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<title>Nine Nines: Dealing with broken clients</title>
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<h1 class="lined-header"><span>Dealing with broken clients</span></h1>
<div class="paragraph"><p>There exists a very large number of implementations for the
HTTP protocol. Most widely used clients, like browsers,
follow the standard quite well, but others may not. In
particular custom enterprise clients tend to be very badly
written.</p></div>
<div class="paragraph"><p>Cowboy tries to follow the standard as much as possible,
but is not trying to handle every possible special cases.
Instead Cowboy focuses on the cases reported in the wild,
on the public Web.</p></div>
<div class="paragraph"><p>That means clients that ignore the HTTP standard completely
may fail to understand Cowboy’s responses. There are of
course workarounds. This chapter aims to cover them.</p></div>
<div class="sect1">
<h2 id="_lowercase_headers">Lowercase headers</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy converts all headers it receives to lowercase, and
similarly sends back headers all in lowercase. Some broken
HTTP clients have issues with that.</p></div>
<div class="paragraph"><p>A simple way to solve this is to create an <code>onresponse</code> hook
that will format the header names with the expected case.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">capitalize_hook</span></span>(<span style="color: #009900">Status</span>, <span style="color: #009900">Headers</span>, <span style="color: #009900">Body</span>, <span style="color: #009900">Req</span>) <span style="color: #990000">-></span>
<span style="color: #009900">Headers2</span> <span style="color: #990000">=</span> [{<span style="font-weight: bold"><span style="color: #000000">cowboy_bstr:capitalize_token</span></span>(<span style="color: #009900">N</span>), <span style="color: #009900">V</span>}
|| {<span style="color: #009900">N</span>, <span style="color: #009900">V</span>} <span style="color: #990000"><-</span> <span style="color: #009900">Headers</span>],
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #009900">Status</span>, <span style="color: #009900">Headers2</span>, <span style="color: #009900">Body</span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Note that HTTP/2 clients do not have that particular issue
because the specification explicitly says all headers are
lowercase, unlike HTTP which allows any case but treats
them as case insensitive.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_camel_case_headers">Camel-case headers</h2>
<div class="sectionbody">
<div class="paragraph"><p>Sometimes it is desirable to keep the actual case used by
clients, for example when acting as a proxy between two broken
implementations. There is no easy solution for this other than
forking the project and editing the <code>cowboy_protocol</code> file
directly.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_chunked_transfer_encoding">Chunked transfer-encoding</h2>
<div class="sectionbody">
<div class="paragraph"><p>Sometimes an HTTP client advertises itself as HTTP/1.1 but
does not support chunked transfer-encoding. This is invalid
behavior, as HTTP/1.1 clients are required to support it.</p></div>
<div class="paragraph"><p>A simple workaround exists in these cases. By changing the
Req object response state to <code>waiting_stream</code>, Cowboy will
understand that it must use the identity transfer-encoding
when replying, just like if it was an HTTP/1.0 client.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">Req2</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set</span></span>(<span style="color: #FF6600">resp_state</span>, <span style="color: #FF6600">waiting_stream</span>)<span style="color: #990000">.</span></tt></pre></div></div>
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