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Finish test for Cache Control Directives

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Tobias Eidelpes 2020-03-28 11:19:46 +01:00
parent b8d6f92bc6
commit e8afe63364
2 changed files with 29 additions and 1 deletions
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1
abbrev/acronym.tex
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@ -23,3 +23,4 @@
\newacronym {ISP} {ISP} {Internet Service Provider}
\newacronym {SQL} {SQL} {Structured Query Language}
\newacronym {CDN} {CDN} {Content Delivery Network}
\newacronym {ETag} {ETag} {Entity Tag}

29
chapters/methods.tex
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@ -779,7 +779,34 @@ attack.
Cache Control Directives can be supplied in the Cache-Control \gls{HTTP} header,
allowing rules about storing, updating and deletion of resources in the cache to
be defined.
be defined. Cache Control Directives make heavy use of \emph{\glspl{ETag}} and
\emph{Last-Modified \gls{HTTP} Headers} to determine whether a cached resource
is stale and needs to be updated. Commonly, a collision-resistant hash function
is used to generate a unique hash of a cached resource which is sent along with
the resource in the first \gls{HTTP} request. The resource and the hash—which is
stored in the \gls{ETag} header—is then cached by the client. On subsequent
retrievals of the same \gls{URL}, the client checks for an expire date on the
requested \gls{URL} via the Cache-Control and Expire headers. If the \gls{URL}
has expired, the client sends a request with the \emph{If-None-Match} field set
with the \gls{ETag}. The server then compares the \gls{ETag} received by the
client with the generated \gls{ETag} of the resource on the server side. If the
two values match (i.e., the resource has not changed), the server can send back
an \gls{HTTP} 304 Not-Modified status. Otherwise, the answer contains a full
\gls{HTTP} response with the modified resource and the newly generated
\gls{ETag}, which the client can cache again. Usage of \glspl{ETag} can
therefore improve performance and cache consistency while at the same time
reducing bandwidth usage.
As with most other tracking methods, unique identifiers can be stored inside the
\gls{ETag} header because it offers a storage capacity of 81864 bits. Once the
identifier has been placed in the \gls{ETag} header, the server can answer
requests to check for an updated resource always with an \gls{HTTP} 301
Not-Modified header, effectively persisting the unique identifier in the
client's cache. During their 2011 survey of QuantCast.com's top 100 U.S. based
websites \citeauthor{ayensonFlashCookiesPrivacy2011} found \texttt{hulu.com} to
be using \glspl{ETag} as backup for tracking cookies that are set by
\texttt{KISSmetrics} (an analytics platform). This allowed cookies to be
respawned once they had been cleared by checking the \gls{ETag} header.
\subsection{DNS Cache}
\label{subsec:dns cache}