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1 Pascal Muetschard John Nagle COEN 150, Spring 03 Prof. JoAnne Holliday Computer Firewalls Introduction The term firewall was originally used with forest fires, as a means to describe the barriers implemented to prevent fires from spreading. A firewall was constructed or cleared as a perimeter so that the fire could not reach beyond the wall and cause further damage against the protected areas. This term has been reused in the automotive industry as the wall between the driver and the engine, which prevents the intense heat of the engine from harming the passengers. In both cases, a firewall serves as a preventive measure that is placed between a type of danger and the protected entity. Its application in the computer world follows suit. A firewall traditionally serves as a gateway between the trusted and the not trusted network or computer user. There is a common misunderstanding that a firewall is an external box that serves as the secure entrance into a private network. While this can be true, a firewall system can also be a router, a personal computer, a host, or a collection of hosts, set up specifically to shield a site or private network from protocols and services that can be abused from hosts outside of that network. A firewall system is usually located at a higher-level gateway, such as a site's connection to the Internet. This is due to the vulnerability that arises when a company chooses to have their network accessible to the Internet, because with this access to the Internet means that that Internet can in

2 return access them. It is easily understood that the firewall, or at least a firewall, should be placed at this access point to add security to its path. This is shown in the graphic below. But as described earlier, the firewall is not limited to the little black box approach. The access to the Internet could be through one of the computers on the network, in which case that computer will need to serve as the firewall. This setup is shown below.

3 There are two main types of regulatory conditions for the setup of a firewall, both offering different advantages and disadvantages to the process. One condition is to allow access to the system unless otherwise specified, and the other is to not allow access to the system unless directly granted permission. The latter of the two proves to be more secure but it is also more tedious and time consuming, whereas the first is easier to use based on its lack of strictness. The general reasoning behind firewall usage is that without a firewall, a network's systems make themselves vulnerable to innately not secure services such as NFS or NIS and to probes and attacks from hosts elsewhere on the network. When firewalls are not implemented in a private network s environment, network security depends totally on host security and that all hosts must cooperate to achieve a consistently high level of security. It is easy to see the many gaps in this approach. The larger the network, the less controllable it is to maintain all hosts at the same security level. As mistakes and slips in security become more common, hack-ins occur not as the result of multifaceted

4 attacks, but because of simple errors in configuration and laughable password protection. Another service that firewalls serve purpose to goes along with the intended goal of keeping the system secure. Firewalls aim at keeping the protected information secure as well as keeping the information that could lead to that information secure. Using a firewall, some users wish to block services such as finger and DNS. Finger is a command that displays information about users such as their last login time, whether they've read mail, and other items. But, finger could reveal information to attackers about how often a system is used, whether the system has active users connected, and whether the system could be attacked without drawing attention. Firewalls can also be used to block DNS information about host systems, thus the names and IP addresses of such systems would not be available to Internet hosts. Some users feel that by blocking this information, they are hiding information that would otherwise be useful to hackers. This type address security can be found in NAT systems. Network Address Translation systems enable a local-area network to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. These systems are usually located where the LAN meets the Internet and makes all necessary IP address translations. Besides serving as a service level firewall by hiding all internal IP address, a NAT also enables a company to use more internal IP addresses. Since they're used only within their network, there is no possibility of conflict with IP addresses used by other companies and organizations. This system also allows a company to combine multiple ISDN connections into a single Internet connection. An

5 example of this configuration is given below. In the above example the following process occurs. A call is made from the Internet to the IP address /24. This source is sent to the destination address but within the NAT this IP is translated to the inner-network s IP /24. Now in reverse the same process occurs. From the network IP , the system translates it to be This address then makes the request to the destination IP It should not be assumed that a NAT can take the place of a firewall. It has a protective aspect but should not be implemented with the assumption that it will give 100% security. Case Study of the Linux Firewall 'iptables' Every Linux system today, comes with a built in firewall called iptables. iptables consists of a set of kernel modules and user-space utilities that allow the configuration and running of a sophisticated firewall. The kernel modules can be inserted into the

6 kernel during boot-up or at runtime. Once inserted, they become part of the kernel and they register themselves with the networking subsystem, where they become part of the TCP stack. In the TCP stack the iptables firewall both filters and or alters the packets moving up or down the stack. The user-space utilities allows the configuration of the firewall by the superuser. These utilities send commands to the kernel modules just like file system utilities send commands to the hard drive controller drivers. iptables consists of different tables, each of which has a different goal. The three most used tables are the filter, mangle and nat tables. The filter table is used to filter packets, i.e. decide whether to accept or block a packet, the mangle table is used to filter/alter packets when routing packets to different hosts, and the nat table is used to perform both source and destination network address translation (SNAT and DNAT). Each of these tables consist of a number of chains. There are built in default chains, (like the input, output, and forward chains) but the superuser can also create new chains. They are called chains, because they are chains of firewall rules. Each of these rules consists of a pattern and an action. When a packet runs through a chain, the patterns of the chain's rules are compared against the packet one at a time, and if a rule matches, the corresponding action is taken. Once the iptables modules are part of the TCP stack they are notified when the packets traverse through the networking subsystem of the kernel, as shown in the following diagram.

7 In each of the hooks the iptables modules may alter the packets and then tell the system what to do with it (accept, drop, queue, repeat,...). There are two places from where the packets may enter the firewall system: either from the outside (through any of the network interfaces, including the loopback device) or from an application running on the box. The packets that enter from the outside (that is packets that are being sent to the system) are first run through the prerouting chains. After that, they are passed through the routing system which decides whether the packets are destined for another interface (i.e. another host on the network) or for a local process. If the packet has reached its destination, it is then run through the input table before it is passed on to its process. If the packet is to be routed, it passes through the forward chains and then through the postrouting chains after which it is put back onto the wire. Packets that are produced by a process on the system (that is packets that are being sent from the system) run first through the output chains after which they run through the routing code to determine over which wire they are to be sent and then they also run through the postrouting chains.

8 Let's now look at the filter table. The filter table should never alter the packets, but just make decisions whether to accept or reject the packets. That is the actions of the rules in the chains of the filter table should only be one of accept, drop, or reject. The difference between reject and drop are that in the case of reject the sender is notified that the packet was rejected, whereas in the case of drop he is not. Usually, drop is used, because then the possible intruder is not notified that his attacks are failing. When the system is started and the iptables modules are loaded, the filter table contains only the empty default chains (input, output and forward) and each chain has a default action of accept. The default action is the action taken when a packet passes through the entire chain without matching any of the rules' patterns. It is usually desirable to change the default action to drop and then create rules to explicitly accept packets that are safe dropping all others. Let us assume that we are running an HTTP server on the system. At this point, however, nobody will be able to connect to the web server, because all packets are being dropped, so we can now say that packets for port 80 (the web server port) are to be accepted. This is done using the iptables user utilities by specifying a pattern, (packets destined to port 80) the table, (the filter table) the chain to add it to, (the input chain) and the action (accept): iptables -t filter -I INPUT -p tcp --dport 80 -j ACCEPT Here, the -t flag tells to use the filter table, -I says to insert it into the input chain, -p specifies the protocol, --dport the destination port and -j (for jump) the action to take. Similar rules would be added to the chain for other services. It is usually desirable to detect spoofed packets. Spoofed packets are packets

9 whose source IP address has been changed, i.e. the packet claims to be from somewhere it actually does not. Spoofing in general is hard to detect, but certain cases are not too hard. For example, any packet destined to the local host network (127.xxx.xxx.xxx) or coming from it may only enter and leave through the loopback interface. Similarly packets claiming to come from the local (usually more trusted) LAN may only enter through the interface connected to the local LAN. Let's assume that the interface eth0 is connected to the local LAN with network address xxx. The two mentioned spoofing tests would be specified like so: iptables -t filter -I INPUT -i!lo -s /8 -j DROP iptables -t filter -I INPUT -i!lo -d /8 -j DROP iptables -t filter -I INPUT -i!eth0 -s /24 -j DROP Here, the -i flag is used to specify the input interface and the '!' means not, so!lo matches any packet entering the system except if it entered through the loopback device. The -s flag is used to specify a source network or host IP address (if it is a network address the number after the '/' indicates the netmask, that is the number of bits of the address that makes up the subnet). In this case we are blocking all packets not entering through the loopback interface, but have source or destination network address 127.xxx.xxx.xxx and all packets not entering through the eth0 interface, but have source network address xxx. So far we have only talked about the input chain of the filter table. The other two chains, the forward and output chains, are rarely used, simply because most systems allow all the packets to leave the system, however you could add rules that would prohibit anybody within the firewall to connect to a certain (dangerous) host. Let's

10 assume that the SCU web server at ( ) is such a host, the following rule would make it impossible for anybody within the firewall to connect to it: iptables -t filter -I OUTPUT -d j DROP The forward chain, on the other hand, is used more often. The forward chain is used to filter packets that are being routed by the system and so a clear application would be to filter packets that would be routed to the local LAN to protect it from unauthorized access. The most (in)famous way to do just that is the Microsoft BackOffice vulnerability. To block that we can simply drop all packets to ports 137 to 139 in the forward chain: iptables -t filter -I FORWARD -p tcp dport 137:139 -j DROP iptables -t filter -I FORWARD -p udp dport 137:139 -j DEOP These two rules block the traffic for both the UDP and the TCP protocols. So far, all the packets entering the system are being checked through all the rules. However, a lot of improvement can be achieved by taking advantage of the iptables' ability to track the connections. Both the UDP and TCP protocols are connectionless, which means given a single packet one cannot know whether a connection has been established or not (here connection means the successful execution of the three-way handshake). However, the connection tracking module allows the firewall rules to check the status of the connection. Therefore, a great increase in performance can be achieved by accepting all packets of an established connection and only let the SYN (connection request) packets traverse through the entire chains. The following rule in both the input and forward chain do the trick:

11 iptables -I INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -I FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT An additional feature of the iptables firewall, is that it provides an easy way to do both SNAT and DNAT. The nat table uses the prerouteing, postrouting, and output chains. In order to do SNAT to allow the local LAN to connect to the Internet only using the router's IP address the following rule can be added: iptables -I POSTROUTING -s /24 -o eth0 -j MASQUERADE iptables -I POSTROUTING -s /21 -o ppp0 -j MASQUERADE This assumes that the interface that connects to the Internet is the ppp0 interface and that the local LAN uses the network address xxx. Hence, any packet coming from the local LAN and destined for the outside world will be masqueraded, that is, its source IP address will be replaced by the IP address of the current system and (thanks to the connection tracking) all the packets sent from the outside world of that connection will have the destination address replaced by the original host. This makes it look from the outside as if the traffic was coming and going to the one system connected, whereas from the inside it looks like everybody is connected to (and present on) the Internet To do DNAT is a little bit more involved. Let's assume that we are running a web server on a machine in the local LAN with address on port 8080, but wish it to be accessible from the outside on port 80 with the IP address given to us by our ISP. Naturally, people cannot connect to the address, but if they use the address of our router, than they will not get our website. However, DNAT allows us to have people connect to the router's IP address, but redirect the traffic to the local

12 machine, without anybody ever noticing. First we need to allow incoming traffic on port 80 and also allow packets to be forwarded to our local system on port 8080 in the filter table: iptables -t filter -I INPUT -p tcp --dport 80 -j ACCEPT iptables -t filter -I FORWARD -p tcp -d dport j ACCEPT Next, in the nat table, we actually need to have the DNAT be performed. iptables allows that easily by providing the DNAT target: iptables -t nat -I PREROUTING -p tcp --dport j DNAT --to :8080 As shown in this paper, it is not that hard to set up a strong firewall under Linux, while at the same time the user is allowed extensive control over what packets to block and allow. The iptables firewall is also very efficient, because it becomes part of the operating system kernel. Furthermore, because it is an open source project, security holes in the firewall can easily be detected and fixed before they become a real threat. It is very important for a system that has a high speed connection to the Internet to have a firewall. When I first set up our Linux server, we did not have one and a friend of mine was able to see all the machines of the local LAN, i.e. the security implemented by the services was not good enough by far. Now, that the firewall is up, this and other kinds of information is well protected. Another reason why firewalls are important: over the last month there have been more than 1000 connection requests dropped by our firewall that tried to take advantage of the Microsoft BackOffice problem plus numerous attempts to connect over FTP!

13 Bibliography Bellovin, Steven M., William R. Cheswick, and Aviel D. Ruben. Firewalls and Internet Security: Repelling the Wily Hacker. 2nd ed. Boston: Addison-Wesley, Firewall Q&A. VicomSoft. 20 May 2003 < Breithaupt, James, and Mark S. Merkow. The Complete Guide to Internet Security. New York : AMACOM, Chapman, Brent D., and Elizabeth D. Zwicky. Building Internet Firewalls. Sebastopol, CA: O\'Reilly and Associates, Inc., Black, Uyless. Internet Security Protocols : Protecting IP Traffic. Upper Saddle River, N.J.: Prentice Hall PTR, Russell, Rusty, and Harald Welte. Linux netfilter Hacking HOWTO. < HOWTO.html>

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