1. Slide #1 CIT 380: Securing Computer Systems TCP/IP

2. Slide #2 Topics 1.TCP/IP Layering 2.Encapsulation 3.Internet Addresses 4.Link Layer Protocols 5.IP 6.Routing 7.TCP and UDP 8.Application Layer Protocols

3. Slide #3 Network Example A1A2A3 B1B2B3 Router External Router

4. Slide #4 TCP/IP Layering Application Transport Network Data Link Physical HTTP, FTP, telnet TCP, UDP IP, ICMP, IGMP PPP, 802.11 Ethernet

5. Slide #5 TCP/IP Layers 1.Physical –NIC, cabling, electrical signaling. 2.Data Link –Single hop transport of packets. –Wired protocols (ethernet, FDDI, PPP) –Wireless protocols (802.11) 3.Network –End to end delivery of packets. –IP: Internet Protocol

6. Slide #6 TCP/IP Layers 4.Transport –Flow of data between two hosts for application layer. –TCP: reliable data flow with acknowledgements, retransmission, and timeouts. –UDP: simpler service with no guarantees. 5.Application –Protocols for particular applications. –ex: FTP, HTTP, SMTP

7. Slide #7 Encapsulation/De-multiplexing Sending: data sent down protocol stack –Each layer prepends a header to data –Ethernet frame sent as bit stream across wire Receiving: data moves up protocol stack –NIC moves bits into memory as ethernet frame –Each layer removes its header from packet

8. Slide #8 Encapsulation

9. Slide #9 De-multiplexing

10. Slide #10 TCP/IP Security TCP/IP has no built-in strong security. –No confidentiality features. –Minimal availability features (ToS options). –Insecure CRC checksums for integrity. –IPsec protocol extension adds security.

11. Slide #11 Data Link Layer IEEE Standards –Ethernet (802.3) –Token Ring (802.5) –Wireless (802.11) Serial Protocols –SLIP and CSLIP –PPP

12. Slide #12 Hubs and Switches Hubs –Broadcast packets received to all interfaces. Switches –Associates MAC addresses with physical interfaces. –Sends packets only to specified interface. –May have SPAN port for network monitoring.

13. Slide #13 Data Link Layer Loopback –Looks like any other link layer device. –Full network processing is performed. –Sends packets to localhost for testing. 48-bit MAC address Maximum Transmission Unit (MTU) –1492 or 1500 bytes, depending on ethernet std

14. Slide #14 Promiscuous Mode All ethernet frames to or from any locally connected host are seen by all hosts. NIC normally filters out frames that are not addressed to its MAC address. In promiscuous mode, NIC processes all ethernet frames, not just ones addressed to it. –Requires administrative access on most OSes.

15. Slide #15 IP: Internet Protocol Unreliable, connectionless datagram service –Packets may arrived damaged, out of order, duplicated or not at all. –Transport/Application layers provide reliability. IPv4 underlies Internet. –32-bit addresses in dotted-quad: 10.17.0.90. –IPv6 is successor with 128-bit addresses. Complexities: addressing, routing

16. Slide #16 IP Header

17. Slide #17 IP Header Protocol version: IPv4 Header length: 5-60 32-bit words Type of service (TOS): –3-bit precedence (ignored today) –4 TOS bits (min delay (telnet), max throughput (ftp), max reliability, min monetary cost) –unused 0 bit

18. Slide #18 IP Header Total length: length of IP datagram (bytes) –maximum size: 65535 bytes –large packets fragmented at data link layer. –small packets may be padded to minimum length. TTL: upper limit on number of router hops. Protocol: which protocol supplied packet data. Header checksum: IP header checksum

19. Slide #19 IP Fragments IP packets may be fragmented by routers for transmission across different media. –Max IP packet size: 65536 –Max Ethernet packet size: 1500 IP headers contain fragment data: –Don’t Fragment Flag: 0=allowed, 1=don’t –More Fragments Flag: 0=last, 1=more fragments –Identification: identifies single packet for reassembly. –Fragment Offset: where contents of fragment go.

20. Slide #20 Internet Addresses 32-bit IPv4 addresses –Dotted decimal notation: ii.jj.kk.ll Divided into two parts –Network ID –Host ID –XOR address with netmask to get Network ID. Network IDHost ID

21. Slide #21 Address Classes Class A: 0.0.0.0-127.255.255.255 8-bit net ID, 24-bit host ID Class B: 128.0.0.0-191.255.255.255 16-bit net ID, 16-bit host ID Class C: 192.0.0.0-223.255.255.255 24-bit net ID, 8-bit host ID Class D: 224.0.0.0-239.255.255.255 28-bit multicast group ID Class E: 240.0.0.0-255.255.255.255 Reserved for future use

22. Slide #22 CIDR Class addressing too inefficient –Still need to aggregate routes to limit routing table size. Example:196.1.1.0/24 –24-bits of Net ID: 196.1.1 –Remaining 8-bits are host ID Not limited to network class sizes –Example: 192.168.128.0/22 –4 class C networks: 192.168.{128,129,130,131}.0

23. Slide #23 Network Address Translation Local network uses IETF reserved addresses. –Non-routable: no router knows how to send packets to. –RFC 1918: 10.x.y.z, 192.168.y.z, 172.16-31.y.z Gateway translates reserved addresses to unique, routable IP addresses. NAT Gateway Src = 10.0.0.1 Dst = 10.0.0.1 Src = 2.3.4.5 Dst = 2.3.4.5 Internal NetworkInternet

24. Slide #24 NAT Techniques One-to-one Mapping –Map each internal IP address to a single external IP addr. –Need as many external IP addresses as have simultaneous connections to Internet. Many-to-one Mapping –Port Address Translation (PAT) –Map all internal IP addresses to a single external IP addr. –NAT device encodes state by rewriting the source port and keeping a state table of the mappings.

25. Slide #25 ARP: Address Resolution Protocol MAC address determines packet destination. How does network layer supply the link layer with a MAC address? ARP: Address Resolution Protocol –Maps 32-bit IP addresses to 48-bit MAC addrs –Data link layer protocol above ethernet –RARP: Reverse ARP

26. Slide #26 ARP Example sftp zappa.nku.edu 1.Obtains IP address via gethostbyname() 2. sftp asks TCP to connect to IP address 3.TCP sends connection request to zappa using an IP datagram 4.Sending host emits ARP broadcast, asking for MAC address of given IP address 5.Destination host’s ARP layer receives broadcast, answers with an ARP reply w/ IP->MAC mapping 6.Sending host constructs ethernet frame with destination MAC address containing IP datagram 7.Sending host sends IP datagram

27. Slide #27 ARP Cache st361m13 (10.1.0.90) > arp -a Net to Media Table: IPv4 Device IP Address Phys Addr ------ -------------------- ------------------ hme0 at_elan.lc3net 00:00:a2:cb:28:5e hme0 10.1.0.79 00:e0:cf:00:0e:92 hme0 st361m13 08:00:20:d8:e0:07 hme0 10.1.7.103 00:90:27:b6:b5:e5 hme0 10.1.0.139 00:e0:cf:00:15:bd

28. Slide #28 ARP Features Proxy ARP –Router can answer ARP requests on network B for a host on network A that doesn’t see broadcast. Gratuitous ARP –Host sends ARP for own IP address at boot. –No reply should be received. –Network misconfiguration if reply received.

29. Slide #29 IP Connectivity No Network –loopback only Single LAN –direct connectivity to hosts Single Router –Direct connectivity to local LAN –Other networks reachable through one router Multiple Routes to Other Networks

30. Slide #30 IP Routing

31. Slide #31 Routing Table Where to send an IP packet to? Use a table lookup: routing table Search Process: 1.Search for a matching host address. 2.Search for a matching network address. 3.Search for a default route. No route to destination: Host or network unreachable error if search fails.

32. Slide #32 Routing Table st361m13 (10.1.0.90) > netstat –rn Routing Table: IPv4 Destination Gateway Flags Ref Use Int ------------- -------------------- ----- ----- 10.1.0.0 10.1.0.90 U 1 4977 hme0 224.0.0.0 10.1.0.90 U 1 0 hme0 default 10.1.0.1 UG 1 66480 127.0.0.1 127.0.0.1 UH 6 798905 lo0

33. Slide #33 Routing Table Destination: final destination host/network Gateway: next host in route to destination Flags U: Route is up G: Route is to a gateway (router) H: Route destination is a host (not a network) D: Route created by a redirect M: Route modified by a redirect

34. Slide #34 Routing Table 10.1.0.0 direct access to local subnet 224.0.0.0 multicast route default forward packets to router at IP 10.1.0.1 127.0.0.1 loopback

35. Slide #35 IP Routing Manual (static) routes Added with the route command. ICMP redirects can alter routes Router sends ICMP redirect when packet should’ve been sent to another router. Routing protocols Routers exchange routes with each other using special routing protocols. Full internet router tables contain ~30,000 routes. Source routing Sender includes routing info in packet header.

36. Slide #36 ICMP (Internet Control Message Protocol) Network layer protocol encapsulated in IP –Communicates error messages and exceptions. –Messages handled by either IP or TCP/UDP. IP Header (20 bytes)ICMP Message 8-bit type8-bit code16-bit checksum Contents (always depend contains on type and code IP header + 8 data bytes)

37. Slide #37 ICMP Message Types Type 0: echo (ping) reply Type 3: destination unreachable Type 4: source quench Type 5: redirect Type 8: echo (ping) request Type 9, 10: router advertisement, solicitation Type 11: time (TTL) exceeded Type 12: parameter (header) problem Type 13: timestamp Type 14: timestamp reply Type 15, 16: information request, reply

38. Slide #38 UDP: User Datagram Protocol Simple datagram transport layer protocol. Each application output generates one UDP datagram, which produces one IP datagram. Trades reliability for speed Sends datagrams directly to unreliable IP layer. 16-bit port numbers Identify sending and receiving processes. Applications DNS, SNMP, TFTP, streaming audio/video

39. Slide #39 UDP Header

40. Slide #40 UDP Example: TFTP Trivial File Transfer Protocol No authentication TFTP Session: sun16 > tftp at204m02 tftp> get readme.txt Received 1024 bytes in 0.2 seconds. tftp> quit

41. Slide #41 TFTP Packet Types Packet types 1)read a file (filename, ascii/binary) 2)write a file (filename, ascii/binary) 3)file data block 4)ACK 5)error

42. Slide #42 TFTP Packet Diagram

43. Slide #43 TFTP Session Trace at204m02 > snoop udp sun16 10.00000 sun16 -> at204m02 TFTP Read "2sun" (netascii) 20.00498 at204m02 -> sun16 TFTP Data block 1 (512 bytes) 30.00136 sun16 -> at204m02 TFTP Ack block 1 40.00010 at204m02 -> sun16 TFTP Data block 2 (300 bytes) (last block) 5 0.00119 sun16 -> at204m02 TFTP Ack block 2

44. Slide #44 TFTP Security Feature: no username/password required TFTP used for diskless hosts to boot. How to protect /etc/passwd ? Limit TFTP server filesystem access. Generally only can access /tftpboot directory.

45. Slide #45 TCP: Transmission Control Protocol Connection-oriented Must establish connection before sending data. 3-way handshake. Reliable byte-stream TCP decides how to divide stream into packets. ACK, timeout, retransmit, reordering. 16-bit source and destination ports. FTP(21), HTTP(80), POP(110), SMTP(25)

46. Slide #46 TCP Reliability 1.Breaks data into best-sized chunks. 2.After sending segment, maintains timer; if no ACK within time limit, resends segment. 3.Sends ACK on receipt of packets. 4.Discards pkts on bad checkum of header and data. 5.Receiver resequences TCP segments so data arrives in order sent. 6.Receiver discards duplicate segments. 7.Flow control: only sends as much data as receiver can process.

47. Slide #47 TCP Header

48. Slide #48 TCP Header Sequence Number: 32-bit segment identifier. Acknowledgment: next sequence number expected by sender of ACK –TCP is full duplex so both sides of connection have own set of sequence numbers Header length: length of header in 32-bit words (20bytes default–60bytes w/ options) Window size: number of bytes receiver is willing to accept (flow control)

49. Slide #49 TCP Header Flags (Code Bits) URG: urgent pointer is valid ACK: acknowledgement number is valid PSH: rcvr should pass data to app asap RST: reset connection SYN: synchronize sequence numbers to initiate a connection FIN: sender is finished sending data

50. Slide #50 TCP Options End of option list (kind=0) NOP (kind=1) Used to pad fields to 32-bit boundary Maximum Segment Size (MSS) (kind=2) Len=4 (length includes kind + len bytes) 16-bit MSS Default: 536 data + 20 TCP hdr + 20 IP hdr Window Scale Factor (kind=3) Timestamp (kind=8)

51. Slide #51 TCP Connections Establishment 3-way handshake Connection Trace Termination Normal Termination Connection Trace Reset

52. Slide #52 Connection Establishment Protocol 1.Requester (client) sends a SYN segment, specifying the port number of the server to which it wants to connect and the client’s initial sequence number (ISN). 2.Server responds with SYN segment containing server’s ISN. Server acknowledges client’s SYN by ACKing the client’s ISN+1. 3.Client acknowledges server SYN by ACKing server’s ISN+1.

53. Slide #53 TCP 3-way Handshake

54. Slide #54 Connection Establishment Test at204m02> /usr/sbin/snoop sun09 at204m02> nc sun09 22 SSH-1.99-OpenSSH_3.7.1p2 ^C If no services running, start your own: at204m02> nc -l -p 8192

55. Slide #55 TCP Connection Trace at204m02 -> sun09 TCP D=22 S=37519 Syn Seq=477982308 Len=0 Win=24820 Options= sun09 -> at204m02 TCP D=37519 S=22 Syn Ack=477982309 Seq=3227257622 Len=0 Win=24820 Options= at204m02 -> sun09 TCP D=22 S=37519 Ack=3227257623 Seq=477982309 Len=0 Win=24820

56. Slide #56 Connection Termination Protocol As TCP is full duplex, each side must terminate half of the connection as follows: Send FIN segment (active close) Other side ACKs w/ FIN sequence number +1 Half-closed connections Side that sent FIN can still receive data. Example: ssh fasthost sort < words.txt

57. Slide #57 TCP Disconnection

58. Slide #58 Connection Termination Test at204m02> /usr/lib/sendmail -bd at204m02> /usr/sbin/snoop port 25 sun09>nc at204m02 25 220 at204m02.lc3net ESMTP Sendmail 8.11.7+Sun/8.11.7; Mon, 29 Mar 2004 14:09:40 -0500 (EST) quit

59. Slide #59 TCP Disconnection Trace at204m02 -> sun09 TCP D=33042 S=25 Fin Ack=3597541820 Seq=872479258 Len=0 Win=24820 sun09 -> at204m02 TCP D=25 S=33042 Ack=872479259 Seq=3597541820 Len=0 Win=24820 sun09 -> at204m02 TCP D=25 S=33042 Fin Ack=872479259 Seq=3597541820 Len=0 Win=24820 at204m02 -> sun09 TCP D=33042 S=25 Ack=3597541821 Seq=872479259 Len=0 Win=24820

60. Slide #60 TCP Reset Connection Refused > telnet at204m02 8192 Trying 10.1.0.90... telnet: Unable to connect to remote host: Connection refused Packet Trace sun09 -> at204m02 TCP D=8192 S=33048 Syn Seq=3848454475 Len=0 Win=24820 Options= at204m02 -> sun09 TCP D=33048 S=8192 Rst Ack=3848454476 Win=0

61. Slide #61 TCP Reset (cont.) Connection Abort Any queued data is thrown away. Other side is informed of abnormal close. Packet Detail: One side sends RST. Other side aborts connection. There is no ACK sent in response.

62. Slide #62 Half-Open Connections Connections where one side has aborted or closed connection w/o knowledge of other. –Client or server host has crashed. –DOS attack: requester sends SYN, doesn’t respond to SYN+ACK.

63. Slide #63 Example List of TCP Ports TCP: IPv4 (netstat –na output) Local Addr Rmt Addr State ---------- -------------------- *.111 *.* LISTEN *.32771 *.* LISTEN *.32772 *.* LISTEN *.32773 *.* LISTEN *.32774 *.* LISTEN *.4045 *.* LISTEN *.22 *.* LISTEN *.2049 *.* LISTEN *.515 *.* LISTEN *.80 *.* LISTEN *.6000 *.* LISTEN *.22 10.17.0.23.32827 ESTABLISHED *.2049 10.17.0.23.799 ESTABLISHED

64. Slide #64 TCP Servers Local Address *.80 means that it will accept connections on any network interface on TCP port 80. Foreign Address *.* means that the server will accept connections from any source host and port. Conn=(src IP, src port, dst IP, dst port) All connections to same server will have same dst IP and port, but will have different source IPs and ports Kernel maintains queue of ~5 incoming connections for each server.

65. Slide #65 Key Points 1.TCP/IP Layers: encapsulation/de-multiplexing 1.Physical/Data Link: ethernet, PPP 2.Network: IP, ICMP 3.Transport: UDP, TCP 4.Application: ftp, http, smtp, telnet, etc. 2.IP 1.Addressing: DNS/IP/MAC, netmasks, CIDR, NAT. 2.Routing: tables, hubs/switches/routers. 3.TCP 1.Connection and Termination: 3-way handshake 2.Addressing: source and destination ports.

66. Slide #66 References 1.K. Egevang and P. Francis, “The IP Network Address Translator (NAT),” RFC 1631, http://www.ietf.org/rfc/rfc1631.txt, 1994.http://www.ietf.org/rfc/rfc1631.txt 2.J.B. Postel, “Internet Protocol,” RFC 791, “http://www.ietf.org/rfc/rfc0791.txt, 1981.http://www.ietf.org/rfc/rfc0791.txt 3.J.B. Postel, “Internet Control Message Protocol,” RFC 792, “http://www.ietf.org/rfc/rfc0792.txt, 1981.http://www.ietf.org/rfc/rfc0792.txt 4.J.B. Postel, “Transmission Control Protocol,” RFC 793, http://www.ietf.org/rfc/rfc0793.txt, 1981. http://www.ietf.org/rfc/rfc0793.txt 5.Ed Skoudis, Counter Hack, Prentice Hall, 2002. 6.Richard Stevens, TCP/IP Illustrated, Vol. 1, Addison-Wesley, 1994. 7.Richard Stevens, UNIX Network Programming, Vol. 1, Prentice- Hall, 1998. 8.Andrew Tannenbaum, Computer Networks, 4 th edition, Prentice- Hall, 2002.