Firewalls 1 Overview Background General Firewall setup Iptables

Firewalls 1

Overview • • • Background General Firewall setup Iptables Introduction Iptables commands “Limit” Function Explanation with icmp and syn floods • Zone Alarm ECE 4112 - Internetwork Security 2

What is a Firewall? • Firewall – a hardware, software, or combination of the two that prevents unauthorized access to or from a private network. ECE 4112 - Internetwork Security 3

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 4112 - Internetwork Security 4

Traffic Control • Three methods used to control traffic flowing in and out of the network § Packet Filtering § Proxy Filtering § Stateful Inspection ECE 4112 - Internetwork Security 5

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 4112 - Internetwork Security 6

What You’re Protected From Security Level External packets allowed HIGH none MIDDLE pre-defined ports (web, ssh) and established connections all packets LOW ECE 4112 - Internetwork Security 7

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 4112 - Internetwork Security 8

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 4112 - Internetwork Security 9

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 4112 - Internetwork Security 10

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” • 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 4112 - Internetwork Security 11

Common Firewall Configurations • • http: //www. firewall. cx/firewall_topologies. php 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 4112 - Internetwork Security 12

Common Firewall Configurations • • • http: //www. firewall. cx/firewall_topologies. php The exposed DMZ configuration depends on two things: § § 1) an external “Internet” router 2) multiple IP addresses. The firewall needs only two network cards. If you control the “Internet” router you have access to a second set of packet-filtering capabilities. If you don't control the “Internet” 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. 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 4112 - Internetwork Security 13

Common Firewall Configurations • • • One solution is to build a second router/firewall. Useful if you're connecting via PPP Exterior router/firewall (Firewall 1) • The other firewall (Firewall 2) • The other solution is to create a three-legged firewall, which is what we are going to talk about next § responsible for creating the PPP connection and controls the access to our DMZ zone § is a standard dual-homed host just like the one we spoke about at the beginning http: //www. firewall. cx/firewall_topologies. php ECE 4112 - Internetwork Security 14

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 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. http: //www. firewall. cx/firewall_topologies. php ECE 4112 - Internetwork Security 15

Lab Setup • Firewall workstations • One firewall host and two virtual machines ECE 4112 - Internetwork Security 16

Iptables Introduction • Iptables is a fourth generation firewall tool for Linux • Requires kernel 2. 3. 15 or above with netfilter framework • Iptables inserts and deletes rules from the kernel’s packet filtering table • Replacement for ipfwadm and ipchains ECE 4112 - Internetwork Security 17

How packets traverse the filters 3 default chains: INPUT, FORWARD, OUTPUT Incoming Routing Decision Outgoing FORWARD OUTPUT INPUT Local Process ECE 4112 - Internetwork Security 18

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 4112 - Internetwork Security 19

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 4112 - Internetwork Security 20

NAT and iptables ECE 4112 - Internetwork Security 21

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 eth 0 –j MASQUERADE ECE 4112 - Internetwork Security 22

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 1. 2. 3. 4 --dport 80 -j / DNAT --to 192. 168. 1. 1: 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 4112 - Internetwork Security 23

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 4112 - Internetwork Security 24

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 4112 - Internetwork Security 25

Iptables commands (contd) • Specifying interface § Specified with the ‘-i’ (input) or ‘-o’ (output) iptables –A INPUT –i eth 0 #check packets coming in on interface eth 0 • Specifying source/destination § Can be specified in 4 ways: name (www. cnn. com), IP (192. 168. 1. 101), group (162. 12. 23. 22/24), using IP/netmask (192. 168. 1. 105/255. 0). Use ‘-s’ for source, and ‘-d’ for destination. iptables –A INPUT –s 192. 168. 1. 101/24 –d 192. 168. 1. 105 ECE 4112 - Internetwork Security 26

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 4112 - Internetwork Security 27

Port Forwarding • Using NAT table, destination address is changed based on the port iptables –A PREROUTING –t nat –d 10. 1 –p tcp --dport 80 –j DNAT --to 192. 168. 1. 3: 80 ECE 4112 - Internetwork Security 28

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. is based on the 1/s limit specified. ECE 4112 - Internetwork Security This 29

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 4112 - Internetwork Security 30

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 4112 - Internetwork Security 31

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 4112 - Internetwork Security 32

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 4112 - Internetwork Security 33

Hardware Firewalls ECE 4112 - Internetwork Security 34

Cisco Firewalls – PIX 515 E • 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 4112 - Internetwork Security 35

Cisco Firewalls – PIX 515 E • ASA – Adaptive Security Algorithm • Data Flow relative to security levels § Security Level 100 – For trusted Inside interface and internal traffic § Security Level 0 – For un-trusted Outside interface § Security Level 1 -99 – Can be assigned to perimeter interfaces like DMZ ECE 4112 - Internetwork Security 36

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 192. 168. 254. 2 § Actual digital console § Controls all routers and other hardware • Need a terminal to the normal lab network ECE 4112 - Internetwork Security 37

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. Hardware Firewalls ECE 4112 - Internetwork Security 38

Acknowledgements “Firewall Topologies”, http: //www. firewall. cx/firewall_topologies. php Russell, Rusty, “Linux 2. 4 Packet Filtering HOWTO” http: //www. netfilter. org/documentation/HOWTO/packet-filtering-HOWTO. html Startup script and basis for rules Stephens , James C. http: //www. sns. ias. edu/~jns/security/iptables/ Steams, William “Adaptive Firewalls with IP Tables” http: //www. ists. dartmouth. edu/IRIA/knowledge_base/adaptive_firewalls. htm Tyson, Jeff, “How Firewalls Work” http: //computer. howstuffworks. com/firewall. htm/ Young, Scott “Designing a DMZ” http: //www. sans. org/rr/firewall/DMZ. php Zone. Alarm tutorial information provided from http: //www. zonelabs. com ECE 4112 - Internetwork Security 39

References • Cisco Secure PIX Firewalls, David Chapman Jr. and Andy Fox. Cisco Press. 2002. • http: //www. cisco. com/univercd/cc/td/doc/ product/iaabu/pix/ • Cisco Security seminar notes. ECE 4112 - Internetwork Security 40