Date: Fri, 09 Jul 1999 16:24:50 +0100 From: Kevin Bracey <kbracey@e-14.com> To: freebsd-net@freebsd.org Subject: Re: interesting connect(2) side effect... Message-ID: <154f91e49%kbracey@kbracey.acorn.co.uk> In-Reply-To: <199907091147.NAA01166@labinfo.iet.unipi.it>
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In message <199907091147.NAA01166@labinfo.iet.unipi.it>
Luigi Rizzo <luigi@labinfo.iet.unipi.it> wrote:
> > > It is in my opinion totally broken with connected datagram sockets,
> > > where the connect has the side effect of in-kernel packet filtering
> > > based on source address (who said that you cannot do that!).
> >
> > It's not a side effect - it's a basic required feature! My TFTP client
> > relies on it to filter incoming multicast TFTP channels, which are keyed
> > by server address+port.
>
> i was considering this a side effect because i can't find documentation
> mentioning this as a feature -- certainly not the connect(2) manpage.
> Not to mention that nothing prevents a second, unprivileged app on
> your server send packets on the same UDP port -- if you want to do
> a safe filtering you do need a better mechanism.
It's not "safe filtering" - it's just the way the protocol works :) It
reduces client size/overhead if the kernel filters out the unwanted
multicasts for you.
Connecting a UDP socket does exactly what I'd expect. It restricts you to
sending to particular addr/port pair, and limits you to receiving from that
addr/port pair. Further, it also starts picking up ICMP messages related to
that addr/port to allow you to detect Destination Unreachable conditions.
If it didn't actually filter on source addr/port, then that's half the
of the meaning of connect() lost. Now, there might be an argument for
saying that connect to a multicast address means you can receive from
anything, but I'm sure an equally strong argument for connecting to a
multicast address meaning you can receive nothing.
--
Kevin Bracey, Senior Software Engineer
Pace Micro Technology plc Tel: +44 (0) 1223 725228
645 Newmarket Road Fax: +44 (0) 1223 725328
Cambridge, CB5 8PB, United Kingdom WWW: http://www.acorn.co.uk/
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