Hello,
I'm troubleshooting an issue with our application's ftp client, which are having timeout issues on the FTP control channel while transferring large files. We have tried to setup a keep-alive mechanism which sends NOOP messages on the control channel periodically, but this seem to cause other issues with FTP servers that ignores messages on the control channel while there is an active transfer on the data channel.
We know that FileZilla manages to upload these large files, so what I did was to monitor traffic using WireShark while uploading a file using FileZilla. The results are a bit strange:
After 1 hour of transfer the server sends a RST,ACK message, closing the control channel. I can even see in FileZilla that it says: "Disconnected from server: ECONNABORTED - Connection aborted". There is no response from the client on this message though. Then about 1 hour later when the transfer finishes it reads the 226 Transfer complete message from the server - on the same control channel, without reconnecting. How does it manage to do that?
You can also see that FileZilla seem to use SO_KEEPALIVE to keep the control channel active during the transfer, and not the NOOP method.
Excerpt from Wireshark, filtering on port 21:
67 38.382093 192.168.1.103 xxxxxxxxx FTP 60 Request: PASV
68 38.403053 xxxxxxxxx 192.168.1.103 FTP 107 Response: 227 Entering Passive Mode (xxxxxxxxx).
69 38.403466 192.168.1.103 xxxxxxxxx FTP 69 Request: STOR test.bin
70 38.424603 xxxxxxxxx 192.168.1.103 FTP 96 Response: 150 Opening BINARY mode data connection.
71 38.465021 192.168.1.103 xxxxxxxxx TCP 54 58981 → 21 [ACK] Seq=118 Ack=408 Win=262144 Len=0
[...]
84 3627.210719 192.168.1.103 xxxxxxxxx TCP 55 [TCP Keep-Alive] 59549 → 21 [ACK] Seq=168 Ack=823
85 3627.231802 xxxxxxxxx 192.168.1.103 TCP 66 [TCP Keep-Alive ACK] 21 → 59549 [ACK] Seq=823 Ack=169
[...]
86 3634.081473 xxxxxxxxx 192.168.1.103 TCP 60 21 → 59549 [RST, ACK] Seq=823 Ack=169 Win=0 Len=0
[FileZilla: Disconnected from server: ECONNABORTED - Connection aborted]
87 3638.504643 192.168.1.103 xxxxxxxxx TCP 55 [TCP Keep-Alive] 58981 → 21 [ACK] Seq=117 Ack=408
88 3638.525476 xxxxxxxxx 192.168.1.103 TCP 66 [TCP Keep-Alive ACK] 21 → 58981 [ACK] Seq=408 Ack=118
[...]
95 6478.177713 xxxxxxxxx 192.168.1.103 FTP 78 Response: 226 Transfer complete.
96 6478.206762 192.168.1.103 xxxxxxxxx FTP 60 Request: PASV
97 6478.227939 xxxxxxxxx 192.168.1.103 FTP 107 Response: 227 Entering Passive Mode (xxxxxxxxx).
98 6478.228310 192.168.1.103 xxxxxxxxx FTP 60 Request: LIST
99 6478.249480 xxxxxxxxx 192.168.1.103 FTP 96 Response: 150 Opening BINARY mode data connection.
100 6478.267052 xxxxxxxxx 192.168.1.103 FTP 78 Response: 226 Transfer complete.
