FTP reply code 421 indicates the server cannot process the request and is terminating the connection. This can appear at any point — as the initial response instead of 220 (server overloaded or shutting down), or mid-session if the server encounters a critical error or resource exhaustion. Unlike most 4xx codes which are temporary failures on specific commands, 421 is terminal for the entire session — the control connection is being closed. The client should wait and reconnect. Common triggers include server shutdown, exceeded connection limits, idle timeout enforcement, or anti-abuse detection (too many failed login attempts from the same IP).
The FTP server has reached its configured maximum number of concurrent connections. New connections are rejected with 421 until existing clients disconnect. This is common on busy public FTP servers.
The client was idle too long (no commands sent within the timeout period, typically 60-300 seconds). The server sends 421 and closes the connection to free resources. Use NOOP to keep connections alive.
The FTP daemon is being stopped or restarted. It sends 421 to all connected clients as part of a graceful shutdown. The server should be available again once maintenance completes.
The server detected too many failed authentication attempts from your IP address and is blocking further connections as a security measure. This is typically an anti-brute-force protection.
Code 421 is a temporary failure. Wait 30-60 seconds and try connecting again. If the server was overloaded, some connections may have freed up.
Verify that port 21 is still reachable. If the connection is refused entirely, the server may be down for maintenance.
Scan PortsConfigure your FTP client to send NOOP commands every 30-60 seconds during idle periods. Most FTP clients have a 'keep-alive' or 'anti-idle' setting for this.
If the FTP server hostname recently changed, your DNS cache may be pointing to the old server. Verify the hostname resolves to the correct IP.
Check DNS RecordsIf the FTP server's DNS records were recently updated, verify the changes have propagated globally.
Check PropagationThe FTP server is not ready yet but will be available shortly. The client should wait and retry.
The FTP server is ready and accepting connections. This is the greeting banner sent upon connection.
The user is not logged in. Authentication is required before this command can be executed.