Firewall Architectures in .NET

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Firewall Architectures
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The cost and overhead of the protection mechanism should be relatively low or the system may become too expensive to run. Network administrators deploy and configure a variety of protection mechanisms; hence it is important to have a clear model of what is being protected. The attacks are constantly changing; hence it should be easy to make changes to the configuration of the protection mechanism. It may be necessary to log input and/or output requests for auditing and defence purposes.
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A PACKET FILTER FIREWALL (405) intercepts all traffic coming and going from a port P and inspects its packets (see the figure below). Those coming from or going to mistrusted addresses are rejected. The mistrusted addresses are determined from a set of rules that implement the security policies of the organization. A client from another network can only access the Local Host if a rule exists authorizing traffic from its address. Specific rules may indicate an address or a range of addresses. Rules may be positive (allow traffic from some address) or negative (block traffic from some address). Most commercial products order these rules for efficiency in checking. Additionally, if a request is not satisfied by any of the explicit rules, then a default rule is applied.
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The concept of the packet filter firewall
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The figure on page 407 shows an external host requesting access to a local host (a server) through a packet filter firewall. The organization policies are embodied in the objects of class Rule collected by the RuleBase. The RuleBase includes data structures
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and operations to manage rules in a convenient way. The rules in this set are ordered, and can be explicit or default.
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We describe the dynamic aspects of the PACKET FILTER FIREWALL (405) using a sequence diagram for one of its basic use cases. There is a symmetric use case, filtering an outgoing request, which we omit for briefness. We also omit use cases for adding, removing, or reordering rules, because they are straightforward. See the figure on page 408.
Filtering a Client s Request
Summary. A host in a remote network wants access to a local host to either transfer or retrieve information. The access request is made through the firewall, which according to its set of rules determines whether to accept or deny the request that is, it filters the access request. Actors. A host on an external network (client). Precondition. An existing set of rules to filter the request must be in place in the firewall.
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Firewall Architectures
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Sequence diagram for filtering a client s request
Description: 1. An external host requests access to the local host. 2. A firewall filters the request according to a set of ordered rules. If none of the explicit rules in the rule set allows or denies the request, a default rule is used for making a decision. 3. If the request is accepted, the firewall allows access to the local host.
Alternate flow. The request is denied. Postcondition. The firewall has accepted the access of a trustworthy client to the local host.
1. Define an organization policy about network access, classifying sites according to our trust in them. 2. Convert this policy into a set of access rules. This can be done manually, which may be complex for large systems. An alternative is using an appropriate commercial product, such as Solsoft [Sol]. 3. Note that the idea of a single point of access is virtual: there may be several physical firewalls deployed at different places. This means that it is necessary to install firewalls at all external boundaries, such as routers or gateways. 4. Write the rules in each firewall. Again, products such as Solsoft and others automatically propagate the rules to each registered firewall.