We have an FTPS (FileZilla) server running and a hundred or so clients connect nightly and upload files in the 20-100MB file range. However, there are 1 or 2 that appear to be able to connect, upload the entire file, then the client reports a failure (winsock 10038 error). The file was actually successfully uploaded. I wrote a simple FTP testing client and it appears that small files (10MB) work just fine. Once we get to around 20MB it starts to fail. I tried bumping up the timeout on the client but that doesn't appear to do anything except increase the amount of time it takes before it reports the failure.

We had them reset their router and test from a few machines at their site and the problem appears on all the machines (so it doesn't appear to be related to the machine). We also checked to make sure they weren't running any anti-virus/firewall software.

To make things even weirder, it occasionally is successful - they're able to upload the file and it completes successfully.

Again, the client connects successfully, can upload the entire file successfully, then it fails.

They appear to be running a Netgear router (don't have the model # yet).

Any ideas on what might be causing this?


first of all, filezilla server cannot provide SFTP, you probably mean FTPS here. Second, the most simple way of debugging this, is to run tcpdump on both the server and the client, reproduce the issue, and follow through the transaction to see the problem


This might me trickier than it might seem. In fact, as you can read in MS's official documentation, winsock error 10038 means that "an operation was attempted on something that is not a socket. Either the socket handle parameter did not reference a valid socket, or for select, a member of an fd_set was not valid".

You may want to use WireShark to dump your TCP traffic and see what is actually happening at the end of your large file transfer. By the way, 20MB should not be considered a "large file transfer", in fact - depending on which "school" you like better, a large file is either a file bigger than 2GB or bigger than 4GB.

One last detail: FileZilla does not implement the SFTP protocol, you're probably referring to FTPS (FTP over SSL/TLS).


A quick way to determine if this is a problem with Filezilla is to get a free trial of another FTP server to see if clients can connect to that. Reputable options include offerings from Ipswitch, Titan, Globalscape or Cerberus. Then you can easily see if this is a Filezilla problem.

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