Outline Definition Point-to-point network denial of service Smurf Distributed denial of service attacks TCP SYN Flooding and Detection

Objectives Understand the concept of DoS attacks and its current threat trends Understand the SYN flooding attacks and be able to detect at the network level and defense them (SYN cookie)

Denial of Service Attack Definition An explicit attempt by attackers to prevent legitimate users of a service from using that service Threat model – taxonomy from CERT Consumption of network connectivity and/or bandwidth Consumption of other resources, e.g. queue, CPU Destruction or alternation of configuration information Malformed packets confusing an application, cause it to freeze Physical destruction or alternation of network components Established in 1988, the CERT® Coordination Center (CERT/CC) is a center of Internet security expertise, located at the Software Engineering Institute, a federally funded research and development center operated by Carnegie Mellon University.

Status DoS attacks increasing in frequency, severity and sophistication August 6, 2009, several social networking sites, including Twitter, Facebook, Livejournal, and Google blogging pages were hit by DDoS attacks Aimed at Georgian blogger "Cyxymu". Internet's root DNS servers attacked on Oct. 22, 2002, 9 out of 13 disabled for about an hour Feb. 6, 2007, one of the servers crashed, two reportedly "suffered badly", while others saw "heavy traffic” An apparent attempt to disable the Internet itself DoS attack on major DNS providers bring Internet to morning crawl (10/21/2016) http://arstechnica.com/security/2016/10/dos-attack-on-major-dns-provider-brings-internet-to-morning-crawl/

Two General Classes of Attacks Flooding Attacks Point-to-point attacks: TCP/UDP/ICMP flooding, Smurf attacks Distributed attacks: hierarchical structures Corruption Attacks Application/service specific Eg, polluting P2P systems

Smurf DoS Attack gateway Send ping request to brdcst addr (ICMP Echo Req) Lots of responses: Every host on target network generates a ping reply (ICMP Echo Reply) to victim Ping reply stream can overload victim 1 ICMP Echo Req Src: Dos Target Dest: brdct addr 3 ICMP Echo Reply Dest: Dos Target gateway DoS Target DoS Source Prevention: reject external packets to brdcst address.

Distributed DOS Stacheldraht is a classic example of a DDoS tool. BadGuy Unidirectional commands Handler Handler Handler Coordinating communication Why such hierarchy? Stacheldraht is a classic example of a DDoS tool. It utilizes a layered structure where the attacker uses a client program to connect to handlers, which are compromised systems that issue commands to the zombie agents, which in turn facilitate the DDoS attack. Agents are compromised via the handlers by the attacker, using automated routines to exploit vulnerabilities in programs that accept remote connections running on the targeted remote hosts. Each handler can control up to a thousand agents.[1] Agent Agent Agent Agent Agent Agent Agent Agent Agent Agent Attack traffic Victim

Can you find source of attack? Hard to find BadGuy Originator of attack compromised the handlers Originator not active when DDOS attack occurs Can try to find agents Source IP address in packets is not reliable Need to examine traffic at many points, modify traffic, or modify routers

Targets of Attack End hosts Critical servers (disrupt C/S network) Web, File, Authentication, Update DNS Infrastructure Routers within org All routers in upstream path

The DDoS Landscape

Attack Tools Over Time Tools High Attackers Low 1980 1985 1990 1995 binary encryption “stealth” / advanced scanning techniques Tools High denial of service packet spoofing sniffers distributed attack tools Intruder Knowledge www attacks automated probes/scans GUI back doors disabling audits network mgmt. diagnostics hijacking sessions burglaries Attack Sophistication exploiting known vulnerabilities password cracking Attackers Low password guessing 1980 1985 1990 1995 2001 Source: CERT/CC Possible Questions: What is “stealth scanning”?

(D)DoS Tools Over Time 1996 - Point-to-point 1997 – Combined w/ multiple tools 1998 - Distributed (small, C/S) 1999 - Add encryption, covert channel comms, shell features, auto-update, bundled w/rootkit trin00, Stacheldraht, TFN, TFN2K 2000 - Speed ups, use of IRC for C&C 2001 - Added scanning, IRC channel hopping, worms include DDoS features Code Red (attacked www.whitehouse.gov) Linux “lion” worm (TFN) 2002 - Added reflection attack 2003 – IPv6 DDoS BNC is an IRC (Internet Relay Chat) proxying server released under the GPL License. It allows users to connect to chat servers by bouncing off the computer which is running BNC. Basically, it forwards the information from the user to the server and vice versa.

Outline Definition Point-to-point network denial of service Smurf Distributed denial of service attacks Trin00, TFN, Stacheldraht, TFN2K TCP SYN Flooding and Detection/Defense

SYN Flooding Attack The most classical DoS attacks Streaming spoofed TCP SYNs Takes advantage of three way handshake Server start “half-open” connections These build up… until queue is full and all additional requests are blocked

TCP Connection Management Three way handshake: Step 1: client host sends TCP SYN segment to server specifies initial seq # no data Step 2: server host receives SYN, replies with SYNACK segment server allocates buffers specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data Recall: TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) client: connection initiator server: contacted by client

TCP Handshake C S SYNC Listening Store data SYNS, ACKC Wait ACKS Connected

TCP segment structure source port # dest port # application data 32 bits application data (variable length) sequence number acknowledgement number Receive window Urg data pnter checksum F S R P A U head len not used Options (variable length) URG: urgent data (generally not used) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data now (generally not used) # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP)

SYN Flooding C S SYNC1 Listening SYNC2 Store data SYNC3 SYNC4 SYNC5

SYN Flooding Explained Attacker sends many connection requests with spoofed source addresses Victim allocates resources for each request New thread, connection state maintained until timeout Fixed bound on half-open connections Once resources exhausted, requests from legitimate clients are denied This is a classic denial of service attack Common pattern: it costs nothing to TCP initiator to send a connection request, but TCP responder must spawn a thread for each request - asymmetry!

Flood Detection System on Router/Gateway Can we maintain states for each connection flow? Stateless, simple detection system on edge (leaf) routers desired Placement: First/last mile leaf routers First mile – detect large DoS attacker Last mile – detect DDoS attacks that first mile would miss What metrics can capture the SYN flooding attacks?

Detection Method (II) SYN – SYN/ACK pair behavior Hard to evade for the attacking source Problems Need to sniff both incoming and outgoing traffic Only becomes obvious when really swamped

Preventing Denial of Service DoS is caused by asymmetric state allocation If responder opens new state for each connection attempt, attacker can initiate thousands of connections from bogus or forged IP addresses Cookies ensure that the responder is stateless until initiator produced at least two messages Responder’s state (IP addresses and ports of the connection) is stored in a cookie and sent to initiator After initiator responds, cookie is regenerated and compared with the cookie returned by the initiator

SYN Cookies C S SYNC SYNS, ACKC ACKS(cookie) Listening… Does not store state Compatible with standard TCP; simply a “weird” sequence number scheme SYNS, ACKC sequence # = cookie Cookie must be unforgeable and tamper-proof Client should not be able to invert a cookie F(source addr, source port, dest addr, dest port, coarse time, server secret) F=Rijndael or crypto hash ACKS(cookie) Recompute cookie, compare with with the one received, only establish connection if they match More info: http://cr.yp.to/syncookies.html

Backup Slides

Attack using Trin00 In August 1999, network of > 2,200 systems took University of Minnesota offline for 3 days scan for known vulnerabilities, then attack with UDP traffic once host compromised, script the installation of the DDoS master agents According to the incident report, took about 3 seconds to get root access http://staff.washington.edu/dittrich/misc/trinoo.analysis.txt Source addresses were not spoofed, so systems running the offending daemons were contacted. However, the attacker responded simply by introducing new daemon machines into the attack.

False Positive Possibilities Many new online users with long-lived TCP sessions More SYNs coming in than FINs An overloaded server would result in 3 SYNs to a FIN or SYN-ACK Because clients would retransmit the SYN

Source Address Validity Spoofed Source Address random source addresses in attack packets Subnet Spoofed Source Address - random address from address space assigned to the agent machine’s subnet En Route Spoofed Source Address - address spoofed en route from agent machine to victim Valid Source Address - used when attack strategy requires several request/reply exchanges between an agent and the victim machine - target specific applications or protocol features

Attack Rate Dynamics Agent machine sends a stream of packets to the victim Constant Rate - Attack packets generated at constant rate, usually as many as resources allow Variable Rate Delay or avoid detection and response Increasing Rate - gradually increasing rate causes a slow exhaustion of the victim’s resources Fluctuating Rate - occasionally relieving the effect - victim can experience periodic service disruptions

Up to 1996 Point-to-point (single threaded) SYN flood Fragmented packet attacks “Ping of Death” “UDP kill”

1997 Combined attacks Targa Rape bonk, jolt, nestea, newtear, syndrop, teardrop, winnuke Rape teardrop v2, newtear, boink, bonk, frag, fucked, troll icmp, troll udp, nestea2, fusion2, peace keeper, arnudp, nos, nuclear, sping, pingodeth, smurf, smurf4, land, jolt, pepsi

1998 fapi (May 1998) fuck_them (ADM Crew, June 1998) UDP, TCP (SYN and ACK), ICMP Echo, "Smurf" extension Runs on Windows and Unix UDP comms One client spoofs src, the other does not Built-in shell feature Not designed for large networks (<10) Not easy to setup/control network fuck_them (ADM Crew, June 1998) Agent written in C; Handler is a shell script ICMP Echo Reply flooder Control traffic uses UDP Can randomize source to R.R.R.R (where 0<=R<=255)

1999 More robust and functional tools trin00, Stacheldraht, TFN, TFN2K Multiple attacks (TCP SYN flood, TCP ACK flood, UDP flood, ICMP flood, Smurf…) Added encryption to C&C Covert channel Shell features common Auto-update

2000 More floods (ip-proto-255, TCP NULL flood…) Pre-convert IP addresses of 16,702 smurf amplifiers Stacheldraht v1.666 Bundled into rootkits (tornkit includes stacheldraht) http://www.cert.org/incident_notes/IN-2000-10.html Full control (multiple users, by nick, with talk and stats) Omegav3 Use of IRC for C&C Knight Kaiten IPv6 DDoS 4to6 (doesn’t require IPv6 support)

Single host in DDoS

2001 Worms include DDoS features Code Red (attacked www.whitehouse.gov) Linux “lion” worm (TFN) Added scanning, BNC, IRC channel hopping (“Blended threats” term coined in 1999 by AusCERT) “Power” bot Modified “Kaiten” bot Include time synchronization (?!!) Leaves worm

Power bot foo: oh damn, its gonna own shitloads foo: on start of the script it will erase everything that it has foo: then scan over foo: they only reboot every few weeks anyways foo: and it will take them 24 hours to scan the whole ip range foo: !scan status Scanner[24]:[SCAN][Status: ][IP: XX.X.XX.108][Port: 80][Found: 319] Scanner[208]:[SCAN][Status: ][IP: XXX.X.XXX.86][Port: 80][Found: 320] . . . foo: almost 1000 and we aren't even close foo: we are gonna own more than we thought foo: i bet 100thousand [11 hours later] Scanner[129]: [SCAN][Status: ][IP: XXX.X.XXX.195][Port: 80][Found: 34] Scanner[128]: [SCAN][Status: ][IP: XXX.X.XXX.228][Port: 80][Found: 67] Scanner[24]: [SCAN][Status: ][IP: XX.XX.XX.42][Port: 80][Found: 3580] Scanner[208]: [SCAN][Status: ][IP: XXX.XXX.XXX.156][Port: 80][Found: 3425] Scanner[65]: [SCAN][Status: ][IP: XX.XX.XXX.222][Port: 80][Found: 3959] bar: cool

2002 Distributed reflected attack tools d7-pH-orgasm drdos (reflects NBT, TCP SYN :80, ICMP) Reflected DNS attacks, steathly (NVP protocol) and encoded covert channel comms, closed port back door Honeynet Project Reverse Challenge binary http://project.honeynet.org/reverse/results/project/020601-Analysis- IP-Proto11-Backdoor.pdf

2003 Slammer worm (effectively a DDoS on local infrastructure) Windows RPC DCOM insertion vector for “blended threat” (CERT reports “thousands”) More IPv6 DoS (requires IPv6 this time) ipv6fuck, icmp6fuck