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Understanding Firewall Types and Filtering Mechanisms in Network Security

Firewalls are crucial network appliances that filter traffic to protect networks from unauthorized access. They operate based on established criteria, examining packet headers and utilizing layers 3-5 of the OSI model. Various types of firewalls include static, dynamic, and stateful packet filtering, each with unique capabilities for managing access control lists (ACLs) and active sessions. Proxy servers may also be employed to reroute messages, adding another layer of security. Understanding these mechanisms is essential for effective network management and protection.

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Understanding Firewall Types and Filtering Mechanisms in Network Security

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  1. Internet Intranet Firewalls  network appliances to filter network traffic  filter on header (largely based on layers 3-5)

  2. Firewall Filtering Criteria Destination IP Source IP Destination Port Source Port Flag (TCP-only) ACK - acknowledge FIN - final PSH - push RST - reset SYN - synchronize URG - urgent

  3. Firewall Types  static packet filtering  dynamic packet filtering  stateful packet filtering  proxy server

  4.  static packet filtering packet  Network manager configures access control lists  Packets are compared to access control lists  Example: block

  5. 0 - echo reply 3 - destination unreachable 4 - source quench (from overloaded router) 5 - redirect (indicates a better path) 6 - echo request 9 - router advertisement (for new routers) 10 - router solicitation (host request for advertisement) 11 - time exceeded (packet header may include time) 12 - parameter problem (catch all for errors) 13 - time stamp request (checking link speed) 14 - time stamp reply Problems with static filtering • Blocking FIN scanning • Difficult to filter ICMP Internet Control Message Protocol - designed for Internet testing/maintenance - does not use ports - has type field

  6.  dynamic packet filtering ACLs packet  Includes all capabilities of static filtering  Maintains an Active Sessions Table  Example: block external FIN scan

  7.  stateful packet filtering ACLs Active Sessions Table packet  Includes all capabilities of dynamic filtering  Also “understands” certain application behavior  Example: better control over UDP, NFS, RPC

  8.  proxy server packet destination  Messages to destination IP are rerouted to a proxy  The proxy communicates on behalf of the destination  The proxy may also communicate with destination

  9. Firewalls in Practice

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