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Firewalls

Learn about the fundamental functions of firewalls, including packet filtering, NAT, and application proxies. Understand firewall rules such as Allow-All and Deny-All to regulate inbound and outbound connections, detect abnormal traffic, and ensure network security.

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Firewalls

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  1. Firewalls • Check incoming and outgoing TCP/IP messages • Try to roughly identify abnormal traffic Regulate Inbound and Outbound connections - Make your machine “invisible” - Alert you to suspicious behavior Adjust Security Settings - Generic “sliding bar” - Precise rule definition Keep Logs - Useful for forensics after the attack - Not so important for home user

  2. Firewall Functions Basic Functions: Packet filtering - check network packet headers before admitting traffic Network Address Translation (NAT) - translate external IP addresses to internal IP addresses Application Proxy - Inspect application-specific header information Data Logging

  3. Firewall Rules • Packet filtering is performed based on a set of rules • Rules can be pre-defined of user-defined Default Strategy Allow-All - Allows all network packets except those explicitly denied by rules Deny-All - Denies all network packets except those explicitly allowed by rules • Deny-All is safer but more annoying • Allow-All may be set as the default, so check it

  4. Firewall Rule Structure Rules are defined based on information that the firewall has access to 1. TCP/IP header information - source address, dest. address, port number, etc. 2. User Information - Filtering may be different for each user 3. Content - Can tell this from the application protocol 4. Time - Some activity may not be allowed at certain times i.e. video downloads during the day

  5. Assume Deny-All Port/Content Users Time Action Port 80/except video All Always Allow Port 80/video Trainers Day Allow • Assume Allow-All Port/Content Users Time Action All ports, except 80 All Always Deny Port 80/video All,not Trainers Always Deny Port 80/video Trainers Night Deny Example Firewall Rules

  6. Weaknesses of Firewalls • Only looks at header information, not packet contents • - Cannot see malicious code in the content • Access control is not precise • - Close off entire ports/applications/addresses • Mistakes can be made in defining rules • - Assume Allow-All and forget to Deny • Good packets may be stopped • - Assume Deny-All and forget to Allow • - Many cryptic warnings may appear • - Need to understand rules to interpret warnings

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