Advanced Network Security – Firewall Implementation and Design

1. COMP 4706 Advanced Network Security – Firewall Implementation and Design Term: January 2005 Dana Epp dana@scorpionsoft.com http://silverstr.ufies.org/blog/

2. Agenda • Review from last class • Group STRIDE analysis • Building a firewall port matrix • Introduction to NMAP • Hands on - Scanning ports with NMAP • Hands on – Creating basic firewall rules on Linux

3. Learning Outcomes On successful completion of this course, students will be able to: • Identify various types of firewalls and their functions, including which firewalls operate at which OSI protocol layer, and the basic variations of firewall architectures • Describe risk mitigation techniques to varying threats with the use of different firewall architectures • Demonstrate the ability to design and deploy policies on a firewall

4. Basic Types of Firewalls • Packet filtering firewalls • Stateful packet inspection firewalls • Application proxies • Hybrids

5. Packet filter A packet filter firewall is the simplest type of firewall. Dealing with each individual packet, the firewall applies its rule set to determine which packet to allow or disallow. The firewall examines each packet based on the following criteria: • Source IP address • Destination IP address • TCP/UDP source port • TCP/UDP destination port

6. Packet Filter - Pros • They are fast because they operate on IP addresses and TCP/UDP port numbers alone, ignoring the data contents (payload) of packets. • Due to the fact that packet payload is ignored, application independence exists. • Least expensive of the three types of firewalls. • Packet filtering rules are relatively easy to configure. • There are no configuration changes necessary to the protected workstations.

7. Packet filters - Cons • Allow a direct connection between endpoints through the firewall. This leaves the potential for a vulnerability to be exploited. • There is no screening of packet payload available. It is impossible to block users from visiting web sites deemed off limits, for example. • Logging of network traffic includes only IP addresses and TCP/UDP port numbers, no packet payload information is available. • Complex firewall policies are difficult to implement using filtering rules alone. • There is a reliance on the IP address for authentication rather than user authentication. • Dynamic IP addressing schemes such as DHCP may complicate filtering rules involving IP addresses.

8. Stateful packet inspection Examines the contents of packets rather than just filtering them; that is, they consider their contents as well as their addresses. Stateful packet inspection firewalls also take into account the state of the connections they handle so that, for example, a legitimate incoming packet can be matched with the outbound request for that packet and allowed in.

9. Stateful packet inspection - Pros • Offers improved security over basic packet filters due to packet examination. • Offers a degree of application independence, based on level of stateful packet examination. • Better logging of activities over basic packet filters. • Good performance. • Configuration changes to the protected workstations are unnecessary.

10. Stateful packet inspection - Cons • Allow a direct connection between endpoints through the firewall. This leaves the potential for a vulnerability to be exploited. • No hiding of your private systems. • Setting up stateful packet examination rules is more complicated. • Only supported protocols at the application layer. • No user authentication.

11. Application proxies An application proxy is a program running on the firewall that emulates both ends of a network connection. One can think of it as a sort of "translator" in-between the two computers communicating.

12. Application proxies - Pros • Firewall does not let end points communicate directly with one another. Thus a vulnerability in a protocol which could slip by a packet filter or stateful packet inspection firewall could be overcome by the proxy program. • Has the best content filtering capability. • Can hide private systems. • Robust user authentication. • Offers the best logging of activities. • Policy rules are usually easier than packet filtering rules.

13. Application proxies - Cons • Performance problems; much slower than the other two • Must have a proxy for every protocol. Failure to have a proxy may prevent a protocol from being handled correctly by the firewall. • TCP is the preferred transport. UDP may not be supported. • Limited transparency, clients may need to be modified. Setting up the proxy server in a browser, for example. • No protection from all protocol weaknesses.

14. OSI – Open System Interconnect

15. TCP/IP Protocol Architecture

16. Three way TCP handshake

17. Common Ports and Services • Windows: %windir%\System32\drivers\etc\services • Linux:/etc/services • Examples: SMTP = port 25 HTTP = port 80 POP3 = port 110 PPTP = port 1723

18. The STRIDE Threat Model • Spoofing identity • Attacker obtains something that enables authentication • Tampering with data • Unauthorized change made to stored or in-transit information • Repudiation • Performing an illegal operation in a system that lacks the ability to trace such operations • Information disclosure • Exposing critical information to unauthorized individuals • Denial of Service (DoS) • Denies service to others • Elevation of privileges • Attacker exploits a weakness to gain greater privileges on a system than were intended

19. Ranking and Prioritizing Threats • Chance of attack occurring • 1 = high 10 = low • How much effort/cost/time is needed to launch the attack? • What is the cost/damage if it occurs? • 1 = little 10 = massive • RISK = Damage / Chance • Goal is to reduce risk • Do high risk items first

20. How to Respond to Threats • Do nothing. • Inform the user of the threat. • Remove the problem. • Fix the problem.

21. Defense in Depth • Assume external systems are insecure • “We’re secure, we have a firewall” *ugh* • Assume your system(s) is the last thing standing • Plan on failure • More layers of security means more work to compromise a target • Threat risk goes down as threat difficulty goes up • Never depend on security through obscurity

22. Group STRIDE Analysis

23. Building a firewall port matrix • Determine trust zones • Determine ports that need opening • Determine packet type (tcp/udp) • Determines direction of packet flow • Determine any limitations you can set on src/dst

24. LUNCH

25. Introduction to NMAP • Can scan networks to find active (online) hosts • Can scan hosts to find open ports • Can send crafted packets to fingerprint the operating system • Can be used defensively to identify weaknesses that need to be corrected, or offensively by an attacker to probe for vulnerabilities to exploit.

26. Interesting NMAP options • -v = Verbose logging • -O = OS fingerprinting • -sS = SYN stealth scan • -P0 = Scan without ping probes • nmap –v –O –sS your.host.com

27. Introduction to iptables • 3rd generation firewall on Linux • Supports basic packet filtering as well as connection state tracking • For our needs for this course, we will use simple/basic packet filtering

28. Introduction to iptables # Sample firewall – incomplete… do not use. For discussion only IPTABLES=/sbin/iptables ANY=“0.0.0.0/0” ETHIP=“10.10.1.1” ADMINNOC=“10.10.1.250” # Flush chains $IPTABLES --flush # Set default policies $IPTABLES -P INPUT ACCEPT $IPTABLES -P OUTPUT ACCEPT $IPTABLES -P FORWARD ACCEPT # Allow SSH from admin NOC $IPTABLES -A INPUT -p tcp -s $ADMINNOC --sport 1024:65534 --dport 22 -j ACCEPT $IPTABLES -A OUTPUT -p tcp -d $ADMINNOC -sport 22 --dport 1024:65534 -j ACCEPT # Allow Web access $IPTABLES -A INPUT -p tcp --dport 80 -j ACCEPT # Allows secure web access $IPTABLES -A INPUT -p tcp --dport 443 -j ACCEPT $IPTABLES -A INPUT -j DROP

29. Dropping vs Rejecting Packets • Rejecting packets COULD resource starve your system • Dropping packets could cause network diagnostic hell for the other end if you don’t respond ‘nicely’ • Dana’s Law: It is better to DROP packets and buy your favorite network admin a beer than to REJECT and have alarms go off at 2 in the morning during a DoS, waking you up.

30. Hands on LAB

31. Good reading • IPTables Packet Filtering HOWTOhttp://netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html • Building Internet FirewallsISBN:1-56592-124-0 • Linux FirewallsISBN: 0-7357-0900-9 • Threat ModelingISBN: 0-7356-1991-3

32. Any Questions?