web bug (a.k.a. tracking image, clear gif, 1-by-1 gif, invisible gif)

What exactly is a Web Bug?

A Web Bug is a graphics on a Web page or in an Email message that is designed to monitor who is reading the Web page or Email message. Web Bugs are often invisible because they are typically only 1-by-1 pixel in size. They are represented as HTML IMG tags.

What information is sent to a server when a Web Bug is viewed?

• The IP address of the computer that fetched the Web Bug
• The URL of the page that the Web Bug is located on
• The URL of the Web Bug image
• The time the Web Bug was viewed
• The type of browser that fetched the Web Bug image
• A previously set cookie value

What kinds of uses does a Web Bug have in an Email message?

• A Web Bug can be used to find out if a particular Email message has been read by someone and if so, when the message was read.
• A Web Bug can provide the IP address of the recipient if the recipient is attempt to remain anonymous.
• Within an organization, A Web Bug can give an idea how often a message is being forwarded and read.

Firefoxban a noscript extension az 1.1.6.11-es verziotol letilthatova teszi (about:config > noscript.blockNSWB)

Tenni Theurer: Maximizing Parallel Downloads in the Carpool Lane

Itt van az egesz, de lenyeg, hogy a HTTP/1.1 specifikacioban leirt ajanlas szerint az IE es a Firefox is max 2 dolgot tolt le parhuzamosan ugyanarrol a hostrol (IP cimet nem nez). Ezt meg lehet valtoztani kliens oldalon.

Az IE-nel a registy manipulalasaval:

1. Start Registry Editor (Regedt32.exe).
2. Locate the following key in the registry:
   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings
3. On the Edit menu, point to New click DWORD Value, and then add the following registry values:
   Value name: MaxConnectionsPer1_0Server
   Value data: 10
   Base: DecimalValue Name: MaxConnectionsPerServer
   Value data: 10
   Base: Decimal
4. Quit Registry Editor.

Firefoxban pedig az about:config oldalon a
network.http.max-persistent-connections-per-server atallitasaval.

Kimertek, hogy mennyit huz a sebessegen, ha tobb hostname aliast hasznalnak amivel arra szorithatjak a browsert, hogy egyszerre tobb elemet toltson le, de az jott ki, hogy 2 aliasnal tobbet nem erdemes hasznalni, mert csak meglassabb lehet az egesz, de gyorsitani biztos nem fog.

Ryan Bran merese szerint viszont siman megeri 3 kulonbozo subdomaint hasznalni.

Honnan tudjuk, hogy az utolag include-olt javascript mar betoltodott. [2]

Az elozo problemara a codingforums.com-on callback-es megoldast javasolnak a firefoxban az element.addEventListener, IE-ben pedig a onreadystatechange event segitsegevel:


function loadScript(src, callback) {
var node = document.createElement("script");
if (node.addEventListener)
node.addEventListener("load", callback, false);
else
node.onreadystatechange = function() {
if (this.readyState == "complete") callback.call(this);
}
node.src = src;
document.getElementsByTagName("head").item(0).appendChild(node);
node = null;
}


// load 30k script.
// Listener.cleanUp is defined at the bottom of drag.js
callback = function() {
var self = this;
alert("Loaded: " + this.src + "\nListener.cleanUp (nested):" + Listener.cleanUp);
};
loadScript("http://dhtmlkitchen.com/editor/js/drag.js", callback);


[Update]
Ahogy Jan Volter is emliti jobb vigyazni ezzel a readyState propertyvel es onreadystatechange eventtel, mert:
The “this.readyState == ‘complete’” test doesn’t actually entirely work. The readyState theoretically goes through a series of states:

0	uninitialized
1	loading
2	loaded
3	interactive
4	complete

But in fact, states may be skipped. In my experience with IE 7, you get either a loaded event or a completed event, but not both. It may have something to do with whether you are loading from cache or not but there seem to be other factors that influence which events you get. Sometimes I get loading or interactive events too, and sometimes I don’t. It’s possible the test should be “this.readyState == ‘loaded’ || this.readyState == ‘complete’“, but that risks triggering twice.