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1 Chapter 8 Security Pt 2 IC322 Fall 2014 Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

2 Network Vulnerability Examples v Application Layer DNS Bandwidth Flooding, Poisoning, Man-in-the-middle Attacks v Transport Layer TCP SYN Flood Attack v Link Layer - Switched LAN Switch Poisoning Attack v Network Layer Datagrams 8.8 Operational security: firewalls and IDS 8-2

3 Application Layer - DNS v DDoS bandwidth-flooding attack sends a deluge of packets to each DNS root server, so the majority of legitimate DNS queries never get answered. v DNS poisoning attack, send bogus replies to a DNS server, tricking the server into accepting bogus records into its cache. v Man-in-the-middle attack intercept queries from hosts and returns bogus replies. v Exploit the DNS infrastructure launch a DDoS attack against a targeted host by causing DNS server to send a deluge of replies to the target. 8-3

4 Transport Layer - TCP v TCP SYN flood attack Server allocates and initializes connection variables and buffers in response to a received SYN. Server sends a SYNACK awaits an ACK. If no ACK received, after set time connection terminated and allocated resource reclaimed. v Attacker sends a large number of SYN segments, without completing the third handshake step. Can be amplified by sending the SYNs from multiple sources, creating a DDoS SYN flood attack v Defense: SYN cookies 8-4

5 Link Layer Switched LAN v Review When node A sends a frame to node B, and there is an entry for node B in the switch table, then the switch will forward the frame only to node B. If node C is running a sniffer, node C will not be able to sniff this A-to-B frame. However, node C can still sniff any broadcast messages. v Switch Poisoning Send tons of packets to the switch with many different bogus source MAC addresses, filling the switch table with bogus entries and leaving no room for the MAC addresses of the legitimate nodes. This causes the switch to broadcast most frames, which can then be picked up by the sniffer 8-5

6 Network Layer - Datagrams v Attackers, knowing the IP address range of your network, can easily send IP datagrams to addresses in your range. v These datagrams can do all kinds of devious things mapping your network with ping sweeps and port scans crashing vulnerable hosts with malformed packets flooding servers with a deluge of lcmp packets infecting hosts by including malware in the packets v Two popular defense mechanisms to malicious packet attacks Firewalls and Intrusion Detection Systems (loss). 8-6

7 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity 8.4 Securing 8.5 Securing TCP connections: SSL 8.6 Network layer security: IPsec 8.7 Securing wireless LANs 8.8 Operational security: firewalls and IDS 8-7

8 Firewalls firewall isolates organization s internal net from larger Internet, allowing some packets to pass, blocking others administered network trusted good guys firewall public Internet untrusted bad guys 8-8

9 Firewalls: why prevent denial of service attacks: v SYN flooding: attacker establishes many bogus TCP connections, no resources left for real connections prevent illegal modification/access of internal data v e.g., attacker replaces CIA s homepage with something else allow only authorized access to inside network v set of authenticated users/hosts three types of firewalls: v stateless packet filters v stateful packet filters v application gateways 8-9

10 Stateless packet filtering Should arriving packet be allowed in? Departing packet let out? v internal network connected to Internet via router firewall v router filters packet-by-packet, decision to forward/drop packet based on: source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits 8-10

11 Stateless packet filtering: example v example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23 result: all incoming, outgoing UDP flows and telnet connections are blocked v example 2: block inbound TCP segments with ACK=0. result: prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside. 8-11

12 Stateless packet filtering: more examples Policy No outside Web access. No incoming TCP connections, except those for institution s public Web server only. Prevent Web-radios from eating up the available bandwidth. Prevent your network from being used for a smurf DoS attack. Prevent your network from being tracerouted Firewall Setting Drop all outgoing packets to any IP address, port 80 Drop all incoming TCP SYN packets to any IP except , port 80 Drop all incoming UDP packets - except DNS and router broadcasts. Drop all ICMP packets going to a broadcast address (e.g ). Drop all outgoing ICMP TTL expired traffic 8-12

13 Access Control Lists v ACL: table of rules, applied top to bottom to incoming packets: (action, condition) pairs action source address allow allow outside of allow allow outside of dest address outside of outside of protocol source port dest port TCP > flag bit any TCP 80 > 1023 ACK UDP > UDP 53 > deny all all all all all all 8-13

14 Stateful packet filtering v stateless packet filter: heavy handed tool admits packets that make no sense, e.g., dest port = 80, ACK bit set, even though no TCP connection established: action source address dest address protocol source port dest port flag bit allow outside of TCP 80 > 1023 ACK v stateful packet filter: track status of every TCP connection track connection setup (SYN), teardown (FIN): determine whether incoming, outgoing packets makes sense timeout inactive connections at firewall: no longer admit packets 8-14

15 Stateful packet filtering v ACL augmented to indicate need to check connection state table before admitting packet action source address allow allow outside of dest address outside of proto source port dest port TCP > flag bit any check conxion TCP 80 > 1023 ACK x allow outside of UDP > allow outside of UDP 53 > x deny all all all all all all 8-15

16 Application gateways host-to-gateway telnet session gateway-to-remote host telnet session v filters packets on application data as well as on IP/TCP/ UDP fields. v example: allow select internal users to telnet outside. application gateway router and filter 1. require all telnet users to telnet through gateway. 2. for authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections 3. router filter blocks all telnet connections not originating from gateway. 8-16

17 Application gateways v filter packets on application data as well as on IP/TCP/UDP fields. host-to-gateway telnet session application gateway router and filter v example: allow select internal users to telnet outside gateway-to-remote host telnet session 1. require all telnet users to telnet through gateway. 2. for authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections 3. router filter blocks all telnet connections not originating from gateway. 8-17

18 Limitations of firewalls, gateways v IP spoofing: router can t know if data really comes from claimed source v if multiple apps need special treatment, each has own app. gateway v client software must know how to contact gateway. e.g., must set IP address of proxy in Web browser v filters often use all or nothing policy for UDP v tradeoff: degree of communication with outside world, level of security v many highly protected sites still suffer from attacks 8-18

19 Intrusion detection systems v packet filtering: operates on TCP/IP headers only no correlation check among sessions v IDS: intrusion detection system deep packet inspection: look at packet contents (e.g., check character strings in packet against database of known virus, attack strings) examine correlation among multiple packets port scanning network mapping DoS attack 8-19

20 Intrusion detection systems v multiple IDSs: different types of checking at different locations firewall internal network Internet IDS sensors Web server FTP server DNS server demilitarized zone 8-20

21 (summary) basic techniques... cryptography (symmetric and public) message integrity end-point authentication. used in many different security scenarios secure secure transport (SSL) IP sec operational security: firewalls and IDS 8-21

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