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Firewalls

Firewalls. Functions and Types (Deployment) Topologies Best Practices. Firewalls - Functions. Firewall is a system that stands on the edge of a private network inspecting all traffic between the private network and the outside (untrusted world).

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Firewalls

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  1. Firewalls • Functions and Types • (Deployment) Topologies • Best Practices

  2. Firewalls - Functions • Firewall is a system that stands on the edge of a private network inspecting all traffic between the private network and the outside (untrusted world). • All communications must pass through firewalls. • Firewalls must permit only authorized traffic. • Firewalls must withstand attacks on themselves. • Firewalls work at Network Layer / IP layer.

  3. Firewalls - Types • packet filters • filtering based on TCP/IP headers (source/dest/ addresses, source/dest. port, ICMP message type etc.) and incoming interface • advantages • good performance and affordable costs • transparency • disadvantages • direct connections permitted • poor scalability • vulnerable to spoofing • large port ranges open (ftp server to client)

  4. Firewalls - Types • application gateways • a.k.a. proxy gateways; proxy servers • decisions based on packet info. at all 7 layer • advantages • application-awareness leads to tighter verification. • disadvantages • slow; • non-transparent • need proxies for each appln.

  5. Firewalls - Types • circuit-level gateways • similar to appln. level gateways but not application-aware. • get info. from clients on application but otherwise act as relays. • suitable for custom clients (e.g. SOCKS proxy)

  6. Stateful firewalls • similar to packet filters but state-aware • filtering based on state info. as well (SYN, ACK, seq. no. and other data in the header). • state of each session is tracked - open/close decisions as required. (connection table to maintain sessions in progress) • essentially combine speed and flexibility of packet filters with application level security of proxies.

  7. Best Practices • Expressly permit and implicitly deny • block outbound public server access • stateful firewalls can do this

  8. Topologies • Basic filtering router (ACLs) • single point of filtering and failure • Classic dual-router DMZ • Internal (detailed ACL) and external (Basic ACL) • Stateful firewall DMZ • stateful filtering instead of internal detailed ACL. • 3-interface firewall • public servers hang off firewall • multi-firewall • graded zone.

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