Troubleshooting Outbound FTP

Outbound FTP connections can fail for several reasons. The sections below cover the most common causes and how to address each one.

Connection and Authentication Failures

If Files.com cannot reach the remote FTP server or cannot log in, start by confirming the hostname, credentials, and port in your remote server configuration.

The default FTP port is 21 (explicit) or 990 (implicit), though a remote server may use a customized port. If the connection information specifies port 22, confirm whether the remote server uses SFTP rather than FTP(S). If it uses SFTP, configure an SFTP remote server instead.

Concurrent Connection Limits

FTP servers support a limited number of simultaneous connections. Enterprise-grade FTP servers allow higher numbers. Smaller FTP servers allow fewer.

If you are seeing connection failures, the remote server may be rejecting connections because its limit has been reached. Reduce the maximum number of connections setting and adjust gradually to find the highest value the remote FTP server can reliably support.

Rate Limits

FTP servers can implement rate limits that cap the number of connections allowed within a specified duration.

Implementing dedicated IP addresses for your Files.com site can help mitigate rate-limit issues.

Timeout Issues

Timeouts can come from several sources.

Intermittent network issues between Files.com and the remote server can prevent FTP commands or their responses from completing within the time allowed by the FTP protocol.

When the remote server is overloaded or runs out of resources, such as CPU or memory hitting 100%, FTP commands take much longer to execute.

When the remote server changes a required configuration item, such as the authentication method or the FTP port, FTP connections time out while attempting to connect using outdated information.

FTP commands fail if the remote FTP server does not respond within 45 seconds. Responses normally take between 5 and 10 seconds. A response time exceeding 45 seconds indicates a problem on the remote server.

Resolve any network issues and confirm that the remote FTP server can support the connection load being sent to it from Files.com.

Remote Server Capacity

If transfers fail intermittently without a clear cause, the remote FTP server itself may not be able to handle the volume of connections being sent to it. Some FTP servers are implemented as a lightweight feature on a larger system and are not designed for sustained or heavy FTP usage.

Confirm that the combined workload from all Syncs, Remote Mounts, and Automations to the remote server stays within its operational limits.

Firewalls

Compare the remote firewall rules with the outbound FTP connection requirements. Confirm that the allowed source addresses match the Remote Server's current routing settings and that the rules cover the control port and the remote server's full passive data-port range.

Ask the remote operator to check for rejected connections from those addresses at the time of a failed attempt. A rule allowing another FTP client or a different Files.com connection does not establish that this connection is allowed.

Partial Connectivity

If login succeeds but listings or transfers fail, check whether the firewall blocks passive data connections. A zero-byte file after an attempted transfer can also indicate that the control connection succeeded but the data connection failed.

A successful listing confirms only the data connection used for that listing; another transfer may use a different port. Have the remote operator compare their server's configured passive-port range with the firewall rules and correct any gaps. Use the connection setup requirements to verify the full range, rather than opening only the port from the failed attempt.

SSL Certificate Issues

If the remote FTPS server uses a certificate that is expired, self-signed, or does not match the hostname, Files.com refuses the connection by default. You can configure the Remote Server to allow non-matching certificates, but this removes the security guarantee that the remote site is who it claims to be.

To check the validity of the remote site's certificate, use an online tool such as the SSL Shopper SSL checker or the DigiCert SSL certificate checker.