Weaponizing BGP
Using Communities
Florian Streibelt, Franziska Lichtblau,
Robert Beverly, Cristel Pelsser, Georgios
Smaragdakis, Randy Bush, Anja Feldmann2018.11.04 Weaponizing BGP

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2018.11.04 Weaponizing BGP

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Ill-Defined Semantics
We have a syntax, */ 0

But there are no formal semantics, justconvention and BCPs
We’re putting semantics in comments

2018.11.04 Weaponizing BGP

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Flavors, We Think• Active• Path prepending• Modify local preference• Remote triggered blackholing• Selective announcements• Passive• Location Tagging

And thenanything athousandkiddies
haveinvented• RTT Tagging
2018.11.04 Weaponizing BGP

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Propagation• RFC 1997: Communities are atransitive optional attribute• RFC 7454: Scrub own, forward
foreign communities• So many people do not expect
them to propagate that widely• I, for one, did not
2018.11.04 Weaponizing BGP

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Only 14% of Transit
ASs propagatecommunities(2.2k of 15.5k)
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Surprise!
• 14% seems small, but AS graph is highly
connected• More than 50% of communities traverse
more than four ASes
• 10% of communities have a hop count of
more than six ASes
• Longest community propagation observed:
through 11 ASes
2018.11.04 Weaponizing BGP

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Fraction of communities (ECDF)

1.0

●

●

●

●

●

●

0.8

●

0.6
●

0.4

●

0.2
0.0●
●
●

024

68

10AS hop count2018.11.04 Weaponizing BGP

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On/Off Path
p1

p
3:666

23

3:666

p
3:666

4

2 and 3 are On Path
4 is Off Path2018.11.04 Weaponizing BGP

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Observed Communities
1.0
0.8
0.6

0.2
0.0

2018.11.04 Weaponizing BGP

1000
100
1200
2000
10
2
3000
0500

0.4
165000
666
100
03000
21000
9498
200

% communities observed

1.2

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So Let’s
Break Things!

2018.11.04 Weaponizing BGP

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Method to our Madness• All experiments first tested in Lab• Impacts were estimated• Validated on the Internet, withoperators' consent, e.g. for hijacks
2018.11.04 Weaponizing BGP

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Remote Triggered Black Hole
Traffic flow
BGP announcements

pAS4

AS3

AS2

AS52:666

X

AS1

AS1 sends p, tagged 2:666
AS2 continues announcing p
Traffic to p is dropped at AS2

Safeguards:• Provider should check customer prefix before accepting RTBH
• Customer may only blackhole own prefixes• Different policies for Customers/Peers• On receiving RTBH, add 
2018.11.04 Weaponizing BGP

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What Can Happen
Traffic flow

Attacker

AS2

BGP announcements

Community
TargetpAS3

AS3:666

AS1p
2018.11.04 Weaponizing BGP

p
Attackeep

AS4X
AS1 announces p
AS2 hijacks p, with AS3:666
Traffic to p is dropped at AS3

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It Works Well
• Works multi-hop and is hard to spot
• Triggering RTBH is possible for attackers
because, e.g.,:• BH prefix is more specific, thus accepted via
exception• Providers check BH community before prefix
filters (bug in NANOG recipe)• No validation for origin of community is possible
2018.11.04 Weaponizing BGP

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Traffic Steering
p4321
p321

p 1

2018.11.04 Weaponizing BGP

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p1

5

4

6:3
p21

p321
6:3

p5432176

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p666
3623
1 21
p16

That’s Not
Realistic

2018.11.04 Weaponizing BGP

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Oh Yeah?
/.
. /-/.

“BGP hijacks made use of BGP communities
to shape route propagation. Although theyalso changed origins, which was thegiveaway.”
2018.11.04 Weaponizing BGP

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It’s the Cloud, Man• ASN value ambiguous: who is ”sender”, ”recipient”• No defined semantics, values can mean anything• Used both for signaling and triggering of actions• No cryptographic protection• Attribution is impossible• It is hard to apply filters or understand what isgoing on2018.11.04 Weaponizing BGP

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I Read it on the Internet• Communities can be modified, added,
removed by every AS• No attribution is possible• No cryptographic protection• Yet operators bet on their ’correctness’• Large communities partially improve thesituation
2018.11.04 Weaponizing BGP

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Don’t Propagate Without
Thinking Very Deeply• On Input – Drop anything not addressed
to you, unless special agreement• On Output – Drop everything except
signals from you to the direct peer
• And Beware Cisco ‘mis-feature’ re wellknown communities
2018.11.04 Weaponizing BGP

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2018.11.04 Weaponizing BGP

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