网络安全英文文献及翻译 第4页
These are also typically the slowest, because more processes need to be started in order to have a request serviced. Figure 5 shows a application gateway.
Packet Filtering
Packet filtering is a technique whereby routers have ACLs (Access Control Lists) turned on. By default, a router will pass all traffic sent it, and will do so without any sort of restrictions. Employing ACLs is a method for enforcing your security policy with regard to what sorts of access you allow the outside world to have to your internal network, and vice versa.
There is less overhead in packet filtering than with an application gateway, because the feature of access control is performed at a lower ISO/OSI layer (typically, the transport or session layer). Due to the lower overhead and the fact that packet filtering is done with routers, which are specialized computers optimized for tasks related to networking, a packet filtering gateway is often much faster than its application layer cousins. Figure 6 shows a packet filtering gateway.
Because we're working at a lower level, supporting new applications either comes automatically, or is a simple matter of allowing a specific packet type to pass through the gateway. (Not that the possibility of something automatically makes it a good idea; opening things up this way might very well compromise your level of security below what your policy allows.)
There are problems with this method, though. Remember, TCP/IP has absolutely no means of guaranteeing that the source address is really what it claims to be. As a result, we have to use layers of packet filters in order to localize the traffic. We can't get all the way down to the actual host, but with two layers of packet filters, we can differentiate between a packet that came from the Internet and one that came from our internal network. We can identify which network the packet came from with certainty, but we can't get more specific than that.
Hybrid Systems
In an attempt to marry the security of the application layer gateways with the flexibility and speed of packet filtering, some vendors have created systems that use the principles of both.
Figure 6: A sample packet filtering gateway
In some of these systems, new connections must be authenticated and approved at the application layer. Once this has been done, the 原文请找腾讯752018766辣'文"论'文.网http://www.751com.cn ackets that are part of an ongoing (already authenticated and approved) conversation are being passed.
Other possibilities include using both packet filtering and application layer proxies. The benefits here include providing a measure of protection against your machines that provide services to the Internet (such as a public web server), as well as provide the security of an application layer gateway to the internal network. Additionally, using this method, an attacker, in order to get to services on the internal network, will have to break through the access router, the bastion host, and the choke router.
Conclusions
Security is a very difficult topic. Everyone has a different idea of what ``security'' is, and what levels of risk are acceptable. The key for building a secure network is to define what security means to your organization . Once that has been defined, everything that goes on with the network can be evaluated with respect to that policy. Projects and systems can then be broken down into their components, and it becomes much simpler to decide whether what is proposed will conflict with your security policies and practices.
Security is everybody's business, and only with everyone's cooperation, an intelligent policy, and consistent practices, will it be achievable.
References
1 The New Lexicon Webster's Encyclopedic Dictionary of the English Language. New York: Lexicon.
2 R.T. Morris, 1985. A Weakness in the 4.2BSD Unix TCP/IP Software. Computing Science Technical Report No. 117, AT&T Bell Laboratories, Murray Hill, New Jersey.
3 S.M. Bellovin. Security Problems in the TCP/IP Protocol Suite. Computer Communication Review, Vol. 19, No. 2, pp. 32-48, April 1989.
4 Y. Rekhter, R. Moskowitz, D. Karrenberg, G. de Groot, E. Lear, ``Address Allocation for Private Internets.'' RFC 1918.
5 J.P. Holbrook, J.K. Reynolds. ``Site Security Handbook.'' RFC 1244.
6 M. Curtin, ``Snake Oil Warning Signs: Encryption Software to Avoid.'' USENET <sci.crypt> Frequently Asked Questions File.
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