iptables, proxy ftp via frox

I am trying to make connections to port 21 go via frox like this:

# iptables -v -t nat -A PREROUTING -s localhost -p tcp -m tcp --dport
21 -j REDIRECT --to-ports 2121
REDIRECT tcp opt -- in * out * 127.0.0.1 -> 0.0.0.0/0 tcp dpt:21
redir ports 2121

But it doesn't seem to work:
# tail /var/log/frox.log
Thu Apr 6 15:12:53 2006 frox[10442] Listening on 0.0.0.0:2121
Thu Apr 6 15:12:53 2006 frox[10442] Bound to device eth0
Thu Apr 6 15:12:53 2006 frox[10442] Dropped privileges

Any idea what's wrong?
jacob