Files of a certain size are not uploaded to the server?

Question

linux

I ran into a very interesting problem - maybe someone else has come across this?
The symptoms are as follows: files of strictly defined sizes are not uploaded to the site.
For example, there is a file with a size of 69,459 bytes.
It is not uploaded to the site in any way at all (i.e. if we send it using the POST method, we get the following entry in the logs:

"POST / HTTP/1.1" 400 0 "/" "Mozilla/5.0 ..."

At the same time, everything is empty in error.log.
If you try to upload it via FTP, the same thing happens, the file is not uploaded (more precisely, VSFTPD creates a file with a size of 0 bytes and that's it)
. The vsftpd logs are empty at the same time (as if there was no upload at all).
The most interesting thing is that if you change this file (for example, resave it through paint) and its size becomes larger or smaller, then it is perfectly uploaded both by the POST method and via FTP.
The server is rented at parking.ru, system debian 6.0.4 squeeze
Linux 2.6.32-5-amd64 #1 SMP Mon Feb 25 00:26:11 UTC 2013 x86_64 GNU/Linux
What could be the problem of non-acceptance of certain files by the server? Where to dig?

Answer 1

Those. it turns out one specific file is not uploaded, the size has nothing to do with it. If the file is changed, it is uploaded. If you upload via SSL, it is uploaded.
Perhaps your antivirus, or some software from your provider, or some parking.ru software analyzes traffic and considers it a security attack. You can try uploading to other hosts, to other parking.ru hosts, to parking.ru from another provider. Or contact parking.ru and describe the situation very specifically.
(an example of such software is NAXSI but, only for HTTP)
There are no other ideas.

Answer 2

All you have to do is listen to traffic on both sides via tcpdump and see what comes up.

Answer 3

I specify - a completely different file of the same size (byte for byte) is uploaded.

Answer 4

What does "Doesn't fill" mean? It's in the "nothing works" category.
We take FileZilla, turn on the logging, upload the FTP file, paste the logs here. (maybe the standard logging level is enough - you don’t even need to turn it on).
If the transfer goes well, then someone is a sucker behind your ftpd or he himself is a sucker.
Try to upload via SFTP to the same place - if it also fails, then the transfer server is on the white list and someone else is goofing (antivirus, for example, or some kind of monitoring), if not, it remains in black.
At the same time, check the transfer mode - set the binaries forcibly.
Possible bug with server ftp - switch active-passive.