SFTPPlus Documentation

Start Page 4. Configuration Instructions 4.13. FTP / Explicit FTPS / Implicit FTPS Service

4.13. FTP / Explicit FTPS / Implicit FTPS Service

This page describes the configuration options available for the FTP / FTPS service.

SFTPPlus provides two ways for securing FTP transfers - explicit FTPS and implicit FTPS.

4.13.1. Explicit FTPS

Explicit FTPS is the most widely used method. The connections for Explicit FTPS are started just like normal FTP connections. After the initial connection, the FTP client can ask the server to switch to the secured Explicit FTPS mode.

The Explicit FTPS service can share the same TCP port with the non-secured FTP service. This is why the FTP and Explicit FTPS protocols are configured in SFTPPlus as a single service using the ftp protocol.

A single service using the FTP protocol can be configured to allow only unsecured FTP connections, to allow only secured Explicit FTPS connections, or to allow both secured and unsecured connections.

4.13.2. Implicit FTPS

Implicit FTPS is a method in which the client is required to initiate a connection using an SSL/TLS handshake. All connections not initiated using an SSL/TLS handshake are dropped. This does not allow sharing the same TCP port with a non-secured FTP service, thus the Implicit FTPS service requires a dedicated port.

For more details, please check the dedicated FTPS article from Wikipedia.

4.13.3. Configuration options

4.13.3.2. passive_port_range

Default value:

All available ports provided by the operating system.

Optional:

Yes

Values:
  • A range of port numbers.
  • MIN_PORT - MAX_PORT
  • 0 - 0 to use random ports.
From version:

1.7.0

To version:

None

Description:

When FTP/FTPS clients are requesting a passive transfer, the server will open a new connection that will be used for sending or receiving files from the client.

By default, the server will use a random port number for the newly created connection.

For example, to set a range of ports starting from 1500 and ending at port 2000, set the value to 1500 - 2000 like the following example:

[services/27b8e2b1-7971-416d-af14-6a8aae2ac46e] passive_port_range = 1500 - 2000

To use all available ports, do not define this or set it to 0 - 0:

[services/27b8e2b1-7971-416d-af14-6a8aae2ac46e] passive_port_range = 0 - 0

Note

passive_port_range is designed to help with firewalled configurations, but to some extent it is also a mechanism to limit the number of concurrent connections to the server. While this option is specific to the FTP protocol, there is a generic option, maximum_concurrent_connections, available to all protocols.

4.13.3.3. active_source_port

Default value:

0

Optional:

Yes

Values:
  • A TCP port number.
From version:

3.6.0

Description:

Source port used by the FTP service to initiate the active connections for the PORT or the EPRT requests.

The source address is the same as address used to listen for incoming control connections.

If you have a set address such as:

[services/27b8e2b1-7971-416d-af14-6a8aae2ac46e]
address = 0.0.0.0

And your deployment requires an explicit outbound / outgoing / egress interface, you will need to configure an explicit address otherwise outbound connections will use the source address automatically provided by the operating system, based on routing table.

When set as 0, a random source port is used.

Note

On Linux and Unix systems, elevated privileges are required to initiate connection from source ports below 1024.

4.13.3.4. passive_address

Default value:

Disabled

Optional:

Yes

Values:
  • 1.2.3.4
  • Disabled to automatically detect server address.
  • Leave empty to automatically detect server address.
From version:

2.9.0

Description:

IP address advertised by the FTP service in the passive PASV request.

When left empty, the service will automatically detect server address.

When set to a single IP address, it will use that address in all PASV requests, regardless of the client’s source IP.

Note

This option is ignored for EPSV requests.

4.13.3.5. idle_data_connection_timeout

Default value:

30

Optional:

Yes

Values:
  • An integer value greater than 0, representing seconds.
From version:

1.8.2

To version:

None

Description:

Specifies the timeout, in seconds, after which an inactive data channel is disconnected. For this number of seconds, the service will only act upon the data channel, without disconnecting the command channel.

4.13.3.6. ssl_domains

Default value:

Empty

Optional:

Yes

Values:
  • Fully qualified domain name (FQDN)
  • Empty
From version:

3.42.0 This configuration option defines the domain for which SFTPPlus will request certificates from the Let’s Encrypt server.

The same domain can be shared by multiple services.

The domain name is handled as a case-insensitive lower case value.

For this option to be used, you need to define a lets-encrypt resource.

4.13.3.7. ssl_certificate

Default value:

no-certificate-defined

Optional:

Yes

Values:
  • Absolute path on local filesystem.
  • Certificate as PEM text format (Since 3.40.0).
  • Disabled
From version:

1.6.0

To version:

None

Description:

This can be defined as an absolute path on the local filesystem to the SSL certificate file used by the component. File content should be encoded in the Privacy-Enhanced Mail (PEM) format.

Note

The path should not be longer than 256 characters.

You can also define the content of the certificate as text in PEM format. In this case the configuration will look as in the following example. It’s important to start each line with at least one space character and keep the number of leading spaces constant:

ssl_certificate = -----BEGIN CERTIFICATE-----
    MIICaDCCAdGgAwIBAgIBDjANBgkqhkiG9w0BAQUFADBGMQswCQYDVQQGEwJHQjEP
    ...
    MORE CERTIFICATE DATA
    ...
    JZQaMjV9XxNTFOlNUTWswff3uE677wSVDPSuNkxo2FLRcGfPUxAQGsgL5Ts=
    -----END CERTIFICATE-----

When the value contains both the certificate and the key, the configuration will look as in the following example:

ssl_certificate = -----BEGIN RSA PRIVATE KEY-----
    MIICXgIBAAKBgQDOoZUYd8KMYbre5zZIwR+V6dO2+cCYVS46BHbRbqt7gczkoIWh
    ...
    MORE KEY DATA
    ...
    Wh+QF3UArO8r8RYv3HRcnBjrGh+yEK93wIifVNGgy63FIQ==
    -----END RSA PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIICaDCCAdGgAwIBAgIBDjANBgkqhkiG9w0BAQUFADBGMQswCQYDVQQGEwJHQjEP
    ...
    MORE CERTIFICATE DATA
    ...
    JZQaMjV9XxNTFOlNUTWswff3uE677wSVDPSuNkxo2FLRcGfPUxAQGsgL5Ts=
    -----END CERTIFICATE-----

This certificate is sent to the remote peer during the SSL/TLS handshake process.

The certificate file can contain both the certificate and the private key, in which case you don’t need to set the path to the private key. Only supported for PEM encoding.

The certificate file can contain the certificate chain. The targeted certificate should be first in the file, followed by the chained certificates. It will advertise the certificate chain in the same order as listed in the file. Only supported for PEM encoding. (Since 3.22.0)

For server-side components using TLS/SSL secure communication, this configuration options is required.

For client-side component using TLS/SSL, you can disable sending the certificate as part of the handshake, by setting this configuration option to Disabled.

4.13.3.8. ssl_key

Default value:

Empty

Optional:

Yes

Values:
  • Absolute path on local filesystem.
  • Key as PEM text format (Since 3.40.0).
  • Empty
From version:

1.6.0

Description:

This can be defined as an absolute path on the local filesystem to the X.509 private key file used by this component.

File content should be encoded in the Privacy-Enhanced Mail (PEM) format.

Note

The path should not be longer than 256 characters.

When the value is defined as PEM text, the configuration will look as in the following example:

ssl_key = -----BEGIN RSA PRIVATE KEY-----
    MIICXgIBAAKBgQDOoZUYd8KMYbre5zZIwR+V6dO2+cCYVS46BHbRbqt7gczkoIWh
    ...
    MORE KEY DATA
    ...
    Wh+QF3UArO8r8RYv3HRcnBjrGh+yEK93wIifVNGgy63FIQ==
    -----END RSA PRIVATE KEY-----

If the value defined in ssl_certificate option already contains the private key, this option can be omitted by leaving it empty.

4.13.3.9. ssl_key_password

Default value:

Disabled

Optional:

Yes

Values:
  • Password as plain text.
  • Disabled
From version:

1.7.19

Description:

This is used to define the password of the private key, when the private X.509 key is stored as an encrypted file.

Set it to Disabled to not use a password for the private key file.

4.13.3.10. ssl_certificate_authority

Default value:

Disabled

Optional:

Yes

Values:
  • Absolute path on local filesystem.
  • Content of the CA chain (Since 3.40.0).
  • ${LETS_ENCRYPT_X3_CA}
  • ${MICROSOFT_IT_CA}
  • Disabled
From version:

1.6.0

Description:

This can be defined as an absolute path on the local filesystem to a file containing the certificates to the Certificates Authority used to validate the remote peer.

You can also define the content of the CA as text in PEM format.

When the value is defined as PEM text, the configuration will look as in the following example:

ssl_certificate_authority = -----BEGIN CERTIFICATE-----
    MIICaDCCAdGgAwIBAgIBDjANBgkqhkiG9w0BAQUFADBGMQswCQYDVQQGEwJHQjEP
    ...
    MORE CERTIFICATE DATA
    ...
    JZQaMjV9XxNTFOlNUTWswff3uE677wSVDPSuNkxo2FLRcGfPUxAQGsgL5Ts=
    -----END CERTIFICATE-----

When a certificate authority is define, this will result in initiating the two-way SSL/TLS authentication/handshake validation. For a successful connection, make sure the client sends a valid certificate. If the connection fails, the event with ID 40009 is emitted.

The certificate authority file should be stored as a file in PEM format. For multiple CA, place all certificates in the same file.

A series of bundle CA are distributed with SFTPPlus. They can be configured together and mixed with other CA certificates. The bundle CAs are available under the following names:

  • ${LETS_ENCRYPT_X3_CA} - For Let’s Encrypt X3 certificate authority.
  • ${MICROSOFT_IT_CA} - For all Microsoft IT CA certificates, used by SharePoint Online and other services provided by Microsoft.

To configure a component to accept peer certificates signed by Microsoft IT CA, which is the CA used by SharePoint Online, you can set the configuration as:

ssl_certificate_authority = ${MICROSOFT_IT_CRL}

Set as Disabled to disable checking the issuer for peer’s certificates. When certificate authority check is disabled, connection peers are not required to send a certificate and will result in a one-way SSL/TLS authentication. If the peer sends a certificate, it is ignored.

This defines the path on local filesystem to a file contain the certificate in PEM format for the single certificate authority or multiple authorities authorities with which this component will communicate.

Only peer connections using certificates signed by one of these certificate authorities will be permitted to communicate to this component.

When this component should communicate with peers holding certificates issued by multiple certificate authorities, put each CA certificate in PEM format inside a single file.

Leave it empty or set it as Disabled to disable checking the issuer of the peer’s certificates.

When certificate authority check is disabled, connection peers are not required to send a certificate. If the peer sends a certificate, it is ignored.

4.13.3.11. ssl_certificate_revocation_list

Default value:

Disabled

Optional:

Yes

Values:
  • Comma separated list of CRL paths or HTTP URLs.
  • crl-distribution-points
  • ${MICROSOFT_IT_CRL}
  • Disabled
From version:

1.6.0

Description:

It defines the locations from where one or more CRLs will be loaded.

Multiple CRLs are defined as a comma separated list.

It supports local files with absolute paths, in either of the following formats:

* ``file:///unix/absolute/test-ca.crl``
* ``file://c:\\windows\\absolute\\test-ca.crl``

Retrieving the CRL over HTTP is also supported. The HTTP request is done using non-persistent HTTP/1.1 connections. The URL will look as follows:

* ``http://example.com/some.crl``

CRL distribution points (CDP) are supported by using the crl-distribution-points configuration value.

When CRL distribution points are configured, the local certificate defined at ssl_certificate needs to have the CDP extension. The CDP advertised in the local certificate is loaded at startup in order to validate the configuration.

The distribution points configuration is mutually exclusive with local file or HTTP url configurations. When the certificate revocation list is configured to use CDP, all other configured CRL location are ignored.

Warning

HTTP redirection is not yet supported for CRL URLs. You have to configure the exact URL for the CRL.

Set it to Disabled, to disable certificate revocation checks.

The certificate revocation list can only be used when the component is configured with CA certificates stored in a single file in PEM format.

When multiple or chained CA certificates are configured the CRL is only checked for the peer’s certificate and not for the CA certificate or for an intermediate CA.

Warning

CDP publishing Delta CRL are not supported.

Note

If the certificate defines multiple HTTP based distribution points in the CDP extension, only the first HTTP URI is used. All non HTTP or the other HTTP URIs are ignored.

The CRL file should be stored in PEM or DER format.

Note

This option is ignored if ssl_certificate_authority is not enabled.

4.13.3.12. ssl_certificate_revocation_list_refresh

Default value:

0

Optional:

Yes

Values:
  • Number of seconds
  • 0
From version:

2.8.0

Description:

This defined the number of seconds after which a configured CRL is reloaded by this component.

When set to 0, the CRL file is initially loaded at startup and then loaded again after the Next Update field advertised in the CRL.

If the Next Publish extension is present in the CRL and this option is set to 0, the CRL will be loaded again at the date and time specified in the Next Publish extension.

If the CRL does not advertise the Next Update field you will have to configure a number of seconds after which the CRL should be reloaded, otherwise you will get a configuration error.

For example, a value of 86400 means that the CRL will be re-read after one day.

For more details about the CRL reloading see the documentation for CRL reloading rules

Note

This option is ignored if ssl_certificate_authority is not enabled.

4.13.3.13. ssl_cipher_list

Default value:

secure

Optional:

Yes

Values:
  • List of SSL/TLS ciphers in OpenSSL format.
  • secure
From version:

1.7.4

Description:

This defined the list of ciphers accepted by this component while communicating over the network.

The special keyword secure contains all the algorithms that we currently consider secure.

Connections are closed if the remote peer has no common cipher in its list of configured ciphers.

More information about the accepted values can be found at the cryptography guide

The format for this value is the same as the one used for defining the OpenSSL cipher list. More information can be found on the OpenSSL site.

4.13.3.14. ssl_allowed_methods

Default value:

tlsv1 tlsv1.1 tlsv1.2

Optional:

Yes

Values:
  • tlsv1.0
  • tlsv1.1
  • tlsv1.2
From version:

1.7.4

Description:

This defines the space separated list of SSL and TLS methods that are accepted by this component during the secure communication handshake.

Currently, the following methods are officially supported:

  • tlsv1 or tlsv1.0, which is TLS 1.0.
  • tlsv1.1, which is TLS 1.1.
  • tlsv1.2, which is TLS 1.2.

Note

SSLv3 is still supported, but highly discouraged, due to the SSLv3 POODLE vulnerability. In the case that you need to interact with an old SSL implementation that only supports SSLv3, it is highly recommended to force the usage of the non-CBC cipher RC4-SHA by configuring as:

[services/681f5f5d-0502-4ebb-90d5-5d5c549fac6b]
ssl_cipher_list = RC4-SHA

Support for SSLv3 will be removed in future versions.

SSLv2 is no longer supported since it is not secure.

In version 2.8.0, the following new methods were added: tlsv1.0 (alias for tlsv1), tlsv1.1 and tlsv1.2

4.13.3.15. idle_connection_timeout

Default value:

300

Optional:

Yes

Values:
  • Number of seconds after which idle connections are disconnected.
  • 0 - To disable timeouts.
  • Disabled - To disable timeouts.
From version:

1.7.19

To version:

None

Description:

The service will close the connection if a client connection is idle for a configurable amount of time.

4.13.3.16. maximum_concurrent_connections

Default value:

10000

Optional:

Yes

Values:
  • Number of maximum concurrent connections accepted by the service.
  • 0 - To disable the limit.
  • Disabled - To disable the limit.
From version:

1.7.19

To version:

None

Description:

Maximum number of allowed concurrent connections for this service.

This limit is imposed by each service, and it is not a global limit for all services active on the server.

4.13.3.17. ftps_explicit_enabled

Default value:

No

Optional:

Yes

Values:
  • Yes - Enable Explicit FTPS protocol.
  • No - Disable Explicit FTPS protocol.
From version:

1.6.0

To version:

None

Description:

If the FTPS protocol is disabled, no secured connections are accepted.

This will only enable Explicit FTPS secure connections, but will not enforce clients to use secured connections. Dedicated configuration options are provided for this purpose.

Not available for the Implicit FTPS protocol.

4.13.3.18. enable_password_authentication

Default value:

Yes

Optional:

Yes

Values:
  • Yes - Enable password-based authentication.
  • No - Disable password-based authentication.
From version:

1.7.4

To version:

None

Description:

Enable authentication based on username and password credentials.

4.13.3.19. enable_ssl_certificate_authentication

Default value:

Yes

Optional:

Yes

Values:
  • Yes - Enable SSL certificate-based authentication.
  • No - Disable SSL certificate-based authentication.
From version:

1.7.4

To version:

None

Description:

SSL certificate-based authentication allows clients to authenticate using a pair of username and SSL certificate credentials. A password is no longer required.

When the SSL certificate-based authentication is disabled, ensure that password-based authentication is enabled, otherwise clients will have no authentication method available.

4.13.3.20. ftps_force_secured_authentication

Default value:

Yes

Optional:

Yes

Values:
  • Yes - Only allow authentication over secured connections.
  • No - Allow authentication over both secured and unsecured connections.
From version:

1.7.18

To version:

None

Description:

The server can be configured to force the user to establish a secured connection before sending account credentials (eg. username and password).

If the server is configured to only allow secured authentication and not to enforce securing generic commands, after a successful authentication, users can send the Clear Command Channel (CCC) command to revert the command channel to an unsecured transport.

This will allow firewall or other tools (eg. FTP audit tools) to parse the commands issued within an FTP session.

Not available for Implicit FTPS protocol, where secured authentication is always enforced due to the protocol’s specification.

4.13.3.21. ftps_force_secured_command_channel

Default value:

No

Optional:

Yes

Values:
  • Yes - Only allow commands over secured connections.
  • No - Allow commands over both secured and unsecured connections.
From version:

1.6.0

To version:

None

Description:

When secure command channel is forced, any attempt to send unencrypted commands will be rejected.

When this is set to Yes, ftps_force_secured_authentication values will be ignored and the server will always require a secured command channel.

This option will be ignored if Explicit FTPS is not enabled.

Not available for Implicit FTPS protocol, where secured command is always enforced due to protocol specification.

4.13.3.22. ftps_force_secured_data_channel

Default value:

Yes

Optional:

Yes

Values:
  • Yes - Only allow data transfers over secured connections.
  • No - Allow data transfers over both secured and unsecured connections.
From version:

1.6.0

Description:

If FTPS secure data channel is forced, any attempt to initiate an unsecured data connection / transfer will be rejected.

This option will be ignored if FTPS is not enabled.

Not available for Implicit FTPS protocol, where data command is always enforced due to protocol specification.

4.13.3.23. ignore_ascii_data_type

Default value:

No

Optional:

Yes

Values:
  • Yes
  • No
From version:

3.8.0

Description:

This option allows configuring the FTP service to pretend that ASCII mode is supported, but to transfer the data in BINARY/IMAGE mode.

4.13.3.24. ascii_data_type_as_default

Default value:

No

Optional:

Yes

Values:
  • Yes
  • No
From version:

3.10.0

Description:

This option defines whether to use ASCII as the data type for the case in which the client does not explicitly ask for specific data type.

Set it to Yes to use ASCII/text as the default data type.

Set it to No to use IMAGE/binary as the default data type.