You should worry if you are below this point Your projected and optimistically projected grades should be in the grade center soon o Projected: Your current weighted score /0 * 100 o Optimistic: (Your current weighted score+70)/100 o Just for your feedback Quiz 1 is posted o Do it before your lab slot but after this week s lab lecture o Open book open notes, unlimited time o You will do the same version again after your lab to be posted soon. Better score counts. Don t allow an individual attack machine to use many of a target s resources Requires: o Authentication, or o 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 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 o But not for others, like public web servers To the extent that approach relies on secrecy, it s fragile o Some such approaches don t require secrecy 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 More advanced versions might use Akamai-like techniques Figure out which machines come from Go to those machines (or near them) and stop the Tracing is trivial if IP source addresses aren t spoofed o 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 1
The basis for most defensive approaches Addresses the core of the problem by limiting the amount of work presented to target Key question is: o What do you drop? Good solutions drop all (and only) attack traffic Less good solutions drop some (or all) of everything Filtering drops packets with particular characteristics o If you get the characteristics right, you do little collateral damage o At odds with the desire to drop all attack traffic Rate limiting drops packets on basis of amount of traffic o Can thus assure target is not overwhelmed o But may drop some good traffic In multiple places? Near the source? In the network core? Near target Near the target? Near target o Easier to detect attack o Sees everything o May be hard to prevent collateral damage o May be hard to handle attack volume Near target o May be hard to detect attack o Doesn t see everything o Easier to prevent collateral damage o Easier to handle attack volume 2
Near target o Easier to handle attack volume o Sees everything (with sufficient deployment) o May be hard to prevent collateral damage o May be hard to detect attack Have database of attack signatures Detect anomalous behavior o By measuring some parameters for a long time and setting a baseline Detecting when their values are abnormally high o By defining which behavior must be obeyed starting from some protocol specification Devise filters that encompass most of anomalous traffic Drop everything but give priority to legitimate-looking traffic o It has some parameter values o It has certain behavior 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 Attacker sends lots of TCP SYN packets o Victim sends an ack, allocates space in memory o Attacker never replies o Goal is to fill up memory before entries time out and get deleted Usually spoofed traffic o Otherwise patterns may be used for filtering o OS at the attacker or spoofed address may send RST and free up memory Effective defense against TCP SYN flood o Victim encodes connection information and time in its SEQ number o Must be hard to craft values that get encoded into the same SEQ number use crypto for encoding o Memory is only reserved when final ACK comes Only the server must change o But TCP options are not supported o And lost SYN ACKs are not repeated
Overwhelm routers o Create a lot of pps o Exhaust CPU o Most routers can t handle full bandwidth s load of small packets No real solution, must filter packets somehow to reduce router load Periodically slam the victim with short, high-volume pulses o Lead to congestion drops on client s TCP traffic o TCP backs off o If loss is large back off to 1 MSS per RTT o Attacker slams again after a few RTTs Solution requires TCP protocol changes o Tough to implement since clients must be changed Generate legitimate application traffic to the victim o E.g., DNS requests, Web requests o Usually not spoofed o If enough bots are used no client appears too aggressive o Really hard to filter since both traffic and client behavior seem identical between attackers and legitimate users Generate service requests to public servers spoofing the victim s IP o Servers reply back to the victim overwhelming it o Usually done for UDP and ICMP traffic (TCP SYN flood would only overwhelm CPU if huge number of packets is generated) o Often takes advantage of amplification effect some service requests lead to huge replies; this lets attacker amplify his attack Pushback Traceback SOS Proof-of-work systems 1 Controlling high bandwidth aggregates in the network, Mahajan, Bellovin, Floyd, Paxson, Shenker, ACM CCR, July 2002 Goal: Preferentially drop attack traffic to relieve congestion Local ACC: Enable core routers to respond to congestion locally by: o Profiling traffic dropped by RED o Identifying high-bandwidth aggregates o 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 4
Even a few core routers are able to control high-volume Separation of traffic aggregates improves current situation o Only traffic for the victim is dropped o 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 + 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 2 6 1 Practical network support for IP Traceback, Savage, Wetherall, Karlin, Anderson, ACM SIGCOMM 2000 Goal: locate the agent machines Each packet header may carry a mark, containing: o EdgeID (IP addresses of the routers) specifying an edge it has traversed o 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 Traceback does nothing to stop DDoS It only identifies attackers true locations o 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 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 Path reassembly is computationally demanding, and is not 100% accurate: o Path information cannot be used for legal purposes o Routers close to the sources can efficiently block attack traffic, minimizing collateral damage + Incrementally deployable + Effective for non-distributed 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 5
1 SOS: Secure Overlay Services, Keromytis, Misra, Rubensteain, ACM SIGCOMM 2002 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 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 o Secret servlet s identity has to be hidden, because their source address is a passport for the realm beyond the firewall o Beacons are nodes that know the identity of secret servlets If a node fails, other nodes can take its role 1 2 SOS successfully protects communication with a private server: o Access points can distinguish legitimate from attack communications o Overlay protects traffic flow o Firewall drops attack packets Redundancy in the overlay and secrecy of the path to the target provide security against DoS on SOS + 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 4 1 Client puzzles: A cryptographic countermeasure against connection depletion, Juels, Brainard, NDSS 1999 Goal: defend against connection depletion When under attack: o Server distributes small cryptographic puzzles to clients requesting service o Clients spend resources to solve the puzzles o Correct solution, submitted on time, leads to state allocation and connection establishment o Non-validated connection packets are dropped Puzzle generation is stateless Client cannot reuse puzzle solutions Attacker cannot make use of intercepted packets 5 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 o Effectively server controls its resource consumption Protocol is safe against replay or interception Other flooding will still work 6 6
+ 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 Will not work against bandwidth consumption (Defense By Offense paper changes this) 7 7