FTP reply code 426 means the data connection was terminated before the file transfer completed. The control connection remains open — only the data channel was lost. This differs from 425 (could not open data connection at all) — with 426, the data connection was established and the transfer began but was interrupted mid-stream. Common causes include network interruptions, firewall timeouts on idle data connections, the server running out of disk space during an upload, or the client sending an ABOR command to cancel the transfer. The transfer should be resumed using REST followed by RETR or STOR.
The network path between client and server was disrupted during the file transfer. Packet loss, routing changes, or intermittent connectivity can cause the data connection to drop while the control connection survives.
Stateful firewalls track connections and may timeout the data connection if there is a pause in data flow. This is common with large transfers over slow links where data rate drops below the firewall's idle threshold.
The client explicitly aborted the transfer by sending ABOR on the control connection. The server terminates the data transfer and closes the data connection, reporting 426 followed by 226 to confirm the abort was processed.
During an upload, the server ran out of disk space. The data connection is closed because the server cannot continue writing the file. The partially uploaded file may remain on the server.
Use the REST command to set the byte offset and then RETR or STOR to resume the transfer from where it left off. Most FTP clients support the -C flag for automatic resume.
curl -C - -O ftp://ftp.example.com/largefile.zip
Run a continuous ping to the server while retrying the transfer to identify network interruptions. If packet loss is high, the transfer will keep failing.
ping ftp.example.com
If your firewall is prematurely closing the data connection, increase the connection tracking timeout for FTP data connections. On Linux, adjust nf_conntrack_ftp timeout settings.
If uploading, the server may have run out of space. Check with the server administrator or try uploading a small test file to confirm the server can still accept data.
The server is ready to transfer the file and is about to open the data connection.
The file transfer completed successfully and the data connection is being closed.
The server could not establish the data connection needed for the file transfer.
The file operation was not completed because the file is temporarily unavailable.