Subject: I-D ACTION:draft-metz-spasv-00.txt

From: [email-masked]
Date: Wed, 14 Jan 1998 09:45:51 -0500

A New Internet-Draft is available from the on-line Internet-Drafts directories.

	Title		: Short Passive (SPASV) Command for FTP
	Author(s)	: C. Metz
	Filename	: draft-metz-spasv-00.txt
	Pages		: 5
	Date		: 13-Jan-98
	
   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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