TCP segmentation and application inspection

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Aug 5th, 2009
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Someone recently complained to me that he could not send SMTP alerts through the ASA 5510 (8.0(3)12) from their IBM ServeRaid application. I set up a packet capture on both the inside and the outside interfaces and found that the ASA is replacing the HELO command with XXXX and sending it on. Upon further investigation, I discovered that the SMTP HELO command had been segmented at the TCP layer such that the first packet contained the "H" and the second packet contained the rest of the command.

This raises a couple of questions. The first question is why in the world the command wasn't transported in a single TCP segment. At this point I am attributing it to a flaky SMTP client. The second question I have is, can this behavior be accommodated? Is there a way to get the ASA to reassemble before inspection takes place?

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suschoud Wed, 08/05/2009 - 12:43
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asa would not be able to reassemble,however u can get rid of inspect esmtp to avoid masking


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