When used in the context of web development, an API is typically a defined set of Hypertext Transfer Protocol (HTTP) request messages, along with a definition of the structure of response messages, which is usually in an Extensible Markup Language (XML) or JavaScript Object Notation (JSON) format. While “Web API” is virtually a synonym for web service, the recent trend (so-called Web 2.0) has been moving away from Simple Object Access Protocol (SOAP) based services towards more direct Representational State Transfer (REST) style communications. Web APIs allow the combination of multiple services into new applications known as mashups.

Adapted from the Wikipedia article Application programming interface, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Shared web hosting can be accomplished in two ways: name-based and IP-based, although some control panels allow a mix of name-based and IP-based on the one server.

Name-based

In name-based virtual hosting, also called shared IP hosting, the virtual hosts serve multiple hostnames on a single machine with a single IP address.

When a web browser requests a resource from a web server using HTTP/1.1 it includes the requested hostname as part of the request. The server uses this information to determine which web site to show the user.

IP-based

In IP-based virtual hosting, also called dedicated IP hosting, each virtual host has a different IP address. The web server is configured with multiple physical network interfaces, or virtual network interfaces on the same physical interface. The web server software uses the IP address the client connects to in order to determine which web site to show the user. The primary reason for a site to use a dedicated IP is to be able to use its own SSL certificate rather than a shared certificate.

Adapted from the Wikipedia article Shared web hosting service, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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mod_proxy is an optional module for the Apache HTTP Server .

This module implements a proxy/gateway/cache for Apache. It implements proxying capability for or AJP13 (Apache JServ Protocol version 1.3), FTP, CONNECT (for SSL), HTTP/0.9, HTTP/1.0, and (since Apache 1.3.23) HTTP/1.1. The module can be configured to connect to other proxy modules for these and other protocols.

One powerful feature of Apache is flexible virtual hosting — multiple virtual hosts on a single machine. This is a convenient way to partition separate websites and applications. With mod_proxy it is possible to set various web framework based applications up as virtual hosts as well .

Mod_proxy can help to improve LAMP security or to strip SSL from HTTP requests .

Adapted from the Wikipedia article Mod proxy, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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There are several different types of software architecture in common use.

Simple control loop

In this design, the software simply has a loop. The loop calls subroutines, each of which manages a part of the hardware or software.

Interrupt controlled system

Some embedded systems are predominantly interrupt controlled. This means that tasks performed by the system are triggered by different kinds of events. An interrupt could be generated for example by a timer in a predefined frequency, or by a serial port controller receiving a byte.

These kinds of systems are used if event handlers need low latency and the event handlers are short and simple.

Usually these kinds of systems run a simple task in a main loop also, but this task is not very sensitive to unexpected delays.

Sometimes the interrupt handler will add longer tasks to a queue structure. Later, after the interrupt handler has finished, these tasks are executed by the main loop. This method brings the system close to a multitasking kernel with discrete processes.

Cooperative multitasking

A nonpreemptive multitasking system is very similar to the simple control loop scheme, except that the loop is hidden in an API. The programmer defines a series of tasks, and each task gets its own environment to “run” in. When a task is idle, it calls an idle routine, usually called “pause”, “wait”, “yield”, “nop” (stands for ”no operation”), etc.

The advantages and disadvantages are very similar to the control loop, except that adding new software is easier, by simply writing a new task, or adding to the queue-interpreter.

Preemptive multitasking or multi-threading

In this type of system, a low-level piece of code switches between tasks or threads based on a timer (connected to an interrupt). This is the level at which the system is generally considered to have an “operating system” kernel. Depending on how much functionality is required, it introduces more or less of the complexities of managing multiple tasks running conceptually in parallel.

As any code can potentially damage the data of another task (except in larger systems using an MMU) programs must be carefully designed and tested, and access to shared data must be controlled by some synchronization strategy, such as message queues, semaphores or a non-blocking synchronization scheme.

Because of these complexities, it is common for organizations to use a real-time operating system (RTOS), allowing the application programmers to concentrate on device functionality rather than operating system services, at least for large systems; smaller systems often cannot afford the overhead associated with a ”generic” real time system, due to limitations regarding memory size, performance, and/or battery life.The choice that a RTOS is required brings in its own issues however as the selection must be done prior to starting to the application development process. This timing forces developers to choose the embedded operating system for their device based upon current requirements and so restricts future options to a large extent.The restriction of future options becomes more of an issue as product life decreases. Additionally the level of complexity is continuously growing as devices are required to manage many variables such as serial, USB, TCP/IP, Bluetooth, Wireless LAN, trunk radio, multiple channels, data and voice, enhanced graphics, multiple states, multiple threads, numerous wait states and so on. These trends are leading to the uptake of embedded middleware in addition to a real time operating system.

Microkernels and exokernels

A microkernel is a logical step up from a real-time OS. The usual arrangement is that the operating system kernel allocates memory and switches the CPU to different threads of execution. User mode processes implement major functions such as file systems, network interfaces, etc.

In general, microkernels succeed when the task switching and intertask communication is fast, and fail when they are slow.

Exokernels communicate efficiently by normal subroutine calls. The hardware, and all the software in the system are available to, and extensible by application programmers.

Monolithic kernels

In this case, a relatively large kernel with sophisticated capabilities is adapted to suit an embedded environment. This gives programmers an environment similar to a desktop operating system like Linux or Microsoft Windows, and is therefore very productive for development; on the downside, it requires considerably more hardware resources, is often more expensive, and because of the complexity of these kernels can be less predictable and reliable.

Common examples of embedded monolithic kernels are Embedded Linux and Windows CE.

Despite the increased cost in hardware, this type of embedded system is increasing in popularity, especially on the more powerful embedded devices such as Wireless Routers and GPS Navigation Systems. Here are some of the reasons:

* Ports to common embedded chip sets are available.

* They permit re-use of publicly available code for Device Drivers, Web Servers, Firewalls, and other code.

* Development systems can start out with broad feature-sets, and then the distribution can be configured to exclude unneeded functionality, and save the expense of the memory that it would consume.

* Many engineers believe that running application code in user mode is more reliable, easier to debug and that therefore the development process is easier and the code more portable.

* Many embedded systems lack the tight real time requirements of a control system. Although a system such as Embedded Linux may be fast enough in order to respond to many other applications.

* Features requiring faster response than can be guaranteed can often be placed in hardware.

* Many RTOS systems have a per-unit cost. When used on a product that is or will become a commodity, that cost is significant.

Exotic custom operating systems

A small fraction of embedded systems require safe, timely, reliable or efficient behavior unobtainable with the one of the above architectures. In this case an organization builds a system to suit.

In some cases, the system may be partitioned into a “mechanism controller” using special techniques, and a “display controller” with a conventional operating system. A communication system passes data between the two.

Additional software components

In addition to the core operating system, many embedded systems have additional upper-layer software components. These components consist of networking protocol stacks like CAN, TCP/IP, FTP, HTTP, and HTTPS, and also included storage capabilities like FAT and flash memory management systems. If the embedded devices has audio and video capabilities, then the appropriate drivers and codecs will be present in the system. In the case of the monolithic kernels, many of these software layers are included. In the RTOS category, the availability of the additional software components depends upon the commercial offering.

Adapted from the Wikipedia article Embedded system, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Before the second browser wars in 2008-2009, the JavaScript engine (also known as JavaScript interpreter or JavaScript implementation) was known simply as an interpreter that read and executed JavaScript source code.

The first JavaScript engine was created by Brendan Eich at Netscape Communications Corporation for the Netscape Navigator web browser. The engine, code named SpiderMonkey, is implemented in C. It has since been updated (in JavaScript 1.5) to conform to ECMA-262 Edition 3. The Rhino engine, created primarily by Norris Boyd (also at Netscape) is a JavaScript implementation in Java. Like SpiderMonkey, Rhino is ECMA-262 Edition 3 compliant. Applications of the technology include Apple Safari 4′s Nitro, Google Chrome’s V8 and Mozilla Firefox 3.5′s TraceMonkey.

By far the most common host environment for JavaScript is a web browser. Web browsers typically use the public API to create “host objects” responsible for reflecting the DOM into JavaScript.

The web server is another common application of the engine. A JavaScript web server exposes host objects representing a HTTP request and response objects, which a JavaScript program then manipulates to dynamically generate web pages. Microsoft’s ASP technology for IIS allows server-side code to be written in VB Script or JScript (Microsoft’s implementation of JavaScript). Jaxer is a web server that runs entirely on JavaScript; this has the benefit of allowing the same code to be shared on the server and on the client.

Adapted from the Wikipedia article JavaScript engine, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Main|Client-side JavaScript

See also|JavaScript engine|Ajax (programming)

The primary use of JavaScript is to write functions that are embedded in or included from HTML pages and that interact with the Document Object Model (DOM) of the page. Some simple examples of this usage are:

* Opening or popping up a new window with programmatic control over the size, position, and attributes of the new window (e.g. whether the menus, toolbars, etc. are visible).

* Validating input values of a web form to make sure that they are acceptable before being submitted to the server.

* Changing images as the mouse cursor moves over them: This effect is often used to draw the user’s attention to important links displayed as graphical elements.

Because JavaScript code can run locally in a user’s browser (rather than on a remote server), the browser can respond to user actions quickly, making an application more responsive. Furthermore, JavaScript code can detect user actions which HTML alone cannot, such as individual keystrokes. Applications such as Gmail take advantage of this: much of the user-interface logic is written in JavaScript, and JavaScript dispatches requests for information (such as the content of an e-mail message) to the server. The wider trend of Ajax programming similarly exploits this strength.

A JavaScript engine (also known as ”JavaScript interpreter” or ”JavaScript implementation”) is an interpreter that interprets JavaScript source code and executes the script accordingly. The first JavaScript engine was created by Brendan Eich at Netscape Communications Corporation, for the Netscape Navigator web browser. The engine, code-named SpiderMonkey, is implemented in C. It has since been updated (in JavaScript 1.5) to conform to ECMA-262 Edition 3. The Rhino engine, created primarily by Norris Boyd (formerly of Netscape; now at Google) is a JavaScript implementation in Java. Rhino, like SpiderMonkey, is ECMA-262 Edition 3 compliant.

A web browser is by far the most common host environment for JavaScript. Web browsers typically use the public API to create “host objects” responsible for reflecting the DOM into JavaScript. The web server is another common application of the engine. A JavaScript webserver would expose host objects representing an HTTP request and response objects, which a JavaScript program could then manipulate to dynamically generate web pages.

Because JavaScript is the only language that the most popular browsers share support for, it has become a target language for many frameworks in other languages, even though JavaScript was never intended to be such a language. Despite the performance limitations inherent to its dynamic nature, the increasing speed of JavaScript engines has made the language a surprisingly feasible compilation target.

Example – use in web pages

A minimal example of a standards-conforming web page containing JavaScript (using HTML 4.01 syntax) would be the following:

simple page

document.write(‘Hello World!’);

Your browser either does not support JavaScript, or you have JavaScript turned off.

Compatibility considerations

Main|Web interoperability

Since JavaScript runs in widely varying environments, an important part of testing and debugging it is testing across browsers.

The DOM interfaces for manipulating web pages are not part of the ECMAScript standard, or of JavaScript itself. Officially, they are defined by a separate standardization effort by the W3C; in practice, browser implementations differ from the standards and from each other, and not all browsers execute JavaScript.

To deal with these differences, JavaScript authors can attempt to write standards-compliant code which will also be executed correctly by most browsers; failing that, they can write code that checks for the presence of certain browser features and behaves differently if they are not available. In some cases, two browsers may both implement a feature but with different behavior, and authors may find it practical to detect what browser is running and change their script’s behavior to match. Programmers may also use libraries or toolkits which take browser differences into account.

Furthermore, scripts may not work for some users. For example, a user may:

* use an old or rare browser with incomplete or unusual DOM support,

* use a PDA or mobile phone browser which cannot execute JavaScript,

* have JavaScript execution disabled as a security precaution,

* use a speech browser due to, for example, a visual disability.

To support these users, web authors can try to create pages which degrade gracefully on user agents (browsers) which do not support the page’s JavaScript. In particular, the page should remain usable albeit without the extra features that the JavaScript would have added.

Accessibility

Main|Web accessibility

Assuming that the user has not disabled its execution, client-side web JavaScript should be written to enhance the experiences of visitors with visual or physical disabilities, and certainly should avoid denying information to these visitors.

Screen readers, used by the blind and partially sighted, can be JavaScript-aware and so may access and read the page DOM after the script has altered it. The HTML should be as concise, navigable and semantically rich as possible whether the scripts have run or not. JavaScript should not be totally reliant on mouse-specific events so as to deny its benefits to users who either cannot use a mouse or who choose to favor the keyboard for whatever reason. Equally, although hyperlinks and webforms can be navigated and operated from the keyboard, accessible JavaScript should not require keyboard events either. There are device-independent events such as onfocus and onchange that are preferable in most cases.

JavaScript should not be used in a way that is confusing or disorientating to any web user. For example, using script to alter or disable the normal functionality of the browser, such as by changing the way the back-button or the refresh event work, is usually best avoided. Equally, triggering events that the user may not be aware of reduces the user’s sense of control as do unexpected scripted changes to the page content.

Often the process of making a complex web page as accessible as possible becomes a nontrivial problem where issues become matters of debate and opinion, and where compromises are necessary in the end. However, user agents and assistive technologies are constantly evolving and new guidelines and relevant information are continually being published on the web.

Security

JavaScript and the DOM provide the potential for malicious authors to deliver scripts to run on a client computer via the web. Browser authors contain this risk using two restrictions. First, scripts run in a sandbox in which they can only perform web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by the same origin policy: scripts from one web site do not have access to information such as usernames, passwords, or cookies sent to another site. Most JavaScript-related security bugs are breaches of either the same origin policy or the sandbox.

Cross-site vulnerabilities

Main|Cross-site scripting|Cross-site request forgery

A common JavaScript-related security problem is cross-site scripting, or XSS, a violation of the same-origin policy. XSS vulnerabilities occur when an attacker is able to cause a target web site, such as an online banking website, to include a malicious script in the webpage presented to a victim. The script in this example can then access the banking application with the privileges of the victim, potentially disclosing secret information or transferring money without the victim’s authorization. A solution to XSS vulnerabilities is to use ”HTML escaping” whenever displaying untrusted data.

Some browsers include partial protection against ”reflected” XSS attacks, in which the attacker provides a URL including malicious script. However, even users of those browsers are vulnerable to other XSS attacks, such as those where the malicious code is stored in a database. Only correct design of Web applications on the server side can fully prevent XSS.

XSS vulnerabilities can also occur because of implementation mistakes by browser authors.

Another cross-site vulnerability is cross-site request forgery or CSRF. In CSRF, code on an attacker’s site tricks the victim’s browser into taking actions the user didn’t intend at a target site (like transferring money at a bank). It works because, if the target site relies only on cookies to authenticate requests, then requests initiated by code on the attacker’s site will carry the same legitimate login credentials as requests initiated by the user. In general, the solution to CSRF is to require an authentication value in a hidden form field, and not only in the cookies, to authenticate any request that might have lasting effects. Checking the HTTP Referrer header can also help.

“JavaScript hijacking” is a type of CSRF attack in which a tag on an attacker’s site exploits a page on the victim’s site that returns private information as JSON or JavaScript. Possible solutions include requiring an authentication token in the POST and GET parameters for any response that returns private JSON (even if it has no side effects); using POST and never GET for requests that return private JSON; and modifying the response so that it can’t be used via a tag (by, for example, wrapping the JSON in a JavaScript comment).

Misplaced trust in the client

Client-server applications, whether they involve JavaScript or not, must recognize that untrusted clients may be under the control of attackers. Thus any secret embedded in JavaScript could be extracted by a determined adversary, and the application author can’t assume that his JavaScript runs as intended, or at all. Some implications:

* Web site authors cannot perfectly conceal how their JavaScript operates, because the code is sent to the client, and obfuscated code can be reverse-engineered.

* JavaScript form validation only provides convenience for users, not security. If a site verifies that the user agreed to its terms of service, or filters invalid characters out of fields that should only contain numbers, it must do so on the server, not only the client.

* Scripts can be selectively disabled, so JavaScript can’t be relied on to prevent operations such as “save image”.

* It would be extremely bad practice to embed a password in JavaScript (where it can be extracted by an attacker), then have JavaScript verify a user’s password and pass “password_ok=1″ back to the server (since the “password_ok=1″ response is easy to forge).

Browser and plugin coding errors

JavaScript provides an interface to a wide range of browser capabilities, some of which may have flaws such as buffer overflows. These flaws can allow attackers to write scripts which would run any code they wish on the user’s system.

These flaws have affected major browsers including Firefox, Internet Explorer, and Safari.

Plugins, such as video players, Adobe Flash, and the wide range of ActiveX controls enabled by default in Microsoft Internet Explorer, may also have flaws exploitable via JavaScript,

and such flaws have been exploited in the past.

In Windows Vista, Microsoft has attempted to contain the risks of bugs such as buffer overflows by running the Internet Explorer process with limited privileges. Google Chrome similarly limits page renderers to an operating-system-enforced “sandbox.”

Sandbox implementation errors

Web browsers are capable of running JavaScript outside of the sandbox, with the privileges necessary to, for example, create or delete files. Of course, such privileges aren’t meant to be granted to code from the web.

Incorrectly granting privileges to JavaScript from the web has played a role in vulnerabilities in both Internet Explorer and Firefox. In Windows XP Service Pack 2, Microsoft demoted JScript’s privileges in Internet Explorer.

Microsoft Windows allows JavaScript source files on a computer’s hard drive to be launched as general-purpose, non-sandboxed programs. This makes JavaScript (like VBScript) a theoretically viable vector for a Trojan horse, although JavaScript Trojan horses are uncommon in practice. (See Windows Script Host.)

Adapted from the Wikipedia article JavaScript, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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mod_ssl is an optional module for the Apache HTTP Server.

Provides strong cryptography for the Apache v1.3 and v2 webserver via the Secure Sockets Layer (SSL v2/v3) and Transport Layer Security (TLS v1) cryptographic protocols by the help of the Open Source SSL/TLS toolkit OpenSSL.

It is possible to provide HTTP and HTTPS with a single server machine, because HTTP and HTTPS use different server ports, so there is no direct conflict between them. Either run two separate Apache server instances (one binds to port 80, the other to port 443) or even use Apache’s virtual hosting facility where you can create two virtual servers which Apache dispatches: one responding to port 80 and speaking HTTP and one responding to port 443 speaking HTTPS.

Adapted from the Wikipedia article Mod ssl, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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FSP never reached the popularity of FTP for legitimate use (although wuarchive and id Software provided FSP service in addition to FTP for some time), but became very popular in the early-to-mid-1990s for underground sites containing pornography and/or warez. Because an FSP server only requires one process (as opposed to one process per client for most FTP servers), it is much harder for a system administrator to notice it in a process list; also, since it uses UDP, it is less likely to be noticed by a network administrator.

Eventually, however, an increased use of firewalls, a decreasing usage of the shell accounts required to run a server or most of the clients, and a lack of FSP support in web browsers caused its use to taper off, and the warez scene moved to HTTP and FXP while pornography moved to publicly-advertised web servers.

Adapted from the Wikipedia article File Service Protocol, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Open Source Components

* Apache

** Webserver

* Qmail with Qpsmtpd

** Mail server

* Djbdns and dnscach

** Domain name system server

* Proftpd

** FTP server

* Samba

** windows file sharing

* SSH, PPTP, HTTP over SSL

** remote administration

* Flexbackup

** tape backups

* Horde

** webmail

Adapted from the Wikipedia article SME Server, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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WebSocket protocol client implementations try to detect if the user agent is configured to use a proxy when connecting to destination host and port and, if it is, uses HTTP CONNECT method to set up a persistent tunnel.

While the WebSocket protocol itself is unaware of proxy servers and firewalls, it features an HTTP-compatible handshake so that HTTP servers can share their default HTTP and HTTPS ports (80 and 443) with a WebSocket gateway or server. The WebSocket protocol defines a ws:// and wss:// prefix to indicate a WebSocket and a WebSocket Secure connection, respectively. Both schemes use an HTTP upgrade mechanism to upgrade to the WebSocket protocol. Some proxy servers are harmless and work fine with WebSocket; others will prevent WebSocket from working correctly, causing the connection to fail. In some cases additional proxy server configuration may be required, and certain proxy servers may need to be upgraded to support WebSocket.

If unencrypted WebSocket traffic flows through an explicit or a transparent proxy server on its way to the WebSocket server, then, whether or not the proxy server behaves as it should, the connection is almost certainly bound to fail today (as WebSocket become more mainstream, proxy servers may become WebSocket aware). Therefore, unencrypted WebSocket connections should be used only in the simplest topologies.

If an encrypted WebSocket connection is used, then the use of Transport Layer Security (TLS) in the WebSocket Secure connection ensures that an HTTP CONNECT command is issued when the browser is configured to use an explicit proxy server. This sets up a tunnel, which provides low-level end-to-end TCP communication through the HTTP proxy, between the WebSocket Secure client and the WebSocket server. In the case of transparent proxy servers, the browser is unaware of the proxy server, so no HTTP CONNECT is sent. However, since the wire traffic is encrypted, intermediate transparent proxy servers may simply allow the encrypted traffic through, so there is a much better chance that the WebSocket connection will succeed if WebSocket Secure is used. Using encryption is not free of resource cost, but often provides the highest success rate.

Unfortunately, the most recent update to the draft (version 76) breaks compatibility with reverse-proxies and gateways because the 8 bytes of data the client must send after the headers is not advertised in a Content-Length header, so the intermediates won’t forward that data until the handshake completes. And since the handshake needs those 8 bytes to complete, the handshake never completes and deadlocks. In current state of affairs, it’s not advisable to modify such intermediate components to support this non-standard HTTP behaviour because doing so would render the components vulnerable to HTTP smuggling attacks, since an attacker would just have to pretend trying to upgrade to the WebSocket protocol in a request to be able to send more data than the target plain HTTP server can parse, possibly bypassing some mandatory security filtering. It is not known if this recent breakage will be worked around in a new draft or not.

Adapted from the Wikipedia article WebSockets, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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