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[KiwiPaulW]

I have a DNS-320L Hardware Version: A3
Current NAS Firmware Version 1.05  Firmware Date   07/16/2015
One hard drive is fitted, 1GB, standard format.
with add-on SoftetherVPN Version 1.00
with settings for SecureNAT and an entry under 'edit the static routing table to push' that enables me to establish a remote connection to file shares mapped to the NAS's ethernet port.

In Windows 7, when I copy (or robocopy) a file of about 1 MByte to the mapped drive that is a NAS file share, the saved file is corrupted about half the time.  The nature of the file corruption is that a 32 KByte block within the file, starting and ending exactly on 32 KByte boundaries, will be totally different to the original.  A number of bytes at the end of the block are null in the corrupted version (and non-null in the original).  I've seen various 32K blocks corrupted (e.g. 0x18000-0x1FFFF or 0x98000-0x9FFFF).

The corruption seems to be more likely when VPN traffic is heavier, such as copying 2 files simultaneously or just viewing the mapped drive in Windows explorer - but that is just my perception.  When using "copy /V" in windows 7 to copy to the mapped NAS drive, there is a high correlation between it returning an error and the destination file being corrupted.

Rebooting the NAS does not fix the problem.  Copying the same file through the MyDlink interface yielded no problem (but it isn't practical for my application).

Any ideas on how to fix or diagnose this further would be appreciated.

[vtverdohleb]

I'm the only developer of add-on SoftEtherVPN for NAS D-Link. Unfortunately already I can not help you. I recommend to reduce the MTU, and it is better to turn to designers SoftEtherVPN.

[KiwiPaulW]

Many thanks vtverdohleb. 

I confirm the problem does NOT go away with 'Secure NAT configuration' - 'MTU value' changed from 1500 to 500.  I also note upgrading from add-on version 1.00 to 1.01 does not help either.

I've posted this with more detail (including some recognisable strings that were inserted into a corrupted file) as http://www.vpnusers.com/viewtopic.php?f=7&t=5445.

[KiwiPaulW]

UPDATE: I have stopped the file corruption issue.

The problem started after loading the add-on SoftetherVPN Version 1.00 into the DNS-320L and was observed as files saved to the NAS over VPN sometimes being corrupted (especially larger files and possibly a VPN client via a marginal wifi connection).  Some consistent observations were made regarding the corrupted part of the saved files.

The problem was repeated using VPN (from 2 different sources) hosted external to the NAS and also continued with the Add-on replaced with version 1.01.  The problem even repeated with the add-on disabled.

I could NOT repeat the problem with a similar setup copying files to the NAS over the local LAN (no VPN).

The problem ceased after deleting the add-on, restoring the NAS configuration to the factory default and re-entering all settings.

During early testing of the add-on (while still learning what was what) I also experienced other NAS mis-behaviours (such as mydlink not working) that was cured by restoring the NAS configuration to a version saved before introduction of the add-on.

I can no longer trust the SoftEther DNS-320L add-on.  SoftEther for the Windows PC is fine.