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Securing a TCP/IP Internetwork
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Security is a concern to all network mangers and administrators, but the level of security that is appropriate for any given internetwork can be determined only by those responsible for that internetwork Obviously, an internetwork servicing financial transactions between banks justifies more security measures than an internetwork providing access to public-domain technical articles When implementing security measures, there often is a tradeoff between security and userfriendliness The risk of a security breach, along with its likely occurrence and its impact on the internetwork owner, must be judged on a case-by-case basis Broadly, there are three areas to be considered when security is designed into an internetwork: physical, network, and application layer issues
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At the Application layer, features such as Application level usernames and passwords can be implemented, and parameters such as the number of concurrent logins for each username, frequency at which a password will be forced to change, and minimum length of passwords can be enforced Typically, all traditional multiuser systems (such as mini- or mainframe hosts, and PC servers like Novell or Windows NT) support these type of features as standard For further information on the Application-level security features of any network operating system, refer to the supplier's documentation As more and more information is made available via the World Wide Web technologies such as HyperText Markup Language (HTML) and HyperText Transport Protocol (HTTP), new Application layer security issues have arisen We will highlight some of the issues associated with implementing security measures at the Application level for the newer Web-based technologies that utilize browsers and Web servers These issues are relevant to securing communications on either an internal intranet or the public Internet It should be noted that the following discussion is intended only to introduce the concepts of Application level security, because the focus of this book is on the Network and lower layers and how Cisco implements the features of these layers Traditional servers (NetWare, NT, etc) authenticate users based on the use of stateful protocols This means that a user establishes a connection to the server and that connection is maintained for the duration of the user's session At any one time, the server will know who is logged on and from where HTTP servers were designed for the rapid and efficient delivery of hypertext documents and therefore use a stateless protocol An HTTP connection has four distinct stages: 1The client contacts the server at the Internet address specified in the URL 2The client requests service, and delivers information about what it can and cannot support 3The server sends the state of the transaction and, if successful, the data requested; if unsuccessful, it transmits a failure code 4Ultimately the connection is closed and the server does not maintain any memory of the i h j k l
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transaction that just took place Because the server does not maintain a connection, it does not know if multiple people are using the same username and password, or from what protocol address they are connecting This is a concern, as users could share usernames and passwords with unauthorized individuals without any loss of service to themselves What is needed is a mechanism that restores the functionality of allowing only one username and password to be used from one location at a time Cookies can help in this situation In HTTP terms, a cookie is a type of license with a unique ID number Cookies give you the ability to force a user to log in again if he or she uses the same username and password from a different workstation Cookies work like this: A user logs in for the first time, in this case his/her browser has no cookie to present The server either issues a cookie associated with this user if the user has logged in before, or issues a new cookie number if the user has not If a request from another workstation using the same username and password comes along, the server issues a new cookie to that connection, making the original cookie invalid The user at the original workstation makes another request, with the cookie number originally received The server sees that a new cookie number already has been issued and returns an "unauthorized" header, prompting the user for the username/password pair This mechanism ensures that each username and password pair can be used only from one location at a time Cookies don't answer all security issues, but they are a piece of the puzzle for providing security features in an environment using Web-based technologies Overview of Cryptography Having illustrated the conceptual difference between the traditional stateful protocols and the Web-based stateless protocols, we will discuss what technologies can address the following concerns: 1How do I authenticate users so that I can be assured users are who they claim to be 2How can I authorize users for some network services and not others 3How can I ensure that store-and-forward (e-mail) applications, as well as direct browser-toserver communications, are conducted privately 4How do I ensure that messages have not been altered during transmission The foundation for supplying answers to these questions is based on cryptography, which is a set of technologies that provides the following capabilities: Authentication to identify a user, particular computer, or organization on the internetwork
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Digital signatures and signature verification to associate a verified signature with a particular user Encryption and decryption to ensure that unauthorized users cannot intercept and read a message before it reaches its destination Authorization, using access control lists to restrict users to accessing specified resources There are two types of cryptography that use a key system to encrypt and decrypt messages, symmetric-key and public-key The following explanations are simplifications of what actually takes place; the implementations use sophisticated mathematical techniques to ensure their integrity Symmetric-key cryptography uses the same key to encrypt and decrypt a message The problem with this method is the need to securely coordinate the same key to be in use at both ends of the communication If it is transmitted between sender and receiver, it is open to being captured and used by a malicious third party Public-key cryptography uses a different approach, wherein each user has a public key and a private key When user 1 wants to send a message to user 2, user 1 uses the public key of user 2 to encrypt the message before it is sent The public key of user 2 is freely available to any other user When user 2 receives the message, it is decrypted using the private key of user 2 Sophisticated mathematical techniques ensure that a message encrypted using the public key can be decrypted only by using the proper private key This enables messages to be securely exchanged without the sender and receiver having to share secret information before they communicate In addition to public-key cryptography, public-key certificates, which are also called digital IDs, can be used to authenticate users to ensure they are who they claim to be Certificates are small files containing user-specific information Defined in an ITU standard (X509), certificates include the following information: A name uniquely identifying the owner of the certificate This name includes the username, company, or organization with which the user is associated, and the user's country of residence The name and digital signature of the device that issued the certificate to the user The owner's public key The period during which the certificate is valid Public-Key Technologies Public-key technology is implemented in industry-standard protocols such as the Secure Sockets Layer (SSL) and the Secure Multipurpose Internet Mail Extensions (S/MIME) protocols These protocols address the issues raised at the beginning of the section as follows: Authentication of users is by digital certificates Authorization, to grant users access to specific resources, is provided by binding users listed in l li ifi d h ki f di i l i
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access control lists to certificates and checking of digital signatures Ensuring privacy of communication between two computers is enabled by the use of public-key technologies Ensuring that messages have not been altered during transmission is covered by the implementation of Digest algorithms, such as Message Digest 5 (MD5) as used by Cisco's implementation of CHAP CHAP will be discussed more fully in the section on Network layer security Typically, browser-to-Webserver communications are based on HTTP, which uses a default port number of 80 for communications SSL uses port 443, so you can force all connections to use SSL and therefore be subject to SSL security features by allowing connections to a Web server only using port 443 This is easily achieved by using Cisco access lists, as shown in Fig 7-9
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Figure 7-9: Implementing a router to force LAN systems to use a Secure Sockets layer connection (port 443)
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