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FTPPortMapper Element Documentation


FTPPortMapper -- Click element; manipulates IPRewriter for FTP



Ports: 1 input, 1 output
Processing: agnostic
Package: app (core)


Expects FTP control packets. Watches packets for PORT commands and installs corresponding mappings into the specified IPRewriter. This makes FTP possible through a NAT-like IPRewriter setup. CONTROL_REWRITER and DATA_REWRITER are the names of IPRewriter-like elements. CONTROL_REWRITER must be a TCPRewriter element, through which the FTP control packets are passed. Packets from FTPPortMapper must pass downstream through CONTROL_REWRITER. DATA_REWRITER can be any IPRewriter-like element; packets from the FTP data port must pass through DATA_REWRITER. CONTROL_REWRITER and DATA_REWRITER might be the same element. DATA_REWRITER_INPUT is a valid input port number for DATA_REWRITER. When a control connection opens a new data port, a mapping is installed in DATA_REWRITER as if a data packet had arrived on DATA_REWRITER_INPUT. Usually DATA_REWRITER_INPUT refers to a pattern specification; see IPRewriter for more information. In summary: Assume that an FTP packet with source address and port and destination address and port contains a command `PORT 1,0,0,2,3,9' (that is, Furthermore assume that the pattern corresponding to DATA_REWRITER_INPUT is ` 9000-14000 - -'. Then FTPPortMapper performs the following actions:

Creates a new mapping using the DATA_REWRITER_INPUT pattern. Say it returns 9000 as the new source port. This installs the following mappings into the rewriter:

? **1.**
! (, 777,, 20) => (, 9000,, 20)
? **2.**
! (, 20,, 9000) => (, 20,, 777)

Rewrites the PORT command to `PORT 1,0,0,1,35,40' (that is,
Updates the packet's IP and TCP checksums.
Updates the downstream CONTROL_REWRITER to reflect the change in sequence numbers introduced by the new PORT command. (The modified packet containing the new PORT command will likely have a different length than the original packet, so some sequence number patching is required.)
Does not change the control packet header's addresses or port numbers. For a PORT command to be recognized, it must be completely contained within one packet, and it must be the first command in the packet. This is usually the case. Also, the destination FTP data port is always assumed to be one less than the destination FTP control port, which is read as the packet's destination port number. This is also usually the case.


IPRewriter, TCPRewriter, IPRewriterPatterns RFC 959, File Transfer Protocol (FTP)

Generated by 'click-elem2man' from '../elements/app/ftpportmapper.hh' on 28/Feb/2010.


elements/ftpportmapper.txt · Last modified: 2010/02/28 13:58 (external edit)
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