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Date:      Sun, 17 Oct 1999 13:40:34 -0400 (EDT)
From:      "Crist J. Clark" <>
To: (Paul Davis)
Cc:        freebsd-questions@FreeBSD.ORG
Subject:   Re: IPFW question
Message-ID:  <>
In-Reply-To: <000101bf1858$6da0b2e0$> from Paul Davis at "Oct 16, 1999 10:31:05 pm"

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Paul Davis wrote,
[Charset iso-8859-1 unsupported, filtering to ASCII...]
> Hi there,  I've got a little problem that I hope someone could help me with.
> I'm running 3.3-Stable with ipfw running along with natd, two NIC cards one
> to the outside world and the other connecting to a hub with a windows98 and
> a SCO UnixWare 7 box.  Firewall type is set to open for right now.  The
> problem I'm having is as soon as I started running ipfw I noticed a HUGH
> amount of netbios udp packets being broadcast out to port 137 and 138 on
> subnet (for example there are a couple of other segments I
> seem to be broadcasting to.)

That network is apparently @Home in Virginia, and if your mail header
is to be believed, you are on the network of @Home in
Texas. The physical separation of your sites does not bother me, but
there is no reason broadcasts on the net should be
leaking into If that is really happening, you should
report it to the network admins.

> I thought possibly I had some windows networking stuff turned on but I don't
> have anything installed on the FreeBSD box that uses netbios, I have ports
> 136-139 turned off in services and inetd.conf.  If I disconnect my internal
> network and reboot FreeBSD it still sends the packets.  I tried setting up
> packet filtering rules to kill out going packet to ports 136-139 but I'm not
> getting the syntax right or something.  The ipfw man page is not quite
> helpful enough, I can't find much about ipfw in the handbook and the FAQ was
> less than helpful.

Wait, your FreeBSD box is spewing out these NetBIOS packets on its
own? You _should_ see lots of NetBIOS trying to get in if we assume
the majority of @Home users on your LAN are sporting WinBoxes. I would
not expect packets trying to get out, even if your internal WinBoxes
are hooked up (since you are using RFC 1918 addresses on that internal
net, right?).

The only reason I can think of for a FreeBSD box to be sending out
NetBIOS packets is if it is running Samba or Sharity-Light. But you
said you turned off all of the NetBIOS using facilities on the box.

> Where can I find some good documentation on using ipfw or could some kind
> soul help me with the syntax to kill packets going out to certain ports?
> BTW I've tried to just deny all netbios packets but that seems to kill natd.
> I don't know help....:)

Killing NetBIOS packets should not do anything to NATd. To block
NetBIOS from coming in or out from the Outside,

# ipfw add deny ip from any 137-139 to any via <external>
# ipfw add deny ip from any to any 137-139 via <external>

Where <exernal> is the name of your external interface. With this
rule, you can still run Samba or Sharity-Light and have it work on
your internal network.

As for good documentation for using ipfw, there is the manpage, the
rc.firewall script is well commented, the FreeBSD Handbook
(, and a many other
websites from FreeBSD users,
Crist J. Clark                 

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