Software Development:
Individual Programming

Symptom

You try to connect to your web space by FTP, but the server denies the connection and notifieds you: "maximum number of clients already connected". Example of an ftp log entry:

Command:USER [Loginname]
Response:331 Password required for [Loginname]
Command:PASS ***************
Response:530 Sorry, the maximum number of clients (3) from your host are already connected.
Error:Critical Error

Cause

Attackers often try to initialize many simultaneous ftp connections. Despite that attackers cannot login or even actually transmit data, many simultaneous connection requests can overload a host and lead to service interruptions. For that reasons the number of simultaneous ftp connections is limited. Malware cannot affect the ftp service any longer.

Some ftp programs use simultaneous connections by default to transfer data. If your ftp program tries to use more than the maximum allowed number of simultaneous connections, the additional connections will by denied.

Solution

Limit your ftp program to the maximum allowed number of simultaneous connections. In FileZilla for example "Edit ..." -> "Settings" -> "Page 'Transfers'" -> "Simultaneous transfers".

If you have disconnected all connections and the server denies new initialization requests although have waited a few seconds, it is possible that a malware on your PC transfers files to your server imperceptibly. In that case your ftp program will not be able to use all simultaneous connections that are normally available.

Overview of frequently asked questions