Re: [squid-users] Compute digest as content is written to cache

From: Amos Jeffries <>
Date: Sun, 12 Aug 2012 18:37:30 +1200

On 11/08/2012 10:21 p.m., Jack Bates wrote:
> On 11/08/12 12:30 AM, Amos Jeffries wrote:
>> On 11/08/2012 7:22 p.m., Jack Bates wrote:
>>> I am interested in intercepting content as it is written to the cache,
>>> and computing a digest from the content. Do you know if this can be
>>> done in some kind of add on, or would it require a change to the core?
>> What type of digest and to what purpose?
> I was thinking of using OpenSSL
> SHA256_Init()/SHA256_Update()/SHA256_Final(). The purpose I have in
> mind is to detect identical content at different URLs
> Given a response with a "Location: ..." header and a "Digest:
> SHA-256=..." header (such as from MirrorBrain), if the URL in the
> "Location: ..." header is not already cached but the "Digest:
> SHA-256=..." header matches the content at some other URL that is
> already cached, then I want to update the "Location: ..." header with
> the cached URL. I think this should redirect clients to mirrors that
> are already cached

Small problem there. The digest is not calculated/known until the object
is finished arriving. By then it is too late to attach new headers. And
way too late to decide whether to ask that source for it.

Received on Sun Aug 12 2012 - 06:37:41 MDT

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