Question

I have an internal proxy that fetches data from my own server and displays to the client. I wanted to keep the proxy side code minimal, and thought that just sending the data got from the content server as-it-is to the client will work for all media types. It is working fine for HTML/TEXT code. However, not for images. I am unable to understand why.

Here is the proxy side code:

$curl_url="http//myserver.com/someimage.jpg";
//Open connection
$curl_handle = curl_init();
curl_setopt($curl_handle,CURLOPT_COOKIE,session_name()."=".session_id().";");
//Set the url, number of POST vars, POST data
curl_setopt($curl_handle, CURLOPT_URL, $curl_url);
curl_setopt($curl_handle, CURLOPT_POST, count($_POST));
curl_setopt($curl_handle, CURLOPT_POSTFIELDS, $_POST);
curl_setopt($curl_handle, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curl_handle, CURLOPT_AUTOREFERER, TRUE);
curl_setopt($curl_handle, CURLOPT_HEADER, 0);
/// curl_setopt($curl_handle, CURLOPT_HTTPHEADER, $curl_request_headers);
/// Above is actually uncommented but omitting details for brevity. They are just 
/// HTTP headers passed by the client
curl_setopt($curl_handle, CURLOPT_ENCODING, "identity");

//Execute post
$result = curl_exec($curl_handle);

//Close connection
curl_close($curl_handle);

echo $result;

Why does above not display images correctly? (Is it not possible to make the proxy like a dummy bridge – that it does not interpret the result sent by the server but just pass it on and still make it work for all content/media types?). Can someone suggest the cleanest solution?

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Notes:

1) The content server handles all files through a php script, and is correctly passing the header using
header('Content-Type: image/jpeg');

It works fine if I access directly from the server. However from the proxy, it does not work (browser displays binary data).

2)
I don’t understand CURLOPT_HEADER very well either.

If I use

curl_setopt($curl_handle, CURLOPT_HEADER, 1);

browser tries to download gzipped data (for text as well as images).

If I use

curl_setopt($curl_handle, CURLOPT_HEADER, 0);

browser displays text/images correctly, but not image data.

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These are the response headers shown by Google Chrome, when I access the image in the browser via the proxy (direct link).

Response Headers
Cache-Control:no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Connection:Keep-Alive
Content-Encoding:gzip
Content-Length:2576
Content-Type:text/html
Date:Mon, 29 Nov 2010 18:03:52 GMT
Expires:Thu, 19 Nov 1981 08:52:00 GMT
Keep-Alive:timeout=15, max=100
Pragma:no-cache
Server:Apache/2.2.14 (Ubuntu)
Vary:Accept-Encoding
X-Powered-By:PHP/5.3.2-1ubuntu4.5

---

The response headers when I access image directly on the content server:

Accept-Ranges:bytes
Connection:Keep-Alive
Content-Length:3669
Content-Type:image/jpeg
Date:Mon, 29 Nov 2010 18:07:25 GMT
ETag:"1802b6-e55-49633c9623e40"
Keep-Alive:timeout=15, max=96
Last-Modified:Mon, 29 Nov 2010 16:44:33 GMT
Server:Apache/2.2.11 (Ubuntu) PHP/5.2.6-3ubuntu4.6 with Suhosin-Patch mod_ssl/2.2.11 OpenSSL/0.9.8g

I have a feeling this is due to the Content-type being GZIP via the proxy. Can someone please help me understand this: Aren’t images GZIPPed by apache by default? (I agree savings might be less). If not, then is CURL (the proxy) Gzipping the data? Shouldn’t CURLOPT_ENCODING, “identity” prevent it? How do I fix?

Answer 1

You need to send the proper headers using header to tell the browser the correct content-type, otherwise it assumes it is just plaintext.