 4893, May 2007
[as4byte] Michaelson, G. and Hustone, G., "Canonical Text Represen-
tation of Four-octet AS Numbers", Work in Progress: draft-
michaelson-4byte-as-representation-05, December 2007
8.2. Informal References
[bates] Bates, T. and Bush, R. and Li, T. and Rekhter, Y., "DNS-
based NLRI origin AS verification in BGP", Expired work in
progress: draft-bates-bgp4-nlri-orig-verif-00, December
1997
[eastlake] Eastlake, D., "Mapping Autonomous Systems Number into the
Domain Name System", Expired work in progress: draft-ietf-
dnssec-as-map-05, July 1997
[liauth] Li, T., "Origin Authentication in BGP", Expired work in
progress: http://www.academ.com/nanog/feb1998/origin.html,
February 1998
[irdb] "The Internet Routing Registry: History and Purpose",
http://www.ripe.net/db/irr.html
[iananum] "Number Resources", http://www.iana.org/numbers/
[sidrwg] "Secure Inter-Domain Routing",
http://tools.ietf.org/wg/sidr/
[rootsign] "IANA (DEMO) DNSSEC Status",
https://ns.iana.org/dnssec/status.html
[youtube] RIPE NCC, "YouTube Hijacking: A RIPE NCC RIS case study",
http://www.ripe.net/news/study-youtube-hijacking.html,
February 2008
[bartels] Bartels, O., "Requirements for a new routing protocol",
Work in Progress: news:6msps3tgjug-
mvlkk1hcr26jpo8nrfhbmj0@4ax.com, March 2008
[iksroot] Donnerhacke, L., "Instructions for a signed root",
https://www.iks-jena.de/leistungen/keys.txt, December 2007
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9. Changes history
This section will not appear in the final document. It does provide
some convenience hints what changed between the document version. It
is not complete nor normative.
Important differences from 00 to 01:
- Added handling of reserved address space using wildcards.
- Added handling of non routable address space using denial of exis-
tence.
- Added classification of multicast address space as non routable
space.
- Added transition phase where information is copied from [irdb] or
verification is explicitly turned off.
- Added recommendation to explicitly announce prepending as self
peering.
- Raised recheck of delayed verifications from SHOULD to MUST.
- Added a section about related work and reasons for design deci-
sions.
10. Acknowledgements
The proposal was developed with the help of Gert Doering and Oliver
Bartels in a USENET News discussion about the YouTube hijacking in
February 2008 [youtube].
Many thanks go to Tony Li for pointing out several historic docu-
ments, and his invaluable comments on the transition phase, reserved
areas, and readvertisement of received prefixes.
Authors' Addresses
Lutz Donnerhacke
IKS GmbH
Leutragraben 1
07743 Jena
Germany
Tel: +49-3641-573561 (1.6.5.3.7.5.1.4.6.3.9.4.e164.arpa. NAPTR)
EMail: lutz@iks-jena.de
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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