[nSLUG] [Somewhat OT] MTA changes header, mssg body lines. What?

David Potter dlpotter at eastlink.ca
Wed Feb 16 08:08:42 AST 2011


That's a nice tidy package... this is why I like this list!

David


On 11-02-16 02:49 AM, Mike Spencer wrote:
> I inquired about my observation that a handcrafted "boundary"
> parameter to the Content-Type: header line in a sent message had been
> changed in the received message.
>
> Aaron Spanik replied:
>
>> Whatever Eastlink's mail system is using to scan the mail for
>> viruses and/or SPAM is the likeliest culprit.
> Okay...
>
>> The reason you're MIME encoding in the first place is because what
>> you're trying to send to the other end isn't (all) naturally 7-bit
>> ASCII text (the encoding technically still at the heart of the SMTP
>> standard).
> Yes.  My script explicitly runs known binary data through mmencode
> and inserts it within appropriate MIME boundaries and (sub)headers.
>
>> In order to scan it, the scanner has to decode it to whatever its
>> declared natural form is...
> application/octet-stream
>
>> ...and scan that.  It can then either rebuild the message, which it
>> appears to be doing, or it can pass on the original.
> Ah-ha!  Let's check that.  Instead of running the binary data through
> mmencode on the fly, we'll manually run it through mmencode>  file;
> manually edit the file to insert some chars illegal in base64
> (tr a+ :_); tell the script to cat that file into the mssg body as the
> base64-encoded block.
>
> You must be right, Aaron.  Because the base64 data block received is
> different from the one sent. They must be unpacking it, presumably
> ignoring the illegal chars (per RFC 1521) and re-encoding it without
> the ignored chars.
>
> They (apparently Eastlink) do insert header lines:
>
>      X-Sieve: CMU Sieve 2.3
>      X-CMAE-Analysis: .....
>
> in addition to the ClamAV header inserted by Tallships.
>
>
> Thanks, Aaron,
>
> - Mike
>
>


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