IP Spoofing CS 236 Advanced Computer Security Peter Reiher April 29, 2008

Lecture 5Page 1CS 236, Spring 2008

IP SpoofingCS 236

Advanced Computer Security Peter Reiher

April 29, 2008

Groups for This Week

1. Golita Benoodi, Nikolay Laptev, Faraz Zahabian2. Darrell Carbajal, Abishek Jain, Peter Wu3. Andrew Castner, Min-Hsieh Tsai, Chen-Kuei Lee4. Chia-Wei Chang, Zhen Huang, Ionnis Pefkianakis5. Chien-Chia Chen, Peter Peterson, Kuo-Yen Lo6. Yu Yuan Chen, Michael Hall, Hootan Nikbakht7. Michael Cohen, Chieh-Ning Lien, Vishwar Goudar8. Jih-Chung Fan, Jason Liu, Sean MacIntyre

Outline

• What is IP spoofing?• What is it used for?• How do you stop it?

The Problem of IP Spoofing

Who sent you the fatal packet?

IP header IP payload

Destination addressSource address

Now we’re getting somewhere!

Now we’ll capture the desperate

criminal!So has someone hacked

Granny’s machine?

No, someone spoofed Granny’s IP address!

What Really Happened

183.11.46.194

76.128.4.33

The dirty liar!

What Is IP Spoofing?

• Existing Internet protocols and infrastructure allow forgery of some IP packet header fields

• In particular, the source address field can often be forged

• If packet causes trouble, can’t determine its true source

• Particularly important for distributed denial of service attacks– But relevant for other situations

What Is Spoofing Used For?

• If attacker forges source address, probably won’t see the response

• So spoofing only useful when attacker doesn’t care about response–Usually denial of service attacks

• This point is not universally true– If attacker can sniff the path . . .

IP Spoofing and Reflector Attacks

• Some network sites accept remote requests and provide answers (or take actions)– E.g., DNS servers, broadcast addresses

• Responses go to whoever’s in the source address of the request

• If response is a lot bigger than the request, the attacker can cause more traffic at victim than attacker must send out

IP Spoofing and Smurf Attacks• Attack on vulnerability in IP broadcasting• Send a ping packet to an IP broadcast address

– With forged IP source address of your target• Ping gets broadcast to all addresses in broadcast group

– Still with forged address• Each broadcast recipient responds to the ping

– Inundating the victim of the attack• Easy to fix at the intermediary

– No IP broadcasts from outside your network• No good solutions for victim

Types of Spoofing

• General spoofing– Attacker chooses a random IP address for

source address• Subnet spoofing

– Attacker chooses an address from the subnet his real machine is on

– With suitable sniffing, can see responses– Harder for some types of filtering

How Much of a Problem Is Spoofing?

• The Spoofing Project suggests 16-25% of Internet is spoofable– Because of ingress filtering

• Methodology based on limited number of volunteers running their code– Arguably the folks most likely to deploy

ingress filtering• Even if they’re right, 20% is a lot

Combating Spoofing• Basic approaches:

1. Authenticate address2. Prevent delivery of packets with spoofed

addresses3. Trace packets with spoofed addresses to

their true source4. Deduce bogosity from other packet header

information5. Deduce bogosity of entire data streams with

shared IP addresses

Authenticate Address

• Probably requires cryptography• Can be done with IPSec• Incurs cryptographic costs• Only feasible when crypto

authentication is feasible• Could we afford to do this for all

packets?

Pushing Authentication Out• Destination node can’t afford to check authentication

– Since, usually, spoofing done at high volumes• Could we push authentication out into the network?

– Enlist core routers to check authentication?• Sounds crazy

– They’re already busy• But maybe they can do it only when needed?• Or maybe it can be built into fast hardware?

Challenges for In-Network Address Authentication

• Large scale authentication problem–Key management, etc.

• Crypto costs• Partial deployment• Costs of updates?

Packet Passports• A simplification of the approach• Destination sends secret stamps to sources it likes• Only packets with the right stamp get delivered

– For their source address• Spoofers don’t know the stamp

– So their packets get dropped• Maybe far out in the network

Issues for Stamping Approaches

• Are stamps related to packet contents?• If not, can attackers “steal” a stamp?• How often do you change stamps?• How to you issue stamps to legitimate

nodes?• Where do you put stamps?• How do you check them fast enough?

Detect Spoofed Addresses

• Recognize that address is spoofed– Usually based on information about:

• Network topology• Addresses

• Simple version is ingress filtering• More sophisticated methods are

possible

Ingress Filtering Example

128.171.192.*

95.113.27.12 56.29.138.2

My network shouldn’t be creating packets with this

source address

Spoofing Detection Approaches

DE F G

Potential Problems With Approaches Requiring Infrastructure Support

• Issues of speed and cost• Issues of trustworthiness• Issues of deployment

–Why will it be deployed at all?–How will it work partially deployed?

SAVE• At each router, build table of proper

“incoming” interface– For source addresses, which interface

should packets arrive?• Kind of a generalization of ingress filtering• But how to get the information?

– Leverage routing table

INCOMING INTERFACE

SAVE Protocol• SAVE builds incoming table at each router through:

– Generating SAVE updates– Processing and forwarding SAVE updates

• Final result is that all routers build proper tables

FORWARDING INTERFACE

ADDRESS

B 3D 3E 3

FORWARDING TABLEB A

ADDRESS

INCOMING TABLE

SAVE Update Generation• Each SAVE router is assigned a source address space (SAS)

• Range of IP addresses that use this router as an exit router for some set of destinations

• Independent of the underlying routing protocol• A periodic SAVE update is generated for every entry in the forwarding table and sent to the next hop • Forwarding table change invokes the generation of triggered SAVE update for the changed entry

• Intermediate routers update their incoming tables

Updates Benefit Multiple Routers

INCOMING INTERFACE

DFORWARDING INTERFACE

ADDRESS

D 3E 3

FORWARDING TABLE

ADDRESS

INCOMING TABLE

INCOMING INTERFACE

ADDRESS

INCOMING TABLE

• Intermediate router can piggyback its incoming interface information to a passing update

Updates Can Be Aggregated

INCOMING INTERFACE

ADDRESS

D 3E 3

FORWARDING TABLE

ADDRESS

INCOMING TABLE

INCOMING INTERFACE

A B 11

ADDRESS

INCOMING TABLE

D A BA

• Addresses in forwarding tables are highly aggregated• At some point, the paths diverge

Sometimes Updates Must Be Split

INCOMING INTERFACE

ADDRESS

DE 3FORWARDING TABLE

DE AA 7

ADDRESS

INCOMING TABLE

INCOMING INTERFACE

A B 11

ADDRESS

INCOMING TABLED 9

ADDRESS

FORWARDING TABLE

D ABD AB

A B 10

ADDRESS

INCOMING TABLE

INCOMING INTERFACE

Did SAVE Work?

• Yes, just fine• In full deployment . . .• In partial deployment, update splitting is

extremely challenging– Since non-deployers won’t split your

updates• Thus, of academic interest

The iSAVE Protocol

• Attempt to solve SAVE’s deployment problem–Designed for partial deployment

• Router proactively send updates when they’re actually sending traffic–Augmented with on-demand requests

from iSAVE routers

iSAVE at Work

iSAVE update

User traffic to X

Send an iSAVE update

to XAB 5

Using the Incoming Table A

Incoming table

X AX AX AX AX AX AX AX AX AX AX AX AX AX A But the incoming table says messages from A come on interface 5,

not interface 6

On-Demand iSAVE Entries

• What if a router gets traffic when it doesn’t have information on the proper interface?

• Might be good traffic or spoofed traffic• So ask the iSAVE router in charge of

the source address for an update

iSAVE’s Little Flaw

• It doesn’t work• Why?• Is it fixable?

Another Possible Approach

• ASPIRE• Use BGP info to validate paths• Essentially, when path chosen, tell

other routers that you chose that path–And that path is the right one for

packets with these addresses

An ASPIRE Deployment

AS 5AS 4

AS 6Source Prefixes: s1, s2, . . . , sn

AS 2Destination Prefix: d1

ASPIRE Capable AS Legacy AS

When ASPIRE First Starts on AS 6

AS 5AS 4

Initially, ASPIRE-capable ASs disallow traffic from s1, s2, . . . , sn to d1 from any neighbour

Disallow incoming

packets from s1, s2, . . . , sn to d1.

AS 2 Sends a BGP Update to AS 6

AS 5AS 4

AS 6 chooses the AS path (3, 1, 2) to route to d1

d1 AS2d1 AS1,AS2d1 AS3,AS1,AS2

ASPIRE Springs Into Action!

AS 5AS 4

s1-snAS3,AS1,AS2d1 s1-snAS3,AS1,AS2d1s1-snAS3,AS1,AS2d1

What Has ASPIRE Achieved Here?

AS 5AS 4

Packets can now flow from s1, . . ., sn to d1 on their proper pathBut all other false paths for these packets are blocked

d1s1 WRONG!!!!

ASPIRE And Partial Deployment

• Non-participating Ases don’t get their traffic protected

• Spoofed traffic can be introduced through non-participating AS–If it’s on the proper path

Why ASPIRE Can Never Work

• You’ll never get full deployment• Security will kill you

–PKI required . . . • The overheads will be unacceptable• These all might (or might not) be true

Packet Tracing• Figure out where the packet really came from• Generally only feasible if there is a continuing

stream of packets– Usually for DDoS

• Challenges when there are multiple sources of spoofed addresses

• For many purposes, the ultimate question is – so what?

Using Other Packet Header Info

• Packets from a particular source IP address have stereotypical header info– E.g., for given destination, TTL probably

is fairly steady• Look for implausible info in such fields• Could help against really random spoofing • Attacker can probably deduce many

plausible values• There aren’t that many possible values

Using TTL To Detect Spoofing

DE F G

323231

A 27BDEFGHI

27265827263030

A 2730

Deducing Spoofing From Data Stream Information

• Streams of packets are expected to have certain behaviors– Especially TCP

• Observe streams for proper behavior– Maybe even fiddle with them a little to see

what happens• Obvious example:

– Drop some packets from TCP stream with suspect address

– Do they get retransmitted?

Diagram for Deducing From Data Stream Information

Packets from 131.179.192.* have been coming in on one interfaceNow packets from those addresses show up on anotherRoute change or spoofing?Drop a few and see what happens

What If It’s Good Traffic?

TCP to the rescue!Receiver tells sender to retransmit “lost” packets

✔✔

Since all dropped packets retransmitted, they weren’t spoofed

What about that other interface?

What If It’s Bad Traffic?

TCP to the rescue!Receiver tells sender to retransmit “lost” packetsBut “sender” never heard of those packets!So it doesn’t retransmitSo knows this interface is wrong

Clouseau• A system designed to do this• Allows router to independently detect spoofing• Doesn’t require crypto

– No PKI!• Must deal with attempted deception• How could you deceive Clouseau?

– How would Clouseau detect it?

Open Questions On Spoofing• Are there entirely different families of

approaches?• How can you actually build tables for detection

approaches?• Can detection approaches work in practical

deployments?• Are crypto approaches actually feasible?• How do you evaluate proposed systems?

IP Spoofing CS 236 Advanced Computer Security Peter Reiher April 29, 2008

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