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 Status
DoS attacks increasing in frequency, severity and sophistication 32% respondents detected DoS attacks (1999 CSI/FBI survey) 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 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 1 ICMP Echo Req Src: Dos Target Dest: brdct addr DoS Source
3 ICMP Echo Reply Dest: Dos Target gateway DoS Target 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 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 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 binary encryption stealth / advanced scanning techniques High
Tools denial of service packet spoofing sniffers Intruder Knowledge GUI distributed attack tools www attacks automated probes/scans
back doors network mgmt. diagnostics disabling audits hijacking burglaries sessions Attack Sophistication exploiting known vulnerabilities password cracking Attackers password guessing
Low 1980 Source: CERT/CC 1985 1990 1995 2001 (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
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 90% of DoS attacks use TCP SYN floods 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: 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 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 TCP Handshake C S SYNC
SYNS, ACKC Listening Store data Wait ACKS Connected TCP segment structure 32 bits URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now
(generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) source port # dest port # sequence number acknowledgement not
U A Pnumber R S F Receive window head len used checksum Urg data pnter Options (variable length) application data (variable length) counting
by bytes of data (not segments!) # bytes rcvr willing to accept SYN Flooding C S SYNC1 SYNC2 SYNC3 SYNC4 SYNC5
Listening Store data 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? TCP Connection Management:
Closing Step 1: client end system sends TCP FIN control segment to server client Step 2: server receives FIN, closing replies with ACK. Closes connection, sends FIN. Step 4: server, receives
ACK. Connection closed. closing FIN timed wait Enters timed wait will respond with ACK to received FINs FIN ACK
Step 3: client receives FIN, replies with ACK. server closed ACK closed TCP Connection Messages Detection Methods (I) Utilize SYN-FIN pair behavior Or SYNACK FIN
Can be both on client or server side However, RST violates SYN-FIN behavior Passive RST: transmitted upon arrival of a packet at a closed port (usually by servers) Active RST: initiated by the client to abort a TCP connection (e.g., Ctrl-D during a telnet session) Often queued data are thrown away So SYN-RSTactive pair is also normal SYN FIN Behavior Generally every SYN has a FIN We cant tell if RST is active or passive Consider 75% active Vulnerability of SYN-FIN Detection
Send out extra FIN or RST with different IP/ port as SYN Waste half of its bandwidth 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 Responders 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
Compatible with standard TCP; simply a weird sequence number scheme SYNS, ACKC Listening Does not store state sequence # = cookie Cookie must be unforgeable F(source addr, source port, and tamper-proof dest addr, dest port, F=Rijndael or crypto hash coarse time, server secret) Client should not be able
to invert a cookie ACKS(cookie) More info: http://cr.yp.to/syncookies.html Recompute cookie, compare with with the one received, only establish connection if they match 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 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 machines 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 victims resources Fluctuating Rate - occasionally relieving the effect - victim can experience periodic service disruptions Up to 1996
1998 fapi (May 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 (doesnt 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:[SCAN][Status: ][IP: XX.X.XX.108][Port: 80][Found: 319] Scanner:[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: [SCAN][Status: ][IP: XXX.X.XXX.195][Port: 80][Found: 34] Scanner: [SCAN][Status: ][IP: XXX.X.XXX.228][Port: 80][Found: 67] Scanner: [SCAN][Status: ][IP: XX.XX.XX.42][Port: 80][Found: 3580]
Scanner: [SCAN][Status: ][IP: XXX.XXX.XXX.156][Port: 80][Found: 3425] Scanner: [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/020601Analysis-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
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