When FTP clients initiate passive file transfer operations, the FTP server uses asynchronous network notification to wait for the actual client connection to a passive data socket. By default, the FTP server allows 60 passive file transfer operations to be queued simultaneously. Once the queue reaches 60, subsequent client requests receive error 451: Too many Passive Users. Client connections remain in this transient state for a short time, between issuing a file transfer command (e.g. RETR, STOR) and connecting the data channel to the passive socket. For this reason, this error can only occur in one of the following scenarios:You may try to work around this problem using non-passive file transfer or changing a registry entry as described here.
-- A DoS attack has issued concurrent file transfer commands without actually connecting to the data socket.
-- An extremely large number of concurrent users are transferring files in passive mode (which is possible on slow networks).
This was first published in May 2005