html header 정의

http://www.w3.org/Protocols/HTTP/HTRQ_Headers.html#z14

User-Agent:

This line if present gives the software program used by the original client. This is for statistical purposes and the tracing of protocol violations. It should be included. The first white space delimited word must be the software product name, with an optional slash and version designator. Other products which form part of the user agent may be put as separate words.

<field>   =   User-Agent: <product>+
        <product> =   <word> [/<version>]
        <version> =   <word>

Example:

User-Agent:  LII-Cello/1.0  libwww/2.5

Referer:
This optional header field allows the client to specify, for the server's 
benefit, the address ( URI ) of the 
document (or element within the document) from which the URI in the request was 
obtained.
This allows a server to generate lists of back-links to documents, for 
interest, logging, etc. It allows bad links to be traced for maintenance.
If a partial URI is given, then it should be parsed relative to the URI of 
the object of the request.
Example:
Referer: http://www.w3.org/hypertext/DataSources/Overview.html