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Date:      Tue, 9 Mar 2021 16:13:53 -0800
From:      Doug Hardie <>
To:        FreeBSD Questions <>
Subject:   Re: IPv6 Startup
Message-ID:  <>
In-Reply-To: <>
References:  <> <>

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> On 9 March 2021, at 12:53, CyberLeo Kitsana <> =
> On 3/9/21 3:58 AM, Doug Hardie wrote:
>> I have two systems on the same ethernet.  One is configured as a =
router, the other as a host.  rtadvd is running on the router, rtsold on =
the host, and route6d on both.  The router was up and running and I =
initiated tcpdump of ip6 packets on the interface.  Then I booted the =
host.  The results are interesting:
> <snip>
>> The question is, why are the host addresses being used before DAD is =
attempted?  It appears there could be some really interesting problems =
if the link-layer address actually was duplicated.  The problems would =
happen before DAD was even attempted?
> I would posit that this is because the fe80:: addresses used in the
> initial solicitation are derived from the MAC address of the =
> and if you have two interfaces with the same MAC address on the same
> subnet you have much bigger problems.

While at first glance that makes sense, there is a problem with that.  =
The fe80:: addresses are no longer supposed to be tied to the MAC =
address.  Mac's and Windows no longer do that.  They use random numbers =
and there could easily be duplicates.  The RFCs still show the MAC usage =
though.  Apparently there is a security issue that if you breach one =
computer in a site, you quickly can get a working address to all the =
others by using the MAC addresses that are easily available in ndp =

Also, if there were no need to do DAD, why bother to do it at all?

-- Doug

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