Firewalls (March 4, 2015) © Abdou Illia – Spring 2015

2 Test your Firewall knowledge Which of the following is true about firewalls? a)A firewall is a hardware device b)A firewall is a software program c)Firewalls could be hardware or software Which of the following is true about firewalls? a)They are used to protect a whole network against attacks b)They are used to protect single computers against attacks c)Both a and b.

3 Test your Firewall knowledge (cont) Which of the following is true about firewalls? a)They are configured to monitor inbound traffic and protect against attacks by intruders b)They are configured to monitor outbound traffic and prevent specific types of messages from leaving the protected network. c)Both a and b

4 Firewall: definition Hardware or software tool used to protect a single host 1 or an entire network 2 by “sitting” between a trusted network (or a trusted host) and an untrusted network Applying preconfigured rules and/or traffic knowledge to allow or deny access to incoming and outgoing traffic 1 Host-based or personal firewall2 network-based firewall Untrusted network Trusted network PC with Host- based Firewall Network-Based Firewall

5 Questions What is the main advantage of having a host-based firewall in addition to having a network-based one? Answer:_________________________________________ What kind of security issue could be associated with having host-based firewall on users PCs? Answer:__________________________________________ Untrusted network Trusted network PC with Host- based Firewall Network-Based Firewall

6 Firewall Architecture Most firms have multiple firewalls. Their arrangement is called the firm’s firewall architecture Internet Main Border Firewall x Subnet Marketing Client on x Subnet Accounting Server on x Subnet Public Webserver SMTP Application Proxy Server HTTP Application Proxy Server External DNS Server Screening Router Firewall Internal Firewall Host Firewall Host Firewall Server on x Subnet Host Firewall Demilitarized Zone (DMZ)

7 Firewall Architecture Internet Main Border Firewall x Subnet Marketing Client on x Subnet Accounting Server on x Subnet Public Webserver SMTP Application Proxy Server HTTP Application Proxy Server External DNS Server Screening Router Firewall Internal Firewall Host Firewall Host Firewall The DMZ is a subnet that includes most vulnerable hosts to attacks; i.e. hosts that provide services to outside users. Common hosts in DMZ: Public web servers, Public DNS servers, public FTP servers, proxy servers. Host in DMZ must be heavily protected. Server on x Subnet Host Firewall Demilitarized Zone (DMZ)

8 Questions What is a DMZ? Why are public web servers usually put in the DMZ? Why are public DNS servers usually put in the DMZ? Which of the following may be placed in a DMZ? a)A SMTP proxy server b)A server that contains files available for downloading by employees c)An File Transfer Protocol server d)A SQL (Structured Query Language) database server What IP addresses should a DNS server in the DMZ be able to find? a) All company’s IP addresses b) Only the IP addresses of the computers in the internal subnet c) Only the IP addresses of the computers in the DMZ You work as the security administrator at King.com. King.com has been receiving a high volume of attacks on the king.com web site. You want to collect information on the attackers so that legal action can be taken. Which of the following can you use to accomplish this? a)A DMZ (Demilitarized Zone). b)A honey pot. c)A firewall. d)None of the above.

9 Basic Firewall Operation Attack Packet 1 1. Internet (Not Trusted) Attacker Log File Dropped Packet (Ingress) Legitimate User Legitimate Packet 1 Attack Packet 1 Internal Corporate Network (Trusted) Border Firewall Passed Legitimate Packet (Ingress) Legitimate Packet 1 Egress filtering: filtering packets leaving to external networks Ingress filtering: filtering packets coming from external networks Legitimate Packet 2 Passed Packet (Egress) Legitimate Packet 2

10 ConnectionSource IPDestination IPState Connection :80TCP opening Connection :80Data transfer ……………. ……… Types of Firewalls Static Packet Filtering Firewalls (1 st generation) Inspect TCP, UDP, IP headers to make filtering decisions Do static filtering of individual packets based on configured ruleset (or Access Control List) Prevent attacks that use IP or port spoofing, etc. Stateful Packet Filtering Firewalls (2 nd generation) Inspect TCP, UDP, IP headers to make filtering decisions Do stateful filtering by checking the firewall’s state table for relation of packets to packets already filtered If packet does not match existing connect, ruleset (static filt.) is used If packet matches existing connection, it is allowed to pass Prevent SYN attacks, teardrops, etc. State Table IP-H TCP-H UDP-HApplication Layer Message

11 Types of Firewalls (cont.) Application Firewalls (3 rd generation) Also called proxy firewalls Inspect the Application Layer message (e.g. HTTP requests, s, etc. Specialized proxy firewalls more effective than general-purpose HTTP proxy firewalls for HTTP requests SMTP proxy firewalls for SMTP s FTP proxy firewall for FTP-based file transfer requests Prevent malware attacks IP-H TCP-H UDP-HApplication Layer Message HTTP Proxy Browser Webserver Application 1. HTTP Request 2. Passed inspected HTTP Request 3. HTTP Response 4. Passed inspected HTTP Response Log File

12 Types of Firewalls (cont.) Network Address Translation Firewall Replace IP address in outgoing message by a spoof IP address Hide internal hosts’ IP address to outsiders Help prevent IP spoofing attacks using internal IP addresses Host IP AddressOutgoing IP AddressRequest ID …….. ………

13 Network Address Translation (Cont) Server Host Client NAT Firewall 1 Internet 2 Sniffer From , Port From , Port IP Addr Port Internal IP Addr Port External Translation Table

14 Network Address Translation (Cont) Server Host Client NAT Firewall 3 Internet 4 Sniffer To , Port To , Port IP Addr Port Internal IP Addr Port External Translation Table

15 Perspective on NAT NAT/PAT NAT does more than network (IP) address translation Also does port number translation Should be called NAT/PAT, but NAT is the common term

16 Firewalls configuration Default configuration (default Rulesets or ACLs) Pass connections initiated by an internal host Deny connections initiated by an external host Can change default configuration with access control lists (ACLs) for ingress and egress filtering ACLs are sets of IF-THEN rules applied in sequential order Internet Automatically Pass Connection Attempt Router Automatically Deny Connection Attempt

17 Ingress ACL 1If Source IP Address = 10.*.*.*, DENY [Private IP Address Range] 2If Source IP Address = *.*, DENY [Private IP Address Range] 3If Source IP Address = *.*, DENY [Private IP Address Range] 4If Destination IP Address = AND TCP Destination Port = 80 or 443, PASS 5If Destination IP Address = *.*, DENY 6If Incoming packet TCP SYN = 1 and ACK = 0, DENY [Attempt to open connection form the outside] 7If TCP Destination Port = 20, DENY 8If TCP Destination Port = 135 Trough 139, DENY 9If UDP Destination Port = 69, DENY 10DENY ALL Untrusted network Trusted network Firewall Port NumberPrimary ProtocolApplication 20TCPFTP Data Traffic 21TCPFTP Supervisory Connection. Passwords sent in the clear 23TCPTelnet. Passwords sent in the clear 25TCPSimple Mail Transfer Protocol (SMTP) 69UDPTrivial File Transfer Protocol (TFTP). No login necessary 80TCPHypertext Transfer Protocol (HTTP) TCPNETBIOS service for peer-to-peer file sharing in older versions of Windows 443TCPHTTP over SSL/TLS

18 Ingress ACL 1If Source IP Address = 10.*.*.*, DENY [Private IP Address Range] 2If Source IP Address = *.*, DENY [Private IP Address Range] 3If Source IP Address = *.*, DENY [Private IP Address Range] 4If Destination IP Address = AND TCP Destination Port = 80 or 443, PASS 5If Destination IP Address = *.*, DENY 6If Incoming packet TCP SYN = 1 and ACK = 0, DENY [Attempt to open connection form the outside] 7If TCP Destination Port = 20, DENY 8If TCP Destination Port = 135 Trough 139, DENY 9If UDP Destination Port = 69, DENY 10DENY ALL Untrusted network Trusted network Firewall What kind of messages does Rule 7 block? Why does Rule 5 have to come after Rule 4? Why does Rule 6 have to come after Rule 4? You work as the security administrator for the trusted network. Employees often download files from a FTP (File Transfer Protocol) server located in the untrusted network. What TCP port do you open in the firewall configuration? a) Open port 69 to all inbound connections. b) Open port 69 to all outbound connections. c) Open port 20/21 to all inbound connections. d) Open port 20/21 to all outbound connections.

19 Typical attacks and firewall config. AttacksTypical configurationComments Ping of deathAny packet with Total Length more than maximum allowed is droppedStateful firewall IP fragmentation- based attacks (e.g. Teardrop) The firewall intercepts all fragments for an IP packet and attempts to reassemble them before forwarding to destination. If any problems or errors are found during reassembly, the fragments are dropped. Stateful firewall Smurf AttackThe firewall drops any ping responses that are not part of an active session. Stateful firewall Attacks that send TCP URG packets Any TCP packets that have the URG flag set are discarded by the firewall. Land AttackAny packets with the same source and destination IP addresses are discarded. IP broadcastPackets with a broadcast source or destination IP address are discarded. TCP SYN/ACK attack TCP Opening segments that have SYN and ACK flags set AND that are not linked to a TCP SYN request are discarded. Stateful firewall Invalid TCP Segment Number The sequence numbers for every active TCP session are maintained in the firewall session database. If the firewall received a segment with an unexpected (or invalid) sequence number, the packet is dropped. Stateful firewall Flag Fields (6 bits) ACKSYNFINRSTURGPSH

20 Firewall Principles Danger of Overload If a firewall is overloaded and cannot handle the traffic, it drops unprocessed packets This is the safest choice, because attack packets cannot enter the network However, this creates a self-inflicted denial- of-service attack

21 Firewall Principles (Continued) Danger of Overload So firewalls must have the capacity to handle the traffic Some can handle normal traffic but cannot handle traffic during heavy attacks Need to regularly check firewalls logs: If too much unchecked packets are dropped, then need to upgrade the firewall.

22 Centralized Firewall Management System Internet Home PC Firewall Management Console Site ASite B Remote Management is needed to reduce management labor Dangerous because if an attacker compromises it, they own the network Remote PCs must be actively managed centrally

23 Firewall Management Firewalls are Ineffective without Planning and Maintenance Planning Asset Assessment: identify all assets and their relative sensitivities Threat Assessment: what threats can attack each asset? Design a Firewall Policy for Each Asset Design a Firewall Architecture

24 Firewall Management (Continued) Implementation Firewall Operating System Hardening Firewall appliances are hardened at the factory Firewall vendors often sell firewalls that are general-purpose computers that have pre- hardened versions of Unix or Windows If a firm purchases a general purpose computer and firewall software, strong actions must be taken to harden the operating system

25 Firewall Management (Continued) Implementation Select Implementation Options e.g., Turn off remote management if not needed Firewall ACL Rule Configuration Complex and therefore error-prone Driven by firewall policies Vulnerability Testing After Configuration Must do vulnerability test even after “trivial” changes Driven by firewall policies

26 Firewall Management (Continued) Maintenance Constantly change firewall policies and ACLs to deal with new threats  Document each change carefully! Read log files daily to understand the current threat environment Read log files daily to detect problems (the dropping of legitimate traffic, etc.) Update the firewall software when there are new releases

27 Firewalls, IDSs, and IPSs FirewallsIDSsIPSs Drops Packets?YesNoYes Logs PacketsYes Sophistication in Filtering MediumHigh Creates Alarms?NoYesSometimes

28 Firewalls, IDSs, and IPSs (Cont) Sophistication in Filtering Message stream analysis, not just individual packets Reassemble fragmented application messages Deep packet inspection: both internet-level headers and application headers