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Principles of Operating Systems

Notes depending on the implementation details of a given client-server facility. Client identification
is more difficult. Clients can be specified by their network name or other identifier, such as IP
address, but these can be spoofed (or imitated). An unauthorized client can spoof the server
into deciding that it is authorized, and the unauthorized client could be allowed access. More
secure solutions include secure authentication of the client to the server via encrypted keys.
Unfortunately, with security comes many challenges, including ensuring compatibility of the
client and server (they must use the same encryption algorithms) and secure key exchanges
(intercepted keys could again allow unauthorized client access). These problems are difficult
enough that, most commonly, unsecure authentication methods are used. In the case of UNIX
and its network file system (NFS), authentication is via the client networking information, by
default. In this scheme, the user IDS must match on the client and server. If not, the server will
be unable to determine access rights to files. Consider the example of a user who has the ID of
1000 on the client and 2000 on the server. A request from the client to the server for a specific
file will not be handled appropriately, as the server will determine if user 1000 has access to the
file, rather than the real user ID of 2000. Access would be granted or denied based on incorrect
authentication information. The server must trust the client to present the correct user ID. The
NFS protocols allow many-to-many relationships. That is, many servers can provide files to many
clients. In fact, a given machine can be both a server to other NFS clients and a client of other
NFS servers. Once the remote file system is mounted, file operation requests are sent on the
behalf of the user, across the network, to the server, via the DFS protocol. Typically, a file open
request is sent along with the ID of the requesting user. The server then applies the standard
access checks to determine if the user has credentials to access the file in the mode requested.
The request is either allowed or denied. If it is allowed, a file handle is returned to the client
application, and the application then may perform read, write, and other operations on the file.
The client closes the file when access is completed. The operating system may apply semantics
similar to those for a local file system mount, or may have different semantics.

6.7.4 Distributed Information Systems
To ease the management of client-server services, distributed information systems, also known
as distributed naming services, have been devised to provide a unified access to the information
needed for remote computing. Domain name system (DNS) provides host-name-to-network-
address translations for the entire Internet (including the World Wide Web). Before DNS was
invented and became widespread, files containing the same information were sent via email
or f t p between all networked hosts. This methodology was not scalable. Other distributed
information systems provide user narne/password/user ID/group ID space for a distributed
facility. UNIX systems have had a wide variety of distributed information methods. Sun
Microsystems introduced yellozv pages (since renamed to network information service (NIS)),
and most of the industry adopted its use. It centralizes storage of user names, host names, printer
information, and the like. Unfortunately, it uses unsecure authentication methods, including
sending user passwords unencrypted (in clear text) and identifying hosts by IP address. Sun’s
NIS+ is a much more secure replacement for NIS, but is also much more complicated and has
not been widely adopted. In the case of Microsoft networks (CIFS), network information is used
in conjunction with user authentication (user name and password) to create a network login
that the server uses to decide whether to allow or deny access to a requested file system. For
this authentication to be valid, the user names must match between the machines (as with NFS).
Microsoft uses two distributed naming structures to provide a single namespace for users. The
older naming technology is domains. The newer technology, available in Windows 2000 and
beyond, is active directory. Once established, the distributed-naming facility is used by all clients
and servers to authenticate users. The industry is moving toward lightweight directory-access
protocol (LDAP) as a secure, distributed naming mechanism. In fact, active directory is based
on LDAP. Sun Microsystems’ Solaris 8 allows LDAP to be used for user authentication as well
as system-wide retrieval of information such as available printers. If the convergence of the use
of LDAP succeeds, then one distributed LDAP directory will be used by an organization to store
all user and resource information for all computers within that organization. The result would

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