Denial of Service

Denial of Service Attacks

• Unlike other forms of computer attacks, goal isn’t access or theft of information or services

• The goal is to stop the service from operating– To deny service to legitimate users– Slowing down may be good enough

• This is usually a temporary effect that passes as soon as the attack stops

How Can a Service Be Denied?

• Lots of ways– Crash the machine– Or put it into an infinite loop– Crash routers on the path to the machine– Use up a key machine resource– Use up a key network resource– Deny another service needed for this one (DNS)

• Using up resources is the most common approach

High-level Attack Categorization

• Floods• Congestion control exploits• Unexpected header values• Invalid content• Invalid fragments• Large packets• Impersonation attacks

Simple Denial of Service

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Simple Denial of Service• One machine tries to bring down another

machine• There is a fundamental problem for the

attacker:– The attack machine must be “more powerful”

than the target machine to overload it OR– Attacker uses approaches other than flooding• The target machine might be a powerful

server

Denial of Service and Asymmetry

• Sometimes generating a request is cheaper than formulating a response e.g. sending a bogus packet is cheaper than decrypting this packet and checking that it’s bogus

• If so, one attack machine can generate a lot of requests, and effectively multiply its power

• Not always possible to achieve this asymmetry• This is called amplification effect

DDoS “Solves” That Problem

• Use multiple machines to generate the workload

• For any server of fixed power, enough attack machines working together can overload it

• Enlist lots of machines and coordinate their attack on a single machine

Distributed Denial-of-Service

Typical Attack Modus Operandi

Is DDoS a Real Problem?

• Yes, attacks happen every day– One study reported ~4,000 per week1

• On a wide variety of targets• Tend to be highly successful• There are very few mechanisms that can stop

certain attacks• There have been successful attacks on major

commercial sites

1”Inferring Internet Denial of Service Activity,” Moore, Voelker, and Savage, Usenix Security Symposium, 2002

DDoS on Twitter• August 2009, hours-long service outage– 44 million users affected

• At the same time Facebook, LiveJournal, YouTube and Blogger were under attack– Only some users experienced an outage

• Real target: a Georgian bloggerImage borrowed from Wired.comarticle. Originally

provided by ArborNetworks

DDoS on Mastercard and Visa• December 2010• Parts of services went down briefly• Attack launched by a group of vigilantes called

Anonymous– Bots recruited through social engineering– Directed to download DDoS software and take

instructions from a master– Motivation: Payback to services that cut their support

of WikiLeaks after their founder was arrested on unrelated charges

• Several other services affected

DDoS on US Banks• September 2012• BofA, Chase and Wells Fargo were among

those attacked– Services were interrupted

• Attack claimed to be launched by a Muslim group Izz ad-Din al Qassam Cyber Fighters– Motivation: outrage about “Innocence of

Muslims” movie

Potential Effects of DDoS Attacks

• Most (if not all) sites could be rendered non-operational

• The Internet could be largely flooded with garbage traffic

• Essentially, the Internet could grind to a halt– In the face of a very large attack

• Almost any site could be put out of business– With a moderate sized attack

Who Is Vulnerable?

• Everyone connected to the Internet can be attacked

• Everyone who uses Internet for crucial operations can suffer damages

But My Machines Are Well Secured!

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Doesn’t matter!The problem isn’t your vulnerability, it’s everyone elses’

But I Have a Firewall!

Doesn’t matter! Either the attacker slips his traffic into

legitimate traffic

Or he attacks the firewall

But I Use a VPN! Doesn’t matter!

The attacker can fill your tunnel with garbageSure, you’ll detect it and discard it . . .

But you’ll be so busy doing so that you’ll have no time for your real work

But I’m Heavily Provisioned

Doesn’t matter!

The attacker can probably get enough resources to overcome any level of resources you buy

Attack Toolkits• Widely available on the net

– Easily downloaded along with source code– Easily deployed and used

• Automated code for: – Scanning – detection of vulnerable machines – Exploit – breaking into the machine – Infection – placing the attack code

• Rootkits– Hide the attack code – Restart the attack code– Keep open backdoors for attacker access

• DDoS attack code

DDoS Attack Code• Attacker can customize:

– Type of attack • UDP flood, ICMP flood, TCP SYN flood, Smurf attack

(broadcast ping flood)• Web server request flood, authentication request flood, DNS

flood– Victim IP address– Duration– Packet size– Source IP spoofing– Dynamics (constant rate or pulsing)– Communication between master and slaves

Implications Of Attack Toolkits

• You don’t need much knowledge or great skills to perpetrate DDoS

• Toolkits allow unsophisticated users to become DDoS perpetrators in little time

• DDoS is, unfortunately, a game anyone can play

DDoS Attack Trends• Attackers follow defense approaches, adjust their

code to bypass defenses• Use of subnet spoofing defeats ingress filtering• Use of encryption and decoy packets, IRC or P2P

obscures master-slave communication• Encryption of attack packets defeats traffic

analysis and signature detection• Pulsing attacks defeat slow defenses and

traceback• Flash-crowd attacks generate legitimate (well-

formed) application traffic • Social-network recruitment

How Come We Have DDoS?• Natural consequence of the way Internet is organized

– Best effort service means routers don’t do much processing per packet and store no state – they will let anything through

– End to end paradigm means routers will enforce no security or authentication – they will let anything through

• It works real well when both parties play fair• It creates opportunity for DDoS when one party cheats

There Are Still No Strong Defenses Against DDoS

• You can make yourself harder to attack• But you can’t make it impossible• And, if you haven’t made it hard enough, there’s not

much you can do when you are attacked– There are no patches to apply– There is no switch to turn– There might be no filtering rule to apply– Grin and bear it

Why Is DDoS Hard to Solve?

1. A simple form of attack2. Designed to prey on the Internet’s strengths3. Easy availability of attack machines4. Attack can look like normal traffic5. Lack of Internet enforcement tools6. Hard to get cooperation from others7. Effective solutions hard to deploy

1. Simplicity Of Attack

• Basically, just send someone a lot of traffic• More complicated versions can add refinements, but

that’s the crux of it• No need to find new vulnerabilities• No need to worry about timing, tracing, etc.• Toolkits are readily available to allow the novice to

perform DDoS• Even distributed parts are very simple

2. Preys On Internet’s Strengths

• The Internet was designed to deliver lots of traffic – From lots of places, to lots of places

• DDoS attackers want to deliver lots of traffic from lots of places to one place

• Any individual packet can look proper to the Internet• Without sophisticated analysis, even the entire flow

can appear proper

Internet Resource Utilization

• Internet was not designed to monitor resource utilization– Most of it follows first come, first served model

• Many network services work the same way• And many key underlying mechanisms do, too• Thus, if a villain can get to the important resources

first, he can often deny them to good users

3. Availability Of Attack Machines

• DDoS is feasible because attackers can enlist many machines

• Attackers can enlist many machines because many machines are readily vulnerable

• Not hard to find 1,000 crackable machines on the Internet– Particularly if you don’t care which 1,000

• Botnets numbering hundreds of thousands of hosts have been discovered

Can’t We Fix These Vulnerabilities?

• DDoS attacks don’t really harm the attacking machines

• Many people don’t protect their machines even when the attacks can harm them

• Why will they start protecting their machines just to help others?

• Altruism has not yet proven to be a compelling argument for for network security

4. Attacks Resemble Normal Traffic

• A DDoS attack can consist of vast number of requests for a web server’s home page

• No need for attacker to use particular packets or packet contents

• So neat filtering/signature tools may not help• Attacker can be arbitrarily sophisticated at mirroring

legitimate traffic– In principle– Not often done because dumb attacks work so well

5. Lack Of Enforcement Tools

• DDoS attackers have never been caught by tracing or observing attack

• Only by old-fashioned detective work– Really, only when they’re dumb enough to boast about

their success• The Internet offers no help in tracing a single attack

stream, much less multiple ones• Even if you trace them, a clever attacker leaves no

clues of his identity on those machines

What Is the Internet Lacking?

• No validation of IP source address• No enforcement of amount of resources used• No method of tracking attack flows

– Or those controlling attack flows

• No method of assigning responsibility for bad packets or packet streams

• No mechanism or tools for determining who corrupted a machine

6. Poor Cooperation In the Internet• It’s hard to get anyone to help you stop or trace or

prevent an attack• Even your ISP might not be too cooperative• Anyone upstream of your ISP is less likely to be

cooperative– ISPs more likely to cooperate with each other, though

• Even if cooperation occurs, it occurs at human timescales– The attack might be over by the time you figure out who to

call

7. Effective Solutions Hard To Deploy• The easiest place to deploy defensive systems is near your

own machine – Defenses there might not work well (firewall example)

• There are effective solutions under research– But they require deployment near attackers or in the Internet

core– Or, worse, in many places

• A working solution is useless without deployment– Hard to get anything deployed if deploying site

gets no direct advantage

Resource Limitations• Don’t allow an individual attack machine to use many

of a target’s resources• Requires:

– Authentication, or– Making the sender do special work (puzzles)

• Authentication schemes are often expensive for the receiver

• Existing legitimate senders largely not set up to handle doing special work

• Can still be overcome with a large enough army of zombies

Hiding From the Attacker

• Make it hard for anyone but legitimate clients to deliver messages at all

• E.g., keep your machine’s identity obscure• A possible solution for some potential targets

– But not for others, like public web servers

• To the extent that approach relies on secrecy, it’s fragile– Some such approaches don’t require secrecy

Resource Multiplication• As attacker demands more resources, supply them• Essentially, never allow resources to be depleted• Not always possible, usually expensive• Not clear that defender can keep ahead of the attacker• But still a good step against limited attacks• More advanced versions might use

Akamai-like techniques

Trace and Stop Attacks• Figure out which machines attacks come from• Go to those machines (or near them) and stop

the attacks• Tracing is trivial if IP source addresses aren’t

spoofed– Tracing may be possible even if they are spoofed• May not have ability/authority to do anything

once you’ve found the attack machines• Not too helpful if attacker has a vast supply of

machines

Filtering Attack Streams• The basis for most defensive approaches• Addresses the core of the problem by limiting the

amount of work presented to target• Key question is:

– What do you drop?• Good solutions drop all (and only) attack traffic• Less good solutions drop some (or all) of everything

Filtering Vs. Rate Limiting• Filtering drops packets with particular characteristics

– If you get the characteristics right, you do little collateral damage

– At odds with the desire to drop all attack traffic• Rate limiting drops packets on basis of amount of

traffic– Can thus assure target is not overwhelmed– But may drop some good traffic

• You can combine them (drop traffic for which you are sure is suspicious, rate-limit the rest) but you gain a little

Where Do You Filter?

Near the target?

Near the source?

In the network core?

In multiple places?

Filtering Location Choices• Near target• Near source• In core

Filtering Location Choices• Near target

– Easier to detect attack– Sees everything– May be hard to prevent collateral damage– May be hard to handle attack volume

• Near source• In core

Filtering Location Choices

• Near target• Near source

– May be hard to detect attack– Doesn’t see everything– Easier to prevent collateral damage– Easier to handle attack volume

• In core

Filtering Location Choices

• Near target• Near source• In core

– Easier to handle attack volume– Sees everything (with sufficient deployment)– May be hard to prevent collateral damage– May be hard to detect attack

How Do You Detect Attacks?• Have database of attack signatures• Detect anomalous behavior

– By measuring some parameters for a long time and setting a baseline• Detecting when their values are abnormally high

– By defining which behavior must be obeyed starting from some protocol specification

How Do You Filter?• Devise filters that encompass most of anomalous

traffic• Drop everything but give priority to legitimate-

looking traffic– It has some parameter values– It has certain behavior

DDoS Defense Challenges• Need for a distributed response • Economic and social factors• Lack of detailed attack information• Lack of defense system benchmarks• Difficulty of large-scale testing• Moving target

TCP SYN Flood• Attacker sends lots of TCP SYN packets– Victim sends an ack, allocates space in memory– Attacker never replies– Goal is to fill up memory before entries time out and get

deleted

• Usually spoofed traffic– Otherwise patterns may be used for filtering– OS at the attacker or spoofed address may send RST and

free up memory

TCP SYN Cookies• Effective defense against TCP SYN flood– Victim encodes connection information and time in ACK

number– Must be hard to craft values that get encoded into the

same ACK number – use crypto for encoding– Memory is only reserved when final ACK comes

• Only the server must change– But TCP options are not supported– And lost SYN ACKs are not repeated

Small-Packet Floods• Overwhelm routers– Create a lot of pps– Exhaust CPU– Most routers can’t handle full bandwidth’s load of small

packets

• No real solution, must filter packets somehow to reduce router load

Shrew Attack• Periodically slam the victim with short, high-volume

pulses– Lead to congestion drops on client’s TCP traffic– TCP backs off– If loss is large back off to 1 MSS per RTT– Attacker slams again after a few RTTs

• Solution requires TCP protocol changes – Tough to implement since clients must be changed

Flash-Crowd Attack• Generate legitimate application traffic to the victim– E.g., DNS requests, Web requests– Usually not spoofed– If enough bots are used no client appears too aggressive– Really hard to filter since both traffic and client behavior

seem identical between attackers and legitimate users

Reflector Attack• Generate service requests to public servers spoofing

the victim’s IP– Servers reply back to the victim overwhelming it– Usually done for UDP and ICMP traffic (TCP SYN flood

would only overwhelm CPU if huge number of packets is generated)

– Often takes advantage of amplification effect – some service requests lead to huge replies; this lets attacker amplify his attack

Sample Research Defenses• Pushback• Traceback• SOS• Proof-of-work systems• Human behavior modeling

Pushback1

• Goal: Preferentially drop attack traffic to relieve congestion

• Local ACC: Enable core routers to respond to congestion locally by:– Profiling traffic dropped by RED– Identifying high-bandwidth aggregates– Preferentially dropping aggregate traffic to enforce

desired bandwidth limit • Pushback: A router identifies the upstream

neighbors that forward the aggregate traffic to it, requests that they deploy rate-limit

1”Controlling high bandwidth aggregates in the network,” Mahajan, Bellovin, Floyd, Paxson, Shenker, ACM CCR, July 2002

Can it Work?• Even a few core routers are able to control high-

volume attacks• Separation of traffic aggregates improves current

situation – Only traffic for the victim is dropped– Drops affect a portion containing the attack traffic

• Likely to successfully control the attack, relieving congestion in the Internet

• Will inflict collateral damage on legitimate traffic

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Advantages and Limitations+ Routers can handle high traffic volumes+ Deployment at a few core routers can affect

many traffic flows, due to core topology+ Simple operation, no overhead for routers+ Pushback minimizes collateral damage by placing

response close to the sources– Pushback only works in contiguous deployment– Collateral damage is inflicted by response, whenever

attack is not clearly separable– Requires modification of existing core routers

Traceback1

• Goal: locate the agent machines• Each packet header may carry a mark, containing:

– EdgeID (IP addresses of the routers) specifying an edge it has traversed

– The distance from the edge• Routers mark packets probabilistically• If a router detects half-marked packet (containing only

one IP address) it will complete the mark• Victim under attack reconstructs the path from the

marked packets

1“Practical network support for IP Traceback,” Savage, Wetherall, Karlin, Anderson, ACM SIGCOMM 2000

Traceback and IP Spoofing

• Traceback does nothing to stop DDoS attacks• It only identifies attackers’ true locations

– Comes to a vicinity of attacker• If IP spoofing were not possible in the Internet,

traceback would not be necessary• There are other approaches to filter out spoofed traffic

Can it Work?• Incrementally deployable, a few disjoint routers can

provide beneficial information• Moderate router overhead (packet modification)• A few thousand packets are needed even for long path

reconstruction• Does not work well for highly distributed attacks• Path reassembly is computationally demanding, and is

not 100% accurate:– Path information cannot be used for legal purposes– Routers close to the sources can efficiently block attack traffic,

minimizing collateral damage

Advantages and Limitations+ Incrementally deployable+ Effective for non-distributed attacks and for highly

overlapping attack paths+ Facilitates locating routers close to the sources– Packet marking incurs overhead at routers, must

be performed at slow path– Path reassembly is complex and prone to errors– Reassembly of distributed attack paths is

prohibitively expensive

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SOS1

• Goal: route only “verified user” traffic to the server, drop everything else

• Clients use overlay network to reach the server• Clients are authenticated at the overlay entrance, their

packets are routed to proxies• Small set of proxies are “approved” to reach the server,

all other traffic is heavily filtered out

1“ SOS: Secure Overlay Services,” Keromytis, Misra, Rubensteain, ACM SIGCOMM 2002

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SOS• User first contacts nodes that can check its legitimacy and

let him access the overlay – access points• An overlay node uses Chord overlay routing

protocol to send user’s packets to a beacon• Beacon sends packets to a secret servlet• Secret servlets tunnel packets to the firewall• Firewall only lets through packets with an IP

of a secret servlet– Secret servlet’s identity has to be hidden, because

their source address is a passport for the realm beyond the firewall

– Beacons are nodes that know the identity of secret servlets• If a node fails, other nodes can take its role

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Can It Work?• SOS successfully protects communication with a

private server:– Access points can distinguish legitimate from attack

communications – Overlay protects traffic flow– Firewall drops attack packets

• Redundancy in the overlay and secrecy of the path to the target provide security against DoS attacks on SOS

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Advantages And Limitations+ Ensures communication of “verified user”

with the victim+ Resilient to overlay node failure+ Resilient to DoS on the defense system– Does not work for public service– Traffic routed through the overlay travels on

suboptimal path– Brute force attack on links leading to the firewall still

possible

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Client Puzzles1

• Goal: defend against connection depletion attacks• When under attack:

– Server distributes small cryptographic puzzles to clients requesting service

– Clients spend resources to solve the puzzles– Correct solution, submitted on time, leads to state

allocation and connection establishment– Non-validated connection packets are dropped

• Puzzle generation is stateless• Client cannot reuse puzzle solutions• Attacker cannot make use of intercepted packets

1“Client puzzles: A cryptographic countermeasure against connection depletion attacks ,” Juels, Brainard, NDSS 1999

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Can It Work?• Client puzzles guarantee that each client has spent a

certain amount of resources• Server determines the difficulty of the puzzle

according to its resource consumption– Effectively server controls its resource consumption

• Protocol is safe against replay or interception attacks• Other flooding attacks will still work

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Advantages And Limitations+ Forces the attacker to spend resources, protects

server resources from depletion+ Attacker can only generate a certain number of

successful connections from one agent machine+ Low overhead on server– Requires client modification– Will not work against highly distributed attacks– Will not work against bandwidth consumption

attacks (Defense By Offense paper changes this)