Firewalls

1. public Internet administered network firewall Firewalls Isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall Network Security

2. Firewalls: Why Prevent denial of service attacks: • Denial-of-Service (DoS) attack: • Send many fake requests to congest link or consume server resource (CPU, memory) • SYN flooding: • attacker sends many SYNs to victim; victim has to allocate connection resource; victim has no resource left for real connection requests any more. • Usually with spoofed source IP address Prevent illegal modification/access of internal data. • e.g., attacker replaces CIA’s homepage with something else Network Security

3. Firewalls: Why Allow only authorized access to inside network • Set of authenticated users • Set of authorized IP addresses Two types of firewalls: • application-level • Checking application level data • packet-filtering • Checking TCP or IP packets only Network Security

4. internal network connected to Internet via router firewall 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 Packet Filtering Should arriving packet be allowed in? Departing packet let out? Network Security

5. Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. All incoming and outgoing UDP flows and telnet connections are blocked. Example 2: Block inbound TCP segments with SYN=1. Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside. Packet Filtering Example of Windows XP service pack 2 firewall (control panelsecurity centerwindows firewall) Network Security

6. Stateless packet filtering: more examples Network Security

7. DMZ A Real Example: Lab setup when I was a PhD student • Gateway: a cheap Linux machine runs Iptables • Web server: Linux machine runs Apache web server • Main server: Linux machine runs SSH, Sendmail (SMTP and IMAPS) • Allow a specific machine outside to print to main server’s printer Network Security

8. DMZ Traffic Allowance Policy: • (1). Internet ==> LAN    To Main Server: SSH, SMTP,IMAP+SSL, LPD request from a trusted IP. • (2). Internet ==> DMZ Web Server ---- SSH, Web request. • (3). Internet ==> Gateway ----    SSH only (for admin) • (4). DMZ Web Server==> LAN, Internet     SSH to Internet and only to Main Server in LAN; DNS request; WWW/FTP request to Internet. • (5). LAN ==> Internet, DMZ WebServer ---- SSH, DNS, WWW/FTP, SMTP, RealPlay. Network Security

9. DMZ • HTTP forwarding from Internet to DMZ web server (port 80) • iptables -t nat -A PREROUTING -p tcp --sport $UNPRIVPORTS -d $out.IP --dport 80 -j DNAT --to-destination $DMZ_SERVER • iptables -A FORWARD -o $eth2 -p tcp --sport $UNPRIVPORTS -d $DMZ_SERVER --dport 80 -m state --state NEW -j ACCEPT • SMTP from internal mail server to external Mail Server • iptables -A FORWARD -i $eth1 -o $eth0 -p tcp -s $LAN_MAINSERVER --sport $UNPRIVPORTS --dport 25 -m state --state NEW -j ACCEPT • Allow Telnet to outside from LAN • iptables -A FORWARD -i $LAN_IF -o $EXT_IF -p tcp --sport $UNPRIVPORTS --dport 23 -m state --state NEW -j ACCEPT Network Security

10. Filters packets on application data as well as on IP/TCP/UDP fields. Example: allow select internal users to telnet outside. Application gateways gateway-to-remote host telnet session host-to-gateway telnet session 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. • Example: block user access to know porn websites • Check if the Web URL is in a “black-list” Network Security

11. IP spoofing: router can’t know if data “really” comes from claimed source SYN flood attack UDP traffic client software must know how to contact application gateway. e.g., must set IP address of proxy in Web browser Speed constraint on high-bandwidth link Application-level firewall is time consuming filters often use all or nothing policy for UDP Usually most incoming UDP ports are blocked The trouble caused to real-time Internet video Limitations of firewalls and gateways Network Security

12. tradeoff: degree of communication with outside world, level of security Trend --- remote office Blurred boundary between inside <-> outside Employee laptop threat many highly protected sites still suffer from attacks Limitations of firewalls and gateways Network Security

13. Internet security threats Mapping: • before attacking: “case the joint” – find out what services are implemented on network • Use ping to determine what hosts have addresses on network • Port-scanning: try to establish TCP connection to each port in sequence (see what happens) • nmap (http://www.insecure.org/nmap/) mapper: “network exploration and security auditing” Countermeasures? Network Security

14. Internet security threats Mapping:countermeasures • record traffic entering network • look for suspicious activity (IP addresses, pots being scanned sequentially) • Firewall to block incoming TCP/SYN to ports or computers not providing the services • Block ping traffic Network Security

15. src:B dest:A payload Internet security threats Packet sniffing: • broadcast media • promiscuous NIC reads all packets passing by • can read all unencrypted data (e.g. passwords) • e.g.: C sniffs B’s packets C A B Countermeasures? Network Security

16. src:B dest:A payload Internet security threats Packet sniffing: countermeasures • all hosts in orgnization run software that checks periodically if host interface in promiscuous mode. • one host per segment of broadcast media (switched Ethernet at hub) C A B Network Security

17. src:B dest:A payload Internet security threats IP Spoofing: • can generate “raw” IP packets directly from application, putting any value into IP source address field • receiver can’t tell if source is spoofed • e.g.: C pretends to be B C A B Countermeasures? Network Security

18. src:B dest:A payload Internet security threats IP Spoofing: egress filtering • routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network) • great, but egress filtering can not be mandated for all networks C A B Network Security

19. SYN SYN SYN SYN SYN SYN SYN Internet security threats Denial of service (DOS): • flood of maliciously generated packets “swamp” receiver • Distributed DOS (DDOS): multiple coordinated sources swamp receiver • e.g., C and remote host SYN-attack A C A B Countermeasures? Network Security

20. SYN SYN SYN SYN SYN SYN SYN Internet security threats Denial of service (DOS): countermeasures • filter out flooded packets (e.g., SYN) before reaaching host • Cooperation with source routers • Detect spoofed SYN based on TTL values • traceback to source of floods (most likely an innocent, compromised machine) C A B Network Security

21. Internet e-mail encryption scheme, de-facto standard. uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. provides secrecy, sender authentication, integrity. inventor, Phil Zimmerman, was target of 3-year federal investigation. ---BEGIN PGP SIGNED MESSAGE--- Hash: SHA1 Bob:My husband is out of town tonight.Passionately yours, Alice ---BEGIN PGP SIGNATURE--- Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- Pretty good privacy (PGP) A PGP signed message: Network Security

22. provides transport layer security to any TCP-based application using SSL services. e.g., between Web browsers, servers for e-commerce (shttp) security services: server authentication, data encryption, client authentication (optional) Secure sockets layer (SSL) Application Application SSL sublayer SSL socket TCP TCP TCP socket IP IP TCP API TCP enhanced with SSL Network Security

23. transport layer security to any TCP-based app using SSL services. used between Web browsers, servers for e-commerce (shttp). security services: server authentication data encryption client authentication (optional) server authentication: SSL-enabled browser includes public keys for trusted CAs. Browser requests server certificate, issued by trusted CA. Browser uses CA’s public key to extract server’s public key from certificate. check your browser’s security menu to see its trusted CAs. Secure sockets layer (SSL) Network Security

24. Encrypted SSL session: Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. Using private key, server decrypts session key. Browser, server know session key All data sent into TCP socket (by client or server) encrypted with session key. SSL: basis of IETF Transport Layer Security (TLS). SSL can be used for non-Web applications, e.g., IMAP. Client authentication can be done with client certificates. Not widely used since too many clients SSL (continued) Network Security

25. 1. Handshake: Bob establishes TCP connection to Alice authenticates Alice via CA signed certificate creates, encrypts (using Alice’s public key), sends master secret key to Alice nonce exchange not shown SSL: three phases TCP SYN TCP SYNACK TCP ACK SSL hello certificate create Master Secret (MS) KA+(MS) decrypt using KA- to get MS Network Security

26. 2. Key Derivation: Alice, Bob use shared secret (MS) to generate 4 keys: EB: Bob->Alice data encryption key EA: Alice->Bob data encryption key MB: Bob->Alice MAC key (message authentication code) MA: Alice->Bob MAC key encryption and MAC algorithms negotiable between Bob, Alice why 4 keys? Different keys by each person Different keys for encryption and integrity checking SSL: three phases Network Security

27. 3. Data transfer . . d d d H(d) H(d) H(d) H( ) H( ) SSL: three phases TCP byte stream b1b2b3 … bn MB d block n bytes together compute MAC EB encrypt d, MAC, SSL seq. # (why ssl#?) SSL seq. # SSL record format Type Ver Len encrypted using EB unencrypted Network Security

28. network-layer secrecy: sending host encrypts the data in IP datagram TCP and UDP segments; ICMP and SNMP messages. network-layer authentication destination host can authenticate source IP address two principal protocols: authentication header (AH) protocol encapsulation security payload (ESP) protocol for both AH and ESP, source, destination handshake: create network-layer logical channel called a security association (SA) each SA unidirectional. uniquely determined by: security protocol (AH or ESP) source IP address 32-bit connection ID IPsec: Network Layer Security Network Security

29. provides source authentication, data integrity, no confidentiality AH header inserted between IP header, data field. protocol field: 51 intermediate routers process datagrams as usual AH header includes: connection identifier authentication data: source- signed message digest calculated over original IP datagram. next header field: specifies type of data (e.g., TCP, UDP, ICMP) IP header data (e.g., TCP, UDP segment) AH header Authentication Header (AH) Protocol Network Security