How to Enable and Verify Automatic Resume in FileZilla for FTP/SFTP Transfers

Enable automatic resume in FileZilla

To have FileZilla continue an interrupted upload or download instead of starting over, set the File exists action to Resume transfer (or Auto resume) and optionally limit retries. This tells the client to send a REST (FTP) or APPEND (SFTP) command with the byte offset of the partially transferred file when the connection is restored.

Where to set the option

1. Open FileZilla.
2. Choose Edit → Settings (Linux/Windows) or FileZilla → Settings (macOS).
3. In the left pane select Transfers → File exists action.
4. From the dropdown choose Resume transfer (labelled Auto resume in some builds).
5. Optionally set Number of retries to 3 to allow automatic reconnection after a temporary drop.
6. Click OK to save.

No restart is required; the setting applies to all subsequent transfers.

How the resume mechanism works

When a transfer is interrupted, FileZilla records the number of bytes already written to the local (download) or remote (upload) file. Upon reconnection it:

1. Checks whether the remote server supports the REST command (FTP) or the APPEND operation (SFTP). This is advertised in the server’s feature list (e.g., 211-REST supported in FTP).
2. If supported, it sends REST <offset> where <offset> is the byte count already transferred.
3. The server replies with 350 Restarting at <offset> (FTP) or acknowledges the append request (SFTP).
4. FileZilla then resumes sending/receiving data from that offset, appending to the existing file.

If the server does not allow appending, or if the file size has changed on the server (indicating modification), FileZilla falls back to a fresh transfer and overwrites the existing file according to the chosen action.

Limits and common mistakes

• Server support required: Some FTP servers disable REST for security reasons, and certain SFTP implementations may not implement append correctly. Without server‑side support, resume will not work and FileZilla will restart the transfer.
• File must be unchanged: If the remote file is modified after the interruption, the byte offset no longer matches the actual data, causing a checksum mismatch. FileZilla detects the size change and treats it as a new file.
• Transfer mode matters: Using ASCII mode for binary data corrupts the file because line‑ending conversions change byte counts, making the offset invalid. Always set the transfer type to Binary (Edit → Settings → Transfers → File type settings) for non‑text files.
• Directory synchronization: The resume feature does not apply when using the Synchronize browsing or Directory comparison functions; those operations treat each file as an atomic unit.
• Firewall/proxy interference: Some NAT devices or proxies alter packet sequencing and may corrupt the offset negotiation, leading to a failed resume.

Verifying that resume works

Follow these steps to confirm that FileZilla is actually sending a resume command and that the resulting file is correct:

1. Enable debug logging: Edit → Settings → Debug → check Enable debug logging and set the log file path.
2. Start a large file transfer (e.g., a 100 MB ISO).
3. Pause the transfer manually via the transfer queue’s context menu → Pause.
4. Disconnect the client (or simulate a network drop) and then reconnect.
5. Observe the debug log: you should see a line similar to Command: REST 12345678 (FTP) or Command: APPEND with offset info (SFTP). The server reply should be 350 Restarting at 12345678 or a success code for append.
6. Allow the transfer to finish.
7. After completion, compute a checksum of the transferred file and compare it to the source (e.g., sha256sum source.iso vs. sha256sum destination.iso). Matching values confirm a successful resume.

If the log shows no REST or APPEND command, the server either does not support it or the setting is not applied.

Rolling back to the default behavior

If you need to revert to the standard Overwrite file action (for example, when troubleshooting), repeat the steps in the Enable automatic resume section and choose Overwrite file from the dropdown. This change takes effect immediately for new transfers; existing transfers in the queue are unaffected.