IPv6 Essentials

FTP

FTP has been designed to work over IPv4 supporting 32-bit addresses. With RFC 2428, “FTP Extensions for IPv6 and NATs,” a specification was made that allows FTP to work over IPv4 and IPv6. During the time in which both protocols coexist (and this will be a long time), it is important that FTP servers have a mechanism to negotiate the network protocol that should be used for a session.

The RFC specifies two new FTP commands to replace the PORT and PASV commands from the earlier FTP specification (RFC 959). The PORT command is used to specify a port different from the default ports used for the data connection. It contains IPv4 address information and therefore cannot be used with IPv6 without modification. The PASV command is used to put the server into passive mode, which means the server listens on a specific data port rather than initiating the transfer. This command includes the host and port address of the FTP server and therefore does not work over IPv6 without modification.

The PORT command is replaced by the EPRT command, which allows the specification of an extended address for the data connection. The extended address specifies the network protocol (IPv4 or IPv6, for instance), as well as the IP address and the port to be used. The EPSV command replaces the PASV command. The EPSV command has an optional argument that allows it to specify the network protocol, if necessary. The server’s reply contains only the port number on which it listens, but the format of the ...