Network Configuration

Setting up network components for FTP is not trivial for use outside your LAN (Local Area Network). Since so many firewalls and routers exist, it is impractical to give detailed step-by-step instructions suitable for every user. It is important to understand the basics of the FTP protocol in order to configure FileZilla and the routers and/or firewalls involved. This documentation describes the history of the FTP protocol and how some aspects of the protocol work. Reading it carefully will save you a lot of trouble setting up FTP.

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Setting up FileZilla Client

If you're running FileZilla 3, it's recommended you run the network configuration wizard. It will guide you through the necessary steps and can test your configuration after set-up.

Obviously, if you want to connect to any server, you need to tell your firewall that FileZilla should be allowed to open connections to other servers. Most normal FTP servers use port 21, SFTP servers use port 22 and FTP over SSL/TLS (implicit mode) use port 990 by default. These ports are not mandatory, however, so it's best to allow outgoing connections to arbitrary remote ports.

Since many servers on the internet are misconfigured and don't support both transfer modes, it's recommended that you configure both transfer modes on your end.

Passive mode

In passive mode, the client has no control over what port the server chooses for the data connection. Therefore, in order to use passive mode, you'll have to allow outgoing connections to all ports in your firewall.

Active mode

In active mode, the client opens a socket and waits for the server to establish the transfer connection.

By default, FileZilla Client asks the operating system for the machine's IP address and for the number of a free port. This configuration can only work if you are connected to the internet directly without any NAT router, and if you have set your firewall to allow incoming connections on all ports greater than 1024.

If you have a NAT router, you need to tell FileZilla your external IP address in order for active mode connections to work with servers outside your local network:

• If you have a fixed external IP address, you can enter it in FileZilla's configuration dialog.
• If you have a dynamic IP address, you can authorize FileZilla to obtain your external IP address from a special website. This will occur automatically each time FileZilla is started. No information will be submitted to the website (regardless of FileZilla version).

If in doubt, use the second option.

If you do not want to allow incoming connections on all ports, or if you have a NAT router, you need to tell FileZilla to use a specific range of ports for active mode connections. You will have to open these ports in your firewall. If you have a NAT router, you need to forward these ports to the local machine FileZilla is installed on. Depending on your router model, you can either forward a range of ports or you need to forward all ports individually.

Valid ports can be from 1 to 65535; however, ports less than 1024 are reserved for other protocols. It is best to choose ports greater than or equal to 50000 for active mode FTP. Due to the nature of TCP (the underlying transport protocol), a port cannot be reused immediately after each connection. Therefore, the range of ports should not be too small to prevent the failure of transfers of multiple small files. A range of 50 ports should be sufficient in most cases.

Setting up and testing FileZilla Server

Setting up the server is very similar to setting up the client, with the main difference being that the roles of active and passive mode are reversed.

A common mistake, especially by users with NAT routers, is in testing the server. If you are within your local network, you can only test using the local IP address of the server. Using the external address from the inside will probably fail, and one of the following may happen:

• It actually works (surprisingly - and it probably means something else is wrong ...)
• The router blocks access to its own external address from the inside, due to identifying it as a possible attack
• The router forwards the connection to your ISP, which then blocks it as a possible attack

Even if the test works, there is no guarantee that an external user can really connect to your server and transfer files. The only reliable way to test your server is to try connecting from an external system, outside of your LAN.

Active mode

Make sure FileZilla Server is allowed to establish outgoing connections to arbitrary ports, since the client controls which port to use.

On the local end of the connection, FileZilla Server tries to use a port one less than that of the control connection (e.g. port 20 if server is listening on port 21). However, this is not always possible - so don't rely on it.

Passive mode

The server configuration is very similar to client configuration for active mode. In passive mode, the server opens a socket and waits for the client to connect to it.

By default, FileZilla Server asks the operating system for the machine's IP address, and for a free port number. This configuration can only work if you are connected to the internet directly without any NAT router and if you have set your firewall to allow incoming connections on all ports greater than 1024.

If you have a NAT router, you need to tell FileZilla Server your external IP address or passive mode connections will not work with clients outside your local network:

• If you have a fixed external IP address, you can enter it in the configuration dialog of FileZilla Server.
• If you have a dynamic IP address, you can let FileZilla Server obtain your external IP address from a special website automatically. Except your version of FileZilla Server, no information will be submitted to that website.

If in doubt, use the second option.

If you do not want to allow incoming connections on all ports, or if you have a NAT router, you need to tell FileZilla Server to use a specific range of ports for passive mode connections. You will have to open these ports in your firewall. If you have a NAT router, you need to forward these ports to the local machine FileZilla Server is installed on. Depending on your router model, you can either forward a range of ports or you need to forward all ports individually.

Valid ports can be from 1 to 65535, however ports less than 1024 are reserved for other protocols. It is best to choose ports >= 50000 for passive mode FTP. Due to the nature of TCP (the underlying transport protocol), a port cannot be reused immediately after each connection. Hence the range of ports should not be too small or transfers of multiple small files can fail. A range of 50 ports should be sufficient in most cases.

Troubleshooting

The following are a few troubleshooting suggestions:

Unfortunately, many personal firewalls and consumer routers are flawed or in some cases, even actively sabotage FTP (e.g. SMC Barricade V1.2). First of all, as with all software, you should keep everything updated. This includes the firewall software as well as the firmware version of your router.

If that does not help, you might want to try to uninstall your firewall to see what happens. Simply disabling your firewall might not work, as some firewalls cannot be fully disabled.

If possible, try to connect directly to the internet without a router.

If you are trying to setup a server and it works fine within your LAN but is not reachable from the outside, try changing the listening port. Some ISPs don't like their customers to host servers and they may block ports with numbers under 1024.

Another issue may occur if you are hosting an FTP server on default port 21. There might be a firewall at the ISP side of your connection which can do odd things like changing the port for PASV commands. Try using another non-default port for your FTP server.

If you encounter "cannot open data connection" on a random basis (i.e., the ftp client can connect to the ftp server without problem for many connections until it encounters this problem), one possible reason may be that your client PC anti-virus software is configured to block outgoing connections on certain ranges of ports. When your ftp connections are running in pasv mode, the client-side outgoing ports are selected randomly and some of those randomly selected ports may be blocked by the anti-virus software. To identify this problem, read your anti-virus log on the client. In general, any software that can block certain ranges of outgoing ports (such as PC firewalls) can cause similar FTP grief.

Timeouts on large files

If you can transfer small files without any issues, but transfers of larger files end with a timeout, a broken router and/or firewall exists between the client and the server and is causing a problem.

As mentioned above, FTP uses two TCP connections: a control connection to submit commands and receive replies, and a data connection for actual file transfers. It is the nature of FTP that during a transfer the control connection stays completely idle.

The TCP specifications do not set a limit on the amount of time a connection can stay idle. Unless explicitly closed, a connection is assumed to remain alive indefinitely. However, many routers and firewalls automatically close idle connections after a certain period of time. Worse, they often don't notify the user, but just silently drop the connection. For FTP, this means that during a long transfer the control connection can get dropped because it is detected as idle, but neither client nor server are notified. So when all data has been transferred, the server assumes the control connection is alive and it sends the transfer confirmation reply. Likewise, the client thinks the control connection is alive and it waits for the reply from the server. But since the control connection got dropped without notification, the reply never arrives and eventually the connection will timeout.

In an attempt to solve this problem, the TCP specifications include a way to send keep-alive packets on otherwise idle TCP connections, to tell all involved parties that the connection is still alive and needed. However, the TCP specifications also make it very clear that these keep-alive packets should not be sent more often than once every two hours. Therefore, with added tolerance for network latency, connections can stay idle for up to 2 hours and 4 minutes.

However, many routers and firewalls drop connections that have been idle for less than 2 hours and 4 minutes. This violates the TCP specifications (RFC 5382 makes this especially clear). In other words, all routers and firewalls that are dropping idle connections too early cannot be used for long FTP transfers. Unfortunately manufacturers of consumer-grade router and firewall vendors do not care about specifications ... all they care about is getting your money (and only deliver barely working lowest quality junk).

To solve this problem, you need to uninstall affected firewalls and replace faulty routers with better-quality ones.

Setting up FileZilla Server with Windows Firewall

If you are having problems with setting up FileZilla Server to run behind Windows Firewall (specifically, it fails on "List" and the client receives a "Failed to receive directory listing" error), you must add the FileZilla Server application to Windows Firewall's Exceptions list. To do this, follow these steps:

1. Open Windows Firewall under Control Panel.
2. If using Vista, click "Change Settings"
3. Select the "Exceptions" tab.
4. Click "Add program..."
5. Do NOT select "FileZilla Server Interface" from the list, instead click on "Browse..."
6. Locate the directory you installed FileZilla Server to (normally "C:\Program Files\FileZilla Server\")
7. Double click or select "FileZilla server.exe" and press open (Once again, NOT "FileZilla Server Interface.exe")
8. Select "FileZilla server.exe" from the list and click "Ok"
9. Verify that "FileZilla server.exe" is added to the exceptions list and that it has a check mark in the box next to it
10. Press "Ok" to close the window

Passive mode should now work. If you are still having problems connecting (from another computer or outside the network), check your router settings or try to add the port number in the Windows Firewall settings located in the Exceptions tab.

See the Microsoft kb article 931130 about running FileZilla with the "Routing and Remote Access" or the "Application Layer Gateway" service enabled. http://support.microsoft.com/kb/931130