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Phrack Inc. Volume 03 Issue 27 File 03
==Phrack Inc.==
Volume Three, Issue 27, File 3 of 12
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<> <>
<> Introduction to MIDNET <>
<> ~~~~~~~~~~~~~~~~~~~~~~ <>
<> Chapter Seven Of The Future Transcendent Saga <>
<> <>
<> A More Indepth Look Into NSFnet <>
<> National Science Foundation Network <>
<> <>
<> Presented by Knight Lightning <>
<> June 16, 1989 <>
<> <>
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Prologue
~~~~~~~~
If you are not already familiar with NSFnet, I would suggest that you read:
"Frontiers" (Phrack Inc., Volume Two, Issue 24, File 4 of 13), and definitely;
"NSFnet: National Science Foundation Network" (Phrack Inc., Volume Three,
Issue 26, File 4 of 11).
Table Of Contents
~~~~~~~~~~~~~~~~~
* Introduction
* The DOD Protocol Suite
* Names and Addresses In A Network
* Telnet (*NOT* Telenet)
* File Transfer
* Mail
Introduction
~~~~~~~~~~~~
MIDNET is a regional computer network that is part of the NSFnet, the National
Science Foundation Network. Currently, eleven mid-United States universities
are connected to each other and to the NSFnet via MIDnet:
UA - University of Arkansas at Fayetteville
ISU - Iowa State University at Ames
UI - University of Iowa at Iowa City
KSU - Kansas State University at Manhattan
KU - University of Kansas at Lawrence
UMC - University of Missouri at Columbia
WU - Washington University at St. Louis, Missouri
UNL - University of Nebraska at Lincoln
OSU - Oklahoma State University at Stillwater
UT - University of Tulsa (Oklahoma)
OU - University of Oklahoma at Norman
Researchers at any of these universities that have funded grants can access the
six supercomputer centers funded by the NSF:
John Von Neuman Supercomputer Center
National Center for Atmospheric Research
Cornell National Supercomputer Facility
National Center for Supercomputing Applications
Pittsburgh Supercomputing Center
San Diego Supercomputing Center
In addition, researchers and scientists can communicate with each other over a
vast world-wide computer network that includes the NSFnet, ARPAnet, CSnet,
BITnet, and others that you have read about in The Future Transcendent Saga.
Please refer to "Frontiers" (Phrack Inc., Volume Two, Issue 24, File 4 of 13)
for more details.
MIDnet is just one of several regional computer networks that comprise the
NSFnet system. Although all of these regional computer networks work the same,
MIDnet is the only one that I have direct access to and so this file is written
from a MIDnet point of view. For people who have access to the other regional
networks of NSFnet, the only real differences depicted in this file that would
not apply to the other regional networks are the universities that are served
by MIDnet as opposed to:
NYSERnet in New York State
SURAnet in the southeastern United States
SEQSUInet in Texas
BARRnet in the San Francisco area
MERIT in Michigan
(There are others that are currently being constructed.)
These regional networks all hook into the NSFnet backbone, which is a network
that connects the six supercomputer centers. For example, a person at Kansas
State University can connect with a supercomputer via MIDnet and the NSFnet
backbone. That researcher can also send mail to colleagues at the University
of Delaware by using MIDnet, NSFnet and SURAnet. Each university has its own
local computer network which connects on-campus computers as well as providing
a means to connecting to a regional network.
Some universities are already connected to older networks such as CSnet, the
ARPAnet and BITnet. In principal, any campus connected to any of these
networks can access anyone else in any other network since there are gateways
between the networks.
Gateways are specialized computers that forward network traffic, thereby
connecting networks. In practice, these wide-area networks use different
networking technology which make it impossible to provide full functionality
across the gateways. However, mail is almost universally supported across all
gateways, so that a person at a BITnet site can send mail messages to a
colleague at an ARPAnet site (or anywhere else for that matter). You should
already be somewhat familiar with this, but if not refer to;
"Limbo To Infinity" (Phrack Inc., Volume Two, Issue 24, File 3 of 13) and
"Internet Domains" (Phrack Inc., Volume Three, Issue 26, File 8 of 11)
Computer networks rely on hardware and software that allow computers to
communicate. The language that enables network communication is called a
protocol. There are many different protocols in use today. MIDnet uses the
TCP/IP protocols, also known as the DOD (Department of Defense) Protocol Suite.
Other networks that use TCP/IP include ARPAnet, CSnet and the NSFnet. In fact,
all the regional networks that are linked to the NSFnet backbone are required
to use TCP/IP. At the local campus level, TCP/IP is often used, although other
protocols such as IBM's SNA and DEC's DECnet are common. In order to
communicate with a computer via MIDnet and the NSFnet, a computer at a campus
must use TCP/IP directly or use a gateway that will translate its protocols
into TCP/IP.
The Internet is a world-wide computer network that is the conglomeration of
most of the large wide area networks, including ARPAnet, CSnet, NSFnet, and the
regionals, such as MIDnet. To a lesser degree, other networks such as BITnet
that can send mail to hosts on these networks are included as part of the
Internet. This huge network of networks, the Internet, as you have by now read
all about in the pages of Phrack Inc., is a rapidly growing and very complex
entity that allows sophisticated communication between scientists, students,
government officials and others. Being a part of this community is both
exciting and challenging.
This chapter of the Future Transcendent Saga gives a general description of the
protocols and software used in MIDnet and the NSFNet. A discussion of several
of the more commonly used networking tools is also included to enable you to
make practical use of the network as soon as possible.
The DOD Protocol Suite
~~~~~~~~~~~~~~~~~~~~~~
The DOD Protocol Suite includes many different protocols. Each protocol is a
specification of how communication is to occur between computers. Computer
hardware and software vendors use the protocol to create programs and sometimes
specialized hardware in order to implement the network function intended by the
protocol. Different implementations of the same protocol exist for the varied
hardware and operating systems found in a network.
The three most commonly used network functions are:
Mail -- Sending and receiving messages
File Transfer -- Sending and receiving files
Remote Login -- Logging into a distant computer
Of these, mail is probably the most commonly used.
In the TCP/IP world, there are three different protocols that realize these
functions:
SMTP -- (Simple Mail Transfer Protocol) Mail
FTP -- (File Transfer Protocol) sending and receiving files
Telnet -- Remote login
How to use these protocols is discussed in the next section. At first glance,
it is not obvious why these three functions are the most common. After all,
mail and file transfer seem to be the same thing. However, mail messages are
not identical to files, since they are usually comprised of only ASCII
characters and are sequential in structure. Files may contain binary data and
have complicated, non-sequential structures. Also, mail messages can usually
tolerate some errors in transmission whereas files should not contain any
errors. Finally, file transfers usually occur in a secure setting (i.e. The
users who are transferring files know each other's names and passwords and are
permitted to transfer the file, whereas mail can be sent to anybody as long as
their name is known).
While mail and transfer accomplish the transfer of raw information from one
computer to another, Telnet allows a distant user to process that information,
either by logging in to a remote computer or by linking to another terminal.
Telnet is most often used to remotely log in to a distant computer, but it is
actually a general-purpose communications protocol. I have found it incredibly
useful over the last year. In some ways, it could be used for a great deal of
access because you can directly connect to another computer anywhere that has
TCP/IP capabilities, however please note that Telnet is *NOT* Telenet.
There are other functions that some networks provide, including the following:
- Name to address translation for networks, computers and people
- The current time
- Quote of the day or fortune
- Printing on a remote printer, or use of any other remote peripheral
- Submission of batch jobs for non-interactive execution
- Dialogues and conferencing between multiple users
- Remote procedure call (i.e. Distributing program execution over several
remote computers)
- Transmission of voice or video information
Some of these functions are still in the experimental stages and require faster
computer networks than currently exist. In the future, new functions will
undoubtedly be invented and existing ones improved.
The DOD Protocol Suite is a layered network architecture, which means that
network functions are performed by different programs that work independently
and in harmony with each other. Not only are there different programs but
there are different protocols. The protocols SMTP, FTP and Telnet are
described above. Protocols have been defined for getting the current time, the
quote of the day, and for translating names. These protocols are called
applications protocols because users directly interact with the programs that
implement these protocols.
The Transmission Control Protocol, TCP, is used by many of the application
protocols. Users almost never interact with TCP directly. TCP establishes a
reliable end-to-end connection between two processes on remote computers. Data
is sent through a network in small chunks called packets to improve reliability
and performance. TCP ensures that packets arrive in order and without errors.
If a packet does have errors, TCP requests that the packet be retransmitted.
In turn, TCP calls upon IP, Internet Protocol, to move the data from one
network to another. IP is still not the lowest layer of the architecture,
since there is usually a "data link layer protocol" below it. This can be any
of a number of different protocols, two very common ones being X.25 and
Ethernet.
FTP, Telnet and SMTP are called "application protocols", since they are
directly used by applications programs that enable users to make use of the
network. Network applications are the actual programs that implement these
protocols and provide an interface between the user and the computer. An
implementation of a network protocol is a program or package of programs that
provides the desired network function such as file transfer. Since computers
differ from vendor to vendor (e.g. IBM, DEC, CDC), each computer must have its
own implementation of these protocols. However, the protocols are standardized
so that computers can interoperate over the network (i.e. Can understand and
process each other's data). For example, a TCP packet generated by an IBM
computer can be read and processed by a DEC computer.
In many instances, network applications programs use the name of the protocol.
For example, the program that transfers files may be called "FTP" and the
program that allows remote logins may be called "Telnet." Sometimes these
protocols are incorporated into larger packages, as is common with SMTP. Many
computers have mail programs that allow users on the same computer to send mail
to each other. SMTP functions are often added to these mail programs so that
users can also send and receive mail through a network. In such cases, there
is no separate program called SMTP that the user can access, since the mail
program provides the user interface to this network function.
Specific implementation of network protocols, such as FTP, are tailored to the
computer hardware and operating system on which they are used. Therefore, the
exact user interface varies from one implementation to another. For example,
the FTP protocol specifies a set of FTP commands which each FTP implementation
must understand and process. However, these are usually placed at a low level,
often invisible to the user, who is given a higher set of commands to use.
These higher-level commands are not standardized so they may vary from one
implementation of FTP to another. For some operating systems, not all of these
commands make equal sense, such as "Change Directory," or may have different
meanings. Therefore the specific user interface that the user sees will
probably differ.
This file describes a generic implementation of the standard TCP/IP application
protocols. Users must consult local documentation for specifics at their
sites.
Names and Addresses In A Network
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In DOD Protocol Suite, each network is given a unique identifying number. This
number is assigned by a central authority, namely the Network Information
Center run by SRI, abbreviated as SRI-NIC, in order to prevent more than one
network from having the same network number. For example, the ARPAnet has
network number 10 while MIDnet has a longer number, namely 128.242.
Each host in a network has a unique identification so other hosts can specify
them unambiguously. Host numbers are usually assigned by the organization that
manages the network, rather than one central authority. Host numbers do not
need to be unique throughout the whole Internet but two hosts on the same
network need to have unique host numbers.
The combination of the network number and the host number is called the IP
address of the host and is specified as a 32-bit binary number. All IP
addresses in the Internet are expressible as 32-bit numbers, although they are
often written in dotted decimal notation. Dotted decimal notation breaks the
32-bit number into four eight-bit parts or octets and each octet is specified
as a decimal number. For example, 00000001 is the binary octet that specifies
the decimal number 1, while 11000000 specifies 192. Dotted decimal notation
makes IP addresses much easier to read and remember.
Computers in the Internet are also identified by hostnames, which are strings
of characters, such as "phrackvax." However, IP packets must specify the
32-bit IP address instead of the hostname so some way to translating hostnames
to IP addresses must exist.
One way is to have a table of hostnames and their corresponding IP addresses,
called a hosttable. Nearly every TCP/IP implementation has such a hosttable,
although the weaknesses of this method are forcing a shift to a new scheme
called the domain name system. In UNIX systems, the hosttable is often called
"/etc/hosts." You can usually read this file and find out what the IP
addresses of various hosts are. Other systems may call this file by a
different name and make it unavailable for public viewing.
Users of computers are generally given accounts to which all charges for
computer use are billed. Even if computer time is free at an installation,
accounts are used to distinguish between the users and enforce file
protections. The generic term "username" will be used in this file to refer to
the name by which the computer account is accessed.
In the early days of the ARPAnet which was the first network to use the TCP/IP
protocols, computer users were identified by their username, followed by a
commercial "at" sign (@), followed by the hostname on which the account
existed. Networks were not given names, per se, although the IP address
specified a network number.
For example, "knight@phrackvax" referred to user "knight" on host "phrackvax."
This did not specify which network "phrackvax" was on, although that
information could be obtained by examining the hosttable and the IP address for
"phrackvax." (However, "phrackvax" is a ficticious hostname used for this
presentation.)
As time went on, every computer on the network had to have an entry in its
hosttable for every other computer on the network. When several networks
linked together to form the Internet, the problem of maintaining this central
hosttable got out of hand. Therefore, the domain name scheme was introduced to
split up the hosttable and make it smaller and easier to maintain.
In the new domain name scheme, users are still identified by their usernames,
but hosts are now identified by their hostname and any and all domains of which
they are a part. For example, the following address,
"KNIGHT@UMCVMB.MISSOURI.EDU" specifies username "KNIGHT" on host "UMCVMB".
However, host "UMCVMB" is a part of the domain "MISSOURI" " which is in turn
part of the domain "EDU". There are other domains in "EDU", although only one
is named "MISSOURI". In the domain "MISSOURI", there is only one host named
"UMCVMB".
However, other domains in "EDU" could theoretically have hosts named "UMCVMB"
(although I would say that this is rather unlikely in this example). Thus the
combination of hostname and all its domains makes it unique. The method of
translating such names into IP addresses is no longer as straightforward as
looking up the hostname in a table. Several protocols and specialized network
software called nameservers and resolvers implement the domain name scheme.
Not all TCP/IP implementations support domain names because it is rather new.
In those cases, the local hosttable provides the only way to translate
hostnames to IP addresses. The system manager of that computer will have to
put an entry into the hosttable for every host that users may want to connect
to. In some cases, users may consult the nameserver themselves to find out the
IP address for a given hostname and then use that IP address directly instead
of a hostname.
I have selected a few network hosts to demonstrate how a host system can be
specified by both the hostname and host numerical address. Some of the nodes I
have selected are also nodes on BITnet, perhaps even some of the others that I
do not make a note of due a lack of omniscent awareness about each and every
single host system in the world :-)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Numerical Hostname Location BITnet
--------- -------- -------- ------
18.72.0.39 ATHENA.MIT.EDU (Mass. Institute of Technology) ?
26.0.0.73 SRI-NIC.ARPA (DDN Network Information Center) -
36.21.0.13 MACBETH.STANFORD.EDU (Stanford University) ?
36.21.0.60 PORTIA.STANFORD.EDU (Stanford University) ?
128.2.11.131 ANDREW.CMU.EDU (Carnegie Mellon University) ANDREW
128.3.254.13 LBL.GOV (Lawrence Berkeley Labrotories) LBL
128.6.4.7 RUTGERS.RUTGERS.EDU (Rutgers University) ?
128.59.99.1 CUCARD.MED.COLUMBIA.EDU (Columbia University) ?
128.102.18.3 AMES.ARC.NASA.GOV (Ames Research Center [NASA]) -
128.103.1.1 HARVARD.EDU (Harvard University) HARVARD
128.111.24.40 HUB.UCSB.EDU (Univ. Of Calif-Santa Barbara) ?
128.115.14.1 LLL-WINKEN.LLNL.GOV (Lawrence Livermore Labratories) -
128.143.2.7 UVAARPA.VIRGINIA.EDU (University of Virginia) ?
128.148.128.40 BROWNVM.BROWN.EDU (Brown University) BROWN
128.163.1.5 UKCC.UKY.EDU (University of Kentucky) UKCC
128.183.10.4 NSSDCA.GSFC.NASA.GOV (Goddard Space Flight Center [NASA])-
128.186.4.18 RAI.CC.FSU.EDU (Florida State University) FSU
128.206.1.1 UMCVMB.MISSOURI.EDU (Univ. of Missouri-Columbia) UMCVMB
128.208.1.15 MAX.ACS.WASHINGTON.EDU (University of Washington) MAX
128.228.1.2 CUNYVM.CUNY.EDU (City University of New York) CUNYVM
129.10.1.6 NUHUB.ACS.NORTHEASTERN.EDU (Northeastern University) NUHUB
131.151.1.4 UMRVMA.UMR.EDU (University of Missouri-Rolla) UMRVMA
192.9.9.1 SUN.COM (Sun Microsystems, Inc.) -
192.33.18.30 VM1.NODAK.EDU (North Dakota State Univ.) NDSUVM1
192.33.18.50 PLAINS.NODAK.EDU (North Dakota State Univ.) NDSUVAX
Please Note: Not every system on BITnet has an IP address. Likewise, not
every system that has an IP address is on BITnet. Also, while
some locations like Stanford University may have nodes on BITnet
and have hosts on the IP as well, this does not neccessarily
imply that the systems on BITnet and on IP (the EDU domain in
this case) are the same systems.
Attempts to gain unauthorized access to systems on the Internet
are not tolerated and is legally a federal offense. At some
hosts, they take this very seriously, especially the government
hosts such as NASA's Goddard Space Flight Center, where they do
not mind telling you so at the main prompt when you connect to
their system.
However, some nodes are public access to an extent. The DDN
Network Information Center can be used by anyone. The server and
database there have proven to be an invaluable source of
information when locating people, systems, and other information
that is related to the Internet.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Telnet
~~~~~~
Remote login refers to logging in to a remote computer from a terminal
connected to a local computer. Telnet is the standard protocol in the DOD
Protocol Suite for accomplishing this. The "rlogin" program, provided with
Berkeley UNIX systems and some other systems, also enables remote login.
For purposes of discussion, the "local computer" is the computer to which your
terminal is directly connected while the "remote computer" is the computer on
the network to which you are communicating and to which your terminal is *NOT*
directly connected.
Since some computers use a different method of attaching terminals to
computers, a better definition would be the following: The "local computer" is
the computer that you are currently using and the "remote computer" is the
computer on the network with which you are or will be communicating. Note that
the terms "host" and "computer" are synonymous in the following discussion.
To use Telnet, simply enter the command: TELNET
The prompt that Telnet gives is: Telnet>
(However, you can specify where you want to Telnet to immediately and bypass
the the prompts and other delays by issuing the command: TELNET [location].)
There is help available by typing in ?. This prints a list of all the valid
subcommands that Telnet provides with a one-line explanation.
Telnet> ?
To connect to to another computer, use the open subcommand to open a connection
to that computer. For example, to connect to the host "UMCVMB.MISSOURI.EDU",
do "open umcvmb.missouri.edu"
Telnet will resolve (i.e. Translate, the hostname "umcvmb.missouri.edu" into an
IP address and will send a packet to that host requesting login. If the remote
host decides to let you attempt a login, it prompts you for your username and
password. If the host does not respond, Telnet will "time out" (i.e. Wait for
a reasonable amount of time such as 20 seconds) and then terminate with a
message such as "Host not responding."
If your computer does not have an entry for a remote host in its hosttable and
it cannot resolve the name, you can use the IP address explicitly in the telnet
command. For example,
TELNET 26.0.0.73 (Note: This is the IP address for the DDN Network Information
Center [SRI-NIC.ARPA])
If you are successful in logging in, your terminal is connected to the remote
host. For all intents and purposes, your terminal is directly hard-wired to
that host and you should be able to do anything on your remote terminal that
you can do at any local terminal. There are a few exceptions to this rule,
however.
Telnet provides a network escape character, such as CONTROL-T. You can find out
what the escape character is by entering the "status" subcommand:
Telnet> status
You can change the escape character by entering the "escape" subcommand:
Telnet> escape
When you type in the escape character, the Telnet prompt returns to your screen
and you can enter subcommands. For example, to break the connection, which
usually logs you off the remote host, enter the subcommand "quit":
Telnet> quit
Your Telnet connection usually breaks when you log off the remote host, so the
"quit" subcommand is not usually used to log off.
When you are logged in to a remote computer via Telnet, remember that there is
a time delay between your local computer and the remote one. This often
becomes apparent to users when scrolling a long file across the terminal screen
nd they wish to cancel the scrolling by typing CONTROL-C or something similar.
After typing the special control character, the scrolling continues. The
special control character takes a certain amount of time to reach the remote
computer which is still scrolling information. Thus response from the remote
computer will not likely be as quick as response from a local computer.
Once you are remotely logged on, the computer you are logged on to effectively
becomes your "local computer," even though your original "local computer" still
considers you logged on. You can log on to a third computer which would then
become your "local computer" and so on. As you log out of each session, your
previous session becomes active again.
File Transfer
~~~~~~~~~~~~~
FTP is the program that allows files to be sent from one computer to another.
"FTP" stands for "File Transfer Protocol".
When you start using FTP, a communications channel with another computer on the
network is opened. For example, to start using FTP and initiate a file
transfer session with a computer on the network called "UMCVMB", you would
issue the following subcommand:
FTP UMCVMB.MISSOURI.EDU
Host "UMCVMB" will prompt you for an account name and password. If your login
is correct, FTP will tell you so, otherwise it will say "login incorrect." Try
again or abort the FTP program. (This is usually done by typing a special
control character such as CONTROL-C. The "program abort" character varies from
system to system.)
Next you will see the FTP prompt, which is:
Ftp>
There are a number of subcommands of FTP. The subcommand "?" will list these
commands and a brief description of each one.
You can initiate a file transfer in either direction with FTP, either from the
remote host or to the remote host. The "get" subcommand initiates a file
transfer from the remote host (i.e. Tells the remote computer to send the file
to the local computer [the one on which you issued the "ftp" command]). Simply
enter "get" and FTP will prompt you for the remote host's file name and the
(new) local host's file name. Example:
Ftp> get
Remote file name?
theirfile
local file name?
myfile
ou can abbreviate this by typing both file names on the same line as the "get"
subcommand. If you do not specify a local file name, the new local file will
be called the same thing as the remote file. Valid FTP subcommands to get a
file include the following:
get theirfile myfile
get doc.x25
The "put" subcommand works in a similar fashion and is used to send a file from
the local computer to the remote computer. Enter the command "put" and FTP
will prompt you for the local file name and then the remote file name. If the
transfer cannot be done because the file doesn't exist or for some other
reason, FTP will print an error message.
There are a number of other subcommands in FTP that allow you to do many more
things. Not all of these are standard so consult your local documentation or
type a question mark at the FTP prompt. Some functions often built into FTP
include the ability to look at files before getting or putting them, the
ability to change directories, the ability to delete files on the remote
computer, and the ability to list the directory on the remote host.
An intriguing capability of many FTP implementations is "third party
transfers." For example, if you are logged on computer A and you want to cause
computer B to send a file to computer C, you can use FTP to connect to computer
B and use the "rmtsend" command. Of course, you have to know usernames and
passwords on all three computers, since FTP never allows you to peek into
someone's directory and files unless you know their username and password.
The "cd" subcommand changes your working directory on the remote host. The
"lcd" subcommand changes the directory on the local host. For UNIX systems,
the meaning of these subcommands is obvious. Other systems, especially those
that do not have directory-structured file system, may not implement these
commands or may implement them in a different manner.
The "dir" and "ls" subcommands do the same thing, namely list the files in the
working directory of of the remote host.
The "list" subcommand shows the contents of a file without actually putting it
into a file on the local computer. This would be helpful if you just wanted to
inspect a file. You could interrupt it before it reached the end of the file
by typing CONTROL-C or some other special character. This is dependent on your
FTP implementation.
The "delete" command can delete files on the remote host. You can also make
and remove directories on the remote host with "mkdir" and "rmdir". The
"status" subcommand will tell you if you are connected and with whom and what
the state of all your options are.
If you are transferring binary files or files with any non-printable
characters, turn binary mode on by entering the "binary" subcommand:
binary
To resume non-binary transfers, enter the "ascii" subcommand.
Transferring a number of files can be done easily by using "mput" (multiple
put) and "mget" (multiple get). For example, to get every file in a particular
directory, first issue a "cd" command to change to that directory and then an
"mget" command with an asterisk to indicate every file:
cd somedirectory
mget *
When you are done, use the "close" subcommand to break the communications link.
You will still be in FTP, so you must use the "bye" subcommand to exit FTP and
return to the command level. The "quit" subcommand will close the connection
and exit from FTP at the same time.
Mail
~~~~
Mail is the simplest network facility to use in many ways. All you have to do
is to create your message, which can be done with a file editor or on the spur
of the moment, and then send it. Unlike FTP and Telnet, you do not need to
know the password of the username on the remote computer. This is so because
you cannot change or access the files of the remote user nor can you use their
account to run programs. All you can do is to send a message.
There is probably a program on your local computer which does mail between
users on that computer. Such a program is called a mailer. This may or may
not be the way to send or receive mail from other computers on the network,
although integrated mailers are more and more common. UNIX mailers will be
used as an example in this discussion.
Note that the protocol which is used to send and receive mail over a TCP/IP
network is called SMTP, the "Simple Mail Transfer Protocol." Typically, you
will not use any program called SMTP, but rather your local mail program.
UNIX mailers are usually used by invoking a program named "mail". To receive
new mail, simply type "mail".
There are several varieties of UNIX mailers in existence. Consult your local
documentation for details. For example, the command "man mail" prints out the
manual pages for the mail program on your computer.
To send mail, you usually specify the address of the recipient on the mail
command. For example: "mail knight@umcvmb.missouri.edu" will send the
following message to username "knight" on host "umcvmb".
You can usually type in your message one line at a time, pressing RETURN after
each line and typing CONTROL-D to end the message. Other facilities to include
already-existing files sometimes exist. For example, Berkeley UNIXes allow you
to enter commands similar to the following to include a file in your current
mail message:
r myfile
In this example, the contents of "myfile" are inserted into the message at this
point.
Most UNIX systems allow you to send a file through the mail by using input
redirection. For example:
mail knight@umcvmb.missouri.edu < myfile
In this example, the contents of "myfile" are sent as a message to "knight" on
"umcvmb."
Note that in many UNIX systems the only distinction between mail bound for
another user on the same computer and another user on a remote computer is
simply the address specified. That is, there is no hostname for local
recipients. Otherwise, mail functions in exactly the same way. This is common
for integrated mail packages. The system knows whether to send the mail
locally or through the network based on the address and the user is shielded
from any other details.
"The Quest For Knowledge Is Without End..."
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