How to fix “You appear to be behind a NAT router” Filezilla Error

Trying to connect to an FTP server using FileZilla, but keep getting the error message "You appear to be behind a NAT router"? This is a common problem that can be fixed with a few simple steps.

FileZilla is a free, easy to use FTP client/server software that implements most of the functions of FTP protocol.

The FileZilla client supports resume on both downloads and uploads, timeout detection, firewall support, SOCKS4/5 and HTTP1.1 support, SSL, SFTP and more, all with an intuitive drag and drop interface.

FileZilla Server is a powerful FTP server with advanced set of features such as SSL encryption, transfer speed limit, remote server administration, etc. "You appear to be behind a NAT router" error only appears in FileZilla Server (not FileZilla Client).

In this article, we’ll cover the possible causes of the You appear to be behind a NAT router" Filezilla Error message. Then we’ll share solutions for each common situation, to help you get your favourite FTP client back running normally.

Why “You appear to be behind a NAT router” Filezilla Error occurs?

Before moving to the solution part, you need to understand what NAT in the error actually means.

First, IP addresses. Every device on the internet has an IP address. This address must be unique, there can be no two devices with the same IP. The number of IP addresses is limited, and mostly used up. There is a group of addresses that have been reserved for use only in internal networks. This works because the devices sharing an address cannot see each other – they are on different networks with no route between them.

NAT (Network Address Translation) is implemented by a device (a computer, a router, or a firewall) that has two network connections, one to the internal network, one to the internet. When a device on the internal network wants to connect outside, it sends its messages (packets) to the NAT device. The NAT device strips the address of the original device and substitutes its own, then sends it out to the internet.

When the reply comes back, the NAT device then strips its own address, substitutes the address of the original device, and sends it to the internal network.

In this way, thanks to NAT, multiple devices can share a single IP address.

"You appear to be behind a NAT router" is rather a warning than an error message, indicates that users outside your local network may not be able to access the server.

Fix “You appear to be behind a NAT router” error

First, open FileZilla Server and select Edit > Setting or directly click the set "gear" button.

Then, select the "Passive mode Settings" option, check the "Use the following IP" box and fill in the server’s IP address.

After this, you should no longer see "You appear to be behind a NAT router" error.

If "Use custom port ranges" option is unchecked, there may be an additional "425 can’t open data connection for transfer of" error when the directory is read by the client. This problem is mainly caused by the Use of Passive Mode. In order to fix this, tick "Use custom port ranges" in the Settings and add the port range to the inbound port of your Windows Firewall.

We hope that this tutorial provides useful information to help you fix the "You appear to be behind a NAT router" error in FileZilla effectively and quickly. If you spot any error in the article, or have a suggestion, please let us know via the comment section below. You might also want to check out our guide on fixing common "command not found" error messages with nodemonjavanpm or composer.

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