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Next: 13. LINUX Resources Up: rute Previous: 11. User Accounts and Contents
Subsections
- 12.1
ssh, nottelnetorrlogin - 12.2
rcpandscp - 12.3
rsh - 12.4 FTP
- 12.5
finger - 12.6 Sending Files by Email
12. Using Internet Services
This chapter summarizes remote access and the various methods of transferring files and data over the Internet.
12.1
ssh, not
telnet or
rlogin
telnet is a program for talking to a UNIX network service. It is
most often used to do a remote login. Try
|
telnet <remote_machine>telnet localhost |
to log in to your remote machine. It needn't matter if there is no physical network; network services always work regardless because the machine always has an internal link to itself.
rlogin is like a minimal version of
telnet
that allows login access only. You can type
|
rlogin -l <username> <remote_machine>rlogin -l jack localhost |
if the system is configured to support remote logins.
These two services are the domain of old world UNIX;
for security reasons,
ssh is now the preferable service for
logging in remotely:
|
ssh [-l <username>] <remote_machine> |
Though
rlogin and
telnet are very convenient,
they should never be used across a public network because
your password can easily be read off the wire as you type it in.
12.2
rcp and
scp
rcp stands for remote copy and
scp is
the secure version from the
ssh package.
These two commands copy files from one machine to another using a similar
notation to
cp.
|
rcp [-r] [<remote_machine>:]<file> [<remote_machine>:]<file>scp [-l <username>] [-r] [<remote_machine>:]<file> [<remote_machine>:]<file> |
Here is an example:
5 10 |
[psheer@cericon]# [psheer@cericon]# The authenticity of host 'divinian.cranzgot.co.za' can't be established.RSA key fingerprint is 43:14:36:5d:bf:4f:f3:ac:19:08:5d:4b:70:4a:7e:6a.Are you sure you want to continue connecting (yes/no)? Warning: Permanently added 'divinian.cranzgot.co.za' (RSA) to the list of known hosts.psheer@divinian's password: psheer 100% |***************************************| 4266 KB 01:18 |
The
-r option copies recursively and
copies can take place in either direction or even between two
nonlocal machines.
scp should always be used instead of
rcp
for security reasons. Notice also
the warning given by
scp for this first-time
connection. See the
ssh documentation for how
to make your first connection securely. All commands
in the
ssh package have this same behavior.
12.3
rsh
rsh (remote shell) is a useful utility for
executing a command on a
remote machine. Here are some
examples:
5 |
[psheer@cericon]# divinian.cranzgot.co.za[psheer@cericon]# tar: Removing leading `/' from member names20+0 records in20+0 records out[psheer@cericon]# |
The first command prints the host name of the remote machine.
The second command backs up my remote home directory to my local
floppy disk. (More about
dd
and
/dev/fd0 come later.)
The last command appends my local mailbox file to a remote mailbox file.
Notice how stdin, stdout, and stderr
are properly redirected to the local
terminal. After reading Chapter 29 see
rsh(8) or
in.rshd(8) to configure this service.
Once again, for security reasons
rsh should
never be available across a public network.
12.4 FTP
FTP stands for File Transfer Protocol. If FTP is set up on your local machine, then other machines can download files. Type
|
ftp metalab.unc.edu |
or
|
ncftp metalab.unc.edu |
ftp is the traditional command-line UNIX FTP
client, [``client'' always indicates
the user program accessing some remote service.]while ncftp is a more powerful client that will not
always be installed.
You will now be inside an FTP session. You will be asked
for a login name and a password. The site
metalab.unc.edu is one that allows anonymous
logins. This means that you can type
anonymous as your
user name, and then anything you like as a password. You will
notice that the session will ask you for an email address as your
password. Any sequence of letters with an
@ symbol will
suffice, but you should put your actual email address out of
politeness.
The FTP session is like a reduced shell. You can type
cd,
ls, and
ls -al to view file lists.
help brings up
a list of commands, and you can also type
help <command>
to get help on a specific command. You can download a file by using
the
get <filename> command, but before you do this, you
must set the transfer type to binary. The
transfer type indicates whether or not newline
characters will be translated to DOS format. Typing
ascii turns on this feature, while
binary turns it off.
You may also want to enter
hash which will print a
# for every 1024 bytes of download. This is useful for
watching the progress of a download. Go to a directory that has a
README
file in it and enter
|
get README |
The file will be downloaded into your current directory.
You can also
cd to the
/incoming directory and
upload files. Try
|
put README |
to upload the file that you have just downloaded. Most FTP sites
have an
/incoming directory that is flushed periodically.
FTP allows far more than just uploading of files, although the administrator has the option to restrict access to any further features. You can create directories, change ownerships, and do almost anything you can on a local file system.
If you have several machines on a trusted LAN (Local Area Network--that is, your private office or home network), all should have FTP enabled to allow users to easily copy files between machines. How to install and configure one of the many available FTP servers will become obvious later in this book.
12.5
finger
finger is a service for remotely listing who is logged in on a remote system.
Try
finger @<hostname> to see who is logged in on
<hostname>. The finger service will often be disabled on
machines for security reasons.
12.6 Sending Files by Email
Mail is being used more and more for transferring files between machines. It is bad practice to send mail messages over 64 kilobytes over the Internet because it tends to excessively load mail servers. Any file larger than 64 kilobytes should be uploaded by FTP onto some common FTP server. Most small images are smaller than this size, hence sending a small JPEG [A common Internet image file format. These are especially compressed and are usually under 100 kilobytes for a typical screen-sized photograph.] image is considered acceptable.
12.6.1
uuencode and
uudecode
If you must send files by mail then you can do it by using
uuencode. This utility packs binary files
into a format that mail servers can handle. If you send a mail
message containing arbitrary binary data, it will more than
likely be corrupted on the way because mail agents are only
designed to handle a limited range of characters.
uuencode represents a binary file with
allowable characters, albeit taking up slightly more space.
Here is a neat trick to pack up a directory and send it to someone by mail.
|
tar -czf - <mydir> | uuencode <mydir>.tar.gz \ | mail -s "Here are some files" <user>@<machine> |
To unpack a
uuencoded file, use the
uudecode
command:
|
uudecode <myfile>.uu |
12.6.2 MIME encapsulation
Most graphical mail readers have the ability to attach files to
mail messages and read these attachments. The way they do this
is not with
uuencode but in a special format known as
MIME encapsulation. MIME (Multipurpose Internet
Mail Extensions) is a way of representing multiple
files inside a single mail message. The way binary data is
handled is similar to
uuencode, but in a format known
as base64.
Each MIME attachment to a mail message has a particular type,
known as the MIME type. MIME types
merely classify the attached file as an image, an audio clip,
a formatted document, or some other type of data. The MIME type
is a text tag with the format
<major>/<minor>.
The
major part is called the major MIME type and the
minor part is called the minor MIME type. Available
major types match all the kinds of files that you would expect to
exist. They are usually one of
application,
audio,
image,
message,
text, or
video.
The
application type means a file format specific to
a particular utility. The minor MIME types run into the hundreds.
A long list of MIME types can be found in
/etc/mime.types.
If needed, some useful command-line utilities in the
same vein as
uuencode can create and extract MIME
messages. These are
mpack,
munpack, and
mmencode
(or
mimencode).
Next: 13. LINUX Resources Up: rute Previous: 11. User Accounts and Contents