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Groups provide a coarse mechanism for sharing information between users. Similar to the /etc/passwd file, the /etc/group file is a table that identifies all the group names on the system, the associated GID, and a comma-separated list of the account names that make up the group membership (Example 2.11). UNIX assumes a limited group set to implement default access privileges. The system administrator can add users to these default groups or create new ones to facilitate additional collaborations. If the user base is small, this can be done relatively easily. For large numbers of users, managing GID sets can be a big chore. Example 2.11 /etc/group system:!:0:root,ops daemon:!:1: bin:!:2:root,bin sys:!:3:root,bin,sys adm:!:4:bin,adm,kenm,root uucp:!:5:uucp mail:!:6: security:!:7:root cron:!:8:root staff:!:10:root,ren,stimpy,daffy,huey,dewey user:!:30:luge,acadmus,gwyneira,bungi
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The Network Information Service (NIS) is an administrative tool that can be used to distribute common system configuration information between machines. NIS and NIS+ are what was commonly known as "Yellow Pages (YP)." NIS is a real boon to system administrators charged with maintaining a large number of machines in a lab or distributed network. The collection of participating machines is called the NIS domain. Each file in the distribution set is converted into ndbm database format and stored as an NIS map file on the server. The collection of NIS maps makes up the NIS database which is distributed periodically to the machines in the NIS domain. As you may have guessed, two files commonly distributed using NIS are the /etc/passwd and /etc/group files. NIS provides a convenient method for synchronizing account information across multiple machines. In cases where only a portion of the full set of accounts and groups need to be kept common between machines, NIS allows for segregating the accounts and groups into local and distributed categories (Table 2.10). Local information is unique to each machine. Distributed information is configuration information administered on the master NIS server and pushed to participating machines for the sake of uniformity. Local data override conflicting distributed data. Table 2.10 NIS Password and Group Files
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/etc/passwd /etc/passwd.yp /etc/passwd.local /etc/group /etc/group.yp /etc/group.local
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There are a number of authentication systems available for UNIX, which augment the functionality and security of the basic password and group architecture. One such system is the Kerberos authentication developed at M.I.T., which is based on the Trusted ThirdParty authentication model. The original design is described in a series of papers presented at the 1988 USENIX Winter Conference. Kerberos is an important technology regarding UNIX and NT coexistence, because it is the network authentication system in Windows 2000. Kerberos assumes that everything in the network is untrustworthy except the Kerberos authentication server itself. The Kerberos authentication server is called the Key Distribution Center (KDC). The KDC acts as an intermediary between the client and the desired services. The client must authenticate itself to the KDC to obtain a ticket, which grants the rights to access distributed services (Figure 2.5). This ticket is called a Ticket Granting Ticket (TGT). It is only necessary to validate yourself once to the KDC rather than once for each service you wish to access. The various Kerberos tickets are also only valid for a given period of time. The Kerberos ticket mechanism eliminates the need to transmit passwords over the network in clear text. The client and server passwords are known and stored by the KDC.
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Figure 2.5: Kerberos ticket exchange
Whenever access to services is requested by an unauthenticated client, a message is sent to the KDC that contains client name and the Kerberos ticket-granting service name. The KDC looks up the names and obtains the encryption key for the client and the ticket server. The encryption key is known only to the owning agent and the KDC. A message is then constructed by the KDC containing the client and ticket server names, address, and a random session key, which it encrypts using the client's encryption key. The message is called a ticket and is sent to the client. The client uses its encryption key to decrypt the ticket and stores the ticket for the duration for which it is valid. The session key is used to encrypt ticket communication with the Kerberos ticket service to gain access to other services and resources. When access to a service is requested, the ticket service provides a new session key along with a ticket encrypted with the services encryption key from the KDC's database. The client uses the new session key to create an authenticator ticket, which identifies the client, and sends it along with the encrypted service ticket to the new service. The service decrypts the ticket using its encryption key. The service ticket contains the session key, which is then used to decrypt the authenticator ticket. Now the client and service know about each other and real work can begin.
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This may seem like a lot of hand waving to inhibit clear text passwords and authentication information from being broadcast over the network. On the other hand, it is extremely easy to eavesdrop on the wire. All this negotiation is taking place under the covers, so it is not visible to the end-user. Remember that Kerberos will not restrict access to someone who has already compromised another user's login ID and password.
Summary
UNIX has a three-tiered architecture: kernel, shell, and command layers. Executing tasks and programs are called processes and are identified by PID number. Server processes are called daemons. Init, PID 1, is responsible for spawning system processes, which are identified in the /etc/inittab file. TCP/IP is a network protocol with four layers: physical, internetwork, transport, and application. Each computer on the network is identified by a four-octet IP number. Groups of networked computers are partitioned using subnets to isolate and manage traffic and to enforce security policies. A hierarchical dynamic network name service called the Domain Name Service (DNS) resolves domain name to IP numbers. The UNIX DNS server is called named. DNS server addresses are identified in /etc/resolv.conf. The inetd server starts network servers when requested by a client defined as /etc/inetd.conf. Well-known service name-to-port mapping is recorded in /etc/services. UNIX file systems are contained in collections of disk blocks called partitions. File system structure within a partition is represented as the superblock, inode table, and data blocks. File systems contain a hierarchical collection of files and directories, each of which is recorded in an inode. All UNIX objects are abstracted as files. File and directory access controls are recorded in the inode indicating read, write, and execute permissions for owner, group, and world. NFS is a stateless networked file system for sharing files and directories.
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Local and remote printing and queuing is managed by the lpd daemon. Printer and queue configuration information is recorded in /etc/printcaps. UNIX users are identified by an account name, UID and GID. User identification information is stored in /etc/passwd. Restricted access to the encrypted password database is enabled through the use of shadow password files. Password access performance can be improved through the use of a dbm password database, /etc/passwd.dir, and /etc/passwd.pag. Group authorization is defined in /etc/groups. NIS can be used to distribute password and group information to multiple machines that share a common user base. Kerberos is a network-based authentication system based on Trusted Third-Party model. Kerberos authentication is supported in Windows 2000.
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