[dkim-ops] How do I know which header is breaking the signature

Murray S. Kucherawy msk at cloudmark.com
Fri Nov 6 09:44:04 PST 2009

I suppose that would depend on which implementation you’re using, but generally I’ve found use of the “z=” tag is a good way to track down such problems.  Failing that, you’ll need to capture the canonicalized message form at each end and use something like “diff” to see what changed.

From: dkim-ops-bounces at mipassoc.org [mailto:dkim-ops-bounces at mipassoc.org] On Behalf Of ram
Sent: Friday, November 06, 2009 4:50 AM
To: dkim-ops at mipassoc.org
Subject: [dkim-ops] How do I know which header is breaking the signature

My messages are being changed in transit  and so the signatures are breaking.
But I dont know which header is being changed

Can I run something to tell me Signature invalid "header-X" has been changed

For eg this message has a broken DKIM  due to Content-Transfer-Encoding ( I found that out by trial and error)

Received: from darkstar.netcore.co.in (unknown []) by
        ho.netcore.co.in (Postfix) with ESMTPA id 3C2026CE04B3 for
        <ram at netcore.co.in>; Fri,  6 Nov 2009 12:09:40 +0530 (IST)
Received: from location.exampledomain.com (list.netcore.co.in
        []) by darkstar.netcore.co.in (Postfix) with ESMTP id
        8C9226680CB for <ram at netcore.co.in>; Fri,  6 Nov 2009 12:09:36 +0530 (IST)
Received: from netcore.co.in (localhost []) by
        location.exampledomain.com (Postfix) with ESMTP id 76B6D52C1C1 for
        <ram at netcore.co.in>; Fri,  6 Nov 2009 12:09:34 +0530 (IST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=netcore.co.in;
        s=default; t=1257489574; bh=GMfT7A4K07OPLdwXHEWtlS9++GG+RNZCfRtWpuTr5yg=;
MIME-Version: 1.0
Content-Disposition: inline
Content-Type: text/plain
X-Mailer: MIME::Lite 3.0105 (F2.74; T1.19; A1.76; B3.07; Q3.07)
Subject: Test Mail
Message-Id: <z8ux1gr9j5i3.1257489574 at netcore.co.in<mailto:z8ux1gr9j5i3.1257489574 at netcore.co.in>>
To: ram at netcore.co.in<mailto:ram at netcore.co.in>
Date: Fri, 06 Nov 2009 12:09:34 +0530
From: idcalerts at netcore.co.in<mailto:idcalerts at netcore.co.in>
Content-Transfer-Encoding: 8bit

This is a Test Mail
Please Ignore

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