We've had our FTP server (Proftpd) running on Ubuntu Server 12.04 for over 12 months now with no connection issues. In the last few days there have been issues with slow connections to the server, with clients reporting their FTP client, Filezilla, hanging on "Connection established, waiting for welcome message..." for up to 20 seconds.
This happens whether I connect to the FTP server with Active or Passive mode. Also, connecting with Explicit FTP over TLS or unencrypted makes no difference.
The only changes to the server configuration during this time was Distro updates - these were mostly OpenSSL security updates and the Bash update (but no updates to Proftpd).
I ran Wireshark on the server while a client was connecting and discovered there were a large number of ACK and PSH, ACK packets between the client and server while waiting for the FTP server to acknowledge the connection. Unfortunately I don't know enough about the protocol to determine the cause of this cycle.
Wireshark data: http://pastebin.com/fZ87rwDR
Note: Proftpd is configured to masquerade as a public IP, however the private IP is shown in the Wireshark output. The client IP has also been changed.
Any tips to help resolve this matter would be greatly appreciated.
