According to the RFC documentation:
The specification for the File Transfer Protocol assumes that the underlying network protocol uses a 32-bit network address (specifically IP version 4). With the deployment of version 6 of the Internet Protocol, network addresses will no longer be 32-bits. This paper specifies extensions to FTP that will allow the protocol to work over IPv4 and IPv6.
If the server supports EPSV and EPRT commands then is there a reason why we should use the original PASV and PORT commands? We can always use the extended commands itself.
I can see that FileZilla uses PASV rather than EPSV.
1 Answer
If the server supports EPSV or EPRT there is in theory no reason to use PASV or PORT since the necessary functionality is possible with EPSV and EPRT already.
But, it might not be clear in the first place if the server supports these extended commands. Additionally there might be some FTP helper in a firewall or NAT device between client and server which dynamically handles states based on the ports given in these commands and responses. And it might be that this helper does not understand the extended commands.
Thus it might be useful to continue to use IPv4-only commands for IPv4 traffic to avoid possible interoperability problems with older equipment.
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