Date: Tue, 21 Jul 1998 14:24:51 +0200 From: Johann Visagie <wjv@cityip.co.za> To: Philippe Regnauld <regnauld@deepo.prosa.dk> Cc: security@FreeBSD.ORG Subject: Re: ipfw & icmp question Message-ID: <19980721142451.A4361@cityip.co.za> In-Reply-To: <19980530234807.14632@deepo.prosa.dk>; from Philippe Regnauld on Sat, May 30, 1998 at 11:48:08PM %2B0200 References: <19980530234807.14632@deepo.prosa.dk>
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On Sat, 30 May 1998 at 23:48 SAT, Philippe Regnauld wrote:
>
> I am a bit puzzled regarding the following situation:
>
> I have a machine with IPFW setup to send "port unreachable" if
> a connection attempt is made on port 113/TCP (identd). The policy
> is default deny. Here is what happens when I do "telnet host 113"
Sorry, can't help you with that one. I just allow queries to 113/tcp and
make sure there's nothing running on the port. *shrug*
> PS: in the /etc/rc.firewall (2.2.6 still), one rule says
> for the "Simple firewall setup":
>
> # Allow DNS queries out in the world
> /sbin/ipfw add pass udp from any 53 to ${oip}
> /sbin/ipfw add pass udp from ${oip} to any 53
>
> This is a but confusing -- from reading the rules, I understand:
>
> "Allow DNS queries, from out in the world, to us", while
> the formulation above says "Allow DNS queries from inside/here
> out into the world".
Nope, your resolver library will originate the query on an arbitrary port
greater than 1023, and connect to 53/udp on the remote nameserver. The
remote nameserver returns the replay from its port 53 to your arbitrary port.
The above rules allows _your_ host to query a remote host, as the comment
indicates. To enable it the other way round:
/sbin/ipfw add pass udp from ${oip} 53 to any
/sbin/ipfw add pass udp from any to ${oip} 53
However, it may not be necessary to add the above rules, since most resolver
libraries will try to query via TCP if UDP failes, i.e. you can simply add:
/sbin/ipfw add pass tcp from any to ${oip} 53 setup
(Assuming you allow already established connections by default elsewhere.)
-- V
Johann Visagie | Email: wjv@CityIP.co.za | Tel: +27 21 419-7878
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