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Network Working Group S. Bellovin Request for Comments: 1579 AT&T Bell Laboratories Category: Informational February 1994

                       Firewall-Friendly FTP

Status of this Memo

 This document provides information for the Internet community.  This
 document does not specify an Internet standard of any kind.
 Distribution of this document is unlimited.

Abstract

 This memo describes a suggested change to the behavior of FTP client
 programs.  No protocol modifications are required, though we outline
 some that might be useful.

Overview and Rational

 The FTP protocol [1] uses a secondary TCP connection for actual
 transmission of files.  By default, this connection is set up by an
 active open from the FTP server to the FTP client.  However, this
 scheme does not work well with packet filter-based firewalls, which
 in general cannot permit incoming calls to random port numbers.

 If, on the other hand, clients use the PASV command, the data channel
 will be an outgoing call through the firewall.  Such calls are more
 easily handled, and present fewer problems.

The Gory Details

 The FTP specification says that by default, all data transfers should
 be over a single connection.  An active open is done by the server,
 from its port 20 to the same port on the client machine as was used
 for the control connection.  The client does a passive open.

 For better or worse, most current FTP clients do not behave that way.
 A new connection is used for each transfer; to avoid running afoul of
 TCP's TIMEWAIT state, the client picks a new port number each time
 and sends a PORT command announcing that to the server.

 Neither scenario is firewall-friendly.  If a packet filter is used
 (as, for example, provided by most modern routers), the data channel
 requests appear as incoming calls to unknown ports.  Most firewalls
 are constructed to allow incoming calls only to certain believed-to-
 be-safe ports, such as SMTP.  The usual compromise is to block only

Bellovin [Page 1]  RFC 1579 Firewall-Friendly FTP February 1994

 the "server" area, i.e., port numbers below 1024.  But that strategy
 is risky; dangerous services such as X Windows live at higher-
 numbered ports.

 Outgoing calls, on the other hand, present fewer problems, either for
 the firewall administrator or for the packet filter.  Any TCP packet
 with the ACK bit set cannot be the packet used to initiate a TCP
 connection; filters can be configured to pass such packets in the
 outbound direction only.  We thus want to change the behavior of FTP
 so that the data channel is implemented as a call from the client to
 the server.

 Fortunately, the necessary mechanisms already exist in the protocol.
 If the client sends a PASV command, the server will do a passive TCP
 open on some random port, and inform the client of the port number.
 The client can then do an active open to establish the connection.

 There are a few FTP servers in existence that do not honor the PASV
 command.  While this is unfortunate (and in violation of STD 3, RFC
 1123 [2]), it does not pose a problem.  Non-conforming
 implementations will return a "500 Command not understood" message;
 it is a simple matter to fall back to current behavior.  While it may
 not be possible to talk to such sites through a firewall, that would
 have been the case had PASV not been adopted.

Recommendation

 We recommend that vendors convert their FTP client programs
 (including FTP proxy agents such as Gopher [3] daemons) to use PASV
 instead of PORT.  There is no reason not to use it even for non-
 firewall transfers, and adopting it as standard behavior will make
 the client more useful in a firewall environment.

 STD 3, RFC 1123 notes that the format of the response to a PASV
 command is not well-defined.  We therefore recommend that FTP clients
 and servers follow the recommendations of that RFC for solving this
 problem.

Discussion

 Given the behavior of most current FTP clients, the use of PASV does
 not cause any additional messages to be sent.  In all cases, a
 transfer operation is preceded by an extra exchange between the
 client and the server; it does not matter if that exchange involves a
 PORT command or a PASV command.

 There is some extra overhead with Gopher-style clients; since they
 transfer exactly one file per control channel connection, they do not

Bellovin [Page 2]  RFC 1579 Firewall-Friendly FTP February 1994

 need to use PORT commands.  If this is a serious concern, the Gopher
 proxy should be located on the outside of the firewall, so that it is
 not hampered by the packet filter's restrictions.

 If we accept that clients should always perform active opens, it
 might be worthwhile enhancing the FTP protocol to eliminate the extra
 exchange entirely.  At startup time, the client could send a new
 command APSV ("all passive"); a server that implements this option
 would always do a passive open.  A new reply code 151 would be issued
 in response to all file transfer requests not preceded by a PORT or
 PASV command; this message would contain the port number to use for
 that transfer.  A PORT command could still be sent to a server that
 had previously received APSV; that would override the default
 behavior for the next transfer operation, thus permitting third-party
 transfers.

Implementation Status

 At least two independent implementations of the modified clients
 exist.  Source code to one is freely available.  To our knowledge,
 APSV has not been implemented.

Security Considerations

 Some people feel that packet filters are dangerous, since they are
 very hard to configure properly.  We agree.  But they are quite
 popular.  Another common complaint is that permitting arbitrary
 outgoing calls is dangerous, since it allows free export of sensitive
 data through a firewall.  Some firewalls impose artificial bandwidth
 limits to discourage this.  While a discussion of the merits of this
 approach is beyond the scope of this memo, we note that the sort of
 application-level gateway necessary to implement a bandwidth limiter
 could be implemented just as easily using PASV as with PORT.

 Using PASV does enhances the security of gateway machines, since they
 no longer need to create ports that an outsider might connect to
 before the real FTP client.  More importantly, the protocol between
 the client host and the firewall can be simplified, if there is no
 need to specify a "create" operation.

 Concerns have been expressed that this use of PASV just trades one
 problem for another.  With it, the FTP server must accept calls to
 random ports, which could pose an equal problem for its firewall.  We
 believe that this is not a serious issue, for several reasons.

 First, there are many fewer FTP servers than there are clients.  It
 is possible to secure a small number of special-purpose machines,
 such as gateways and organizational FTP servers.  The firewall's

Bellovin [Page 3]  RFC 1579 Firewall-Friendly FTP February 1994

 filters can be configured to allow access to just these machines.
 Further precautions can be taken by modifying the FTP server so that
 it only uses very high-numbered ports for the data channel.  It is
 comparatively easy to ensure that no dangerous services live in a
 given port range.  Again, this is feasible because of the small
 number of servers.

References

 [1] Postel, J., and J. Reynolds, "File Transfer Protocol", STD 1, RFC
     959, USC/Information Sciences Institute, October 1985.

 [2] Braden, R., Editor, "Requirements for Internet Hosts -
     Application and Support", STD 3, RFC 1123, USC/Information
     Sciences Institute, October 1989.

 [3] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
     D., and B. Alberti, "The Internet Gopher Protocol (a distributed
     document search and retrieval protocol)", RFC 1436, University of
     Minnesota, March 1993.

Author's Address

     Steven M. Bellovin
     AT&T Bell Laboratories
     600 Mountain Avenue
     Murray Hill, NJ  07974

     Phone: (908) 582-5886
     EMail: smb@research.att.com

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