Configuring and Using an FTP Proxy

Running a public FTP site securely can be difficult. Taking full advantage of the security features supported by your FTP server application of choice can be a chore, and even then there's a good chance that sooner or later vulnerabilities will come to light making all that work for naught. So what else can you do?

One important technique is to run an FTP proxy on your firewall. Whereas the standard Netfilter code in the Linux kernel only inspects packets, an FTP proxy lets your firewall act as an intermediary in all FTP transactions. This increases your protection against buffer overflows and many other kinds of FTP attacks. It also allows you to restrict which FTP commands are executed by FTP clients.

This month I explain how to run SuSE's free (and non-SuSE-Linux-specific) Proxy-Suite FTP proxy on your Linux firewall, adding transparent but strong protection to all your FTP transactions.

If you run SuSE Linux, you can install the package proxy-suite, which installs a binary copy of ftp-proxy along with its configuration file and startup script. If you wish to use ftp-proxy as a transparent proxy, or if you want ftp-proxy to perform LDAP authentication, you'll need the latest version (1.9 as of this writing).

To run the latest version or use ftp-proxy on non-SuSE distributions, your best bet is to compile it yourself from source code, available at ftp.suse.com/pub/projects/proxy-suite/src.

Complete instructions on building and installing ftp-proxy are provided in the file INSTALL. By default, the configure script will check for libwrap, libldap and whether your system supports regular expressions. On my Red Hat 7.3 system, libwrap was present but caused a compile-time error, so I disabled libwrap like this:

# ./configure --without-libwrap

and ftp-proxy compiled properly. However, this wasn't necessary when I compiled ftp-proxy on my SuSE 7.1 system (obviously, SuSE's and Red Hat's libwrap packages differ).

After building ftp-proxy and installing it and its documentation, you'll probably want a startup script for your new proxy. Included with ftp-proxy's source (in the directory ftp-proxy/) is a sample script, rc.script, which is explained in the accompanying file rc.script.txt.

On SuSE systems, you simply can copy rc.script to /etc/init.d and optionally create a symbolic link to it from /usr/sbin. Rename the script /etc/init.d/ftp-proxy, and name the symbolic link /usr/sbin/rcftp-proxy. If you run SuSE 7.x, you'll also need to add this line to /etc/rc.config:

START_FTP_PROXY="yes"

For non-SuSE distributions, the example rc.script will need to be heavily tweaked, because much of it is SuSE-specific. Look at other scripts in your distribution's init.d directory for examples. Once you've figured out how, I strongly encourage you to send your hacked script to Marius Tomaschewski (mt@suse.de), one of the major contributors to FTP-Proxy, so others may benefit from your brilliance.

Once you've installed ftp-proxy from source or from a SuSE package, it's time to configure it. Most configurable parameters are kept in /etc/proxy-suite/ftp-proxy.conf (or, if you installed from source, in /usr/local/etc/proxy-suite/ftp-proxy.conf). Before diving into ftp-proxy.conf, however, you've got a couple of odds and ends to attend to.

First, you need a new, unprivileged user account for the proxy dæmon to use. On my system I created such a user, ftpproxy, like this:

bash-# useradd -u 65500 -g nogroup -d
/var/ftp-proxy/rundir -s /bin/false ftpproxy

No one should log in as this user, so be sure also to put an asterisk in the password field of the proxy user's line in /etc/shadow:

ftpproxy:*:12345:0:99999:7:0::

bash-# /etc/init.d/ftp-proxy chroot

To point your syslog dæmon to the chrooted log device, simply add an -a parameter to its startup script so that syslog is started:

syslog -a /var/ftp-proxy/rundir/dev/log

On SuSE systems the customary way to do this is in /etc/rc.config via the SYSLOGD_PARAMS variable. You can specify multiple -a statements if, for example, you're also receiving logs from a chrooted named.

Firewall Primers