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Question:User-Agent definition

This document tries to describe how to identify the HTTP user agent (browser or robot) examining the User-Agent string in the HTTP header of the request to the WEB server. The User-Agent string format is described by RFC 1945 and RFC 2068 . Here are some excerpts from RFC 2068, please refer to the original RFCs for a complete reference: Many HTTP/1.1 header field values consist of words separated by LWS or special characters. These special characters MUST be in a quoted string to be used within a parameter value. token = 1* tspecials = "(" | ")" | "<" | ">" | "@" | "," | ";" | ":" | "\" | <"> | "/" | "[" | "]" | "?" | "=" | "{" | "}" | SP | HT Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing "comment" as part of their field value definition. In all other fields, parentheses are considered part of the field value. comment = "(" *( ctext | comment ) ")" ctext = 3.8 Product Tokens Product tokens are used to allow communicating applications to identify themselves by software name and version. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by whitespace. By convention, the products are listed in order of their significance for identifying the application. product = token ["/" product-version] product-version = token Examples: User-Agent: CERN-LineMode/2.15 libwww/2.17b3 Server: Apache/0.8.4 Product tokens should be short and to the point -- use of them for advertising or other non-essential information is explicitly forbidden. Although any token character may appear in a product- version, this token SHOULD only be used for a version identifier (i.e., successive versions of the same product SHOULD only differ in the product-version portion of the product value). 14.42 User-Agent The User-Agent request-header field contains information about the user agent originating the request. This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations. User agents SHOULD include this field with requests. The field can contain multiple product tokens (section 3.8) and comments identifying the agent and any subproducts which form a significant part of the user agent. By convention, the product tokens are listed in order of their significance for identifying the application. User-Agent = "User-Agent" ":" 1*( product | comment ) Example: User-Agent: CERN-LineMode/2.15 libwww/2.17b3 Unfortunately the standard is not followed by some browsers, and often each browser uses its own format to compose the User-Agent string, resulting in a jungle of different formats. Moreover many browsers identify themselves as another browser, for compatibility reasons, and this makes it harder for a piece of software the recognition of the actual name and version of the agent.

asked Sep 13, 2013 in Java Interview Questions by ashish singh
edited Sep 12, 2013
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