Background General Firewall setup Iptables Introduction Iptables commands Limit Function Explanation with icmp and syn floods Zone Alarm

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1 Firewalls 1

2 Overview Background General Firewall setup Iptables Introduction Iptables commands Limit Function Explanation with icmp and syn floods Zone Alarm ECE Internetwork Security 2

3 What is a Firewall? Firewall a hardware, software, or combination of the two that prevents unauthorized access to or from a private network. ECE Internetwork Security 3

4 Benefits Uninhibited internal LAN traffic Ability to leave internal ports open without fear of those ports being abused Sense of security by filtering WAN interface for expected traffic ECE Internetwork Security 4

5 Traffic Control Three methods used to control traffic flowing in and out of the network! Packet Filtering! Proxy Filtering! Stateful Inspection ECE Internetwork Security 5

6 Firewall Configuration Rules/filters can be defined to look for a number of things, some of these are:! IP addresses! Domain names! Protocols - IP TCP HTTP FTP UDP ICMP SMTP SNMP Telnet! Ports! Specific words and phrases ECE Internetwork Security 6

7 What You re Protected From Security Level HIGH MIDDLE LOW External packets allowed none pre-defined ports (web,ssh) and established connections all packets ECE Internetwork Security 7

8 What You re Protected From We allow traffic that is expected! The firewall is responsible for inspecting connections and packet headers We allow all traffic on a few specific ports! Certain ports are forwarded to a server ECE Internetwork Security 8

9 Expected Traffic Protects you from floods of packets! TCP/SYN, PING/REPLY, IP SPOOFING Protects you from scans! Port scans and vulnerability probes Blocks unwanted connections! Telnet, SSH, FTP, and others can be regulated ECE Internetwork Security 9

10 Port Forwarding Biggest security hole in our firewall Opened ports to allow traffic to servers! All incoming data on this specific port is allowed in, and forwarded to server Hackers could exploit this open port Hackers could exploit a bug in the software on the server ECE Internetwork Security 10

11 Demilitarized Zone (DMZ) Frontline of protection A network added between a protected network and external network in order to provide an additional layer of security -SI Security Does not allow external networks to directly reference internal machines Acts as system of checks and balances to make sure that if any one area goes bad that it cannot corrupt the whole ECE Internetwork Security 11

12 Common Firewall Configurations Firewall takes care of passing packets that pass its filtering rules between the internal network and the Internet, and vice versa. May use IP masquerading but that's all it does. Also known as a dual-homed host The two "homes" refer to the two networks that the firewall machine is part of! one interface connected to the outside home! the other connected to the inside home. ECE Internetwork Security 12

13 Common Firewall Configurations The firewall needs only two network cards. If you control the router you have access to a second set of packet-filtering capabilities. If you don't control the router, your DMZ is totally exposed to the Internet. Hardening a machine enough to live in the DMZ without getting regularly compromised can be tricky. The exposed DMZ configuration depends on two things:! 1) an external router! 2) multiple IP addresses. If you connect via PPP (modem dial-up), or you don't control your external router, or you want to masquerade your DMZ, or you have only 1 IP address, you'll need to do something else. There are two straightforward solutions to this, depending on your particular problem. ECE Internetwork Security 13

14 Common Firewall Configurations One solution is to build a second router/firewall. Useful if you're connecting via PPP Exterior router/firewall (Firewall 1)! responsible for creating the PPP connection and controls the access to our DMZ zone The other firewall (Firewall 2)! is a standard dual-homed host just like the one we spoke about at the beginning The other solution is to create a three-legged firewall, which is what we are going to talk about next ECE Internetwork Security 14

15 Common Firewall Configurations Need an additional network adapter in your firewall box for your DMZ. Firewall is configured to route packets between the outside world and the DMZ differently than between the outside world and the internal network. You can masquerade the machine or machines in the DMZ too, while keeping them functionally separate from protected internal machines. The primary disadvantage to the threelegged firewall is the additional complexity. Access to and from the DMZ and to and from the internal network is controlled by one large set of rules. It's pretty easy to get these rules wrong if you're not careful! On the other hand, if you don't have any control over the Internet router, you can exert a lot more control over traffic to and from the DMZ this way. It's good to prevent access into the DMZ if you can. And I think that just about completes our discussion of Firewall Topologies! ECE Internetwork Security 15

16 Lab Setup Firewall workstations One firewall host and two virtual machines ECE Internetwork Security 16

17 Iptables Introduction Iptables is a fourth generation firewall tool for Linux Requires kernel 2.35 or above with netfilter framework Iptables inserts and deletes rules from the kernel s packet filtering table Replacement for ipfwadm and ipchains ECE Internetwork Security 17

18 How packets traverse the filters 3 default chains: INPUT, FORWARD, OUTPUT Incoming Routing Decision FORWARD Outgoing INPUT OUTPUT Local Process ECE Internetwork Security 18

19 How packets traverse the filters (continued) When a packet reaches a circle, that chain determines the fate of the packet The chain can say to DROP the packet or ACCEPT it. If no rules match in chain, the default policy is used (usually to DROP) ECE Internetwork Security 19

20 Network Address Translation The table of NAT rules invoked by iptables t nat contains PREROUTING and POSTROUTING chains PREROUTING Routing Decision POSTROUTING Local Process ECE Internetwork Security 20

21 NAT and iptables PREROUTING Routing Decision FORWARD POSTROUTING INPUT OUTPUT Local Process ECE Internetwork Security 21

22 Masquerading Special form of Source NAT Dynamically changes source address to that of the firewall Simple one-line rule iptables A POSTROUTING t nat o eth0 j MASQUERADE ECE Internetwork Security 22

23 Creating your own rules Adding/Deleting rules:! Append a new rule to an existing chain: iptables A <chain> iptables -A PREROUTING -t nat -p tcp -d dport 80 -j \ DNAT --to 19268:80! Deleting a rule from an existing chain: iptables D <chain> <rule info> iptables -D INPUT --dport 80 -j DROP, iptables -D INPUT 1 Changing chains:! Creating a new chain: iptables N <name> iptables N PERMISSION ECE Internetwork Security 23

24 Creating your own rules (contd)! Delete an empty chain: iptables X <name> iptables X PERMISSION! List the rules of a chain: iptables L <name> iptables L PERMISSION! Flush a chain (delete all rules in a chain): iptables F <name> iptables F PERMISSION ECE Internetwork Security 24

25 More iptables commands Specifying jump! If a packet matches a specified rule, jump (-j option) to another chain: iptables A INPUT j DROP Specifying protocol! Used to specify the protocol, tcp, udp, or icmp (case sensitive) using p option. iptables A INPUT p icmp Specifying inversion! Used to invert any rules using the! option iptables A INPUT p! tcp ECE Internetwork Security 25

26 Iptables commands (contd) Specifying interface! Specified with the -i (input) or -o (output) iptables A INPUT i eth0 #check packets coming in on interface eth0 Specifying source/destination! Can be specified in 4 ways: name ( IP ( ), group ( /24), using IP/netmask ( / ). Use -s for source, and -d for destination. iptables A INPUT s /24 d ECE Internetwork Security 26

27 State matching Different states are checked to analyze packets (need to have ip_conntrack module loaded). The states that are checked are:! NEW: A packet that creates a new connection.! ESTABLISHED: A packet belonging to an existing connection (reply or outgoing packet).! RELATED: A packet that is related to, but not part of an existing connection (ICMP error).! INVALID: A packet that could not be identified. ECE Internetwork Security 27

28 Port Forwarding Using NAT table, destination address is changed based on the port iptables A PREROUTING t nat d 10.0 p tcp \ --dport 80 j DNAT --to :80 ECE Internetwork Security 28

29 Defending against ICMP Ping Floods and tcp syn attack Using limit module specified with -m limit packets can be restricted based on rate of matches iptables A INPUT p icmp -icmp-type echo-request \ m limit -limit 1/s -limit-burst 5 j ACCEPT Limit burst recharges 1 packet every second. This is based on the 1/s limit specified. ECE Internetwork Security 29

30 Zone Alarm Firewall for the Windows OS. Several types of alerts:! New program alerts: Accept/deny programs to access the internet.! Repeat program alerts: grant access permission to program that has already requested before.! Server program alerts: grant server permission to a program. Caution: Some Trojan horses require server access to execute.! Changed program alerts: If a program has been changed since the last time it access the internet. ECE Internetwork Security 30

31 What is a zone? Zone Alarm classifies computer and networks that you communicate with into good, bad, and unknown zones. 3 types:! Internet Zone: is the unknown zone. All computers and networks belong to this zone until you move them to one of the other zones.! Trusted Zone: is the good zone. Contains all computers you trust.! Blocked Zone: is the bad zone. Contains all computers you distrust (only available in Zone Alarm Pro and Zone Alarm Plus version). ECE Internetwork Security 31

32 What is a zone? (contd.) When another computer wants to communicate with your computer Zone Alarm looks at what zone it belongs to and decides what to do. ECE Internetwork Security 32

33 Summary Firewalls filter unwanted traffic. Port Forwarding: big security hole. Network Address Translation. Use iptables to setup filters. State checking. Zone Alarm: Firewall for Windows OS. ECE Internetwork Security 33

34 Acknowledgements Firewall Topologies, Russell, Rusty, Linux 2.4 Packet Filtering HOWTO Startup script and basis for rules Stephens, James C. Steams, William Adaptive Firewalls with IP Tables Tyson, Jeff, How Firewalls Work Young, Scott Designing a DMZ ZoneAlarm tutorial information provided from ECE Internetwork Security 34

35 Hardware Firewalls A hardware firewall usually has 3 interfaces! Inside Trusted area of the internetwork.! Outside Untrusted area of the internetwork! DMZ Isolated area of the internetwork with limited access to Outside users. ECE Internetwork Security 35

36 Hardware Firewalls ECE Internetwork Security 36

37 Cisco Firewalls PIX 515E Different modes of configuration! Unprivileged Mode! Privileged Mode! Configuration Mode! Monitor Mode Can type unique short forms of commands in each mode! Example: config t for configure terminal, write t for write terminal ECE Internetwork Security 37

38 Cisco Firewalls PIX 515E ASA Adaptive Security Algorithm Data Flow relative to security levels! Security Level 100 For trusted Inside interface and internal traffic! Security Level 0 For untrusted Outside interface! Security Level 1-99 Can be assigned to perimeter interfaces like DMZ ECE Internetwork Security 38

39 PIX Lab Network Setup Need to get an ECE UNIX account! Can only access firewall from ECE machines ssh into digiconsole.ece-int.gatech.edu ssh into ! Actual digital console! Controls all routers and other hardware Need a terminal to the normal lab network ECE Internetwork Security 39

40 PWR OK WI C 0 ACT/CH0 ACT/CH1 WI C 0 ACT/CH0 ACT/CH1 ETH ACT COL Lab Network - Mini-Net GTISC Mini-Net NETWORK/MASK:VLAN Autonomous System RIP OSPF BGP Version 9 January 19, 2004 Accounting-rtr Cisco 1720 # /24: #10 Engineering-rtr Cisco 1720 ENTERPRISE AS Cisco UNIVERSITY AS Georgia Tech /24: /24: /24: /24:108 OSPF 0 R3 51 Gatech Webserver Redhat Apache R10.2 #8 Gateway-rtr Cisco 1760-K /24:103 R2 Terminal R /24:102 #6 Edge1-rtr Cisco 1760-K /24:210.3 Cisco-dns Dell Poweredge TIER 1 AS Abilene /24:101 Edge-fwall Cisco PIX-515E /24: /30:308 EBGP 9 OSPF 0.4 EBGP /30:309 Virtual IP Addresses Edge2-rtr Cisco 1760-K9 # Abilene-rtr Cisco 2621-XM Cisco Web Server Redhat Apache #7.2 R1 EBGP EBGP /30:300 EBGP /30: /30: /30:303 R /24:161 Bellsouth-dns Dell Poweredge /24:153.2 #2 Uunet1-rtr 8 Cisco 2621-XM /30:306 EBGP # /24: /24: Cingular-hq-rtr Cisco 1760-K9+NAT.49 # /30: Bellsouth-rtr.50 #13 Cisco EMI (L3) /30:304 IBGP Uunet2-rtr Cisco 3550 #14 #15 Cingular-site1-rtr 8.34 Cisco 1760-K9+NAT 64.06/28: /24: Root1-dns Dell Poweredge /30:307 R10 TIER 1 AS UUNET /24:163 OSPF /28: /24: /24:164 Cingular-site2-rtr Cisco 1760-K9+NAT /24: R10 Earthlink-dns Dell Poweredge /24:160 Cingular-intr1-rtr Cisco /24:159 # /30: /30:156 Cingular-intr2-rtr Cisco 1720 # /24:158 OSPF 1 GOOD ISP AS Bellsouth.net BAD ISP AS EarthLink CoC1-rtr Cisco # /24: /24: R4 R7 CoC Webserver CoC Ftp Server Redhat Apache Redhat Gateway2-rtr.2 EBGP Cisco EMI (L3) # /24: /24:201 R10 #26.4 CoC-vpn Cisco VPN Conc Gatech-dns Admin-rtr Cisco 1760-K9 Dell Poweredge /24:202 # /24: /24: /24: /24:204 R /24: R11 #21 Admin Webserver Printer NAS CoC2-rtr MS IIS /24:206 OSPF 0 Dell Network Cisco Attached Storage RIP /24: EBGP /30:251 #22 7 #25 Earthlink-rtr Cisco EMI (L3) Joe-travel-rtr /24:255 Cisco /30:253.2 StorageRus-rtr /30: K9 8 #23 # /24:258 ADSL-rtr Cisco /24:256 R /24: /24:257 StrRus Webserver MS IIS W1 W20 ECE Internetwork Security 40

41 References Cisco Secure PIX Firewalls,David Chapman Jr. and Andy Fox. Cisco Press product/iaabu/pix/ Cisco Security seminar notes. ECE Internetwork Security 41