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the user for its user id and password; when the user supplies this information, the gateway checks to see if the user has permission to Telnet to the outside world If not, the gateway terminates the Telnet session If the user has permission, then the gateway (1) prompts the user for the hostname of the external host to which the user wants to connect, (2) sets up a Telnet session between the gateway and the external host, (3) relays to the external host all data arriving from the user, and relays to the user all data arriving from the external host Thus the Telnet application gateway not only performs user authorization but also acts as a Telnet server and a Telnet client Note that the filter will permit step (2) because the application gateway initiates the Telnet connection
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Figure 85-1: Firewall consisting of an application gateway and a filter Internal networks often have multiple application gateways, for example, gateways for Telnet, HTTP, FTP and e-mail In fact, an organization's mail server (see Section 24) and Web cache (see Section 29) are application gateways Application gateways do not come without their disadvantages First, you need a different application gateway for each application, which requires installing and configuring a new server for each application Second, either:
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the client software must know how to contact the gateway instead of the external server when the user makes a request, and must know how to tell the gateway what external server to connect to; or the user must explicitly connect to the external server through the gateway
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We conclude this section by mentioning that firewalls are by no means a panacea for all security problems They introduce a tradeoff between the degree of communication with the outside world and level of security Because filters can't stop spoofing of IP addresses and port numbers, filters often use an all or nothing policy (for example, banning all UDP traffic) Gateways can have software bugs, allowing attackers to penetrate them Also, firewalls or even less effective if the internal users have wireless communication with the external world For these reasons and others, firewalls remain controversial, with many security experts and network administrators being reluctant to use them References Two excellent references are [Chapman 1995][Cheswick 1994] Readers are also encouraged to read white papers in the Web sites for major manufacturers of firewalls (eg, [Checkpoint 1999]) [Cert 1999] CERT, "CERT Summaries," http://wwwcertorg/summaries/ [Chapman 1995] DE Chapman and ED Zwicky, "Building Internet Firewalls," O'Reilly and Associates, Sebastopol, CA, 1995 [Cheswick 1994] WR Cheswick and S M Bellovin, "Firewalls and Internet Security," AddisonWesley, Reading, MA, 1994 [Checkpoint 1999] Checkpoint Software Technologies Ltd homepage, http://wwwcheckpointcom [Newman 1998] D Newman, H Holzbar, M Carter, "Firewalls: Tough Enough", Data Communications Magazine, April, 1998
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Copyright 1999-2000 Keith W Ross and Jim Kurose All rights reserved
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Our study of network management, and indeed of all of networking, is now complete! In this final chapter on network management, we began by motivating the need for providing appropriate tools for the network administrator - the person whose job it is to keep the network "up and running - for monitoring, testing, polling, configuring, analyzing, evaluating and controlling the operation of the network Our analogies with the management of complex systems such as power plants, airplanes, and human organization helped motivate this need We saw that the architecture of network management systems revolve around five key components - (i) a network manager, (ii) a set of managed remote (from the network manager) devices, (iii) the management information bases (MIBs) at these devices, containing data about the device's status and operation, and (iv) remote agents that report MIB information and take action under the control of the network manager, and (v) a protocol for communicating between the network manager and the remote devices We then delved into the details of the Internet Network Management Framework, and the SNMP protocol in particular We saw how SNMP instantiates the five key components of a network management architecture, and spent considerable time examining MIB objects, the SMI - the data definition language for specifying MIB's, and the SNMP protocol itself Noting that the SMI and ASN1 are inextricably tied together, and that ASN1 plays a key role in the presentation layer in the ISO/OSI seven layer reference model, we then briefly examined ASN1 Perhaps more important than the details of ASN1 itself, was the noted need to provide for translation between machine-specific data formats in a network While the ISO/OSI reference model explicitly acknowledges the important of this service by the existence of the presentation layer, we noted that this layer is absent in the Internet protocol stack Finally, we concluded this chapter with a discussion of firewalls - a topic that falls within the realms of both security and network management We saw how packet filtering and application-level gateways can be used to provide the network with some level of protection against unwanted intruders, perhaps allow the network manager to sleep better at night, knowing the network is relatively safe from these intruders It is also worth noting that there are many topics in network management that we chose not to cover topics such as fault identification and management, proactive anomaly detection , alarm correlation, and the larger issues of service management (eg, as opposed to network management) While important, these topics would form a text in their own right and we refer the reader to the references noted in section 81
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