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I have two routers set up I my home, the default Verizon router that comes with my ISP and a Linksys EA6350 which I set to cascade my Verizon router. The linksys router also comes with a built in ftp server that I am using with my external hard drive. The problem with this is that since this router is not connected to the Internet, the ftp server is for my LAN only. When i tried to port forward on my Verizon router, I forwarded port 21 and to 192.168.1.4 which is the LAN ip address of the linksys router. But now when I test if the port is open on canyouseeme.org,its showing that port number 21 is blocked. Can someone help me fix this please?

  • Is Verizon connected to Linksys yellow port? bit.ly/1NCsx8O In that case, you probably need to open port(s) of Linksys to&fro WAN. – SΛLVΘ Nov 8 '15 at 17:23
  • The Verizon is connected to the Lan port of the linksys. It is a Lan to lan connection – Vinay Nov 8 '15 at 18:38
  • Yes I can connect internally using Chrome. I'm away from home at the moment but the router model is mi424wr Rev e – Vinay Nov 8 '15 at 19:42
  • For what it's worth, the port forwarding is working after a closed inspection of the Verizon logs. The problem lies in the linksys router. – Vinay Nov 8 '15 at 20:26
  • FTP uses two ports, 21 to open communication with the server, and 20 to transfer data. – Ron Maupin Nov 9 '15 at 3:20
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Thoughts

  • Something may be blocking port 21 for some reason. From what I can google, Verizon doesn't block port 21 but you may wish to try a different control port (e.g. 2222) for the outside connection anyway. This may add a tiny layer of security as well from people scanning port 21 for hackable FTP servers.

  • Toggle connection modes of your client. There is "active" (PORT) and "passive" (PASV) mode FTP and this essentially governs who gets to tell whom what address/ports to use (the client or the server). This makes a difference. For FTP to work behind Firewalls/NAT, PASV mode is often necessary/preferred.

  • You may need to open more ports. Port 21 is just a control port. Most data transfer takes place on a range of higher ports (e.g. 9000-10000) which you may have to forward in your router as well (this occurs in "PASV" mode). With "PORT" mode, you may need to open/forward Port 20 for additional data communications.

  • According to this scheme server's port 20 starts communication, thus there shouldn't be need for port forwarding. Moreover, that port is needed just on active FTP, is this the case? – SΛLVΘ Nov 9 '15 at 6:49
  • While it's is true that outgoing connections are unlikely to be blocked, note step 4 where the client responds to port 20. I would say this warrants perhaps considering opening/forwarding the port (since incoming communications are often blocked). Perhaps I worded this too strongly. Note this scheme for PASV -- slacksite.com/images/ftp/passiveftp.gif -- where port 20 is omitted. A random port substitutes for the data communication that normally occurs on port 20 for active FTP from the server (so no, it isn't needed). – Anaksunaman Nov 9 '15 at 8:13
  • Correct, port 20 is involved just for active FTP. According to TCP, reply should go thru the router, as communication started from LAN. – SΛLVΘ Nov 9 '15 at 8:17

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