FTP Error 125: Data Connection Already Open, Transfer Starting
FTP reply code 125 indicates that the data connection between client and server was already open when the transfer command (RETR, STOR, LIST, etc.) was issued, and the transfer is beginning immediately. In normal FTP operation, the server sends 150 to indicate it is about to open the data connection. Code 125 skips that step because the connection was pre-established. This typically occurs when the client issued a PORT or PASV command earlier and the data connection was set up before the transfer command. It is functionally equivalent to 150 but indicates the data channel was already waiting.
Status
125 Data Connection Already Open, Transfer StartingExample
220 ftp.example.com FTP server ready
USER alice
331 Password required
PASS ****
230 User alice logged in
PASV
227 Entering Passive Mode (203,0,113,10,195,80)
RETR report.pdf
125 Data connection already open; transfer starting
226 Transfer completeHow to Troubleshoot
No action needed — transfer is proceeding
Code 125 is a normal informational reply. The file transfer has started and will complete with a 226 response when finished. No troubleshooting is required.
Verify data connection mode
If you are seeing unexpected behavior, check whether your client is using active (PORT) or passive (PASV) mode. Passive mode is generally more reliable through firewalls and NAT.
curl -v --ftp-pasv ftp://ftp.example.com/file.txt
Common Causes
Data connection pre-established via PORT or PASV
highThe client set up the data connection (active mode via PORT or passive mode via PASV/EPSV) before issuing the transfer command. Since the connection is already established, the server responds with 125 instead of 150 and begins transferring immediately.
Persistent data connection between commands
mediumSome FTP clients and servers support keeping the data connection open between consecutive transfers. When the next transfer command is issued, the existing data connection is reused and the server sends 125.
Related Error Codes
File Status Okay, About to Open Data Connection
The server is ready to transfer the file and is about to open the data connection.
Closing Data Connection, Transfer Complete
The file transfer completed successfully and the data connection is being closed.
Can't Open Data Connection
The server could not establish the data connection needed for the file transfer.
Connection Closed, Transfer Aborted
The data connection was closed unexpectedly and the file transfer was aborted.
References
| Specification | Section |
|---|---|
| RFC 959 | RFC 959 §4.2 |
Frequently Asked Questions
This reference was compiled from official RFCs, protocol specifications, and hands-on troubleshooting experience. AI tools were used primarily for formatting and organizing the content on the page.