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Firewalls

Firewalls. Securing the Network Perimeter. Network Preliminaries. Layers Headers IP addresses Ports. Many Steps. Layers. Application. Transport. Network. Link. Physical. Layers. Package and Process. Congestion control, reliability. Routing, Fragmentation.

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Firewalls

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  1. Firewalls Securing the Network Perimeter

  2. Network Preliminaries • Layers • Headers • IP addresses • Ports

  3. Many Steps

  4. Layers Application Transport Network Link Physical

  5. Layers Package and Process Congestion control, reliability Routing, Fragmentation Collision handling, error correction Data to signals and back

  6. Layers HTTP, SMTP, DNS TCP, UDP IP, BGP Ethernet, 802.11 Physical

  7. Eth IP TCP Headers <html>blah blah TCP Header: Sequence no., ACK no. IP Header: IP address, port no. Eth Header: Ethernet address

  8. A B C D 192 . 118 . 64 . 13 A B C D 192 . 118 . 64 . * IP Addresses • 32 bits • Four parts • Ownership • A C-class network • Starting bits: 110 • What values?

  9. Classes ipindex.net • Class A 0. 0. 0. 0 = 00000000.00000000.00000000.00000000 127.255.255.255 = 01111111.11111111.11111111.11111111 0nnnnnnn.HHHHHHHH.HHHHHHHH.HHHHHHHH • Class B 128. 0. 0. 0 = 10000000.00000000.00000000.00000000 191.255.255.255 = 10111111.11111111.11111111.11111111 10nnnnnn.nnnnnnnn.HHHHHHHH.HHHHHHHH • Class C 192. 0. 0. 0 = 11000000.00000000.00000000.00000000 223.255.255.255 = 11011111.11111111.11111111.11111111 110nnnnn.nnnnnnnn.nnnnnnnn.HHHHHHHH Also reserve two addresses: first and last

  10. Address Mask • Rules • Class A • First bit 0 • 8 total bits • Class B • First bits 10 • 16 total bits • Class C • First bits 110 • 24 total bits • How many networks? • Available addresses per n/w?

  11. Ports • Different services • Web, email, ssh, etc. • Known server ports • HTTP (80), ssh (22), DNS (53) • Client ports • Anything between 1024 and 65535

  12. Perimeter Security • Barriers keep others out • Examples?

  13. Network Firewalls

  14. Benefits • Enforcement point • Corporate policies • Security policies • A single point of observation • Logging and auditing • Forensics • IDS

  15. Benefits • Cost & Complexity • A single point of focus • Reduced need for specialized security internally • Simple security principle • Don’t give away more information than necessary • Is this security by obscurity?

  16. Limitations • Obvious work-arounds • physical media • other networks (POTS) • attacks over allowed protocols • insider attacks • Central point of failure • Hard tasks • malware ID

  17. Policies • Heart of firewall • What services are available? • In which directions? • Basics • Default Allow • Explicitly dis-allow “dangerous” services • Far too easy to circumvent • Default Deny • Explicitly allow OK services

  18. Methods • Packet Filtering (low level) • Proxy service (high level) • The firewall handles transactions with the outside

  19. Basic Packet Filtering • Direct Access • simple to setup • But rules can be complex • Hard to verify • Exceptions are hard to manage • Flexible rule sets needed

  20. Filtering Example • Security policy • Allow FTP access to the download server • Otherwise no FTP access is allowed. • Rules • Deny list: * * 21 • Allow list: * 128.119.245.66 21 • Outgoing • * *.badstuff.com *

  21. mail, dns 195.55.55.10 Internet UNIX Filtering Class C network

  22. UNIX Filtering ipfwadm -F -f ipfwadm -F -p deny ipfwadm -F -i m -b -P tcp -S 0.0.0.0/0 1024:65535 -D 195.55.55.10 25 ipfwadm -F -i m -b -P tcp -S 0.0.0.0/0 1024:65535 -D 195.55.55.10 53 ipfwadm -F -i m -b -P udp -S 0.0.0.0/0 1024:65535 -D 195.55.55.10 53 ipfwadm -F -a m -S 195.55.55.0/24 -D 0.0.0.0/0 -W eth0 flush (-f) the Forwarding (-F) rules Policy is set to default deny Match bi-dir. Input: masq. TCP Source Ports Destination Append

  23. mail, dns 195.55.55.10 Internet http ssh 195.55.55.20 Making a change • New Web server Class C network 131.114.55.*

  24. Internet http ssh 195.55.55.20 Let’s update the rules ipfwadm -F -f ipfwadm -F -p deny ipfwadm -F -i m -b -P tcp -S 0.0.0.0/0 1024:65535 -D 195.55.55.10 25 ipfwadm -F -i m -b -P tcp -S 0.0.0.0/0 1024:65535 -D 195.55.55.10 53 ipfwadm -F -i m -b -P udp -S 0.0.0.0/0 1024:65535 -D 195.55.55.10 53 ipfwadm -F -a m -S 195.55.55.0/24 -D 0.0.0.0/0 -W eth0 PUT COMMANDS HERE 131.114.55.*

  25. The End

  26. Cisco Rules • access-list 101 permit tcp any host 195.55.55.10 eq smtp • access-list 101 deny ip 195.55.55.0 0.0.0.255 Match bi-dir. Input: masq. TCP Source

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