Tài liệu TCP/IP Network Administration- P11 docx

[Chapter 11] 11.9 Simple Network Management Protocol useful information for spotting usage trends and potential trouble spots. Every agent supports MIBI or MIBII. Some systems also provide a private MIB in addition to the standard MIBII. Private MIBs add to the monitoring capability by providing system-specific information. Most UNIX systems do not provide private MIBs. Private MIBs are most common on network hardware like routers, hubs, and switches. No matter what MIBs are provided by the agents, it is the monitoring software that displays the information for the system administrator. A private MIB won't do you any good unless your network monitoring software also supports that MIB. For this reason, most administrators prefer to purchase a monitor from the vendor that supplies the bulk of their network equipment. Another possibility is to select a monitor that includes a MIB compiler, which gives you the most flexibility. A MIB compiler reads in the ASN.1 description of a MIB and adds the MIB to the monitor. A MIB compiler makes the monitor extensible because if you can get the ASN.1 source from the network equipment vendor, you can add the vendor's private MIB to your monitor. MIB compilers are only part of the advanced features offered by some monitors. Some of the features offered are: Network maps Some monitors automatically draw a map of the network. Colors are used to indicate the state (up, down, etc.) of the devices on the network. At a glance, the network manager sees the overall state of the network. Tabular data displays Data displayed in tables or rendered into charts is used to make comparisons between different devices. Some monitors output data that can then be read into a standard spreadsheet or graphing program. Filters Filters sift the data coming in from the agents in order to detect certain conditions. Alarms Alarms indicate when "thresholds" are exceeded or special events occur. For example, you may want an alarm to trigger when your server exceeds some specified number of transmit errors. Don't be put off by the jargon. All of this detail is necessary to formally define a network management scheme that is independent of the managed systems, but you don't need to memorize it. You need to know that a MIB is a collection of management information, that an NMS is the network management station, and that an agent runs in each managed device in order to make intelligent decisions when selecting an SNMP monitor. This information provides that necessary background. The features available in network monitors vary widely; so does the price. Select an SNMP monitor that is suitable for the complexity of your network and the size of your budget. file:///C|/mynapster/Downloads/warez/tcpip/ch11_09.htm (4 of 5) [2001-10-15 09:18:52] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 11] 11.9 Simple Network Management Protocol Previous: 11.8 Protocol Case Study TCP/IP Network Administration Next: 11.10 Summary 11.8 Protocol Case Study Book Index 11.10 Summary [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] file:///C|/mynapster/Downloads/warez/tcpip/ch11_09.htm (5 of 5) [2001-10-15 09:18:52] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. file:///C|/mynapster/Downloads/warez/tcpip/ch11_10.htm Previous: 11.9 Simple Network Management Protocol Chapter 11 Troubleshooting TCP/IP Next: 12. Network Security 11.10 Summary Every network will have problems. This chapter discusses the tools and techniques that can help you recover from these problems, and the planning and monitoring that can help avoid them. A solution is sometimes obvious if you can just gain enough information about the problem. UNIX provides several built-in software tools that can help you gather information about system configuration, addressing, routing, name service and other vital network components. Gather your tools and learn how to use them before a breakdown occurs. In the next chapter, we talk about another task that is important to the maintenance of a reliable network: keeping your network secure. Previous: 11.9 Simple Network Management Protocol TCP/IP Network Administration Next: 12. Network Security 11.9 Simple Network Management Protocol Book Index 12. Network Security [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] file:///C|/mynapster/Downloads/warez/tcpip/ch11_10.htm [2001-10-15 09:18:53] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 12] Network Security Previous: 11.10 Summary Chapter 12 Next: 12.2 User Authentication 12. Network Security Contents: Security Planning User Authentication Application Security Security Monitoring Access Control Encryption Firewalls Words to the Wise Summary Hosts attached to a network - particularly the worldwide Internet - are exposed to a wider range of security threats than are unconnected hosts. Network security reduces the risks of connecting to a network. But by nature, network access and computer security work at cross-purposes. A network is a data highway designed to increase access to computer systems, while security is designed to control access. Providing network security is a balancing act between open access and security. The highway analogy is very appropriate. Like a highway, the network provides equal access for all - welcome visitors as well as unwelcome intruders. At home, you provide security for your possessions by locking your house, not by blocking the streets. Likewise, network security generally means providing adequate security on individual host computers, not providing security directly on the network. In very small towns, where people know each other, doors are often left unlocked. But in big cities, doors have deadbolts and chains. In the last decade, the Internet has grown from a small town of a few thousand users to a big city of millions of users. Just as the anonymity of a big city turns neighbors into strangers, the growth of the Internet has reduced the level of trust between network neighbors. The ever-increasing need for computer security is an unfortunate side effect. Growth, however, is not all bad. In the same way that a big city offers more choices and more services, the expanded network provides increased services. For most of us, security consciousness is a small price to pay for network access. file:///C|/mynapster/Downloads/warez/tcpip/ch12_01.htm (1 of 7) [2001-10-15 09:18:54] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 12] Network Security Network break-ins have increased as the network has grown and become more impersonal, but it is easy to exaggerate the extent of these security breaches. Over-reacting to the threat of break-ins may hinder the way you use the network. Don't make the cure worse than the disease. The best advice about network security is to use common sense. RFC 1244, Site Security Handbook, by Holbrook, Reynold, et al., states this principle very well: Common sense is the most appropriate tool that can be used to establish your security policy. Elaborate security schemes and mechanisms are impressive, and they do have their place, yet there is little point in investing money and time on an elaborate implementation scheme if the simple controls are forgotten. This chapter emphasizes the simple controls that can be used to increase your network's security. A reasonable approach to security, based on the level of security required by your system, is the most cost-effective - both in terms of actual expense and in terms of productivity. 12.1 Security Planning One of the most important network security tasks, and probably one of the least enjoyable, is developing a network security policy. Most computer people want a technical solution to every problem. We want to find a program that "fixes" the network security problem. Few of us want to write a paper on network security policies and procedures. However, a well-thought-out security plan will help you decide what needs to be protected, how much you are willing to invest in protecting it, and who will be responsible for carrying out the steps to protect it. 12.1.1 Assessing the Threat The first step toward developing an effective network security plan is to assess the threat that connection presents to your systems. RFC 1244 identifies three distinct types of security threats usually associated with network connectivity: Unauthorized access A break-in by an unauthorized person. Disclosure of information Any problem that causes the disclosure of valuable or sensitive information to people who should not have access to the information. Denial of service Any problem that makes it difficult or impossible for the system to continue to perform productive work. Assess these threats in relation to the number of users who would be affected, as well as to the file:///C|/mynapster/Downloads/warez/tcpip/ch12_01.htm (2 of 7) [2001-10-15 09:18:54] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 12] Network Security sensitivity of the information that might be compromised. For some organizations, break-ins are an embarrassment that can undermine the confidence that others have in the organization. Intruders tend to target government and academic organizations that will be embarrassed by the break-in. But for most organizations, unauthorized access is not a major problem unless it involves one of the other threats: disclosure of information or denial of service. Assessing the threat of information disclosure depends on the type of information that could be compromised. While no system with highly classified information should ever be directly connected to the Internet, systems with other types of sensitive information might be connected without undue hazard. In most cases, files such as personnel and medical records, corporate plans, and credit reports can be adequately protected by standard UNIX file security procedures. However, if the risk of liability in case of disclosure is great, the host may choose not to be connected to the Internet. Denial of service can be a severe problem if it impacts many users or a major mission of your organization. Some systems can be connected to the network with little concern. The benefit of connecting individual workstations and small servers to the Internet generally outweighs the chance of having service interrupted for the individuals and small groups served by these systems. Other systems may be vital to the survival of your organization. The threat of losing the services of a mission-critical system must be evaluated seriously before connecting such a system to the network. In his class on computer security, Brent Chapman classifies information security threats into three categories: threats to the secrecy, availability, and integrity of data. Secrecy is the need to prevent the disclosure of sensitive information. Availability means that you want information and information processing resources available when they are needed; a denial-of-service attack disrupts availability. The need for the integrity of information is equally obvious, but its link to computer security is more subtle. Once someone has gained unauthorized access to a system, the integrity of the information on that system is in doubt. Furthermore, some intruders just want to compromise the integrity of data. We are all familiar with cases where intruders gain access to a Web server and change the data on the server in order to embarrass the organization that runs the Web site. Thinking about the impact network threats have on your data can make it easier to assess the threat. Network threats are not, of course, the only threats to computer security, or the only reasons for denial of service. Natural disasters and internal threats (threats from people who have legitimate access to a system) are also serious. Network security has had a lot of publicity, so it's a fashionable thing to worry about; but more computer time has probably been lost because of fires than has ever been lost because of network security problems. Similarly, more data has probably been improperly disclosed by authorized users than by unauthorized break-ins. This book naturally emphasizes network security, but network security is only part of a larger security plan that includes physical security and disaster recovery plans. Many traditional (non-network) security threats are handled, in part, by physical security. Don't forget to provide an adequate level of physical security for your network equipment and cables. Again, the investment in physical security should be based on your realistic assessment of the threat. 12.1.2 Distributed Control file:///C|/mynapster/Downloads/warez/tcpip/ch12_01.htm (3 of 7) [2001-10-15 09:18:54] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 12] Network Security One approach to network security is to distribute responsibility for, and control over, segments of a large network to small groups within the organization. This approach involves a large number of people in security, and runs counter to the school of thought that seeks to increase security by centralizing control. However, distributing responsibility and control to small groups can create an environment of small networks composed of trusted hosts. Using the analogy of small towns and big cities, it is similar to creating a neighborhood watch to reduce risks by giving people connection with their neighbors, mutual responsibility for one another, and control over their own fates. Additionally, distributing security responsibilities formally recognizes one of the realities of network security - most security actions take place on individual systems. The managers of these systems must know that they are responsible for security, and that their contribution to network security is recognized and appreciated. If people are expected to do a job, they must be empowered to do it. 12.1.2.1 Use subnets to distribute control Subnets are a possible tool for distributing network control. A subnet administrator should be appointed when a subnet is created. She is then responsible for the security of the network and for assigning IP addresses to the devices connected to the networks. Assigning IP addresses gives the subnet administrator some control over who connects to the subnet. It also helps to ensure that she knows each system connected and who is responsible for that system. When the subnet administrator gives a system an IP address, she also delegates certain security responsibilities to the system's administrator. Likewise, when the system administrator grants a user an account, the user takes on certain security responsibilities. The hierarchy of responsibility flows from the network administrator, to the subnet administrator, to the system administrator, and finally to the user. At each point in this hierarchy the individuals are given responsibilities and the power to carry them out. To support this structure, it is important for users to know what they are responsible for and how to carry out that responsibility. The network security policy described in the next section provides this information. 12.1.2.2 Use mailing lists to distribute information If your site adopts distributed control, you must develop a system for disseminating security information to each group. Mailing lists for each administrative level can be used for this purpose. The network administrator receives security information from outside authorities, filters out irrelevant material, and forwards the relevant material to the subnet administrators. Subnet administrators forward the relevant parts to their system administrators, who in turn forward what they consider important to the individual users. The filtering of information at each level ensures that individuals get the information they need, without receiving too much. If too much unnecessary material is distributed, users begin to ignore everything they receive. At the top of this information structure is the information that the network administrator receives from outside authorities. In order to receive this, the network administrator should join the appropriate mailing lists and newsgroups and browse the appropriate Web sites. A few places to start looking for file:///C|/mynapster/Downloads/warez/tcpip/ch12_01.htm (4 of 7) [2001-10-15 09:18:54] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 12] Network Security computer security information are the following: Your UNIX Vendor Many vendors have their own security information mailing lists. Security Newsgroups The comp.security newsgroups - comp.security.unix, comp.security.firewalls, comp.security.announce, and comp.security.misc - contain some useful information. Like most newsgroups, they contain lots of unimportant and uninteresting material. But they also contain an occasional gem. FIRST Mailing List The Forum of Incident Response and Security Teams (FIRST) is a worldwide organization of computer security response teams. FIRST provides a public mailing list, first-info@first.org, for computer security information. To subscribe to this list, send email to first- majordomo@first.org that contains the line: subscribe first-info YOUR-EMAIL-ADDRESS where YOUR-EMAIL-ADDRESS is literally your email address. NIST Computer Security Alerts The National Institute of Standards and Technology's Computer Security Division maintains a Web site with pointers to security-related Web pages all over the world. As a single source for security alerts from several different organizations, the site http://csrc.nist.gov/secalert/ can't be beat. Computer Emergency Response Team (CERT) Advisories The CERT advisories provide information about known security problems, and the fixes to these problems. You can retrieve these advisories from ftp://info.cert.org/pub/cert_advisories. The CERT Web site is also worth a visit: http://www.cert.org. DDN Security Bulletins These bulletins are very similar in content to the CERT advisories, though DDN bulletins do occasionally add information. DDN bulletins and CERT advisories deal primarily with network security threats. DDN bulletins can be viewed online with your Web browser at http://nic.ddn.mil/SCC/bulletins.html. Risks Forum The risks forum discusses the full range of computer security risks. The forum is available on the Web at http://catless.ncl.ac.uk/Risks. Computer Virus Information file:///C|/mynapster/Downloads/warez/tcpip/ch12_01.htm (5 of 7) [2001-10-15 09:18:54] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 12] Network Security The VIRUS-L list deals primarily with computer viruses - a threat usually associated with PCs. You can retrieve the VIRUS-L archive from ftp://ftp.infospace.com/pub/virus-l. An equally important document, at http://ciac.llnl.gov/ciac/CIACHoaxes.html, provides information about computer virus hoaxes. False rumors about computer viruses can waste as much time as tracking down real viruses. 12.1.3 Writing a Security Policy Security is largely a "people problem." People, not computers, are responsible for implementing security procedures, and people are responsible when security is breached. Therefore, network security is ineffective unless people know their responsibilities. It is important to write a security policy that clearly states what is expected and who it is expected from. A network security policy should define: The network user's security responsibilities The policy may require users to change their passwords at certain intervals, to use passwords that meet certain guidelines, or to perform certain checks to see if their accounts have been accessed by someone else. Whatever is expected from users, it is important that it be clearly defined. The system administrator's security responsibilities The policy may require that every host use specific security measures, login banner messages, and monitoring and accounting procedures. It might list applications that should not be run on any host attached to the network. The proper use of network resources Define who can use network resources, what things they can do, and what things they should not do. If your organization takes the position that email, files, and histories of computer activity are subject to security monitoring, tell the users very clearly that this is the policy. The actions taken when a security problem is detected What should be done when a security problem is detected? Who should be notified? It is easy to overlook things during a crisis, so you should have a detailed list of the exact steps that a system administrator, or user, should take when a security breach has been detected. This could be as simple as telling the users to "touch nothing, and call the network security officer." But even these simple actions should be in the policy so that they are readily available. Connecting to the Internet brings with it certain security responsibilities. RFC 1281, A Guideline for the Secure Operation of the Internet, provides guidance for users and network administrators on how to use the Internet in a secure and responsible manner. Reading this RFC will provide insight into the information that should be in your security policy. A great deal of thought is necessary to produce a complete network security policy. The outline file:///C|/mynapster/Downloads/warez/tcpip/ch12_01.htm (6 of 7) [2001-10-15 09:18:54] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [Chapter 12] Network Security shown above describes the contents of a network policy document, but if you are personally responsible for writing a policy, you may want more detailed guidance. I also recommend that you read RFC 1244. It is a very good guide for developing a security plan. Security planning (assessing the threat, assigning security responsibilities, and writing a security policy) is the basic building block of network security, but a plan must be implemented before it can have any effect. In the remainder of this chapter, we'll turn our attention to implementing basic security procedures. Previous: 11.10 Summary TCP/IP Network Administration Next: 12.2 User Authentication 11.10 Summary Book Index 12.2 User Authentication [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] file:///C|/mynapster/Downloads/warez/tcpip/ch12_01.htm (7 of 7) [2001-10-15 09:18:54] Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... passwords are not enough Passwords are transmitted across the network as clear text Intruders use protocol-analyzer software to spy on network traffic and steal passwords If a thief steals your password, it does not matter how good the password was The thief can be on any network that handles your TCP/IP packets If you log in through your local network you have to worry only about local snoops But if you... allows you to check all of the systems on the network from one central system If, however, you are responsible for only one system and you use SATAN to probe the other systems on your network, you will irritate all of the other system administrators on the network who will view the SATAN probes as attempted break-ins Use SATAN only to test systems on your own network that you have officially recognized... That "more" is usually some technique for limiting access between systems connected to the network, or for limiting access to the data the network carries In the remainder of this chapter we look at various security techniques that limit access Previous: 12.3 Application Security 12.3 Application Security TCP/IP Network Administration Book Index Next: 12.5 Access Control 12.5 Access Control Please purchase... For those occasions, one-time passwords, as provided by OPIE, are still essential Previous: 12.1 Security Planning 12.1 Security Planning TCP/IP Network Administration Book Index Next: 12.3 Application Security 12.3 Application Security [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] Please purchase PDF Split-Merge on www.verypdf.com to remove this... and then add back to the file only those daemons that you decide you really need 12.3.2 Keep Software Updated Vendors frequently release new versions of network software for the express purpose of improving network security Use the latest version of the network software offered by your vendor Track the security alerts, CERT advisories, and bulletins to know what programs are particularly important to... security fixes Make sure that the vendor knows that security is important to you Previous: 12.2 User Authentication 12.2 User Authentication TCP/IP Network Administration Book Index Next: 12.4 Security Monitoring 12.4 Security Monitoring [ Library Home | DNS & BIND | TCP/IP | sendmail | sendmail Reference | Firewalls | Practical Security ] Please purchase PDF Split-Merge on www.verypdf.com to remove this... Analyzing Networks (SATAN) SATAN's introduction was met by near hysteria in the popular press, largely because of the tool's name Despite its name, SATAN is just another security tool SATAN does have some unique features While COPS is intended for use on an individual system, SATAN is designed to test entire networks of systems This is both a feature and a problem If you are the administrator of your network, ...[Chapter 12] 12.2 User Authentication Previous: 12.1 Security Planning Chapter 12 Network Security Next: 12.3 Application Security 12.2 User Authentication Good passwords are one of the simplest parts of good network security Passwords are used to log in to systems that use password authentication Popular mythology says that network security breaches are caused by sophisticated security crackers who discover... unseen listeners from any number of unknown networks The rlogin command is not vulnerable to this type of attack rlogin does not send the password over the network, because user authentication is done only on the local host The remote host accepts the user because it trusts the local host However, trust should be extended only to UNIX hosts on your local network that you really do trust Never extend... tcpd Make tcpd and then install it in the same directory as the other network daemons Edit /etc/inetd.conf and replace the path to each network service daemon that you wish to place under access control with the path to tcpd The only field in the /etc/inetd.conf entry affected by tcpd is the sixth field, which contains the path to the network daemon For example, assume that the entry for the finger daemon . a reliable network: keeping your network secure. Previous: 11.9 Simple Network Management Protocol TCP/IP Network Administration Next: 12. Network Security. Previous: 11.9 Simple Network Management Protocol Chapter 11 Troubleshooting TCP/IP Next: 12. Network Security 11.10 Summary Every network will have problems.