In computer networks, a proxy server is a server (a computer system or an application program) that services the requests of its clients by forwarding requests to other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server. The proxy server provides the resource by connecting to the specified server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it would 'cache' the first request to the remote server, so it could save the information for later, and make everything as fast as possible.
A proxy server that passes all requests and replies unmodified is usually called a gateway or sometimes tunneling proxy.
A proxy server can be placed in the user's local computer or at various points between the user and the destination servers or the Internet.
Caching proxy server
A caching proxy server accelerates service requests by retrieving content saved from a previous request made by the same client or even other clients. Caching proxies keep local copies of frequently requested resources, allowing large organizations to significantly reduce their upstream bandwidth usage and cost, while significantly increasing performance. Most ISPs and large businesses have a caching proxy. These machines are built to deliver superb file system performance (often with RAID and journaling) and also contain hot-rodded versions of TCP. Caching proxies were the first kind of proxy server.
The HTTP 1.0 and later protocols contain many types of headers for declaring static (cacheable) content and verifying content freshness with an original server, e.g. ETAG (validation tags), If-Modified-Since (date-based validation), Expiry (timeout-based invalidation), etc. Other protocols such as DNS support expiry only and contain no support for validation.
Some poorly-implemented caching proxies have had downsides (e.g., an inability to use user authentication). Some problems are described in RFC 3143 (Known HTTP Proxy/Caching Problems).
Another important use of the proxy server is to reduce the hardware cost. In organization there may be many systems working in the same network or under control of one server, now in this situation we can not have individual connection for all systems with internet. We can simply connect those systems with one proxy server and proxy server with the main server.
Web proxy
A proxy that focuses on WWW traffic is called a "web proxy". The most common use of a web proxy is to serve as a web cache. Most proxy programs (e.g. Squid) provide a means to deny access to certain URLs in a blacklist, thus providing content filtering. This is usually used in a corporate environment, though with the increasing use of Linux in small businesses and homes, this function is no longer confined to large corporations. Some web proxies reformat web pages for a specific purpose or audience (e.g., cell phones and PDAs).
AOL dialup customers used to have their requests routed through an extensible proxy that 'thinned' or reduced the detail in JPEG pictures. This sped up performance, but caused trouble, either when more resolution was needed or when the thinning program produced incorrect results. This is why in the early days of the web many web pages would contain a link saying "AOL Users Click Here" to bypass the web proxy and to avoid the bugs in the thinning software.
Content-filtering web proxy
A content-filtering web proxy server provides administrative control over the content that may be relayed through the proxy. It is commonly used in commercial and non-commercial organizations (especially schools) to ensure that Internet usage conforms to acceptable use policy.
Some common methods used for content filtering include: URL or DNS blacklists, URL regex filtering, MIME filtering, or content keyword filtering. Some products have been known to employ content analysis techniques to look for traits commonly used by certain types of content providers.
A content filtering proxy will often support user authentication, to control web access. It also usually produces logs, either to give detailed information about the URLs accessed by specific users, or to monitor bandwidth usage statistics. It may also communicate to daemon based and/or ICAP based antivirus software to provide security against virus and other malware by scanning incoming content in real time before it enters the network.
Anonymizing proxy server
An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. These can easily be overridden by site administrators, and thus rendered useless in some cases. There are different varieties of anonymizers. One of the more common variations is the open proxy. Because they are typically difficult to track, open proxies are especially useful to those seeking online anonymity, from political dissidents to computer criminals.
Access control: Some proxy servers implement a logon requirement. In large organizations, authorized users must log on to gain access to the web. The organization can thereby track usage to individuals.
Hostile proxy
Proxies can also be installed in order to eavesdrop upon the dataflow between client machines and the web. All accessed pages, as well as all forms submitted, can be captured and analyzed by the proxy operator. For this reason, passwords to online services (such as webmail and banking) should always be exchanged over a cryptographically secured connection, such as SSL.
Intercepting proxy server
An intercepting proxy (also known as a "transparent proxy") combines a proxy server with a gateway. Connections made by client browsers through the gateway are redirected through the proxy without client-side configuration (or often knowledge).
Intercepting proxies are commonly used in businesses to prevent avoidance of acceptable use policy, and to ease administrative burden, since no client browser configuration is required.
It is often possible to detect the use of an intercepting proxy server by comparing the external IP address to the address seen by an external web server, or by examining the HTTP headers on the server side.
Transparent and non-transparent proxy server
The term "transparent proxy" is most often used incorrectly to mean "intercepting proxy" (because the client does not need to configure a proxy and cannot directly detect that its requests are being proxied). Transparent proxies can be implemented using Cisco's WCCP (Web Cache Control Protocol). This proprietary protocol resides on the router and is configured from the cache, allowing the cache to determine what ports and traffic is sent to it via transparent redirection from the router. This redirection can occur in one of two ways: GRE Tunneling (OSI Layer 3) or MAC rewrites (OSI Layer 2).
However, RFC 2616 (Hypertext Transfer Protocol -- HTTP/1.1) offers different definitions:
"A 'transparent proxy' is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification".
"A 'non-transparent proxy' is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering".
Forced proxy
The term "forced proxy" is ambiguous. It means both "intercepting proxy" (because it filters all traffic on the only available gateway to the Internet) and its exact opposite, "non-intercepting proxy" (because the user is forced to configure a proxy in order to access the Internet).
Forced proxy operation is sometimes necessary due to issues with the interception of TCP connections and HTTP. For instance interception of HTTP requests can affect the usability of a proxy cache, and can greatly affect certain authentication mechanisms. This is primarily because the client thinks it is talking to a server, and so request headers required by a proxy are unable to be distinguished from headers that may be required by an upstream server (esp authorization headers). Also the HTTP specification prohibits caching of responses where the request contained an authorization header.
Open proxy server
Because proxies might be used to abuse, system administrators have developed a number of ways to refuse service to open proxies. Many IRC networks automatically test client systems for known types of open proxy. Likewise, an email server may be configured to automatically test e-mail senders for open proxies.
Groups of IRC and electronic mail operators run DNSBLs publishing lists of the IP addresses of known open proxies, such as AHBL, CBL, NJABL, and SORBS.
The ethics of automatically testing clients for open proxies are controversial. Some experts, such as Vernon Schryver, consider such testing to be equivalent to an attacker portscanning the client host. [1] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.
Reverse proxy server
A reverse proxy is a proxy server that is installed in the neighborhood of one or more web servers. All traffic coming from the Internet and with a destination of one of the web servers goes through the proxy server. There are several reasons for installing reverse proxy servers:
• Encryption / SSL acceleration: when secure web sites are created, the SSL encryption is often not done by the web server itself, but by a reverse proxy that is equipped with SSL acceleration hardware. See Secure Sockets Layer. Furthermore, a hoster can provide a single "SSL proxy" to provide SSL encryption for an arbitrary number of hosts; removing the need for a separate SSL Server Certificate for each host, with the downside that all hosts behind the SSL proxy have to share a common DNS name or IP address for SSL connections.
• Load balancing: the reverse proxy can distribute the load to several web servers, each web server serving its own application area. In such a case, the reverse proxy may need to rewrite the URLs in each web page (translation from externally known URLs to the internal locations).
• Serve/cache static content: A reverse proxy can offload the web servers by caching static content like pictures and other static graphical content.
• Compression: the proxy server can optimize and compress the content to speed up the load time.
• Spoon feeding: reduces resource usage caused by slow clients on the web servers by caching the content the web server sent and slowly "spoon feeds" it to the client. This especially benefits dynamically generated pages.
• Security: the proxy server is an additional layer of defense and can protect against some OS and WebServer specific attacks. However, it does not provide any protection to attacks against the web application or service itself, which is generally considered the larger threat.
• Extranet Publishing: a reverse proxy server facing the Internet can be used to communicate to a firewalled server internal to an organization, providing extranet access to some functions while keeping the servers behind the firewalls. If used in this way, security measures should be considered to protect the rest of your infrastructure in case this server is compromised, as its web application is exposed to attack from the Internet.
Circumventor
A circumventor is a method of defeating blocking policies implemented using proxy servers. Ironically, most circumventors are also proxy servers, of varying degrees of sophistication, which effectively implement "bypass policies".
A circumventor is a web-based page that takes a site that is blocked and "circumvents" it through to an unblocked web site, allowing the user to view blocked pages. A famous example is elgooG, which allowed users in China to use Google after it had been blocked there. elgooG differs from most circumventors in that it circumvents only one block.
Students are able to access blocked sites (games, chatrooms, messenger, offensive material, internet pornography, social networking, etc.) through a circumventor. As fast as the filtering software blocks circumventors, others spring up. However, in some cases the filter may still intercept traffic to the circumventor, thus the person who manages the filter can still see the sites that are being visited.
Circumventors are also used by people who have been blocked from a web site.
Another use of a circumventor is to allow access to country-specific services, so that Internet users from other countries may also make use of them. An example is country-restricted reproduction of media and webcasting.
The use of circumventors is usually safe with the exception that circumventor sites run by an untrusted third party can be run with hidden intentions, such as collecting personal information, and as a result users are typically advised against running personal data such as credit card numbers or passwords through a circumventor.
An example of one way to circumvent a content-filtering proxy server is by tunnelling through to another proxy server, usually controlled by the user, which has unrestricted access to the internet. This is often acheived by using a VPN type tunnel, such as VPN itself or SSH, through a port left open by the proxy (eg. Port 443 is nearly always left open to allow the use of HTTPS). Through the use of encryption, tunnelling to a remote proxy server, provided the remote proxy server is itself secure, is not only difficult to detect, but also difficult to intercept.
Content filter
Many work places, schools and colleges restrict the web sites and online services that are made available in their buildings. This is done either with a specialized proxy, called a content filter (both commercial and free products are available), or by using a cache-extension protocol such as ICAP, that allows plug-in extensions to an open caching architecture.
Requests made to the open internet must first pass through an outbound proxy filter. The web-filtering company provides a database of URL patterns (regular expressions) with associated content attributes. This database is updated weekly by site-wide subscription, much like a virus filter subscription. The administrator instructs the web filter to ban broad classes of content (such as sports, pornography, online shopping, gambling, or social networking). Requests that match a banned URL pattern are rejected immediately.
Assuming the requested URL is acceptable, the content is then fetched by the proxy. At this point a dynamic filter may be applied on the return path. For example, JPEG files could be blocked based on fleshtone matches, or language filters could dynamically detect unacceptable language. If the content is rejected then an HTTP fetch error is returned and nothing is cached.
Most web filtering companies use an internet-wide crawling robot that assesses the likelihood that a content is a certain type (i.e. "This content is 70% chance of porn, 40% chance of sports, and 30% chance of news" could be the outcome for one web page). The resultant database is then corrected by manual labor based on complaints or known flaws in the content-matching algorithms.
Unfortunately, web filtering proxies are not able to peer inside secure sockets HTTP transactions. As a result, users wanting to bypass web filtering will typically search the internet for an open and anonymous HTTPS transparent proxy. They will then program their browser to proxy all requests through the web filter to this anonymous proxy. Those requests will be encrypted with https. The web filter cannot distinguish these transactions from, say, a legitimate access to a financial website. Thus, content filters are only effective against unsophisticated users.
A special case of web proxies is "CGI proxies". These are web sites that allow a user to access a site through them. They generally use PHP or CGI to implement the proxy functionality. These types of proxies are frequently used to gain access to web sites blocked by corporate or school proxies. Since they also hide the user's own IP address from the web sites they access through the proxy, they are sometimes also used to gain a degree of anonymity, called "Proxy Avoidance".
Suffix proxy
A suffix proxy server allows a user to access web content by appending the name of the proxy server to the URL of the requested content (e.g. "en.wikipedia.org.6a.nl").
Suffix proxy servers are easier to use than regular proxy servers. The concept appeared in 2003 in form of the IPv6Gate and in 2004 in form of the Coral Content Distribution Network, but the term suffix proxy was only coined in October 2008 by "6a.nl"[citation needed].
Risks of using anonymous proxy servers
In using a proxy server (for example, anonymizing HTTP proxy), all data sent to the service being used (for example, HTTP server in a website) must pass through the proxy server before being sent to the service, mostly in unencrypted form. It is therefore possible, as has been demonstrated, for a malicious proxy server to record everything sent to the proxy: including unencrypted logins and passwords.
By chaining proxies which do not reveal data about the original requester, it is possible to obfuscate activities from the eyes of the user's destination. However, more traces will be left on the intermediate hops, which could be used or offered up to trace the user's activities. If the policies and administrators of these other proxies are unknown, the user may fall victim to a false sense of security just because those details are out of sight and mind.
The bottom line of this is to be wary when using proxy servers, and only use proxy servers of known integrity (e.g., the owner is known and trusted, has a clear privacy policy, etc.), and never use proxy servers of unknown integrity. If there is no choice but to use unknown proxy servers, do not pass any private information (unless it is properly encrypted) through the proxy.
An important fact is that even secure https (SSL) connections can not protect from a sniffing proxy. It has been demonstrated that the SSL handshake can be intercepted on the proxy. The browser will show a secure, encrypted connection but the proxy is able to read everything in clear text. When encrypting email traffic through a proxy, one more technique is frequently used to stop the encryption: The proxy intercepts the email servers response to the encryption (TLS, SSL) request and fakes a negative response. Almost all email clients use a default setting that uses cleartext transfer in such a case. Using a proxy is always dangerous if you do not own or trust it, keep this in mind. Even high anonymity and privacy services like Tor or Cloakfish can not protect from these risks. Whenever a proxy is used you are at risk that someone "in the middle" can read your data.
In what is more of an inconvenience than a risk, proxy users may find themselves being blocked from certain Web sites, as numerous forums and Web sites block IP addresses from proxies known to have spammed or trolled the site.
Minggu, 11 Januari 2009
Proxy server
Rabu, 07 Januari 2009
run kursor without mouse
How To Running cursor withaout mouse
* Click Start.
* Click Run.
* Type “access.cpl“(tanpa quotes).
* Click pada ‘mouse‘ tab.
* Check ‘use mouse keys‘.
* Click OK.
Don't forget Activated NUMLOCK,you can running cursor by 9,3,1 or 7
Internet Explorer Keyboard Shortcuts & Tips
F1 Display the Internet Explorer Help, or when in a dialog box, display context help on an item
F11 Toggle between Full Screen and regular view of the browser window
TAB Move forward through the items on a Web page, the Address bar, and the Links bar
SHIFT+TAB Move back through the items on a Web page, the Address bar, and the Links bar
ALT+HOME Go to your Home page
ALT+RIGHT ARROW Go to the next page
ALT+LEFT ARROW or
BACKSPACE Go to the previous page
SHIFT+F10 Display a shortcut menu for a link
CTRL+TAB or
F6 Move forward between frames
SHIFT+CTRL+TAB Move back between frames
UP ARROW Scroll toward the beginning of a document
DOWN ARROW Scroll toward the end of a document
PAGE UP Scroll toward the beginning of a document in larger increments
PAGE DOWN Scroll toward the end of a document in larger increments
HOME Move to the beginning of a document
END Move to the end of a document
CTRL+F Find on this page
F5 or
CTRL+R Refresh the current Web page only if the time stamp for the Web version and your locally stored version are different
CTRL+F5 Refresh the current Web page, even if the time stamp for the Web version and your locally stored version are the same
ESC Stop downloading a page
CTRL+O or
CTRL+L Go to a new location
CTRL+N Open a new window
CTRL+W Close the current window
CTRL+S Save the current page
CTRL+P Print the current page or active frame
ENTER Activate a selected link
CTRL+E Open Search in Explorer bar
CTRL+I Open Favorites in Explorer bar
CTRL+H Open History in Explorer bar
CTRL+click In History or Favorites bars, open multiple folders
Using the Address bar
ALT+D Select the text in the Address bar
F4 Display the Address bar history
CTRL+LEFT ARROW When in the Address bar, move the cursor left to the next logical break (. or /)
CTRL+RIGHT ARROW When in the Address bar, move the cursor right to the next logical break (. or /)
CTRL+ENTER Add "www." to the beginning and ".com" to the end of the text typed in the Address bar
UP ARROW Move forward through the list of AutoComplete matches
DOWN ARROW Move back through the list of AutoComplete matches
Working with favorites
CTRL+D Add the current page to your favorites
CTRL+B Open the Organize Favorites dialog box
ALT+UP ARROW Move selected item up in the Favorites list in the Organize Favorites dialog box
ALT+DOWN ARROW Move selected item down in the Favorites list in the Organize Favorites dialog box
Editing
CTRL+X Remove the selected items and copy them to the Clipboard
CTRL+C Copy the selected items to the Clipboard
CTRL+V Insert the contents of the Clipboard at the selected location
CTRL+A Select all items on the current Web page
Selasa, 06 Januari 2009
Spam: prevention is better than cure!
Nowadays almost everyone who heard about the internet knows what spam is. And it is no longer surprise to receive a letter from God-knows-who advertising God-knows-what. Spam has become an annoying part of everyday life for an email user.
So, is it possible to get rid of spam? The answer is apparently negative because there will always exist people who read spam and react on it (you must have heard stories about 'Nigerian letters').
However, what you can do is protect your personal mailbox from the flow of spam. Spam is like rain - you cannot stop it but you can take an umbrella to stay dry and enjoy your walk.
There are two main ways to resist spam: filtering out the incoming spam and taking care not to receive it at all.
The first method requires using various client and server spam filters while the second is just a matter of being careful with your personal email address. Let's talk about the second one.
Web pages
First of all you need to realize that you receive spam because somehow spammers managed to find out your email address.
Very often spammers use special programs that send queries to search systems like Google, Yahoo or MSN in order to find web pages containing email addresses. Then the addresses are saved to a database and here it comes - spam.
This simply means that if your address is published somewhere in the internet then sooner or later you will get spam.
As you are unlikely to be happy about this you should find the web pages with your email address using different search systems.
If your address is unimportant in the context of the web page you should better delete it. Otherwise it is sensible to write the address in a different way: 'name(@)domain.com' or 'name(nospam)domain.com', for example.
When a person reads such an address, they can easily understand what this means but to a program this will not be a valid email address.
Instant messengers
Another source of email addresses are the various internet messengers like ICQ, Yahoo messenger and so on, where during the registration the user is usually asked to enter their email address.
After that the address becomes available to every user of the messenger service. So spammers use fake user names to get access to other users' personal information, particularly email addresses.
Fortunately, in more recent messenger versions users personal information is not available for everyone but it is sometimes still possible to trick the system - and many users still use older unprotected versions.
If you are using a messenger and your email address is in your personal information then your steps should be as above - delete your address or write it in a way inappropriate for automatic recognition.
Form filling
Finally, probably the most common way for an email address to get to spammers databases is when people carelessly enter their emails on some websites.
Often during the registration process for a product/service on the internet the company asks the client for their email address. And then it all depends on the company whether the address is used only to contact the client.
If the company is reliable and cares about its image then your email address will be kept in secret. In this case the only thing you need worry about is that you will be automatically subscribed to the company news-list.
If you are not interested in the newsletters you can always unsubscribe from the list. Unfortunately, there exist companies that do not really care about their clients and the safety of their personal data.
If you are unlucky enough to deal with such a company then quite soon your email address will be in various spammers databases.
A few times I myself have been in the situation where I started receiving spam on one of my email addresses. Remarkably, the address was new and known only to the company - one that seemed to be trustworthy at the beginning.
How to eliminate spam
Industry analysts estimate that spam currently accounts for close to 80 percent of email messages sent and causes close to £5 billion in economic losses annually.
The problem with spam is very similar to that of pollution: spammers profit from their activity at the expense of the rest of the population, just like polluters of the environment profit while annoying or endangering others.
So it seems reasonable that our understanding of the economics of pollution may give us some insight into the economics of spam. The work of Nobel laureate Ronald Coase is particularly useful for this.
In 1991, Coase was awarded the Bank of Sweden Prize in Economic Sciences in Memory of Alfred Nobel for his contributions to understanding how property rights and transaction costs affect the structure and functioning of an economy.
Coase showed that if we assume that transaction costs are negligible, as long as property rights are clearly defined, the equilibrium that a market will reach does not depend on who initially owns the affected property. All that will change is who profits from the transactions that lead to the equilibrium.
An example of this principle is a locomotive whose coal-burning engine showers sparks over the land that it passes. Reducing the level of sparks emitted is possible, but requires that the owner of the train incur the additional costs to purchase some sort of spark-reduction equipment.
To quantify this, let’s assume that a train normally produces 10 units of sparks, but these can be reduced at a cost of £200 per unit eliminated, and that each unit of sparks does £300 of damage to the land that it passes.
So if the train produces 10 units of sparks there will be no additional costs for the owner of the train, and the train will do £3,000 of damage to the land that it passes. If the train produces no sparks at all, there will be additional costs of £2,000 for the owner of the train, but the train will do no damage to the land that it passes.
If the owner of the train is free to shower sparks over the land that his train passes, the owners of the land will be willing to pay the owner of the train £300 for each unit of sparks that they eliminate.
This situation will reach an equilibrium where the owners of the land will pay the owner of the train £1,200 to reduce the sparks down to only four units, which the owner of the train will use to finance the modifications to his trains that the reduction in sparks requires.
On the other hand, if the owner of the land is free to deny the owner of the train the right to shower sparks on his land, then the train owner will be happy to pay the landowner £1,200 to compensate him for his inconvenience, and the landowner will then be happy to endure four units of sparks.
The end result that we arrive at is the same in both cases, with the only difference being who is paying whom. And since both parties prefer the arrangement where four units of sparks are produced to any other, it will be the state that this market eventually reaches.
Coase showed that this will always happen as long as there are no transaction costs. So as long as we have clearly-established property rights, we will reach an equilibrium between a polluter and the victims of the pollution, and the equilibrium that we will reach will be the same no matter who owns the property rights to the environment.
If we apply this model to spam, we see that spammers are analogous to the train owners and recipients of email are analogous to the owners of the land that the train will damage with its sparks. But in the case of spam, there is no way for spammers and recipients of email to reach an agreement that limits the amount of spam to a mutually-acceptable level.
First, there are no property rights to enforce; neither the spammers nor the legitimate users of email can claim any exclusive right to use the internet for messaging.
Next, there is no efficient way for spammers to reach an agreement with their victims. Because of this, the amount of spam sent remains unchecked by market forces, as does the annoyance suffered by users of email.
Thus Coase's result provides an easy solution to the problem of spam: define ownership of the internet and the rights to use it. Once we do this, market forces will then drive the amount of spam that is sent to an acceptable level, with slight inefficiencies possible due to the transaction costs involved.
And since the equilibrium that the market will reach does not depend on to whom we assign ownership of the internet, we will even end up with the same reduction in spam if we decide to assign the ownership of the internet to the spammers – a truly remarkable result.
Boot your Computer from a Bootable USB Device
Time Required: Booting from a USB device usually takes less than 10 to 20 minutes.
Here's How:
1. Change the BIOS boot order so the USB device option is listed first. The BIOS is rarely setup this way by default.
f the USB port is not first in the boot order, your PC will start "normally" (i.e. boot from your hard drive) without even looking at any boot information that might be on your USB device.
Note: After setting your USB device as the first boot device, your computer will check it for boot information each time your PC starts. Leaving your computer configured this way shouldn't cause problems unless you plan on leaving the USB device attached all the time.
2. Attach the USB device to your computer via any available USB port. How do you know if a USB device is bootable? The easiest way to find out if a flash drive or external hard drive is bootable is to plug it in and follow the rest of these instructions.
Note: Creating a bootable flash drive or configuring an external hard drive as bootable is a task in itself. Follow any instructions given with the data that should be be bootable.
3. Restart your computer.
4. Watch for a Press any key to boot from external device... message.
On some bootable devices, you may be prompted with a message to press a key before the computer will boot to the flash drive or other USB device.
If you do nothing, your computer will check for boot information on the next boot device in the list in BIOS (see Step 1) which will probably be your hard drive.
Note: Most of the time when trying to boot to a USB device there is no key-press prompt. The boot process usually starts immediately.
5. Your computer should now boot from the USB flash drive or USB based external hard drive.
Note: What happens now depends on what the bootable USB device was intended for. If you're booting to an old version of MS-DOS on a flash drive, MS-DOS will load. If you're booting to the DSL version of Linux, it will start. You get the idea.
If you tried the above steps but your computer did not boot from the USB device, check out some of the tips below.
Tips:
1. Recheck the boot order in BIOS (Step 1). The number one reason a bootable flash drive or other USB device won't boot is because BIOS is not configured to check the USB port first.
2. Didn't find a "USB Device" boot order listing in BIOS? If your computer was manufactured around 2001 or before, it may not have this ability. If your computer is newer, check for some other ways that the USB option might be worded. In some BIOS versions, it's called "Removable Devices" or "External Devices".
3. Switch to another USB port. The BIOS on some motherboards only check the first few USB ports. Switch to another USB port and restart your computer.
4. Copy the files to the USB device again. If you created the bootable flash drive or external hard drive yourself, which you probably did, repeat whatever steps you took again. You may have made a mistake before or missed an important file.