CS155 Firewalls. Simon Cooper CS155 - Firewalls 23 May of 30

Transcription

1 CS155 Firewalls Simon Cooper CS155 - Firewalls 23 May of 30

2 Plug! Building Internet Firewalls 2nd Edition, O Reilly Elizabeth D. Zwicky, Simon Cooper, D. Brent Chapman 2 of 30

3 What Is A Firewall? Literally? Prevents fire from spreading! The Castle Moat Analogy Restricts access from the outside Prevents attackers from getting too close Restricts people from leaving Logically; a separator, a restrictor and an analyzer Rarely a single physical object Any place where internal and external data can meet 3 of 30

4 Why Firewalls? There are a lot of people on the Internet Millions of people together; bad things happen True for cities; it is true for the Internet Exchange of information; Education, Business Recreation, Social and Political Want to do something useful with your computer However; Unsolicited attention and bugs 4 of 30

5 Bugs, Bugs, Bugs All programs contain bugs Larger programs contain more bugs Network protocols contain design weaknesses and implementation flaws Careful (defensive) programming & protocol design is hard 5 of 30

6 Where Do You Put A Firewall? Between insecure systems & the Internet To separate test or lab networks For networks with more sensitive data; Financial records Student grades Secret Projects Partner or joint venture networks 6 of 30

7 Firewall Design & Architecture Issues Least Privilege Defense in Depth Choke Point Weakest Link Fail-Safe Stance Universal Participation Diversity of Defense Simplicity 7 of 30

8 Firewall Architectures Internet Routes or blocks packets, as determined by site s security policy. Screening Router Internal Network Using A Screening Router to do Packet Filtering 8 of 30

9 Packet Filtering IP Packet version length type of service 16-bit total length (in bytes) 16-bit identification flags 13-bit fragmentation offset 8-bit Time To Live 8-bit protocol 16-bit header checksum 32-bit source IP address 32-bit destination IP address IP options (if any) 9 of 30

10 Packet Filtering UDP Packet Structure version length type of service 16-bit total length (in bytes) 16-bit identification flags 13-bit fragmentation offset 8-bit Time To Live 8-bit protocol 16-bit header checksum 32-bit source IP address 32-bit destination IP address IP options (if any) 16-bit source port number 16-bit destination port number 16-bit UDP length 16-bit UDP checksum Data (if any) 10 of 30

11 Packet Filtering TCP Packet Structure version length type of service 16-bit total length (in bytes) 16-bit identification flags 13-bit fragmentation offset 8-bit Time To Live 8-bit protocol 16-bit header checksum 32-bit source IP address 32-bit destination IP address IP options (if any) 16-bit source port number 16-bit destination port number 32-bit sequence number 32-bit acknowledgement number h length reserved Flags 16-bit window size 16-bit TCP checksum 16-bit urgent pointer 11 of 30

12 Packet Filtering Summary IP Source Address IP Destination Address Protocol (TCP, UDP, ICMP, etc.) TCP or UDP Source & Destination Ports TCP Flags (SYN, ACK, etc.) ICMP message type Packet Size 12 of 30

13 Router Knowledge Interface packet arrives on Interface packet will go out Is the packet in response to another one? How many packets have been seen recently? Is the packet a duplicate? Is the packet an IP fragment? 13 of 30

14 Filtering Example Inbound SMTP SMTP Client TCP Internet Firewall Internal Network 2 SMTP Server TCP Packet Direction Source Address Dest Address Protocol Dest Port 1 In TCP 25 2 Out TCP of 30

15 Filtering Example Outbound SMTP SMTP Server TCP 25 4 Internet Firewall Internal Network 3 SMTP Client TCP Packet Direction Source Address Dest Address Protocol Dest Port 3 Out TCP 25 4 In TCP of 30

16 Stateful or Dynamic Packet Filtering Client Dynamic Packet Filter Server Filter remembers this information Matches outgoing, so allowed in No match, so not allowed in SP = source port SA = source address DP = destination port DA = destination address 16 of 30 UDP SP = 3264 SA = DP = 1525 DA = UDP SP = 1525 SA = DP = 3264 DA = UDP SP = 1525 SA = DP = 2049 DA =

17 Network Address Translation (NAT) Port and Address Translation (PAT) Client : 1024 Client 2 Address Translation System : : 2027 Server : 1024 Inside Outside 17 of 30

18 Normal Fragmentation IP TCP DATA DATA IP TCP DATA IP...DATA IP DATA 18 of 30

19 Abnormal Fragmentation Normal IP TCP DATA IP MORE DATA Overlapping data Overlap IP TCP DATA IP DATA Overlapping headers Overlap IP IP TCP Fake TCP DATA DATA 19 of 30

20 Firewall Architectures Internet Firewall Screening Router Internal Network Bastion Host Screened Host Architecture 20 of 30

21 Bastion Host A secured system Disable all non-required services; keep it simple Install/modify services you want Run security audit to establish baseline Connect system to the network Be prepared for system to be compromised 21 of 30

22 Firewall Architectures Internet Bastion Host Exterior Router Perimeter Network Firewall Interior Router Internal Network Screened Subnet Architecture Using Two Routers 22 of 30

23 Firewall Architectures Internet Bastion Host Attacker Exterior Router X Packet Source: (claims to be) Destination: Perimeter Network Interior Router X Internal Network Source/Destination Address Forgery 23 of 30

24 Firewall Architectures Internet Firewall Dualhomed Host Internal Network Dual Homed Host Architecture 24 of 30

25 Proxies Application level; Dedicated proxy Circuit level; generic proxy Some protocols are natural to proxy SMTP ( ) NNTP (Netnews) DNS (Domain Name System) NTP (Network Time Protocol) SOCKS - a generic proxy WinSock - almost generic proxy for Microsoft 25 of 30

26 Firewall Architectures Customer traffic Internet Employee traffic Backup Exterior Router 1 Exterior Router 2 Bastion Host Perimeter Network Firewall Web Server Mail 1 Mail 2 Perimeter Network Interior Router 1 Firewall Perimeter Network Interior Router 2 Internal Network An Intricate Firewall Setup 26 of 30

27 Firewall Architectures Internet Web Server Firewall Real Database Server Database Server Internal Network A web server using a database on a perimeter network 27 of 30

28 Firewall Architectures Internet Firewall Warning: We recommend against this configuration. Web Server Database Server Internal Network A web and database server on an internal network 28 of 30

29 Problems With Firewalls They interfere with the Internet They don t solve the real problems; buggy software bad protocols Denial of Service They are becoming more complicated Many commercial firewalls permit very complex configurations 29 of 30

30 CS155 - Firewalls Simon Cooper <sc@sgi.com> Elizabeth D. Zwicky, Simon Cooper, D. Brent Chapman 30 of 30