FTP 125 Data Connection Open vs 503 Bad Sequence
FTP 125 (Data Connection Open) is a 1xx Positive Preliminary response, while 503 (Bad Sequence) is a 5xx Permanent 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, 503 means that bad sequence of commands. The command is valid but was sent in the wrong order relative to other commands.
Description
The data connection is already open and the transfer is beginning. No need to open a new data connection.
When You See It
When the server reuses an existing data connection for a file transfer instead of opening a new one.
How to Fix
No fix needed — the transfer is starting immediately. Your client should begin reading or writing data on the existing connection.
Description
Bad sequence of commands. The command is valid but was sent in the wrong order relative to other commands.
When You See It
When you skip a required step, like sending PASS before USER, or RNTO without first sending RNFR.
How to Fix
Follow the correct command sequence. Common sequences: USER then PASS for login, RNFR then RNTO for rename, PASV then RETR for transfer.
Key Differences
125 is a 1xx Positive Preliminary response, while 503 is a 5xx Permanent Negative response.
FTP 125: The data connection is already open and the transfer is beginning. No need to open a new data connection.
FTP 503: Bad sequence of commands. The command is valid but was sent in the wrong order relative to other commands.
You encounter 125 when when the server reuses an existing data connection for a file transfer instead of opening a new one.
You encounter 503 when when you skip a required step, like sending PASS before USER, or RNTO without first sending RNFR.
When to Use Which
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 503 (Bad Sequence): Follow the correct command sequence. Common sequences: USER then PASS for login, RNFR then RNTO for rename, PASV then RETR for transfer.