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I have configured Core FTP mini-sftp application in my server to use SFTP. I observe some unknown IP's connecting to my server in the connected list. By tracing the ips I found they are some chinese and thai websites. I use JSCH to connect SFTP. Suggest me on how to avoid these IP address connecting and make secure.

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migrated from stackoverflow.com Aug 31 '12 at 12:22

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Use a separate firewall to restrict connections to your ftp server to certain ip (whitelisting or blacklisting). –  Andreas_D Aug 31 '12 at 5:19
@Andreas_D I'm doing that. My wonder is how can they connect without knowing my username and password ? And any permanent solution with out doing it for individual ips. –  Samrat Aug 31 '12 at 5:21
Your firewall is not configured properly. Like @Andreas_D suggested, you should use the whitelist approach, and to allow access only to some known ip addresses, everything else is denied. –  dan Aug 31 '12 at 5:24
Ah, so it's not about preventing them from reaching the server, they actually can login and use it. If they can go through your firewall and have access to the server, then both systems need a configuration update an all (!) passwords should be changed immediately. –  Andreas_D Aug 31 '12 at 5:25
If they can login with a username, then they know the password for this user. –  Andreas_D Aug 31 '12 at 5:26
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