Conventional rewalls rely on topology restrictions and controlled network entry points to enforce traAEc ltering. Furthermore, a rewall cannot lter traAEc it does not see, so, e ectively, everyone on the protected side is trusted. While this model has worked well for small to medium size networks, networking trends such as increased connectivity, higher line speeds, extranets, and telecommuting threaten to make it obsolete. To address the shortcomings of traditional rewalls, the concept of a \distributed rewall" has been proposed. In this scheme, security policy is still centrally de ned, but enforcement is left up to the individual endpoints. IPsec may be used to distribute credentials that express parts of the overall network policy. Alternately, these credentials may be obtained through out-of-band means. In this paper, we present the design and implementation of a distributed rewall using the KeyNote trust management system to specify, distribute, and resolve policy, and OpenBSD, an open source UNIX operating system.
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