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FTP packet tracing

Posted on 2001-08-08
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Last Modified: 2013-12-07
Normally we use use data compression (zip) before FTP'ing a file. This allows us to check for data integrity
simply by uncompressing. Failure to uncompress implies data corruption.

However, we now need to FTP to a destination where data compression is not wanted. We are executing
the FTP from SCO UNIX to a Window 2000 PC. We are looking for a way to test data integrity without zipping
the data prior to FTP'ing.  From the FTP prompt, if I type the word 'trace' it toggles between 'Packet
tracing on' and 'Packet tracing off'. The manual seems to imply that packet tracing is not implemented
at this time. Does anyone know if there is a way to use packet tracing, or another way to verify that data corruption doesn't occur during the FTP process?
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Question by:teemartin
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by:jjeff1
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I think you over-estimate how much data corruption will occur. FTP is used all the time with no problems. Sites like Walnut Creek serve up Terabytes per day, all FTP, all with no errors. How often were you seeing corrupted FTP files when using ZIP?

If you want to do simple error checking, you can write a program, or find one, to do file checksums.

This is a free one I found:
http://www.geocities.com/SiliconValley/Heights/3222/ffv/

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by:newmang
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Packet tracing per se is not going to provide a data integrity check unless you do it manually. I would have thought that the standard TCP packet checksumming would provide a fairly good check.

When you say that the data receivers don't "want" compression does this imply that they do not want any post-transfer processing at all?

If they were not averse to post transfer processing and you wanted some form of verification presumably you could write an UNIX based program to "wrap" the data to be transferred into a file containing a checksum on the data and a Windows based program to "unwrap" the data and, at the same time check the "unwrapped" data against the checksum contained in the file.

Cheers - Gavin
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by:Droby10
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data integrity is built into tcp...if a packet is lost or corrupted it is resent...so extra measures aren't really needed...but if you want to verify data integrity after transmission, then i would create an md5 hash before and after the upload and compare the two.
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SteveJ earned 100 total points
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We FTP gigabytes of data per day without corruption but occasionally a link will drop or a machine will lockup or reboot and AIX's FTP doesn't really supply a good way of confirming that a given transfer was complete (there's more detail, but not worth going into). So, I have a script to FTP files that simply writes the local file size and remote file size after the transfer into temporary files that I then "diff". If they're alike, there's a really good possibility that the xfer was ok and I move on to the next file transfer.

Droby10's solution is a little more elegant but I put my script together in 5 minutes and I don't rely on anything other than FTP access to the remote machine and a 10 line shell script.

If you have corruption problems, you may have hardware or physical layer problems. It isn't likely that FTP is the cause.

Good luck.
Steve
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by:teemartin
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Thanks for the input, I'd give you all 25 points if I knew how. SteveJ's will work for us because we can check file size on both sides thru FTP commands. I'm sure the other solutions would work as well, if I had the time to look into algorithms.
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