ftp with NAT

From: Max Gribov (mgribov@kplab.com)
Date: Tue Jun 13 2000 - 17:27:31 EST

we have a very strange problem with our nat.
sometimes, when a machine behind the nat tries to ftp into some machine on
the internet, it will get 421, service not availabe. we know that alot of
those machines (like ftp.ssh.org) has ftp running all the time, and it is
definitely our problem. and it is a nat problem, because machines which
bypass the nat do not have the same problem. both windows and linux machines
are affected. why might this happen? nat masq's everything else, and only
some ftp sites are unaccessible (we were not able to determine a pattern in
unaccessible sites either)
this is what we did to set up masq on the nat:
/sbin/depmod -a
/sbin/modprobe ip_masq_ftp
echo "1" >/proc/sys/net/ipv4/ip_always_defrag
/sbin/ipchains -M -S 7200 10 160
ip forwarding is also enabled.
the nat has two outgoing feeds - t1 and dsl, but there is default route -
t1 - and we are not running neither routed nor gated.
so that should take care of masq's, but doesnt for some reason.

our nat runs RH6.2, kernel 2.2.14-5.0 on intel pentium.

can anyone help us out with this one? or should there be anymore input data
on my part?

Max Gribov
Systems Engineer
KPL, inc.

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