Short Passive (SPASV) Command for FTP
Expired & archived
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft is available in these formats:
RFC 1639[Pis94] documents experimental long port (LPRT) and long passive (LPSV) commands that many IP Version 6 implementations are using as the replacement for the PORT and PASV commands in FTP [PR85]. The author believes that this is the incorrect direction to be heading and that the replacement for PORT and PASV should carry less information instead of more. The passive command (SPASV) is a replacement for the PASV command. It only carries port numbers and does not carry addresses. This makes it usable with IPv4 and IPv6. A benefit of not carrying addresses is that pure network address translators (NAT) do not have to do a search-and-replace on the TCP stream, which is an expensive operation. This also eliminates three-way FTP, which is a rarely used mode of operation that leaves most existing FTP servers wide open to the FTP Bounce Attack [Hob95]. Because the FTP PORT command is unfriendly to some kinds of firewall configurations [Bel94] and that unfriendliness is there to support three-way FTP, there is no replacement for the PORT command -- all transfers should use passive mode instead. The author's inet6-apps kit (available on ftp.ipv6.inner.net and ftp.inner.net) includes a client and server that supports the current version of these commands. Those FTP servers implement this command.
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