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<!DOCTYPE html> <html> <head> <title>ProFTPD: FXP (Site-to-Site Transfers)</title> </head> <body bgcolor=white> <hr> <center><h2><b>ProFTPD: FXP (Site-to-Site Transfers)</b></h2></center> <hr> <p> <b>FXP</b><br> What is FXP? FXP is part of the name of a popular Windows FTP client: <pre> <a href="http://www.flashfxp.com">http://www.flashfxp.com</a> </pre> This client has made the name "FXP" commonly used as a synonym for site-to-site FTP transfers, for transferring a file between two remote FTP servers without the transfer going through the client's host. Sometimes "FXP" is referred to as a protocol; in fact, it is not. The site-to-site transfer capability was deliberately designed into FTP. <p> <b>Site-to-Site Transfers</b><br> In a site-to-site transfer, the client logs in to two servers (server A and server B). It then arranges for a file transfer, telling one server (server A) that it will be a passive transfer, and the other server (server B) that it will be an active transfer. For a passive transfer, server A will return an address/port (via response to the <code>PASV</code> command) to which the client is to connect. The client then passes that address/port in a <code>PORT</code> command to server B. Then, the client sends a <code>RETR</code> to one of the servers and a <code>STOR</code> to the other, thus starting the transfer. The data does not pass to the client machine at all. <p> Any firewalls protecting either (or both) servers involved may need to allow active and passive FTP data transfers, depending on which server is told to be active, which is told to be passive. <p> <b>Example Site-to-Site Transfer</b><br> In the example below, italicized represent responses to the given FTP commands. Lines in blue show communications to <font color=blue>server A</font>, while those in red are to <font color=red>server B</font>. Black lines are informational messages displayed by the FXP client. <pre> <font color=blue>TYPE I <i>200 Type set to I.</i></font> <font color=red>TYPE I <i>200 Type set to I.</i></font> <font color=blue>PASV <i>227 Entering Passive Mode (1,2,3,4,130,161).</i></font> <font color=red>PORT 1,2,3,4,130,161 <i>200 PORT command successful</i></font> <font color=blue>STOR file.mp3 <i>150 Opening BINARY mode data connection for file.mp3</i></font> <font color=red>RETR file.mp3 <i>150 Opening BINARY mode data connection for file.mp3 (15000000 bytes)</i></font> <font color=blue><i>226 Transfer complete.</i></font> <font color=red><i>226 Transfer complete.</i></font> Transferred: file.mp3 14.31 MB in 2.38 (6,147.06 KBps) <font color=blue>TYPE A <i>200 Type set to A.</i> PASV <i>227 Entering Passive Mode (5,6,7,8,168,183).</i> LIST <i>150 Opening ASCII mode data connection for file list</i> <i>226 Transfer complete.</i></font> <font color=red>TYPE A <i>200 Type set to A.</i> PASV <i>227 Entering Passive Mode (1,2,3,4,130,162).</i> LIST <i>150 Opening ASCII mode data connection for file list</i> <i>226 Transfer complete.</i></font> Transfer queue completed Transferred 1 file totaling 14.31 MB in 3.42 (6,147.06 KBps) </pre> The client instructs <font color=blue>server A</font> to expect a passive data transfer, and <font color=red>server B</font> to initiate an active data transfer. The address and port returned in <font color=blue>server A</font>'s response to the <code>PASV</code> command is passed to <font color=red>server B</font> as arguments in the <code>PORT</code> command. The transfer is then started by telling <font color=blue>server A</font> to expect a file to be uploaded, and <font color=red>server B</font> to download the file. The following <code>PASV/LIST</code> sequences are the client confirming that the file was transferred successfully. <p> This example also illustrates that site-to-site transfers use both active and passive data transfers; for sites that operate behind firewalls and NAT, passive transfers may require extra configuration to operate properly (<i>i.e.</i> use of the <a href="../modules/mod_core.html#MasqueradeAddress"><code>MasqueradeAddress</code></a> and <a href="../modules/mod_core.html#PassivePorts"><code>PassivePorts</code></a> configuration directives). <p> <b>"FTP Bounce" Attacks and <code>AllowForeignAddress</code></b><br> So, what does this mean for ProFTPD? By default, ProFTPD <b>does not allow</b> site-to-site transfers, for by allowing them, the server also allows a type of attack known as the "FTP bounce" attack: <pre> <a href="http://www.cert.org/advisories/CA-1997-27.html">http://www.cert.org/advisories/CA-1997-27.html</a> </pre> The protection against this attack is to enforce the requirement that, from the server's point of view, the remote address of a control connection matches the remote address of a data connection. If the addresses do not match, the data connection is treated as from a foreign client, and thus rejected. <p> However, some site administrators do want to allow their servers to support site-to-site transfers. ProFTPD must be explicitly configured to allow these by using the <a href="../modules/mod_core.html#AllowForeignAddress"><code>AllowForeignAddress</code></a> configuration directive. <p> Note that even if <code>AllowForeignAddress</code> is enabled, you may still encounter trouble with site-to-site transfers involving your server. These issues usually arise from firewall configurations, or with upstream ISPs performing filtering on the FTP port. <p> <hr> <font size=2><b><i> © Copyright 2017 The ProFTPD Project<br> All Rights Reserved<br> </i></b></font> <hr> </body> </html>