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Active FTP vs. Passive FTP, Appendix 1


Introduction

This appendix will describe methods used to configure various popular FTP servers to limit the number of passive ports they will listen on. As mentioned in the main text, FTP server admins will almost definitely need to support passive FTP in order to allow the greatest number of clients to access their FTP resources. In order to support passive FTP, however, a large number of high-numbered ports on the server must be opened through a firewall. Luckily, most FTP servers allow this port range to be specified so as to limit exposure to attacks.


ProFTPd

ProFTPd, http://www.proftpd.net, is an increasingly popular FTP server due to its modularity and Apache-style configuration directives. ProFTPd also supports virtual hosts "out of the box", causing it to become one of the most common FTP servers used by web hosting companies.

As of version 1.20RC3 and later (current version as of this writing is 1.2.4), ProFTPd supports a directive called PassivePorts. The PassivePorts directive is usually used in a global context in the proftpd.conf file (the location of which varies depending on how ProFTPd was configured and installed). PassivePorts takes two arguments, the minimum port number and the maximum port number, as in the below example:

PassivePorts 51000 51999

The ProFTPd documentation has the following to say about the PassivePorts directive:

PassivePorts restricts the range of ports from which the server will select when sent the PASV command from a client. The server will randomly choose a number from within the specified range until an open port is found. Should no open ports be found within the given range, the server will default to a normal kernel-assigned port, and a message logged.

The port range selected must be in the non-privileged range (eg. greater than or equal to 1024); it is STRONGLY RECOMMENDED that the choosen range be large enough to handle many simultaneous passive connections (for example, 49152-65534, the IANA-registered ephemeral port range).

If you are attempting to use SSH port forwarding to securely tunnel the FTP command channel over an SSH connection (so that passwords are not sent in clear text), be aware that you must set the AllowForeignAddress directive to "on" in the proftpd.conf file. If this is not set and a tunnelled connection is attempted, ProFTPd will log a message similar to the following:

SECURITY VIOLATION: Passive connection from a.b.c.d rejected

Important Note: Please read and understand the documentation about the AllowForeignAddress directive before implementing it. This can open your FTP server up to bounce attacks. It is strongly recommended that this option not be set on systems being used as anonymous FTP servers.