Re: Dantz Retrospect, Piton multicast, security, and XP SP2

From: John Faughnan (jfaughnan_at_spamcop.net)
Date: 08/28/04 

Date: 28 Aug 2004 06:50:27 -0700

Barry Margolin <barmar@alum.mit.edu> wrote in message news:<barmar-77A5E6.02124728082004@comcast.dca.giganews.com>...
> In article <5c0dbfb4.0408272029.3db72aa9@posting.google.com>,
> jfaughnan@spamcop.net (John Faughnan) wrote:
>
> > Barry Margolin <barmar@alum.mit.edu> wrote in message
> > news:<barmar-CC9A0D.18004527082004@comcast.dca.giganews.com>...
> > > In article <5c0dbfb4.0408250646.757268a9@posting.google.com>,
> > > jfaughnan@spamcop.net (John Faughnan) wrote:
> > > > Retrospect apparently clients communicate their IP addresses to Dantz
> > > > (224.1.0.38) over port 497...

> > mine of course. I pinged 224.1.0.38 and it resolves to the internal
> > address of my iBook!
> >
> > Thanks for helping me see the light.

> > "Address 224.1.0.38 does not refer to any node on your network. It is
>
> What they mean is that it's not an IP address assigned specifically to a
> device on your network. It's a multicast address that all Retrospect
> servers adopt when they're running.

Yes, I see that now. The Dantz explanation reads like it was written
by someone with a good technical understanding -- not necessarily
someone writing for relative novices.

> > There should be no traffic on the Internet using port 497 that's not
> > using Retrospect."
> > "The range of addresses between 224.0.0.0 and 224.0.0.255, inclusive,
> > is reserved for the use of routing protocols and other low-level
... Multicast routers should not forward
> > any multicast datagram with destination addresses in this range,
> > regardless of its TTL."

> The Dantz address isn't in that range. However, unless you've
> specifically arranged with your ISP for multicast routing on their
> backbone, your Internet router won't forward it because it doesn't know
> where to send it to.
> And I expect that Retrospect sets the TTL to 1 to prevent their traffic
> from being routed anyway (or maybe they set it to a low number, to
> support enterprises with internal routing, and the Retrospect server on
> a different subnet from the clients).

With your hints it was easy to find this referece.

http://www.sgr.nada.kth.se/mac/docs/retrospect_ttl.html.en saying they
use a TTL of 1.

So they use a multicast IP address, but the packets will either die on
my AirPort router, or at my ISP's router. They mention "port 497"
packets on the Internet -- would that fit with an enterprise doing
backup across the Internet? The enterprise would configure their
router to pass that packet to another router they owned across the
internet?

> > Multicasting is a topic beyond the old TCP/IP books in my home, on the
> > net ...
> There's a decent discussion of it in the TCP/IP Guide
> <http://www.tcpipguide.com/free/>.

Super! Esp. http://www.tcpipguide.com/free/t_IPMulticasting.htm. I'm
surprised this doesn't show higher in Google.

This discussion will be a handy reference for me and others going
forward -- thanks be to google for indexing usenet.

john
meta: jfaughnan, jgfaughnan, dantz, backup, multicasting, TTL, TCP/IP,
tutorial, reference, IGMP

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