✯✯✯ Wesley Lefebvre Strengths And Weaknesses

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Wesley Lefebvre Strengths And Weaknesses



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Abstract Conventional firewalls rely on the notions of restricted topology and controlled entry points to function. More precisely, they rely on the assumption that everyone on one side of the entry point--the firewall--is to be trusted, and that anyone on the other side is, at least potentially, an enemy. The vastly expanded Internet connectivity in recent years has called that assumption into question. A distributed firewall preserves central control of access policy, while reducing or eliminating any dependency on topology.

So-called "extranets" can allow outsiders to reach the "inside" of the firewall; on the other hand, telecommuters' machines that use the Internet for connectivity need protection when encrypted tunnels are not in place. Other trends are also threatening firewalls. For example, some machines need more access to the outside than do others. Conventional firewalls can do this, but only with difficulty, especially as internal IP addresses change. End-to-end encryption is another threat, since the firewall generally does not have the necessary keys to peek through the encryption. Some people have suggested that the proper approach is to discard the concept of firewalls.

Firewalls, they feel, are obsolete, or are not needed if cryptography is used. We disagree. Firewalls are still a powerful protective mechanism. Most security problems are due to buggy code--in , 9 of 13 CERT advisories concerned buffer overflows, and two of the rest were cryptographic bugs--and cannot be prevented by encryption or authentication. A firewall shields most such applications from hostile connections. Firewalls are also useful at protecting legacy applications. While applications that require strong authentication should provide their own, there are too many older protocols and implementations that do not.

Saying that strong cryptography should be used is true but irrelevant; in the context of such applications, it is simply unavailable. More subtly, firewalls are a mechanism for policy control. That is, they permit a site's administrator to set a policy on external access. Just as file permissions enforce an internal security policy, a firewall can enforce an external security policy. To solve these problems while still retaining the advantages of firewalls, we propose a distributed solution. In such a scheme, policy is still centrally defined; enforcement, however, takes place on each endpoint. We thus retain the advantages of firewalls while avoiding most of the problems we have described, most notably the dependency on topology.

The basic idea is simple. A compiler translates the policy language into some internal format. The system management software distributes this policy file to all hosts that are protected by the firewall. And incoming packets are accepted or rejected by each "inside" host, according to both the policy and the cryptographically-verified identity of each sender. SMTP from the outside can only reach the machine with a certificate identifying it as the mail gateway; it, in turn, can speak SMTP to all inside machines. NTP--a low-risk protocol that has its own application- level protection--can be distributed from a given IP address to all inside machines.

Finally, all outgoing calls are permitted. The exact nature is not crucial, though clearly the language must be powerful enough to express the desired policy. A sample is shown in Figure 1. What is important is how the inside hosts are identified. Today's firewalls rely on topology; thus, network interfaces are designated "inside", "outside", "DMZ", etc. We abandon this notion but see Section 5 , since distributed firewalls are independent of topology. A second common host designator is IP address. That is, a specified IP address may be fully trusted, able to receive incoming mail from the Internet, etc. Distributed firewalls can use IP addresses for host identification, though with a reduced level of security.

Certificates can be a very reliable unique identifier. They are independent of topology; furthermore, ownership of a certificate is not easily spoofed. If a machine is granted certain privileges based on its certificate, those privileges can apply regardless of where the machine is located physically. In a different sense, policies can be "pulled" dynamically by the end system. For example, a license server or a security clearance server can be asked if a certain communication should be permitted.

A conventional firewall could do the same, but it lacks important knowledge about the context of the request. End systems may know things like which files are involved, and what their security levels might be. Such information could be carried over a network protocol, but only by adding complexity. Instead, to simplify system administration and to permit some level of central control, a system management package is used to administer individual machines. Patches can be installed, new software distributed, etc. We use the same mechanisms, which are likely present in any event, to control a distributed firewall. Policy is enforced by each individual host that participates in a distributed firewall.

The security administrator--who is no longer necessarily the "local" administrator, since we are no longer constrained by by topology--defines the security policy in terms of host identifiers. The resulting policy probably, though not necessarily, compiled to some convenient internal format is then shipped out, much like any other change. This policy file is consulted before processing incoming or outgoing messages, to verify their compliance. It is most natural to think of this happening at the network or transport layers, but policies and enforcement can equally well apply to the application layer.

For example, some sites might wish to force local Web browsers to disable Java or Javascript. Policy enforcement is especially useful if the peer host is identified by a certificate. If so, the local host has a much stronger assurance of its identity than in a traditional firewall. In the latter case, all hosts on the inside are in some sense equal. If any such machines are subverted, they can launch attacks on hosts that they would not normally talk to, possibly by impersonating trusted hosts for protocols such as rlogin. With a distributed firewall, though, such spoofing is not possible; each host's identity is cryptographically assured. This is most easily understood by contrasting it to traditional packet filters [ Mog89 ]. Consider the problem of electronic mail.

Because of a long-standing history of security problems in mailers, most sites with firewalls let only a few, designated hosts receive mail from the outside. They in turn will relay the mail to internal mail servers. Traditional firewalls would express this by a rule that permitted SMTP port 25 connections to the internal mail gateways; access to other internal hosts would be blocked. On the inside of the firewall, though, access to port 25 is unrestricted. With a distributed firewall, all machines have some rule concerning port The mail gateway permits anyone to connect to that port; other internal machines, however, permit contact only from the mail gateway, as identified by its certificate.

Note how much stronger this protection is: even a subverted internal host cannot exploit possible mailer bugs on the protected machines. Distributed firewalls have other advantages as well. The most obvious is that there is no longer a single chokepoint. From both a performance and an availability standpoint, this is a major benefit. Throughput is no longer limited by the speed of the firewall; similarly, there is no longer a single point of failure that can isolate an entire network. Some sites attempt to solve these problems by using multiple firewalls; in many cases, though, that redundancy is purchased only at the expense of an elaborate and possibly insecure firewall-to-firewall protocol.

A second advantage is more subtle. Today's firewalls don't have certain knowledge of what a host intends. Instead, they have to rely on externally-visible features of assorted protocols. Thus, an incoming TCP packet is sometimes presumed legitimate if it has the "ACK" bit set, since such a packet can only be legitimate if it is part of an ongoing conversation--a conversation whose initiation was presumably allowed by the firewall. But spoofed ACK packets can be used as part of "stealth scanning".

Similarly, it is hard for firewalls to treat UDP packets properly, because they cannot tell if they are replies to outbound queries, and hence legal, or if they are incoming attacks. The sending host, however, knows. Relying on the host to make the appropriate decision is therefore more secure. By default, FTP clients use the PORT command to specify the port number used for the data channel; this port is for an incoming call that should be permitted, an operation that is generally not permitted through a firewall. Today's firewalls--even the stateful packet filters--generally use an application-level gateway to handle such commands. With a distributed firewall, the host itself knows when it is listening for a particular data connection, and can reject random probes.

The most important advantage, though, is that distributed firewalls can protect hosts that are not within a topological boundary. Consider a telecommuter who uses the Internet both generically and to tunnel in to a corporate net. How should this machine be protected? A conventional approach can protect the machine while tunneled. But that requires that generic Internet use be tunneled into the corporate network and then back out the Internet. Apart from efficiency considerations, such use is often in violation of corporate guidelines. Furthermore, there is no protection whatsoever when the tunnel is not set up. By contrast, a distributed firewall protects the machine all of the time, regardless of whether or not a tunnel is set up.

Corporate packets, authenticated by IPSEC, are granted more privileges; packets from random Internet hosts can be rejected. And no triangle routing is needed. That is, one can combine the techniques described here with traditional firewalls, achieving adequate functionality at lower cost, especially until IPSEC support becomes ubiquitous. In a hybrid implementation, some hosts are behind a traditional firewall, while other hosts live on the outside. Whether this gateway is inside the traditional firewall, outside it, in parallel with it, or even integrated with it is largely irrelevant to this discussion. This configuration is common at companies with a major central site and some number of telecommuters.

Traffic from inside machines to the remote nodes is similarly protected. What is distinct is that traffic from remote nodes to the rest of the Internet is governed by the central site's security policy. That is, the firewall administrator distributes a security policy to the remote nodes, as we have described. Ideally, of course, this same policy statement is used to control the traditional firewall, thus ensuring a consistent security policy. In this situation, address-dependent policy rules are distributed to, and enforced by, every individual host within a site. Many newer systems support such functionality in the kernel.

While address-based authentication is quite weak, if a simple router prevents address-spoofing from the outside the security should comparable to that of traditional firewalls. Here, we use system management techniques to ensure consistent policy. We also rely on topology, thus forfeiting the ability to protect remote hosts. However, we still eliminate the single chokepoint and point of failure.

A final hybrid scheme combines the two previous hybrid schemes. Again, a simple router prevents address-spoofing by outside machines that talk to inside nodes. IPSEC is used to tunnel traffic from inside notes to remote nodes. When he was faced with health struggles he kept fighting to be a good person to all. He is always there for me no matter what. He is by far the funniest man I know. He is very dependable and he will do anything he can to help someone out. My dad and Beowulf are two completely different people, but they do have a lot of things in common. Everyone who knew Beowulf considered him the most reliable man, and that. Being honorable means doing whatever it takes to achieve the goal you have set. An honorable person does what they can and more to accomplish their goal.

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