c# printing barcode An event showing that a TGT was requested and the result was successful in .NET

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An event showing that a TGT was requested and the result was successful
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Monitoring Network Protocol Security 11-43
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The member server caches the TGT and can use it when necessary to request access to services. In fact, that is exactly what happens next. The server requests service tickets from the KDC. If you examine additional records in the security log near the TGT request, you will find that a request for a service ticket is successful as well. At this point, the server uses the TGT to obtain a service ticket for access to its own resources. The Kerberos packets in the capture do not reveal any interesting informa tion the data is encrypted. You should also note the connection to download repre sentative policy modules. Kerberos at User Logon Next, when a domain user in this case, the Administrator logs on, the process repeats. Credentials are presented and a TGT is requested. If the credentials are approved, the TGT is issued. In the Network Monitor log, more UDP frames are bound for port 88 on the domain controller, followed by a response. Check the time of these frames (Figure 11-15) and follow up with a look at the Security Event log for this time.
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Security Event log showing a TGT request
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Figure 11-16 shows the successful TGT request for the Administrator.
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Figure 11-16
Security Event log confirming successful request
Now life gets interesting. The TGT for the Administrator account, like the TGT for the system, is stored in the Kerberos ticket cache. It is used when the Administrator account requests access to services. You can examine your ticket cache by using the Kerbtray.exe utility. To use Kerbtray.exe, you run the self-installing file by doubleclicking it and then clicking the executable to run the tool. This procedure places an icon on the taskbar, which can then be opened by clicking it to reveal the tickets in the cache. Figure 11-17 shows the list of tickets in the cache and the Administrator account s TGT ticket.
Figure 11-17 Using the Kerbtray.exe utility to view Kerberos tickets in the ticket cache of the local machine
Lesson 2
Monitoring Network Protocol Security 11-45
One of the first requests is for the services of the local computer. In Figure 11-17, notice that tickets are listed for several services: Host, IAM$, Common Internet File System (CIFS), and LDAP. All these services were requested at logon and represent the Admin istrator s ability to access the local computer (Host and IAM$), to access a share on the domain controller (CIFS), and to make LDAP queries to the directory service (LDAP). Requests for service tickets include the TGT and another authenticator. Because the TGT is cached, you might wonder whether it could be captured and possibly used in a replay attack. The use of a new authenticator protects the KDC. Because the time on the server has changed, the authenticator message, the timestamp, will always be current, and the KDC can check that it is within the time skew policy of its domain. (Time skew is the difference between the KDC s time and the client s; if it is off by more than the policy skew time, the Kerberos request is rejected.) Kerberos Role in Authorization Kerberos is an authentication protocol, but it does play a role in authorization. If you map a drive to a share on a computer, the Kerberos TGT requests a session ticket for the CIFS Server service on the computer, imple mented using the CIFS protocol. If a Network Monitor capture is made, you can trace the steps in accessing the share. However, the service ticket does not give users access to the share. The ticket authen ticates users only to the server. It says, in essence, that the users are who they say they are, and the server does not have to check in with the domain controller to verify this information. A portion of the service ticket is encrypted using the password hash for the server, so the server can decrypt it. Remember, the DC stores password hashes for computers as well as users. Because the server can decrypt the ticket, the server rec ognizes that it is valid and must come from a DC, because the DC is the only other entity that has the server s password. Where does the authorization information come from The service ticket, although it is only validation of user identity, does contain information useful for authorization. This information is the same as that collected by the KDC when the user first presented domain credentials, the user s security identifier (SID), and the SIDs of the groups of which the user is a member. The file server uses this information to create the access token, and then the file server can determine whether the user has the proper permis sion to access the share and the folders and files underneath. You can use the Security Event log to determine whether access was allowed or denied, and Kerbtray.exe shows the CIFS service ticket in the cache. Note that the CIFS service ticket in the cache is issued for a specific server. If the user attempts access to a share on this server or a reconnection if the connection has been broken this ser vice ticket can be used. If the user attempts access to another server, a new service ticket must be obtained. Once again, the service ticket requests are not viewable. However, Kerbtray.exe can be used to verify that a service ticket is issued. Figure 11-17 shows a CIFS service ticket.
11-46
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