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COEN 252 Computer Forensics

COEN 252 Computer Forensics. Network Protocols. Network Protocols: Layering. TCP/IP stack has four levels. OSI has seven. Network Protocols: Layering. Network Protocols: Layering. Each layer adds a header. Application TCP IP Link. Link Layer. Network Interface Cards (NIC)

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COEN 252 Computer Forensics

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  1. COEN 252 Computer Forensics Network Protocols

  2. Network Protocols: Layering • TCP/IP stack has four levels. • OSI has seven.

  3. Network Protocols: Layering

  4. Network Protocols: Layering • Each layer adds a header. • Application • TCP • IP • Link

  5. Link Layer • Network Interface Cards (NIC) • Unique Medium Access Control (MAC) number • Format 48b written as 6B in hex. • NICs either select based on MAC address or are in promiscuous mode (capture every packet).

  6. Link Layer • Address Resolution Protocol (ARP) • Resolves IP addresses to MAC addresses • RFC 826

  7. Link Layer: ARP Resolution Protocol • Assume node A with IP address 10.10.10.100 and MAC 00:01:02:03:04:05 wants to talk to IP address 10.10.10.101. • Sends out a broadcast who-has request: 00:01:02:03:04:05; ff:ff:ff:ff:ff:ff; arp 42 who-has 10.10.10.101 • All devices on the link capture the packet and pass it to the IP layer. • 10.10.10.101 is the only one to answer: a0:a0:a0:a0:a0:a0; 00:01:02:03:04:05; arp 64; arp reply 10.10.10.101 is-at a0:a0:a0:a0:a0:a0 • A caches the value in its arp cache.

  8. IP • Uses IP addresses of source and destination. • IP datagrams are moved from hop to hop. • “Best Effort” service. • Corrupted datagrams are detected and dropped.

  9. IP • Addresses contain IP address and port number. • IPv4 addresses are 32 bit longs • IPv6 addresses are longer.

  10. IP: ICMP • Internet Control Message Protocol • Created to deal with non-transient problems. • Fragmentation is necessary, but the No Frag flag is set. • UPD datagram sent to a non-listening port. • Ping.

  11. IP: ICMP • ICMP error messages should not be sent, • For any but the first fragment. • A source address of broadcast or loopback address. • Are probably malicious, anyway.

  12. IP: ICMP • ICMP errors are not sent, • In response to an ICMP error message. • Otherwise, craft a message with invalid UDP source and destination port. Then watch ICMP ping-ponging. • A destination broadcast address. • Don’t answer with destination unreachable for a broadcast. Otherwise, this makes it trivial to scan a network.

  13. Transport Layer: TCP and UDP • Transmission Control Protocol (TCP) • Reliable • Connection-Oriented. • Slow • User Datagram Protocol (UDP) • Unreliable • Connectionless. • Fast.

  14. TCP • Only supports unicasting. • Full duplex connection. • Message numbers to prevent loss of messages.

  15. TCP:Three Way Handshake • Initiator to responder: Syns • Responder to initator: Acks, Synt • Initiator to responder: Ackt • Sets up two connections with initial message numbers s and t.

  16. TCP:Three Way Handshake • 20:13:34.972069 IP Bobadilla.scu.edu.1316 > server8.engr.scu.edu.23: S 2882650416:2882650416(0) win 16384 <mss 1460,nop,nop,sackOK> (DF) • 20:13:34.972487 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1316: S 1012352000:1012352000(0) ack 2882650417 win 32768 <mss 1460> (DF) • 20:13:34.972500 IP Bobadilla.scu.edu.1316 > server8.engr.scu.edu.23: . ack 1 win 17520 (DF)

  17. TCP:Terminating Connections • Graceful shutdown • Party 1 to Party 2: Fin • Party 2 to Party 1: Ack • Party 2 to Party 1: Fin • Party 1 to Party 2: Ack • Abrupt shutdown • Party 1 to Party 2: Res

  18. TCP:Shutting down a connection • 20:48:45.221851 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: P 4:5(1) ack 5 win 16958 (DF) • 20:48:45.226300 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: P 5:7(2) ack 5 win 32768 (DF) • 20:48:45.231650 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: P 7:23(16) ack 5 win 32768 (DF) • 20:48:45.231666 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: . ack 23 win 16940 (DF) • 20:48:45.235303 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: F 23:23(0) ack 5 win 32768 (DF) • 20:48:45.235331 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: . ack 24 win 16940 (DF) • 20:48:45.235494 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: F 5:5(0) ack 24 win 16940 (DF) • 20:48:45.236027 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: . ack 6 win 32767 (DF)

  19. TCPExchanging Data • Each packet has a sequence number. • (One for each direction.) • Initial sequence numbers are created during initial three way handshake. • NMap uses the creation of these sequence numbers to determine the OS. • OS are now much better with truly random sequence numbers.

  20. TCP Exchanging Data • Party that receives packet sends an acknowledgement. • Acknowledgement consists in • Ack flag. • Sequence number of the next package to be expected.

  21. TCP Exchanging Data • If a package is lost, then the ack number will not change: • “Duplicate acknowledgement” • Depending on settings, sender will resend, after at most three stationary ack numbers. • Also, resend after timeout.

  22. TCP Exchanging Data • 20:48:45.087563 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: . ack 4 win 16959 (DF) • 20:48:45.087583 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: P 3:4(1) ack 4 win 16959 (DF) • 20:48:45.096443 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: P 4:5(1) ack 4 win 32768 (DF) • 20:48:45.221851 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: P 4:5(1) ack 5 win 16958 (DF) • 20:48:45.226300 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: P 5:7(2) ack 5 win 32768 (DF) • 20:48:45.231650 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: P 7:23(16) ack 5 win 32768 (DF) • 20:48:45.231666 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: . ack 23 win 16940 (DF)

  23. TCP flags • Part of TCP header • F : FIN - Finish; end of session • S : SYN - Synchronize; indicates request to start session • R : RST - Reset; drop a connection • P : PUSH - Push; packet is sent immediately • A : ACK - Acknowledgement • U : URG - Urgent • E : ECE - Explicit Congestion Notification Echo • W : CWR - Congestion Window Reduced

  24. UDP • “Send and pray” • No connection. • No special header like TCP. • Protocol field in the IP header is 0x11 • Another field in the IP header contains UDP specific header information

  25. Fragmentation • IP datagram can come across smaller maximum transmission units than its own size. • Resender chops up the IP datagram into many IP datagrams, the fragments.

  26. Fragmentation • Fragments are reassembled at the destination. • Fragments carry: • Fragment identifier • Offset in original data portion • Length of data payload in fragment • Flag that indicates whether or not this is the final fragment.

  27. Fragmentation Example • Large Echo Request • ping -l 1480 129.218.19.198 • Assume MTU is 1500

  28. Fragmentation

  29. Fragmentation: First Fragment

  30. Fragmentation: Second Fragment

  31. Fragmentation: Last Fragment

  32. Fragmentation ping –l 65500 129.218.19.198 12:02:18.256066 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp 1472: echo request seq 6400 (frag 10712:1472@0+) 12:02:18.257282 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@1472+) 12:02:18.258498 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@2944+) 12:02:18.258502 IP dhcp-19-115.engr.scu.edu.137 > 129.210.19.255.137: udp 50 12:02:18.259714 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@4416+) 12:02:18.261177 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@5888+) 12:02:18.262389 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@7360+) 12:02:18.263604 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@8832+) 12:02:18.264820 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@10304+) 12:02:18.266037 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@11776+) 12:02:18.267495 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@13248+) 12:02:18.268712 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@14720+)

  33. Fragmentation • DF (Don’t Fragment) Flag • If forwarding node finds that the datagram needs to be fragmented but that the DF flag is set, it should respond with ICMP host unreachable – need to fragment. • Useful to find minimum MTU on a link.

  34. Fragmentation • Stateless firewalls look only at individual packages. • Protocol header is only in the first fragment. • “Stealth attacks / scans” have evil payload only in the second and following fragments.

  35. Fragments:Teardrop and Friends • Teardrop (1997) • Fragments with overlapping offset fields. • Many contemporary OS crash, hang, reboot. • Jolt2 • Single fragment with non-zero offset. • Receiving system allocates resources to reconstruct a datagram that never arrives.

  36. Fragments:Teardrop and Friends • Create fragments that seem to come from a GB datagram. • Trusting OS tries to allocate memory and dies. • Ping of Death • Win95 allowed to send a ping that was just a tad too long. Receiving host would crash. • Unnamed Attacks • Missing fragments lead to resource allocation.

  37. ICMP • ICMP has no port numbers. • No acks, no message delivery guarantee • http://www.iana.org/assignments/icmp-parameters • First Byte Type • Second Byte Code

  38. ICMP • Mapping Techniques. • Detect up host. • Detect OS through responses.

  39. ICMP Tireless Mapper • Sends ICMP echo requests messages to all possible IP addresses • Many IDS might not capture this scan if the number of packages per hour is small. • Firewalls should filter incoming ping requests.

  40. ICMP Efficient Mapper • Use the ICMP echo request with a broadcast address. • Ping 129.210.19.255

  41. ICMP Clever Mapper • Use a different ICMP message such as ICMP address mask. • Determines the class of the network

  42. ICMP Normal messages • Host unreachable • Port unreachable • Admin prohibited • Need to fragment • Time exceeded in transit

  43. Malicious ICMP: Smurf Attack Smurf attack on victim 129.219.19.198 • Step 1: Send ICMP echo request to a broadcast address with spoofed IP of 129.219.19.198 • Step 2: Router allows in ICMP echo request to broadcast address • Step 3: All live hosts respond with ICMP echo reply to real source IP

  44. Malicious ICMP: Smurf Attack • Denial of Service Attack. • Effort of Attacker << Effort of Victim. • Uses ICMP replies from network as an amplifier. • Works well if victim has a slow connection.

  45. Malicious ICMP: Tribal Flood Network • Based on Smurf • Creates zombies out of compromised machines • Compromised machines use a trigger to start bombarding a victim with requests • Many variations on this theme

  46. Malicious ICMP:Winfreeze (obsolete) • Uses the ICMP redirect message. • Legal use is to update routing information. • Flood of redirect message causes the victim (Win95 / Win98) to redirect traffic to itself via random hosts. • Victim spends too much time updating routing table.

  47. Malicious ICMP: Loki • Uses ICMP packages for covert channel • A compromised host with a Loki server responds to requests from a Loki client. • Requests are sent via ping messages with data embedded in ICMP pings. • Originally used bytes 6 and 7.

  48. Malicious ICMP: Conclusions • Limit ICMP messages at the firewall. • Leads to inefficiencies, such as trying a TCP connection to a host that is down. • Need to admit path MTU discovery. • Log those that are let through.

  49. FTP • Uses TCP • Active / Passive FTP • Both use port 21 to issue FTP commands. • Active FTP: • Uses port 20 for data. • FTP server establishes connection to client

  50. FTP: Active FTP Example: • Command channel between server8.engr.scu.edu.21 and dhcp-19-211.engr.scu.edu.3268 • Dir command creates a new connection between server8.engr.scu.edu.20 and dhcp-19-211.engr.scu.edu.5003

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