SFTPPlus Documentation

Start Page 6. Client-side Usage Instructions 6.4. FTP/FTPS Client Side Operation Mode

6.4. FTP/FTPS Client Side Operation Mode

6.4.1. Introduction

This page contains information about the usage and behaviour of the FTP commands implemented by the FTP location/client.

The FTP/FTPS implementation is designed to follow the RFCs as much as possible.

The client-side is designed to work with old and new FTP implementation. It will use the FEAT command to discover and make use of the advance functionalities provided by the server. In the absence of the FEAT command it will assume that the FTP server is a bare minimum FTP server-side implementation.

6.4.2. UTF-8 support

SFTPPlus will always use the UTF-8 encoding for file names and paths.

The FTP client informs the server about UTF-8 support by sending the OPTS UTF8 command, if UTF-8 support is advertised by the server.

If server supports the CLNT command, the client will also issue the CLNT command, before issuing the OPTS command.

6.4.3. Date and Time

The FTP protocol, as designed tens of years ago, doesn’t include time zone information or support for detecting the time zone used by the server.

Beside that, the standard LIST command implemented by all FTP servers will return the date and time in a format which is not defined by the FTP standard.

For the moment SFTPPlus will assume that the server and client are on the same timezone. Due to this, for correct operation the client and server need to have the same timezone and have the time synchronized.

It will try to extract the date and time from the LIST command, assuming that it it similar to the result presented by the Unix ls command.

When running on Windows, HP-UX and AIX SFTPPlus will not be able to handle dates before the year 1970 and after the year 2038.

6.4.4. Validating the identity of the remote FTPS server

When no certificate authority is defined, SFTPPlus will not check the identity of the remote server.

When a certificate authority is defined for a FTPS server, SFTPPlus will check that the certificate presented by the server is signed by the trusted certificate authority and that the certificate was issued for the CN or Alternate Name matching the name used in the SFTPPlus configuration file.

Server identity validation is only supported when the server address is defined as DNS names / FQDN. It is not supported when the server is defined as an IP address.

On top of that, the certificate is not accepted when outside of its validity period.

If a certificate revocation list is defined, it will also check whether the certificate presented by the server was not revoked.

When the certificate presented by the server has the Subject Alternative Name defined, the CN field is ignored.

6.4.5. Authenticating FTPS with username and X.509 certificate

SFTPPlus can use the username and an X.509 certificate as credentials during the authentication process.

This assumes that the remote server supports this type of credentials.

When the certificate is accepted as the credential, the configured password is ignored and not sent to the server.

If the certificate is rejected as the credentials, it will fall back to using the password credentials and will send the password to the server.

6.4.6. Checking the existence of remote paths

For servers which support the MLST command as defined in RFC 3659, SFTPPlus will use the command to determine the existence of remote files and folders.

The MLST command provides the best performance as it only uses the already established command channel. No new connection or SSL/TLS handshake is required.

When MLST is missing, SFTPPlus will use the LIST command and check if the parent folder contains the targeted path.

Without the MLST command and with parent folders containing tens of members, the exist operation will be significantly slower. For each operation a new data channel is created, and for FTPS this means that a new SSL/TLS handshake is performed.

When the remote server returns the error code 550 SFTPPlus will consider that the file does not exists.


Most FTP server implementation will return the same error code 550 for Path not found and Permission denied error. When listing of the folder is not permitted but file write operation is permitted this can lead to the file being overwritten. The list operation fails and lets SFTPPlus know that file does not exist, but then the write operation will succeed as write is allowed.