The Web Application Hacker’s Handbook Discovering and Exploiting Security Flaws

cached copy, and the 

Etag

string, which the server provided with that copy of

the resource. The 

Etag

is a kind of serial number that the server assigns to

each cacheable resource and that it updates each time the resource is

modified. If the server possesses a newer version of the resource than the date

specified in the 

If-Modified-Since

header, or if the 

Etag

of the current

version does match the one specified in the 

If-None-Match

header, then the

server will respond with the latest version of the resource. Otherwise, it will

return a 304 response as shown here, informing the browser that the resource

has not been modified and that the browser should use its cached copy.