FileZilla Forums

I have just migrated from 0.9.60.2 to 1.6.7.0 and stumbled into problems with amount of connections to the server.

In 0.9.60.2 the amount of clients which can connect til FileZilla Server is "unlimited" by default
In 1.6.7.0 there seems to be a limit of maximum 10 connections.

I managed to raise this limit to 256 connections by changing the parameter "Number of threads" in Protocol settings -> Performance.
According to parameter 256 is the maximum limit that can be entered.

However I found this a bit misleading;
In 0.9.60.2 "Number of threads" represented how many CPU ressources the application could occupy, in 1.6.7.0 there is no balloon help text explaining the meaning of this parameter, and maybe I am just too simple a person that feels "Number of threads" is not a text I would link to max amount of connections.

Anywho, is this limit a new feature in FileZilla Server, a bug in the release and is there any method where I ran raise this limit beyond 256 which is the current maximum.

Why do you say that in "1.6.7.0 there seems to be a limit of maximum 10 connections"? What do you see?

In 1.6.7.0 I saw a lot of connections being established until the limit of 10 connections is being reached, further connections beyond this point is not ACKed by the FileZilla server.
At that moment we discovered the issue there where around 40 incoming connections towards FTP server.

When changing the mentioned "Number of threads" to the maximum of 256, all connections beyond the already 10 established got connected correctly.

We cannot reproduce it here, even during high load with many sessions already open, so we need to know more details about a few things:

1) Are you using TLS?
2) What CPU is the server running on (to understand whether it possesses some instructions that are used by TLS)?
3) What exactly do you mean by "not ACKed"? Are the connections waiting to be accepted on the server side, or are they accepted but login is not happening at the pace you expect? Logs would be welcome.

- Plaintext FTP
- VM ESXi 7.0 (CPU tough Xeon Gold 6338, VMTools installed)
- What was experienced was that clients beyond the 10 fully connected clients were connecting to FTP server, but never received response back (the SYN-ACK)

I have tried recreating the scenario too on 2 other different VM guests on same VM platform (just to eliminate hardware) and cannot recreate the scenario either;
Tried both on 1, 2 and 10 thread setting on the server.
Tested with 30 simultaneous downloads through local loopback to server with above thread setting.
Nothing could re-create the scenario.

I am sorry to have bothered with something we cannot recreate and pinpoint, issue closed.

The SYN+ACK is sent by Windows, before the server even calls accept() on the socket. If you don't get SYN+ACK, something else must be blocking the connection. Check for firewalls.