FTP 125 Data Connection Open vs 421 Service Not Available
FTP 125 (Data Connection Open) is a 1xx Positive Preliminary response, while 421 (Service Not Available) is a 4xx Transient Negative response. 125 indicates that the data connection is already open and the transfer is beginning. No need to open a new data connection. In contrast, 421 means that service not available, closing control connection. The server is shutting down or has reached its connection limit and cannot serve this request.
説明
The data connection is already open and the transfer is beginning. No need to open a new data connection.
このコードが表示される場合
When the server reuses an existing data connection for a file transfer instead of opening a new one.
解決方法
No fix needed — the transfer is starting immediately. Your client should begin reading or writing data on the existing connection.
説明
Service not available, closing control connection. The server is shutting down or has reached its connection limit and cannot serve this request.
このコードが表示される場合
When the FTP server is overloaded, shutting down for maintenance, or has reached its maximum number of concurrent connections.
解決方法
Wait a few minutes and try reconnecting. If persistent, check if the server has a connection limit and whether your IP is being rate-limited.
主な違い
125 is a 1xx Positive Preliminary response, while 421 is a 4xx Transient Negative response.
FTP 125: The data connection is already open and the transfer is beginning. No need to open a new data connection.
FTP 421: Service not available, closing control connection. The server is shutting down or has reached its connection limit and cannot serve this request.
You encounter 125 when when the server reuses an existing data connection for a file transfer instead of opening a new one.
You encounter 421 when when the FTP server is overloaded, shutting down for maintenance, or has reached its maximum number of concurrent connections.
どちらをいつ使うか
For 125 (Data Connection Open): No fix needed — the transfer is starting immediately. Your client should begin reading or writing data on the existing connection. For 421 (Service Not Available): Wait a few minutes and try reconnecting. If persistent, check if the server has a connection limit and whether your IP is being rate-limited.