Querystring

A typical URL containing a query string is as follows:

http://server/path/program?query_string

When a server receives a request for such a page, it runs a program (if configured to do so), passing the query_string unchanged to the program. The question mark is used as a separator and is not part of the query string.A link in a web page may have a URL that contains a query string. However, the main use of query strings is to contain the content of an HTML form, also known as web form. In particular, when a form containing the fields field1, field2, field3 is submitted, the content of the fields is encoded as a query string as follows:

field1=value1&field2=value2&field3=value3...

  • The query string is composed of a series of field-value pairs.
  • The field-value pairs are each separated by an equal sign.
  • The series of pairs is separated by the ampersand, '&'.

You can use the fields to create arrays of values. In which case the syntax is:

 field[]=value1&field[]=value2&field[]=value3...  

This will create an array, field with 3 values. You can also add the keys yourself in the square brackets if required.For each field of the form, the query string contains a pair field=value. Web forms may include fields that are not visible to the user; these fields are included in the query string when the form is submitted.This 'name then equal sign then value then ampersand' convention is a W3C recommendation. They also provide a further appendix entry that recommends the use of a semicolon instead of an ampersand.Technically, the form content is only encoded as a query string when the form submission method is GET. The same encoding is used by default when the submission method is POST, but the result is not sent as a query string, that is, is not added to the action URL of the form. Rather, the string is sent as the body of the request.Security

A URL query string allows for flexibility in retrieving data from a web server and possibly from the database used to populate pages for that web server. A read only data store, such as a weather mapping service, is one example where URL query strings can be used with great flexibility.In some circumstances, a URL query string may expose security issues because it can be edited by a user to retrieve data that they do not have access to. In particular, a URL query string containing a username and password could be used with a dictionary attack to guess at valid login credentials to a particular web site. Most secure webservers use at least MD5 hash checking, or more powerful encoding methods to validate all given strings.

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