I tried to use "receive file over SFTP" smart service to connect this

When logging in to the designer portal on the "Home" tab there's a left navigation menu in which you can find a link called "Access System Logs", the application server log will be the one called: jboss1-stdOut.log.2012-MM-DD.

at $Proxy294.onMessage(Unknown Source)
          at com.appiancorp.ra.workpoller.WorkItem.run(WorkItem.java:41)
          at org.jboss.resource.work.WorkWrapper.execute(WorkWrapper.java:205)
          at org.jboss.util.threadpool.BasicTaskWrapper.run(BasicTaskWrapper.java:260)
          at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
          at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
          at java.lang.Thread.run(Thread.java:662)
Caused by: com.jcraft.jsch.JSchException: connection is closed by foreign host
          at com.jcraft.jsch.Session.connect(Session.java:244)
          at com.jcraft.jsch.Session.connect(Session.java:158)
          at com.appiancorp.a2.process.runtime.activities.sftp.ReceiveFileOverSFTP.run(ReceiveFileOverSFTP.java:67)
          ... 61 more
Could you help to take a look?What's the problem?
Thank you so much!

It seems the SFTP server is closing the connection. Could it be you need to setup a VPN to connect from your app server to this SFTP? Do you know if the SFTP is only accessible within your network and if you already have a VPN tunnel configured in your environment?

Not yet, So you meant I should ask the server admin to set VPN for me?
Thanks a lot!

If your SFTP server is only available in your internal network and not from the outside (private IP), for it to be accessible from your Cloud server there should be a VPN tunnel enabled between your site and your internal network.

Per the documentation, this can be coordinated with Appian Technical Support and the authorized support contact from your company (forum.appian.com/.../Cloud_VPN_Integration)

But the http address which I mentioned above is the SFTP test for public. I don't think it needs to set VPN to access. In my localhost, I can access the address via FTP command under CMD like "ftp->open XX.XX.XX.XX->user ID->passward->get /home/test/test1.txt". I think Appian could server can also access the address without VPN setting? Could you try to access the test address on your PC.

I can't even connect using WinSCP. What port is it supposed to use? (It doesn't work with the default 22)

The port is 990.
Thank you so much!

Please do not post passwords or usernames in any public forum. I've removed that information from your post, I was just interested in the port. I need to check if this plug-in allows other port different from 22, I think it doesn't have that feature yet.

Can you try using WinSCP to connect? When I use the the right port and credentials I get: "Server unexpectedly closed the connection" which is similar to what Appian is throwing. Try WinSCP and let me know the results, I don't think this is an Appian issue.