Bandwidth DoS Attacks and Defenses

Robert Morris

Frans Kaashoek, Hari Balakrishnan, Students

MIT LCS

What is a Denial of Service Attack?

• Goal: make a service unusable.

• How: overload a server, router, network link.

• Focus: bandwidth attacks (“trinoo”, “tfn”).

Logical View of Attack NetAttacker

Master

Victim

Slave SlaveSlave SlaveSlave

Control Traffic

Attack Traffic

Attack Targets

App

O/S

OtherCustomers

OtherISPs

Customer’s LAN

Customer’sRouter

ISPRouter Link

Host

Attacks use IP Packets

IP Header: Source Address Destination Address

User Data

• Routers forward each packet independently.• Routers don’t know about connections.• Complexity is in end hosts; routers are simple.

Outline

• Case study: Yahoo.– What happened.– Analysis.

• Our framework for defense: RON.

Case Study: Yahoo Attack

• Early February 2000.

• Took Yahoo off the net for hours.

Yahoo’s Point of View

ISPRouter

Yahoo’sRouter

1 Gbit/secondof Ping Response

packets.

www.yahoo.com

Yahoo Attack Overview

Yahoo’s ISP

Yahoo

Other ISPs

Co-locationCenters

Attack Packet Generation

Co-locationCenter

M S1 S2 … Sn

Internet

Ping,DST=bcast,SRC=Yahoo

Ping Responses,DST=Yahoo

LeaderSlaves

What did the attack depend on?

• Pervasive insecure hosts.

• Fake IP source addresses.

• Use of hosts as amplifiers.

• Weak router software.

• Difficulty of diagnosis.

Pervasive Insecure Hosts

• Required for disguise and to generate enough traffic.• How do they break in?

– Buffer overruns.– Typically Solaris and Linux.– Highly automated.

• Defenses?– Better programming practices.– Disable services by default.– Firewalls, intrusion detection.– Motivation for deployment is not strong.

Fake IP Source Addresses

• Two uses:– Hide the source of attack.– Part of weapon.

• Example: SYN flooding.

• Defense:– Ingress/egress filtering.– But motivation for deployment is not strong.

Ingress Filtering

ISP 1

ISP 3

ISP 2

Site 1

Victim

Site 2

AttackerSRC=Site2

Use of Hosts as Amplifiers

• Attackers need this:– To avoid using their own machines.– To generate lots of traffic.– To avoid detection via load monitoring.

• Two approaches:– Break into 1000s of machines.– Trick legitimate machines into generating

traffic.

Weak Router Software

• Routers themselves are often victims.

• Why?– Forwarding and management compete for CPU.– Control and data traffic compete for net b/w.

• Solutions?– Simplify and partition.

Difficulty of Diagnosis

• Very little automatic support for traffic analysis and correlation.– Is the high load legitimate?– What does the attack consist of?– Where does the attack come from?– How ask upstream routers to discard attack

packets?

• Defense: distributed analysis system.

Why are these attacks easy?

• Internet built around end-to-end principle:– Most functions done by end hosts.

– Examples: reliable delivery.

• Advantages:– Simplifies network core.

• Example: IP packet forwarding.

• Example: it’s easy to start an ISP.

– Anyone can introduce new services.

• Result: lots of innovation.

Why is defense hard?

• End-to-end principle conflicts with:– Centralized control.– Centralized monitoring.– Separation of data from control traffic.– Mandatory authentication.– Mandatory accounting.

RON Project

• End-to-end framework for:– Cooperative statistics collection.– Cooperative reaction to attacks.– Fault-tolerant control and data routing.

• How: resilient overlay network (RON).

• Funded by DARPA/IA/FTN.

What is an Overlay Network?

ISP1

ISP2

N1

N4

N3

N5

N2

• Better routing functions built in end hosts.• Can be used to build distributed defenses.

Why Distributed Defenses?

• Presence of attack obvious near victim.– Not obvious near sources of attack.– But control is easier near sources.

• Identifying attackers requires cooperation.– Asymmetric routing.– Fake source addresses.

Why Distribution is Hard

• RON itself is a target.

• Authorized communication between RON nodes.

• Bandwidth attacks on RON nodes.

• Application-level DoS attacks.

• Political / deployment problems.– Needs cooperation? Or single-organization?

Monitoring Scenario

Backbone B1

Backbone B2

N1

N4

N3

N5

N2

Attacker

Victim

1. Measure

2. Communicate

3. Control

Fault-Tolerant Routing

• Use Internet to connect multiple sites.

• Inter-ISP routing:– Ignores link quality.– Ignores many available paths due to policy.– Chooses only one path.– Reacts slowly.

• RON allows end-system control of routing.

Fault-tolerant Routing (2)

Backbone B1

Backbone B2

PeeringPoint Q

N1

N4

N3

N5

PeeringPoint P

N2

Attacker

Peer-to-Peer Networking

• Multi-organization overlays.

• Early work: Gnutella and FreeNet.– Data replicated at many sites.– Queries traverse reliable overlay.– Explicit protection of virtual infrastructure.

Summary

• Raise the bar:– Improve host security.– Make it hard to fake IP addresses.

• Experiment with RON-like and peer-to-peer architectures.