exceptionfactory opened a new pull request #5655:
URL: https://github.com/apache/nifi/pull/5655
#### Description of PR
NIFI-6266 Corrects proxy connection handling for FTP components and adds
support for SOCKS proxy servers with authentication.
The current `FTPTransfer` class attempted FTP server host resolution using
`InetAddress.getByName()` in order to pass an instance of `InetAddress` to
Commons Net `FTPClient.connect()`. Although `FTPClient.connect()` accepts a
`String` for the host parameter, the `connect()` implementation performs the
same `InetAddress.getByName()` resolution. In environments where the NiFi
system cannot resolve the destination FTP server address, the
`InetAddress.getByName()` throws an exception.
To resolve this problem, a new `ProxyFTPClient` extends the standard
`FTPClient` and overrides the `connect(String host, int port)` method to create
an `InetSocketAddress` that allows the host to be unresolved. This approach
allows a proxy socket to pass the unresolved host to the proxy server, relying
on the proxy server to perform name resolution. Proxy server connections
require FTP passive mode, which is the default setting for the `Connection
Mode` property. In addition to overriding the `FTPClient.connect()` method,
disabling the Passive NAT Workaround Hostname Resolver is also necessary.
Calling `FTPClient.setPassiveNatWorkaroundStrategy()` with a `null` value
disables potential hostname resolution that could also cause transfers to fail
when connecting through a proxy server.
Additional changes include refactoring `FTPClient` creation using a new
`FTPClientProvider` that follows the same approach as the `SSHClientProvider`.
The `StandardFTPClientProvider` handles configuring processor properties and
also disconnects the `FTPClient` on initial failures, such as authentication
problems. These changes also provided the opportunity to remove several unused
utility classes.
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