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Transcript
CSC 382: Computer Security TCP/IP CSC 382: Computer Security Slide #1 Topics 1. 2. 3. 4. 5. 6. 7. 8. TCP/IP Layering Encapsulation Internet Addresses Link Layer Protocols IP Routing TCP and UDP Application Layer Protocols CSC 382: Computer Security Slide #2 Network Example A1 A2 A3 Router B1 B2 CSC 382: Computer Security External Router B3 Slide #3 TCP/IP Layering Application HTTP, FTP, telnet Transport TCP, UDP Network IP, ICMP, IGMP Data Link Physical CSC 382: Computer Security PPP, 802.11 Ethernet Slide #4 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 CSC 382: Computer Security Slide #5 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 CSC 382: Computer Security Slide #6 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 CSC 382: Computer Security Slide #7 Encapsulation CSC 382: Computer Security Slide #8 De-multiplexing CSC 382: Computer Security Slide #9 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. CSC 382: Computer Security Slide #10 Data Link Layer IEEE Standards – Ethernet (802.3) – Token Ring (802.5) – Wireless (802.11) Serial Protocols – SLIP and CSLIP – PPP CSC 382: Computer Security Slide #11 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. CSC 382: Computer Security Slide #12 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 CSC 382: Computer Security Slide #13 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. CSC 382: Computer Security Slide #14 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 CSC 382: Computer Security Slide #15 IP Header CSC 382: Computer Security Slide #16 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 CSC 382: Computer Security Slide #17 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 CSC 382: Computer Security Slide #18 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. CSC 382: Computer Security Slide #19 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 ID CSC 382: Computer Security Host ID Slide #20 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 CSC 382: Computer Security Slide #21 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 CSC 382: Computer Security Slide #22 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.y.z Gateway translates reserved addresses to unique, routable IP addresses. – NAT: Dynamic mapping to pool of routable IP addresses. • 10.0.0.1 -> 4.2.3.5 • 10.0.0.2 -> 4.2.3.6 – NAPT: Dynamic mapping to IP addresss/pool of src ports. • 10.0.0.1 -> 4.2.3.5:1 • 10.0.0.2 -> 4.2.3.5:2 CSC 382: Computer Security Slide #23 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 CSC 382: Computer Security Slide #24 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 brahms 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 CSC 382: Computer Security Slide #25 ARP Cache at204m02 (10.1.0.90) > arp -a Net to Device -----hme0 hme0 hme0 hme0 hme0 Media Table: IPv4 IP Address -------------------at_elan.lc3net 10.1.0.79 at204m02 10.1.7.103 10.1.0.139 CSC 382: Computer Security Phys Addr -----------------00:00:a2:cb:28:5e 00:e0:cf:00:0e:92 08:00:20:d8:e0:07 00:90:27:b6:b5:e5 00:e0:cf:00:15:bd Slide #26 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. CSC 382: Computer Security Slide #27 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 CSC 382: Computer Security Slide #28 IP Routing CSC 382: Computer Security Slide #29 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. CSC 382: Computer Security Slide #30 Routing Table at204m02 (10.1.0.90) > netstat –rn Routing Table: IPv4 Destination Gateway Flags Ref ------------- -------------------10.1.0.0 10.1.0.90 U 1 224.0.0.0 10.1.0.90 U 1 default 10.1.0.1 UG 1 127.0.0.1 127.0.0.1 UH 6 CSC 382: Computer Security Use Int ----- ----4977 hme0 0 hme0 66480 798905 lo0 Slide #31 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 CSC 382: Computer Security Slide #32 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 CSC 382: Computer Security Slide #33 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. CSC 382: Computer Security Slide #34 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 type 8-bit code 16-bit checksum Contents (always depend contains on type and code IP header + 8 data bytes) CSC 382: Computer Security Slide #35 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 CSC 382: Computer Security Slide #36 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 CSC 382: Computer Security Slide #37 UDP Header CSC 382: Computer Security Slide #38 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 CSC 382: Computer Security Slide #39 TFTP Packet Types Packet types 1) 2) 3) 4) 5) read a file (filename, ascii/binary) write a file (filename, ascii/binary) file data block ACK error CSC 382: Computer Security Slide #40 TFTP Packet Diagram CSC 382: Computer Security Slide #41 TFTP Session Trace at204m02 > snoop udp sun16 1 0.00000 sun16 -> at204m02 (netascii) TFTP Read "2sun" 2 0.00498 at204m02 -> sun16 (512 bytes) TFTP Data block 1 3 0.00136 TFTP Ack block 1 4 0.00010 at204m02 -> sun16 (300 bytes) (last block) 5 sun16 -> at204m02 0.00119 sun16 -> at204m02 CSC 382: Computer Security TFTP Data block 2 TFTP Ack block 2 Slide #42 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. CSC 382: Computer Security Slide #43 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) CSC 382: Computer Security Slide #44 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. CSC 382: Computer Security Slide #45 TCP Header CSC 382: Computer Security Slide #46 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) CSC 382: Computer Security Slide #47 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 CSC 382: Computer Security Slide #48 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) CSC 382: Computer Security Slide #49 TCP Connections Establishment 3-way handshake Connection Trace Termination Normal Termination Connection Trace Reset CSC 382: Computer Security Slide #50 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. CSC 382: Computer Security Slide #51 TCP 3-way Handshake CSC 382: Computer Security Slide #52 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 CSC 382: Computer Security Slide #53 TCP Connection Trace at204m02 -> sun09 TCP D=22 S=37519 Syn Seq=477982308 Len=0 Win=24820 Options=<nop,nop,sackOK,mss 1460> sun09 -> at204m02 TCP D=37519 S=22 Syn Ack=477982309 Seq=3227257622 Len=0 Win=24820 Options=<nop,nop,sackOK,mss 1460> at204m02 -> sun09 TCP D=22 S=37519 Ack=3227257623 Seq=477982309 Len=0 Win=24820 CSC 382: Computer Security Slide #54 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 CSC 382: Computer Security Slide #55 TCP Disconnection CSC 382: Computer Security Slide #56 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 CSC 382: Computer Security Slide #57 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 CSC 382: Computer Security Slide #58 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=<nop,nop,sackOK,mss 1460> at204m02 -> sun09 TCP D=33048 S=8192 Rst Ack=3848454476 Win=0 CSC 382: Computer Security Slide #59 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. CSC 382: Computer Security Slide #60 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. CSC 382: Computer Security Slide #61 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 CSC 382: Computer Security Slide #62 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. CSC 382: Computer Security Slide #63 Key Points 1. TCP/IP Layers: encapsulation/de-multiplexing 1. 2. 3. 4. Physical/Data Link: ethernet, PPP Network: IP, ICMP Transport: UDP, TCP 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. CSC 382: Computer Security Slide #64 References 1. 2. 3. 4. 5. 6. 7. 8. K. Egevang and P. Francis, “The IP Network Address Translator (NAT),” RFC 1631, http://www.ietf.org/rfc/rfc1631.txt, 1994. J.B. Postel, “Internet Protocol,” RFC 791, “http://www.ietf.org/rfc/rfc0791.txt, 1981. J.B. Postel, “Internet Control Message Protocol,” RFC 792, “http://www.ietf.org/rfc/rfc0792.txt, 1981. J.B. Postel, “Transmission Control Protocol,” RFC 793, http://www.ietf.org/rfc/rfc0793.txt, 1981. Ed Skoudis, Counter Hack, Prentice Hall, 2002. Richard Stevens, TCP/IP Illustrated, Vol. 1, Addison-Wesley, 1994. Richard Stevens, UNIX Network Programming, Vol. 1, PrenticeHall, 1998. Andrew Tannenbaum, Computer Networks, 4th edition, PrenticeHall, 2002. CSC 382: Computer Security Slide #65