11.4. FTP/FTPS Client Side Operation Mode¶
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.
Below is the list of FTP commands required to be implemented on the FTP server-side, for the FTP location to operate:
PASS (optional for FTPS, as FTPS can have certificate authentication)
RETR (required when location is the transfer source)
LIST (required when location is the transfer source)
STOR (required when location is the transfer destination)
Below is the list of command required for Implicit FTPS:
The Explicit FTPS locations requires the same set of commands as for FTP and Implicit FTPS with the addition of:
To transfer a file to a FTP destination with temporary name, these FTP commands are required:
To verity that a file on destination is not overwritten, the following commands are required:
MSLT (if the server supports it, otherwise LIST is used)
The following command are also used by SFTPPlus, but they are not required. SFTPPlus will fallback to a default behaviour, when the server doesn't supports them:
CLNT - only if advertised in FEAT
OPTS UTF8 ON - only if advertised in FEAT
PWD - used to detect remote filesystem separator. defaults to Unix paths.
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.
The FTP client implementation from SFTPPlus will always use the same IP address for the command and data channel.
The IP address returned by the PASV command is ignored.
In this way, the PASV command implementation is similar to the ESPV (Extended PASV) command.
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, SFTPPlus will not be able to handle dates before the year 1970 and after the year 2038.
It is important to define a certificate authority that is associated to the peer. The role of a CA (or certificate authority) is not only to be the issuer for digital certificates, but it is also central within the PKI (public key infrastructure) in building the trust required for both the subject/owner of the certificate (such as a local peer) and the recipient/or party that is reliant on the certificate (such as a remote peer).
Without defining a CA, the trust model is incomplete and the remote serve certificates are not validated.
To define a certificate authority, the administrator must add the following to the location configuration:
[locations/234a-bc34-9812] ssl_certificate_authority: path/to/ca-cert.pem adderss: same.name-as-CN-or-SAN.tld
Once a CA is defined, SFTPPlus will check that the certificate presented by the remote peer is signed by the trusted CA and that the certificate was issued for the CN or if relevant, the Subject Alternative Name.
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. To link a certificate revocation list, please add the following:
ssl_certificate_revocation_list: path/to/crl-distribution-points ssl_certificate_revocation_list_refresh = value
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.
For client validation, the client proving its identity to the remote server, ensure that the client key and certificate specified by the ssl_key and ssl_certificate configuration options.
When the SFTPPlus client-side does not send its certificate due to a misconfiguration, the server-side might not accept the SSL/TLS connection and will emit an SSL handshake error message.
Once 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.
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.