分类： 系统运维
2012-06-21 10:49:28
Many sites do not run a Finger server for two reasons. First, a
programming error in an earlier version of the server was one of the
entry points used by the infamous Internet worm of 1988. (RFC 1135
[Reynolds 1989] and [Curry 1992] describe the worm in more detail.)
Second, the Finger protocol can reveal detailed information on users
(login names, phone numbers, when they last logged in, etc.) that many
administrators consider private. Section 3 of RFC 1288 details the
security aspects of this service.
From a protocol perspective,
the Finger server has a well-known port of 79. The client does an active
open to this port and sends a one-line query The server processes the
query, sends back the output, and closes the connection. The query and
response are NVT ASCII, similar to what we saw with FTP and SMTP.
Whois Protocol
The
Whois protocol is another information service. Although any site can
provide a Whois server, the one at the InterNIC, rs.internic.net, is
most commonly used. This server maintains information about all
registered DNS domains and many system administrators responsible for
systems connected to the Internet. (Another server is provided at
nic.ddn.mil, but contains information only about the MILNET.)
Unfortunately the information can be out of data or incomplete. RFC 954
[Harrenstien, Stahl, and Feinler 1985] documents the Whois service.
From a protocol perspective, the Whois server has a well-known TCP port of 43. It accepts connection requests from clients, and the client sends a one-line query to the server. The server responds with whatever information is available and then closes the connection. The requests and replies are transmitted using NVT ASCII. This is almost identical to the Finger server, although the requests and replies contain different information.
The common Unix client is the whois(1) program, although we can
use Telnet and type in the commands ourself. The starting place is to
send a request consisting of just a question mark, which returns more
detailed information on the supported client requests.
White Pages
Using the VRFY command of SMTP, along with the Finger protocol and
the Whois protocol to locate users on the Internet is similar to using
the white pages of a telephone book to find someone's phone number. At
the present time ad hoc tools such as these are all that's widely
available, but research is under way to improve this type of service.
Archie
Many
of the resources used in this text were obtained using anonymous FTP.
The problem is finding which FTP site has the program we want. Sometimes
we don't even know the exact filename, but we know some keywords that
probably appear in the filename.
Archie provides a directory of
thousands of FTP servers across the Internet. We can access this
directory by logging into an Archie server and searching for files whose
name contains a specified regular expression. The output is a list of
servers with matching filenames. We then use anonymous FTP to that site
to fetch the file.
There are many Archie servers across the world. One starting point is
to use Telnet to ds.internic.net, login as archie, and execute the
command servers. This provides a list of all the Archie servers, and
their location.
WAIS: Wide Area Information Servers
Archie
helps us locate filenames that contain keywords, but sometimes we're
looking for a file or database that contains a keyword. That is, we want
to search for a file that contains a keyword, not a filename containing
a keyword.
WAIS knows about hundreds of databases that contain information on
both computer-related topics and other general topics. To use WAIS we
select the databases to search and specify the keywords. To try WAIS
Telnet to quake.think.com and login as wais.
Gopher
Gopher
is a menu-driven front end to other Internet resource services, such as
Archie, WAIS, and anonymous FTP Gopher is one of the easiest to use,
since its user interface is the same, regardless of which resource
service it's using.
To use Gopher, Telnet into is.internic.net and login as gopher.
Veronica: Very Easy Rodent-Oriented Netwide Index to Computerized Archives
Just as Archie is an index of anonymous FTP servers. Veronica is an index of titles of Gopher items. A Veronica search typically searches hundreds of Gopher servers.
To access Veronica we must go through a Gopher client. Select the
Gopher menu item "Beyond InterNIC: Virtual Treasures of the Internet"
and then select Veronica from the next menu.
WWW: World Wide Web
World
Wide Web lets us browse a large, worldwide set of services and
documents using a tool called hypertext. As information is displayed,
certain keywords are highlighted, and we can select more information on
those keywords. To access WWW, Telnet to info.cern.ch.
X Window System
The
X Window System, or just X, is a client-server application that lets
multiple clients (applications) use the bit-mapped display managed by a
server. "The server is the software that manages a display, keyboard,
and mouse. The client is an application program that runs on either the
same host as the server or on a different host. In the latter case the
common form of communication between the client and server is TCP,
although other protocols such as DECNET can be used. In some instances
the server is a dedicated piece of hardware (an X terminal) that
communicates with clients on other hosts. In another instance, a
stand-alone workstation, the client and server are on the same host and
communicate using interprocess communication on that host, without any
network involvement at all. Between these two extremes is a workstation
that
supports clients on the same host and clients on other hosts.
X
requires a reliable, bidirectional stream protocol, such as TCP. (X was
not designed for an unreliable protocol such as UDP.) The communication
between the client and server consists of 8-bit bytes exchanged across
this connection. [Nye 1992] gives the format of the more than 150
messages exchanged between the client and server across their TCP
connection.
On a Unix system, when the X client and X server are on the same
host, the Unix domain protocols are normally used instead of TCP,
because there is less protocol processing than if TCP were used. The
Unix domain protocols are a form of interprocess communication that can
be used between clients and servers on the same host.
