File eXchange Protocol

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File eXchange Protocol (FXP) is a method of data transfer which uses the FTP protocol 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 and a single 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.

1 Risk
2 FXP over SSL
3 Technical
4 References
5 See also

[edit] Risk

Enabling FXP support, however, can make a server vulnerable to an exploit known as FTP bounce. FXP is also frequently used for warez exchange. As a result of this, FTP server software often has FXP disabled by default.

[edit] FXP over SSL

Some FTP clients such as FlashFXP and FTP Servers such as glFTPd and RaidenFTPd 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 SSL or TLS connection. However, both methods - CPSV and SSCN - are susceptible to Man-in-the-Middle attacks, since the two FTP servers do not verify each other's SSL certificates.

[edit] Technical

Although FXP may be classed as its own "protocol", it is in fact merely an extension of FTP and is specified in 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

[edit] References

This "protocol" is standardized as a subset of RFC 0959 by the IETF as:

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