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
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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
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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
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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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