1. Firewalls

8. Packet Filters

10. Packet-Filtering Examples Connection to our SMTP port * * 25 OUR-GW Allow We don’t trust these people * SPIGOT * * Block comment Port Theirhost Port Ourhost Action

11. default * * * * Block comment Port Theirhost Port Ourhost Action Connection to their SMTP 25 * * * Allow comment Port Theirhost Port Ourhost Action

14. Application Level Gateway (Proxy server) Internal host (private n/w) Application level GW Inside connection External host (part of internet) Outside connection User’s illusion (HTTP,FTP,TELNET,SMTP)

16. Circuit Level Gateway out out out in in in Inside host Inside connection Outside host Outside connection Circuit-level gateway

20. Firewall Configurations

31. Access Control Matrix

33. Access control list Decomposition of the matrix by columns Access control list for Segment B: Process2(Read) Access control list for Segment A: Process1(Read,Write) Access control list for program1: Process1(Read,Executre)

35. Capability list Decomposition of the matrix by rows Capability list for process2: Segment B (Read) Capability list for process1: Program1(Read,Executre) Segment A (Read, Write)