1 Computer System Evolution Central Data Processing System: - with directly attached peripherals (card reader, magnetic tapes, line printer). Local Area Networks: - connects PC’s (in “terminal emulation” mode), remote terminals (next building) and mini-computers. Premises Network: - connects LANs and LAN-attached devices to each other. Enterprise-wide Network: - leased data lines (T1, DS-3) connect various offices. Internet Connectivity: - initially for , now for Web access, e-commerce,.... Makes the world accessible, but now the world also has access to you.

2 Agency Virtual Private Network LANs at Agency Offices across Georgia State WWW Gateway State Internet Citizens Contractors City & County Governments Agency Gateway & Web Server Non-Agency State Server Private Virtual Connection Agency Server Schools Libraries Kiosks Connectivity Provided by the Georgia Backbone Network Other Agencies WWW

Agency Firewall - Protects Agency Subnets from Unwanted Connections Agency Firewall - Protects Agency Subnets from Unwanted Connections Subnet 1 Subnet 2 Gate- way WAN Firewalls (and many routers) can reject: Packets with certain source and destination addresses Packets with certain high-level protocols (UDP, Telnet) Proxy Servers - for specific applications messages assembled and inspected, then passed to internal server machine. Prevent Cyber Loafing - Exploring the Internet for fun. Gate- way 3

4 Application Layer (HTTP) Transport Layer (TCP,UDP) Network Layer (IP) E'net Data Link Layer Ethernet Phys. Layer Network Layer E'net Data Link Layer E'net Phys. Layer Network Layer Web Server Browser Router-Firewall can drop packets based on source or destination, ip address and/or port Application Layer (HTTP) Transport Layer (TCP,UDP) Network Layer (IP) Token Ring Data-Link Layer Token Ring Phys. Layer IP Address IP Address Port 80 Port Segment No. Token Ring Data Link Layer Token Ring Phys. Layer

Application Layer (HTTP, FTP, TELNET, SMTP) Transport Layer (TCP, UDP) Network Layer (IP) E'net Data Link Layer E'net Phys. Layer Transport Layer (TCP, UDP) Network Layer (IP) E'net Data Link Layer E'net Phys. Layer Process Transport or App.-Layer Gateway, or Proxy Application Layer (HTTP(HTTP, FTP, TELNET, SMTP) Transport Layer (TCP,UDP) Network Layer (IP) TR Data Link Layer TR Phys. Layer Transport Layer (TCP, UDP) Network Layer (IP) TR Data Link Layer TR Phys. Layer 5

Policy No outside Web access. Outside connections to Public Web Server Only. Prevent Web-Radios from eating up the available bandwidth. Prevent your network from being used for a Smuft DoS attack. Prevent your network from being tracerouted or scanned. Firewall Setting Drop all outgoing packets to any IP, Port 80 Drop all incoming TCP SYN packets to any IP except 130:207: , port 80 Drop all incoming UDP packets - except DNS and Router Broadcasts. Drop all ICMP packets going to a “broadcast” address ( or ). Drop all incoming ICMP, UDP, or TCP echo- request packets, drop all packets with TTL < 5. 6

Firewall Attacks IP Internal-Address Spoofing. Source Routing (External Spoof). Tiny Fragment Attacks. 2nd-Fragment Probes. SYN-ACK Probes. Firewall Defense Drop all incoming packets with local address. Drop all IP packets with Source-Routing Option. Drop all incoming packets with small offset. Assemble IP fragments (hard work). Be “Stateful” -keep track of TCP outgoing SYN packets (start of all TCP connections) (hard work). 7

A Firewall is a single point that a Network Administrator can control, even if individual computers are managed by workers or departments Over half of corporate computer misfeasance is caused by employees who are already behind the main firewall. Solution 1 - isolate subnets with firewalls (usually routers or Ethernet switches with “filter” capabilities). Protect Finance from Engineering. Solution 2 - implement “IP Chains” to limit access to individual computers at the lowest protocol level possible, to specific hosts and subnets. 8

IP Chains /etc/hosts.deny ALL:ALL /etc/hosts.allow in.telnetd: in.ftpd: UNIX and Linux computers allow network contact to be limited to individual hosts or subnets ( means any). Above, telnet connection is available to all on the subnet, and a single off-subnet host, FTP service is available to only to two local hosts,.19 and.102. The format for each line is “daemon:host-list” 9

Router Setup with Network Address Translation (NAT) Addresses and reserved for private networks.

Internet Router with NAT that Masquerades could be a “dual-homed bastion host” Host Host Host Host Web Server port 80 FTP Server port 23 FTP Client To :x from :23 To :x from :23 To :23 from :x To :23 from :x Note: x is a high port number, ,535

Internet Router with NAT that Masquerades Host Web Client Host Host Web Server port 80 FTP Server port 23 Web Host To :80 from :x To :80 from :x To :x from :80 To :x from :80