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

Groups for This Week Golita Benoodi, Nikolay Laptev, Faraz Zahabian Darrell Carbajal, Abishek Jain, Peter Wu Andrew Castner, Min-Hsieh Tsai, Chen-Kuei Lee Chia-Wei Chang, Zhen Huang, Ionnis Pefkianakis Chien-Chia Chen, Peter Peterson, Kuo-Yen Lo Yu Yuan Chen, Michael Hall, Hootan Nikbakht Michael Cohen, Chieh-Ning Lien, Vishwar Goudar 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 IP header IP payload Now we’ll capture the desperate criminal! Destination address Source address So has someone hacked Granny’s machine? Who sent you the fatal packet? No, someone spoofed Granny’s IP address! Now we’re getting somewhere!

What Really Happened The dirty liar! 183.11.46.194 183.11.46.194 76.128.4.33 183.11.46.194 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: Authenticate address Prevent delivery of packets with spoofed addresses Trace packets with spoofed addresses to their true source Deduce bogosity from other packet header information 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 95.113.27.12 56.29.138.2 My network shouldn’t be creating packets with this source address 128.171.192.*

Spoofing Detection Approaches B J C H D G F E

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

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 C 4 5 RE 1 2 10 RC 6 E A B A RA 3 RD FORWARDING INTERFACE 11 ADDRESS RB 7 8 D C 2 B 3 9 INCOMING INTERFACE ADDRESS D 3 E 3 A 7 FORWARDING TABLE INCOMING TABLE B 23

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 24

Updates Benefit Multiple Routers Intermediate routers update their incoming tables C 4 5 RE 1 2 10 RC 6 E A D A D A RA 3 D A RD FORWARDING INTERFACE 11 ADDRESS RB 7 8 D C 2 B 3 9 D 3 INCOMING INTERFACE INCOMING INTERFACE E 3 ADDRESS ADDRESS FORWARDING TABLE A 7 A 11 B INCOMING TABLE INCOMING TABLE 25

Updates Can Be Aggregated Intermediate router can piggyback its incoming interface information to a passing update C 4 5 RE 1 2 10 RC 6 E A D A RA 3 D A B RD D A B ADDRESS FORWARDING INTERFACE 11 RB 7 8 D C 2 B 3 9 D 3 INCOMING INTERFACE INCOMING INTERFACE E 3 ADDRESS ADDRESS FORWARDING TABLE A 7 A B 11 B INCOMING TABLE INCOMING TABLE 26

Sometimes Updates Must Be Split Addresses in forwarding tables are highly aggregated At some point, the paths diverge INCOMING INTERFACE ADDRESS C A B 10 4 5 INCOMING TABLE 1 2 10 RC 6 E AB E A DE A RE RA 3 RD E AB FORWARDING INTERFACE 11 ADDRESS RB D AB 7 8 D C 2 D AB B 3 9 DE 3 FORWARDING TABLE INCOMING INTERFACE ADDRESS FORWARDING INTERFACE B ADDRESS INCOMING INTERFACE ADDRESS A B 11 E 8 A 7 INCOMING TABLE D 9 INCOMING TABLE FORWARDING TABLE 27

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

Send an iSAVE update to X iSAVE at Work User traffic to X 1 2 3 4 A B 5 7 6 8 X AB iSAVE update Y Send an iSAVE update to X AB 5 X

Using the Incoming Table X A A X A X A X A X A X A X A X A X A X A B X A X A X A X A 5 7 6 8 X A X A But the incoming table says messages from A come on interface 5, not interface 6 Y AB 5 Incoming table X

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 3 AS 2 AS 6 AS 1 AS 4 AS 5 ASPIRE Capable AS Destination Prefix: d1 AS 6 AS 1 AS 6 Source Prefixes: s1, s2, . . . , sn AS 4 AS 5 ASPIRE Capable AS Legacy AS

When ASPIRE First Starts on AS 6 Disallow incoming packets from s1, s2, . . . , sn to d1. AS 3 AS 2 AS 2 Destination Prefix: d1 AS 6 AS 1 AS 6 Source Prefixes: s1, s2, . . . , sn AS 4 AS 5 Initially, ASPIRE-capable ASs disallow traffic from s1, s2, . . . , sn to d1 from any neighbour

AS 2 Sends a BGP Update to AS 6 AS3,AS1,AS2 d1 AS1,AS2 d1 AS2 AS 2 Destination Prefix: d1 AS 6 AS 1 AS 6 Source Prefixes: s1, s2, . . . , sn AS 4 AS 5 AS 6 chooses the AS path (3, 1, 2) to route to d1

ASPIRE Springs Into Action! s1-sn AS3,AS1,AS2 d1 s1-sn AS3,AS1,AS2 d1 s1-sn AS3,AS1,AS2 d1 AS 6 AS 1 AS 2 Destination Prefix: d1 AS 6 Source Prefixes: s1, s2, . . . , sn AS 4 AS 5

What Has ASPIRE Achieved Here? WRONG!!!! d1 s1 AS 4 AS 5 Packets can now flow from s1, . . ., sn to d1 on their proper path But all other false paths for these packets are blocked

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 32 32 31 I A 29 30 28 27 B J A 27 A 27 B D E F G H I 26 58 30 30 C H D G F E

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 interface Now packets from those addresses show up on another Route change or spoofing? Drop a few and see what happens

What If It’s Good Traffic? AS ✔ ✔ 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? AS TCP to the rescue! Receiver tells sender to retransmit “lost” packets But “sender” never heard of those packets! So it doesn’t retransmit So 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?