How to Reuse the Same TLS Session for Data Connections in FTPS using Apache Commons Net?

How to establish a data connection with the same TLS session using FTPS?

Maintaining the same TLS session for both control and data connections is a crucial security measure in FTPS. Some FTP servers enforce this requirement, causing issues when the data connection attempts to establish a separate TLS session.

In Apache Commons Net, this issue can arise due to its default configuration, which assumes a new session for each host/port combination. To resolve this, consider the following measures:

1. Verify Server Compliance: Ensure that the FTPS server enforces the same TLS session requirement. Check the server documentation or use a client like FileZilla to confirm.
2. Customize Apache Commons Net: Implement a custom FTPSClient that overrides the "_prepareDataSocket_" method. Refer to Cyberduck's FTPClient.java code for guidance on how to set up TLS session caching and reuse.
3. Override Spring Integration DefaultFtpsSessionFactory: Create a custom subclass of DefaultFtpsSessionFactory and override the "createClientInstance()" method to return your modified FTPSClient instance.
4. Disable Extended Master Secret Extension (JDK 8u161 and above): Add the following line to your code to disable the extended master secret extension, which may cause compatibility issues:

System.setProperty("jdk.tls.useExtendedMasterSecret", "false");

5. Use the sessionsByHostAndPort Field: For newer Java versions, use the "sessionsByHostAndPort" field instead of "sessionHostPortCache" to access the TLS session cache.
6. Bypass SDK Restrictions on Android: On Android devices, you may need to bypass SDK restrictions to access private fields using reflection.

By implementing these measures, you can enable Apache Commons Net to reuse the TLS session for the data connection, ensuring compatibility with FTPS servers that require it.

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