dr.-ing. michael roßberg fachgebiet telematik/rechnernetze quo vadis, virtual … ·...

Quo Vadis, Virtual Private Network?

Dr.-Ing. Michael Roßberg Fachgebiet Telematik/Rechnernetze Technische Universität Ilmenau

Overview

•  Configuration of VPN infrastructures •  Objectives to auto-configuration •  Existing approaches & systems •  The SOLID system

– Problems & basic approach – Achieved goals & properties – Selected features of our prototype

•  Résumé & outlook 6

Constructing global VPN infrastructures •  Security gateways and road warriors

connect internal networks over untrustworthy networks

•  Usually IPsec or SSL/TLS •  Smartcards used as

trust anchors •  Public & private IP

address ranges (IPv4 or IPv6)

•  Nested networks •  Multiple networks per gateway •  Multiple gateways per network •  Cycles in the network ⇢  High complexity

7

Private Network

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Private Network

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Internet

Private Network

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Private Network

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Private Network

10.2.4.0/24

Private Network

10.2.3.0/24 Private Network

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Private Network

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Private Network

172.16.1.0/16

Private Network

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Problems with the configuration of large VPNs (I) •  Usually infrastructures configured statically &

manually � Problems with scalability

– Required labor increases – Susceptibility to errors increases

� Problems with agility – No direct connections between mobile users – No reaction to failures and attacks

8

Problems with the configuration of large VPNs (II)

9

Objectives to automatic VPN configuration •  Self-configuration •  Support for

–  Nested networks –  Private IP address ranges

•  Scalability & Agility •  Confidentiality, integrity & authentication •  DoS-resistance / resilience •  ... ⇢ Development of a number of very different

approaches 10

Example 1: Tunnel Endpoint Discovery (TED) •  Reactive search of IPsec

gateways by IKE messages with destination address of target client

•  Shortcomings –  Requires public IP addresses

for all clients –  No nested networks –  Covert channel to arbitrary

hosts possible –  Addresses not attested

BlackNetwork

Red Net 1

Red Net 2

11

Example 2: Group Encrypted Transport VPN (GET) •  Central servers distribute symmetric keys •  All IPsec gateways use the same security association

(incl. traffic keys) •  (Some) Shortcomings:

–  No protection against internal attackers

–  No Perfect-Forward- Secrecy

–  Availability hard to guarantee

12

PrivateNetwork

PublicTransportNetwork

PrivateNetwork

CentralKey Server

PrivateNetwork

Backup Key Server

[RoSc09] Rossberg, Michael; Schaefer, Guenter: Ciscos Group Encrypted Transport VPN – Eine kritische Analyse, DACH security, 2009

Example 3: Dynamic Multipoint VPN (DMVPN) •  VPNs consist of „Hubs” and „Spokes“ •  OSPF-Routing between static hubs •  Dynamic spokes contact pre-configured hub •  Additionally “Spoke-to-Spoke”-connections •  Shortcomings:

–  Configuration- overhead

–  Internal attackers –  Fixed hubs critical

for DoS-resistance

13

Private Network

Private Network

Private Network Private

Network

Private Network

Private Network

Related work

14 [RoSc11] Rossberg, Michael; Schaefer, Guenter: A Survey on Automatic

Configuration of Virtual Private Networks, Computer Networks, June 2011

TopologyTopology centralizedcentralizedcentralized decentralizeddecentralizeddecentralizeddecentralizeddecentralized distributeddistributeddistributeddistributeddistributeddistributeddistributeddistributeddistributed

ApproachApproach

Protocol Layer of VPNProtocol Layer of VPN

3 3 4 3 4 4 4 3 3 3 3 3 3 7 4 4 4

Protocol Layer of Forwarded DataProtocol Layer of Forwarded Data

3 3 3 3 3 3 2 3 3 3 3 3 3 4 3 2/3 2/3

Simple Config.Simple Config. Ø Ø + + + + Ø - + + Ø Ø Ø + + - Ø

Gateway FunctionGateway Function n n 1 0 0 0 n n 0 0 n n n 0 0 n 0Private AddressesPrivate Addresses + - + - + + + - - - - + + + + +NestingNesting - - - - - 1 1 1/n - - - - - - n n nUni-/MulticastUni-/Multicast u u u/m u u u u u u u u u m u u u uNAT TraversalNAT Traversal Ø - + - + + Ø Ø - - - - - - Ø + Ø

RobustnessRobustness - - - - - + + Ø Ø Ø Ø Ø Ø Ø + Ø Ø

ScalabilityScalability - + Ø + Ø + - Ø + + + + Ø - - - -

EfficiencyEfficiency + + Ø + Ø + + + + + + + - - - - -

E2E- Protection

- - + + + + + - - Ø + + + - - - -

PFS + - + + - + - + + + + + + - - - +Covert-Channel Resistance

+ - + NA NA NA + + NA NA Ø Ø + NA NA + +

Infrastruc-ture Hiding

- - - - - + - Ø NA NA Ø Ø - + Ø + +

Entity AuthenticationEntity Authentication

+ - ? + - + - Ø - Ø + + Ø Ø - + Ø

Data Integrity/ AuthenticationData Integrity/ Authentication

Ø - ? + ? + - Ø + + + + + - - Ø Ø

Static Access ControlStatic Access Control

+ + + - + + Ø + - - + + + Ø Ø + +

Dynamic Access ControlDynamic Access Control

+ - - - - + - Ø - - + + - + - - -

DoS-Resistance

- - Ø - - Ø - Ø + + + + - Ø Ø Ø Ø

GracefulDegradation

- - - + + + - - + + + + - - - Ø -

DoS-Recovery

- - - - - - - - + Ø - - - + + - Ø

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•  Survey of 17 approaches

•  All tailored for a special scenario

•  Many weaken security

•  None address –  Nested tunnels –  DoS-resistance –  Internal

attackers

Secure OverLay for IPsec Discovery (SOLID) Derived research questions: •  How can a scalable and robust VPN be

constructed automatically? •  How can we construct efficient VPN structures

with as few associations as possible? •  How can topology knowledge be kept local? •  How can security challenges like internal

attackers be encountered? •  How can DoS-resistance be achieved?

15

Main approach •  Routing by a structured

overlay network •  Gateways ordered by

internal addresses •  Gateways may be

inserted multiple times •  Routing information

is held within the topology

⇢ Combination of routing and dynamic topology control

Private Network 110.2.0.0/24


Private Network 6

172.16.1.0/16



Private Network 5

172.16.0.0/16


Private Network 810.2.4.0/24 Private

Network 710.2.3.0/24

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[RSS10] Rossberg, Michael; Strufe, Thorsten; Schaefer, Guenter: Distributed Automatic Configuration of Complex IPsec-Infrastructures. Journal of Network and Systems Management, Volume 18, Issue 3, pp. 300-326, 2010

10.5.0.0/16

10.4.0.0/1610.3.0.0/16

10.0.0.0/16

10.31.0.0/16

10.30.0.0/16

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Ring topology Guarantees discovery in O(n) steps

Net 6

Net 2

Public Network

Net 5

Net 4

Net 3

Net 1

Net 7

Net 8

Net 9

Embedding of the overlay structure

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•  Embedding of the ring into the transport network

⇢ Efficient embedding with local knowledge?

10.5.0.0/16

10.4.0.0/1610.3.0.0/16

10.0.0.0/16

10.31.0.0/16

10.30.0.0/16

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10.2.0.0/16

Ring topology

Guarantees discovery in O(n) steps

•  Tunnels are indirect at first

•  Later optimization

Public or Private Network Private

Network

Optimization of forwarding paths

•  Indirect connections will be optimized:

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⇢ Optimal path in common transport networks ⇢ Only usage of local knowledge

Overlay path lengths

21

•  HOT router topology with cycles

⇢ Optimization algorithms might find only local minima

•  Despite extreme assumption: –  Average influence

barely measureable with significance

–  Worst-case: sub-linear increase

Cross-connections (aka fingers) Discovery in O(log n) steps

22

⇢ Scalable VPN with very few connections

Efficiency of fingers (I)

23

Direct scenario •  Comparison between

Sample-based and SkipGraphs

•  Efficiency of SkipGraph asymptotically equal

•  But sample based better as more exact

Search E�ciency =Ø Overlay–Hops with Random Fingers

Ø Overlay–Hops with Network under Test

Efficiency of fingers (II)

24

HOT graph •  Comparison nested vs. direct scenario: –  Sample-based a little bit

worse –  SkipGraphs way worse

•  Main cause: Samples allow more flexible selection of targets

⇢ Much more efficiency especially in nested scenarios

Non-functional properties

•  Minimal configuration: Only a certificate with IP addresses and bootstrapping information required

•  Scalability: Support for many thousand nodes expected

•  Robustness: Tolerates partitioning, partial connectivity problems, and high packet loss rates

•  Agile: Dynamic actions cause local effects only, MOBIKE support

25

Level of security Dynamic contruction of associations leads to new threats? � External attackers: always IPsec protection � Internal attackers: end-to-end security

26

Intermediate 1Source TargetIntermediate 2

Assessment of security against internal attackers

•  Only thing possible: attacker does not optimize routes & initiates many security associations � Attacker controls more connections � Traffic flow analysis, grey- & blackhole

attacks •  However: attack difficult to coordinate & a

general problem of todays routing algorithms ⇢ High resistance against internal attackers

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DoS-resistance

28

DoS-attacks

Resource Destruction

ResourceExhaustion

CPU

Memory

Bandwidth

✔

✔

✔

?

•  No exposed instances •  Fast repair process with possibility to re-route •  Proactive planning of backup paths •  VPN tunnels reduce attack vector

0.5

0.3

0.1

0.4

0.60.9

0.8

0.2

0.7

Basic bandwidth-attacker model

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•  Attacker observes node set •  Attacks identified neighbors by bandwidth exhaustion •  Possibly different probabilities of observation

X

pv

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Planning attacks

•  Assumptions: – Attackers know topology – Only network addresses unknown –  Independent observations

•  Attacker may choose observation points: – Randomly – Greedy – Optimally

30

Planning optimal attacks (I)

•  Optimal attack for a “budget” :

•  Vulnerability against optimal attackers

31

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(G) =

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Pmin=0D

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Planning optimal attacks (II)

32

0

20

40

60

80

100

0.0 0.2 0.4 0.6 0.8 1.0

P(X) of Attack

Affe

cted

End

-to-E

nd C

onne

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ns [%

]

Vulnerability

Resistance

Planning optimal attacks (III)

•  Finding optimal attacks is NP-hard ! –  Reduction to Vertex Cover –  Without relying on different probabilities

•  But: –  May be approximated (even though not well) –  For smaller networks possible optimally "

•  Used binary linear optimization, e.g., by branch-and-cut

•  Runtime heavily depends on graph structure

33

Constructing resilient topologies

•  Optimal topologies � Bi-level Optimization Problem

•  Operator:

•  Attacker:

•  Only solvable for very small instances ⇢ Heuristics & simple rules required

34

min

x

{attackGain(x, y) + c · costs(x),

for feasible topologies x}max

y

{attackGain(x(y), y),

for feasible attacks y}

[RGS12] Rossberg, Michael; Girlich, Franz; Schaefer, Guenter: Analyzing and Improving the Resistance of Overlay-Networks against Bandwidth Exhaustion Attacks, RNDM 2012.

Availability zones

35

[BRS09] Brinkmeier, Michael; Rossberg, Michael; Schaefer, Guenter: Towards a Denial-of-Service Resilient Design of Complex IPsec Overlays, International Conference on Communications (ICC), 2009

•  Arrange nodes in zones

•  Only neighboring zones may communicate

•  Reduces observability ⇢ Constrains external &

internal DoS attacks ⇢ Requires support from

key exchange protocol

Increase of DoS-resistance •  Direct scenario •  50 nodes, p uniform

(0,1) •  Monotone zone

distribution by probabiltiy

•  24h observation ⇢ Despite the strong

attacker significant increase

36

[RSSM09] Rossberg, Michael; Steudel, Wolfgang; Schaefer, Guenter; Martius, Kai: Eine Software-Architektur zur Konstruktion flexibler IPsec-Infrastrukturen. 11. Deutscher IT-Sicherheitskongress, 2009

INET

OMNeT++

simLib

Architecture of the prototype

37

netfilter

uDHCPd

libnet

Charon

XFRM rtnetlinkioctl

strongDaemon

TUN

iptables

ipt_solid

IPIP TunnelD

evice

Routing

IPsec Monitoring

DBusstroke

UD

P

init

Packetreinjection

UDPUDPPackets without

active SA

Dynam

icFirew

alling

Sockets

libnlC

reation of C

UG

associations

Linux Kernel

coreLib posixLib

soLib

⇢ Same base system in simulator und prototype

Inserting a new node Simulation Lab experiment

38

⇢ Reusage makes simulation extremely significant

Conclusion •  SOLID is a

•  A distributed auto-configuration approach for complex IPsec-VPN that

•  Does not reduce the achievable level of security in comparison to a manual configuration and that

•  Uses dynamic topology reconfiguration to cope with failures and attacks and that

•  Is implemented and evaluated in both - a prototypic and a simulative - environments

39

Outlook •  Further development of the prototype

–  Optimizations –  Stabilization –  Resolve kernel IPv6 issues "

•  Management aspects –  Monitoring data:

•  Collection? •  Querying? •  Visualization?

–  Deployment of certificates & keys?

40

41

Thanks for listening!

Dr.-Ing. Michael Roßberg Technische Universität Ilmenau

[email protected]

dr.-ing. michael roßberg fachgebiet telematik/rechnernetze quo vadis, virtual … ·...

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