Editing File eXchange Protocol

{{Short description|Method of data transfer}}
{{Refimprove|date=March 2010}}
{{IPstack}}
'''File eXchange Protocol''' ('''FXP''' or '''FXSP''') is a method of data transfer which uses [[File Transfer Protocol|FTP]] to transfer data from one remote server to another ([[inter-server]]) without routing this data through the client's connection. Conventional FTP involves a single [[Server (computing)|server]] and a single [[client (computing)|client]]; all data transmission is done between these two. In the FXP session, a client maintains a standard FTP connection to two servers, and can direct either server to connect to the other to initiate a data transfer. The advantage of using FXP over FTP is evident when a high-bandwidth server demands resources from another high-bandwidth server, but only a low-bandwidth client, such as a network administrator working away from location, has the authority to access the resources on both servers.

==Risk==
Enabling FXP(.RVL) support can make a server vulnerable to an [[exploit (computer science)|exploit]] known as [[FTP bounce attack|FTP bounce]]. As a result of this, [[FTP server]] software often has FXP disabled by default. Some sites restrict IP addresses to trusted sites to limit this risk.

==FXP over SSL==
Some [[FTP Server]]s such as [[glFTPd]], cuftpd, RaidenFTPD, drftpd, and wzdftpd support negotiation of a secure data channel between two servers using either of the FTP protocol extension commands; CPSV or SSCN. This normally works by the client issuing CPSV in lieu of the PASV command—or by sending SSCN prior to PASV transfers—which instructs the server to create either a [[Secure Sockets Layer|SSL]] or [[Transport Layer Security|TLS]] connection. However, both methods—CPSV and SSCN—may be susceptible to [[man-in-the-middle attack]]s, if the two FTP servers do not verify each other's SSL certificates. SSCN was first introduced by RaidenFTPD and [[SmartFTP]] in 2003 and has been widely{{Citation needed|date=July 2010}} adopted.{{When|date=July 2010}}

==Technical==
Although FXP is often considered a distinct protocol, it is in fact merely an extension of the FTP protocol and is specified in {{IETF RFC|959}}:

         User-PI - Server A  (Dest)              User-PI - Server B  (Source)
         ------------------                      ------------------
        
         C->A : Connect                          C->B : Connect
         C->A : PASV
         A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
                                                 C->B : PORT A1,A2,A3,A4,a1,a2
                                                 B->C : 200 Okay
         C->A : STOR                             C->B : RETR

                    B->A : Connect to HOST-A, PORT-a

==References==

This "protocol" is [[Standardization|standardized]] as a subset of [[Request for Comments|RFC]] 959 by the [[IETF]] as:

* {{IETF RFC|959}} File Transfer Protocol (FTP). J. Postel, J. Reynolds. Oct-1985. This obsoleted the preceding {{IETF RFC|765}} and earlier FTP RFCs back to the original {{IETF RFC|114}}.

==See also==
* [[File Transfer Protocol]] (FTP)
* [[Comparison of FTP client software]]
* [[List of FTP server software]]
* [[Trivial File Transfer Protocol]] (TFTP)
* [[SSH file transfer protocol]] (sftp), a protocol running over [[secure shell|SSH]]
* [[FTPS]] (FTPS), FTP run over SSL
* [[Simple File Transfer Protocol]] (SFTP), the historical protocol {{IETF RFC|913}}

[[Category:File Transfer Protocol]]
[[Category:Internet protocols]]

[[ru:FTP#FXP]]