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Wednesday, 18 October 2017 17:04

Viruses, Worms & Trojans

A computer virus, according to Webster's Collegiate Dictionary, is "a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs or files, and that usually performs a malicious action (such as destroying data)". Two categories of viruses, macro viruses and worms, are especially common today. Computer viruses are never naturally occurring; they are always man-made. Once created and released, however, their spread is not directly under human control.

Macro viruses
A macro is a piece of code that can be embedded in a data file. Some word processors (e.g., Microsoft Word) and spreadsheet programs (e.g., Microsoft Excel) allow you to attach macros to the documents they create. In this way, documents can control and customize the behavior of the programs that created them, or even extend the capabilities of the program. For example, a macro attached to a Microsoft Word document might be executed every time you save the document and cause its text to be run through an external spell checking program.

A macro virus is a virus that exists as a macro attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to data files (i.e., documents) rather than executable programs. Many people do not think that viruses can reside on simple document files, but any application which supports document-bound macros that automatically execute is a potential haven for macro viruses. By the end of the last century, documents became more widely shared than diskettes, and document-based viruses were more prevalent than any other type of virus. It seems highly likely that this will be a continuing trend.

Worms are very similar to viruses in that they are computer programs that replicate functional copies of themselves (usually to other computer systems via network connections) and often, but not always, contain some functionality that will interfere with the normal use of a computer or a program. The difference is that unlike viruses, worms exist as separate entities; they do not attach themselves to other files or programs. Because of their similarity to viruses, worms are often also referred to as viruses.

Trojan horse?
Named after the wooden horse the Greeks used to infiltrate Troy, a Trojan horse is a program that does something undocumented which the programmer intended, but that the user would not approve of if he or she knew about it. According to some people, a virus is a particular case of a Trojan horse, namely one which is able to spread to other programs (i.e., it turns them into Trojans too). According to others, a virus that does not do any deliberate damage (other than merely replicating) is not a Trojan. Finally, despite the definitions, many people use the term "Trojan" to refer only to a non-replicating malicious program.

SSH and SSH2

The program SSH (Secure Shell) is a secure replacement for telnet and the Berkeley r-utilities (rlogin, rsh, rcp, and rdist). It provides an encrypted channel for logging into another computer over a network, executing commands on a remote computer, and moving files from one computer to another. SSH provides strong host-to-host and user authentication as well as secure encrypted communications over an insecure Internet.

SSH2 is a more secure, efficient, and portable version of SSH that includes SFTP, which is functionally similar to FTP, but is SSH2 encrypted. At Indiana University, UITS has upgraded its central systems to SSH2 (usually the OpenSSH version), and encourages those concerned with secure communications to connect using an SSH2 client.

Mac OS X comes with OpenSSH built in.

When connecting to a server for the first time, SSH presents you with a host key fingerprint for that server and asks you to confirm that you wish to save the new host key to the local database. Before agreeing, you should compare this fingerprint with one you obtain by some other means (e.g., by telephone) from the server administrators to avoid connecting to an imposter server. To avoid this message the next time you connect, click Yes.

Rather than validating identities via passwords, SSH2 can also use public key encryption to authenticate remote hosts. For example, if you were to connect to a remote host called global.conspiracy.org (also running SSH2), SSH2 would use this system to verify that the remote system is the real global.conspiracy.org and not a computer set up to imitate it. If you wish, you can set up SSH2 to use public key authentication rather than passwords for logging into your other accounts, much like the Unix rlogin program. For more information on how to set up SSH and SSH2 to use public key authentication, see In SSH and SSH2 for Unix, how do I set up public key authentication?

Files can also be transferred between the SSH client and server using protocols such as SCP and SFTP, both of which run on top of SSH. While SCP is essentially the old Unix rcp utility transplanted onto a different transport, SFTP is a very flexible remote file manipulation protocol that can be used for a wide variety of purposes. It is also much better standardized. If you find yourself in doubt over which one of these protocols to use, use SFTP. (Note that, apart from the name, SFTP carries virtually no semblance to the FTP protocol that everybody knows and uses. Technically, the protocols are completely different.)

Finally, SSH also provides a service known as the exec request, which is conceptually very similar to a remote console, only without the console. The exec request executes a program on the server like a remote console does, but the program's input and output are sent raw, without any terminal encoding. Exec requests are very useful for network automation purposes.


SMTP stands for Simple Mail Transfer Protocol. When someone speaks of a SMTP server they are referring to an outgoing mail server. Most hosting companies do not offer access to SMTP servers as they can be difficult to maintain and defend and the want to cut corners with their customers.

The ability to provide E-mail services to clients includes two critical functions: SMTP and POP3. Together, they provide the means for clients to send outgoing mail and check for new incoming mail, respectively.

SMTP service is the side of e-mail that allows clients to send outgoing e-mail messages to any valid e-mail address. The SMTP server performs two basic but important functions. First, it verifies that anyone attempting to send outgoing e-mail through the SMTP server has the right to do so. Secondly, it sends the outgoing mail and if undeliverable, sends the message back to the sender.

Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.

You need to configure your e-mail client so that it knows what SMTP server to use for sending outgoing e-mail messages. In order to send mail through the proper SMTP server, configure your e-mail client to access the SMTP Server: yourdomain.com or IP address.


SFTP, or secure FTP, is a program that uses SSH to transfer files. Unlike standard FTP, it encrypts both commands and data, preventing passwords and sensitive information from being transmitted in the clear over the network. It is functionally similar to FTP, but because it uses a different protocol, you can't use a standard FTP client to talk to an SFTP server, nor can you connect to an FTP server with a client that supports only SFTP.

There are two ways you can use SFTP: graphical SFTP clients and command line SFTP.

Graphical SFTP clients
Using graphical SFTP clients simplifies file transfers by allowing you to transmit files simply by dragging and dropping icons between windows. When you open the program, you will have to enter the name of the host (e.g., steel.ucs.indiana.edu) and your username and password. Two common SFTP programs are MacSFTP (for Mac OS and Mac OS X) and SSH Secure Shell (for Windows). If you are a student, faculty member, or staff member at Indiana University, you can download these programs from IUware Online. To download the software, you must be connected to the Internet either from an on-campus Ethernet connection, through the IU modem pools, or using IU's remote VPN.

Command line SFTP
You can use command line SFTP from your Unix account, or from your Mac OS X or Unix workstation. To start an SFTP session, at the command prompt, enter: sftp username@host

Some standard commands for command line SFTP include:

cd:  Change the directory on the remote computer  
chmod:  Change the permissions of files on the remote computer  
chown:  Change the owner of files on the remote computer  
dir (or ls):  List the files in the current directory on the remote computer  
exit (or quit):  Close the connection to the remote computer and exit SFTP  
get:  Copy a file from the remote computer to the local computer  
help (or ?):   Get help on the use of SFTP commands  
lcd:   Change the directory on the local computer  
lls:  See a list of the files in the current directory on the local computer  
lmkdir:  Create a directory on the local computer  
ln (or symlink):  Create a symbolic link for a file on the remote computer  
lpwd:  Show the current directory (present working directory) on the local computer  
lumask:  Change the local umask value  
mkdir:  Create a directory on the remote computer  
put: Copy a file from the local computer to the remote computer  
pwd:  Show the current directory (present working directory) on the remote computer  
rename:  Rename a file on the remote host  
rm:  Delete files from the remote computer  
rmdir:  Remove a directory on the remote host (the directory usually has to be empty)  
version:  Display the SFTP version  
!:  In Unix, exit to the shell prompt, where you can enter commands. Enter exit to get back to SFTP. If you follow  !  with a command (e.g., !pwd), SFTP will execute the command without dropping you to the Unix prompt.


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