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Internet and Intranet Fundamentals

Internet and Intranet Fundamentals. Class 9 Session A. Topics. Firewalls (continued). Firewalls (Continued). Bastion Hosts Packet Filtering. Bastion Hosts. Public Presence on the Internet The “Lobby” Analogy Public Exposure Implies Increased Security Requirements

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Internet and Intranet Fundamentals

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  1. Internet and Intranet Fundamentals Class 9 Session A

  2. Topics • Firewalls (continued)

  3. Firewalls(Continued) • Bastion Hosts • Packet Filtering

  4. Bastion Hosts • Public Presence on the Internet • The “Lobby” Analogy • Public Exposure Implies Increased Security Requirements • focus special attention on building a Bastion host • host security • some principles apply to other hosts as well

  5. Bastion HostsVarious Types • Non-routing Dual-homed Hosts • make sure they are non-routing! • Victim Machines • sacrificial goat • don’t let users put valuables on them • Internal, semi-Bastion Hosts • inside the firewall • communicate with external bastion

  6. Bastion HostsGeneral Design Guidelines • Minimize the Number of Services Provided • keep it simple, scholar • server software may have bugs that can be exploited • Expect Bastion Host to be Compromised • expect the worst and plan for it • most likely to be attacked • bastion host considered untrusted host

  7. Bastion Hosts • What Platform? • Unix, NT, etc. ? • Criteria • your experience • firewall tools availability • Class of Machine • minimal • not a supercomputer • RAM more important than CPU

  8. Bastion HostsLocation • Physical Location • safe • Network Location • preferably on a perimeter network • or a network not susceptible to spoofing • ATM, Ethernet switch

  9. Bastion HostServices • Proxy and Relay Services • HTTP Proxy • SMTP Server • NNTP Server • FTP Server • Public Services • HTTP • SMTP

  10. Bastion HostsConstruction Steps • Secure the Machine • start with minimal, clean operating system • fix all known system bugs • use a security checklist • safeguard the system logs • requires lots of logging

  11. Bastion HostsConstruction Steps • Disable Non-required Services • Install or Modify Services • Reconfigure Machine from Development to Deployment • Perform Security Audit • Connect Machine to Network

  12. Packet FilteringTopics • What is it? • Advantages and Disadvantages • Configuring a Packet Filtering Router • Various Kinds of Filtering

  13. Packet FilteringWhat is it? • Selectively reject IP packets based on: • source address • destination address • incoming physical port • tcp application port

  14. Packet FilteringAdvantages and Disadvantages • Advantages • one router protects an entire network • doesn’t require user knowledge or cooperation • widely available • Disadvantages • current filtering tools not perfect • can be hard to configure, test, and maintain • may have bugs • some protocols don’t lend themselves to filtering

  15. Packet FilteringConfiguring a PF Router • Protocols Bidirectional • Inbound vs. Outbound Semantics • packets vs. services • think “packets” • Default Security Policy • permit or deny? • Returning ICMP Error Codes • destination unreachable, for example

  16. Various Kinds of Filtering • Rules • Direction • Source Address • Destination Address • ACK Set • Action

  17. Various Kinds of FilteringRules

  18. Various Kinds of FilteringRisks of Address Filtering • Address Forgery • source • does not hope to get any packets back • man-in-the-middle • must intercept return packets • must alter network topology to get in the middle

  19. Various Kinds of FilteringFiltering by Service • More Complicated • TELNET • outgoing • local host’s IP source address • remote host’s IP destination address • TCP packet type • TCP destination port is 23 • content: your keystrokes

  20. Various Kinds of FilteringFiltering by Service • TELNET • incoming • remote host’s IP source address • local host’s IP destination address • TCP packet type • TCP source port is 23 • TCP destination port is same as prior source port • ACK set

  21. Various Kinds of FilteringFiltering by Service • TELNET • Rules • permit output on port 23 • permit inbound on port 23 if ACK is set • deny both outbound and inbound for everything else • default rule • Risks • some other service on port 23?

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