Apache VFS SFTP Connection hangs - ssh

Question

I am using Apache VFS to upload a file to an SFTP server, if the file is newer than the file on the server or doesn't exist there yet. The server connection uses SSH Keys for Authentication.
I am using the following java code (plus error handling etc.) to connect to the server and check the file modification date-time:
DefaultFileSystemManager manager = new DefaultFileSystemManager();
manager.addProvider("sftp", new SftpFileProvider());
manager.init();
FileSystemOptions opts = createDefaultOptions();
BytesIdentityInfo identityInfo = new BytesIdentityInfo(server.sshKey.getBytes(), null);
SftpFileSystemConfigBuilder.getInstance().setIdentityProvider(opts, identityInfo);
remoteFileObject = manager.resolveFile(new URI("sftp",server.UserName,server.HostName,server.Port,remoteFilePath,null,null).toString(), createDefaultOptions(server.Key));
FileContent content = remoteFileObject.getContent();
return content.getLastModifiedTime();
The SSH key is in the format -----BEGIN RSA PRIVATE KEY----- etc.; as exported by puttyGen under Conversions -> Export OpenSSH Key (i.e. the old format of OpenSSH key, not the new one).
I have tested this code on Windows, with a locally hosted SFTP server (i.e. also on the same Windows machine), and it works successfully.
I am now wanting to use this in a Linux environment (RHEL), connecting to an AWS Transfer SFTP server, secured using SSH keys as described.
I can connect successfully using the SFTP command from the Linux OS shell:
sftp -oIdentityFile=/path/to/test.ppk USER#xxx.xxx.xxx.xxx
But, when I try to run the java code, the code hangs on the call to manager.resolveFile.
After half an hour (I think - this might not be related), I get the following in /var/log/messages:
systemd-logind[1297]: Session 115360 logged out. Waiting for processes to exit.
systemd[1]: session-115360.scope: Succeeded.
systemd-logind[1297]: Removed session 115360.
I don't have SELinux enabled, so I don't think that's interfering in any way.
Can anyone help suggest what might be causing this?

Answer 1

There were a couple of things, as it turns out:
Timeout
The timeout can be set when you configure the SftpFileSystemConfigBuilder, by using the .setSessionTimeout(FileSystemOptions, Duration) method call. This reduces the timeout which, if nothing else, makes the issue easier to debug.
The Session comments in the messages log were not related to the issue. Instead, the issue happened because the exec channel is disabled on the SFTP server, but VFS is trying to use it. At a simple level, this can be disabled using setDisableDetectExecChannel on the SftpFileSystemConfigBuilder object - but you should know the implications of this before doing so.