Автор работы: Пользователь скрыл имя, 21 Мая 2010 в 01:34, Не определен
What a virus is?
A virus is a self-replicating program that spreads by inserting copies of itself into programs or documents that already exist on a computer. The name comes from an analogy with biological viruses. These cannot reproduce by themselves but make use of the functions of infected cells to spread. Similarly, a computer virus makes use of the executable code in legitimate programs to carry out its purposes. A virus may be designed to be destructive to a system or to be a prank. In either case, the virus will rapidly reproduce itself until the system may be overwhelmed. Viruses spread to other systems when infected programs are copied to another machine. Documents with executable code like Word macros can also be vectors of infection. A very common method of spreading viruses is by attachments to email . Today a variant of a virus known as a worm is more often used.
Another method used by spammers is the “dictionary” attack. By combining all common words and names (with variations like joe1, joe2, joe3, etc.) with all the common providers such as AOL, Hotmail, MSN, Earthlink, computer programs can generate millions of possible email addresses. Many of these will be legitimate and the spammer doesn’t care about the ones that bounce. The cost of mailing to a lot of incorrect addresses is too small to be any deterrent. Thus some people advise using uncommon combinations of symbols for your email address.
Everyone should have several disposable junk email address that they use where public exposure is likely. One of the free services like Hotmail or My Yahoo serves admirably for this purpose. If an address starts to attract spam, it can just be discarded.
You
can also “munge” your address in places like Newsgroups. To “Munge”
is to add easily recognized extra characters to your address along with
the accompanying phrase “remove xyz to obtain address”. Thus myname@myISP.com
becomes myname@mynospamISP.com. The only trouble is that address harvesting
software can be programmed to strip out obvious strings like nospam
although many times they don't bother.
Blocking Spam
One method of dealing with spam is to block or filter mail from known spammers or that contain particular subjects or key words. This can be done either on your email program or with special software. The common email programs like Outlook Express allow for setting up rules that apply to categories like senders, subjects, and textual content. Check your particular email client for the details. For example, in Outlook Express go to the menu under Tools-Message Rules. The problem is that spammers keep changing or faking their ostensible names and addresses as well as using phony subjects. Personally, I have found that rules and filtering within my email program may keep out some spam but that it is only a partial answer to the problem. You can also install some extra software. There are a slew of utilities devoted to stopping spam. The best types of programs use a statistical technique known as Bayesian filtering. These programs set up filtering rules based on actual experience and "learn" how to improve filters from the email that you receive. See the sidebar for references on this technique and on various software programs.
Businesses and those who are big users of email will need some heavy-duty methods of filtering spam but average PC users who receive only a few emails each day can use a program like MailWasher Pro. Also, ISPs are getting better at filtering and may also provide some way for individual users to create filtering rules.
There are also services that will filter your mail. By collecting large databases of known spammers and using their client’s emails to keep up with the latest tricks and twists of the spammers, these services can be better at stopping spam than software located on your own computer. These services naturally slow down the processing of your mail since it has to go through their server. Several are listed in the sidebar.
Note that no matter whether you filter mail with software on your own computer or use an external service, some spam will get through and some legitimate mail will get blocked.
Although there are many ways to try to block spam from arriving in your mailbox by using software or filtering services, my experience is that spam has reached the point where one of the best defenses is to have more than one email address. You can reserve one address for friends and relatives and have a second throwaway address that is changed fairly regularly. This second address would be the one that is used whenever it might be subject to public exposure. Many ISPs allow for an account to have multiple mailboxes and one can be set aside for junk. If the volume builds up, the box can be discarded and replaced by a new one. Another route is to use one of the free Internet email services like Yahoo or Hotmail. Yet another approach is to use one of the services that provide email addresses with a limited lifetime. For example, SpamGourmet will give you addresses good for a certain number of uses only.
The
last and perhaps best defense is common sense and the “delete” key.
Don’t open obvious spam messages and be very careful about responding
to “Remove me from this list” type of addresses. That may very well
just get you on more lists. Also note that formatted spam may contain Web Bugs that tell the spammer if you have
opened that mail.
REFERENCES