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Date:      Tue, 9 Mar 2021 01:58:18 -0800
From:      Doug Hardie <bc979@lafn.org>
To:        FreeBSD Questions <freebsd-questions@freebsd.org>
Subject:   IPv6 Startup
Message-ID:  <3F059A72-F45B-43B1-8EE3-0176EE072054@sermon-archive.info>

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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:

22:26:17.963393 IP6 (hlim 255, next-header ICMPv6 (58) payload length: =
16) fe80::aa60:b6ff:fe1d:8dbc > ff02::2: [icmp6 sum ok] ICMP6, router =
solicitation, length 16
	  source link-address option (1), length 8 (1): =
a8:60:b6:1d:8d:bc
	0x0000:  6000 0000 0010 3aff fe80 0000 0000 0000
	0x0010:  aa60 b6ff fe1d 8dbc ff02 0000 0000 0000
	0x0020:  0000 0000 0000 0002 8500 a2b8 0000 0000
	0x0030:  0101 a860 b61d 8dbc

22:26:17.997455 IP6 (hlim 255, next-header ICMPv6 (58) payload length: =
56) fe80::3e18:a0ff:fe44:765b > ff02::1: [icmp6 sum ok] ICMP6, router =
advertisement, length 56
	hop limit 64, Flags [none], pref medium, router lifetime 1800s, =
reachable time 0ms, retrans timer 0ms
	  source link-address option (1), length 8 (1): =
3c:18:a0:44:76:5b
	  prefix info option (3), length 32 (4): fec2::/64, Flags =
[onlink, auto], valid time 2592000s, pref. time 604800s
	0x0000:  6000 0000 0038 3aff fe80 0000 0000 0000
	0x0010:  3e18 a0ff fe44 765b ff02 0000 0000 0000
	0x0020:  0000 0000 0000 0001 8600 8356 4000 0708
	0x0030:  0000 0000 0000 0000 0101 3c18 a044 765b
	0x0040:  0304 40c0 0027 8d00 0009 3a80 0000 0000
	0x0050:  fec2 0000 0000 0000 0000 0000 0000 0000

22:26:18.011402 IP6 (hlim 255, next-header ICMPv6 (58) payload length: =
32) :: > ff02::1:ff00:210: [icmp6 sum ok] ICMP6, neighbor solicitation, =
length 32, who has fec2::210
	  unknown option (14), length 8 (1):=20
	  0x0000:  cc1a 5611 b76a
	0x0000:  6000 0000 0020 3aff 0000 0000 0000 0000
	0x0010:  0000 0000 0000 0000 ff02 0000 0000 0000
	0x0020:  0000 0001 ff00 0210 8700 9025 0000 0000
	0x0030:  fec2 0000 0000 0000 0000 0000 0000 0210
	0x0040:  0e01 cc1a 5611 b76a

22:26:18.098774 IP6 (hlim 255, next-header ICMPv6 (58) payload length: =
32) :: > ff02::1:ff1d:8dbc: [icmp6 sum ok] ICMP6, neighbor solicitation, =
length 32, who has fec2::aa60:b6ff:fe1d:8dbc
	  unknown option (14), length 8 (1):=20
	  0x0000:  c983 4557 f1d3
	0x0000:  6000 0000 0020 3aff 0000 0000 0000 0000
	0x0010:  0000 0000 0000 0000 ff02 0000 0000 0000
	0x0020:  0000 0001 ff1d 8dbc 8700 f218 0000 0000
	0x0030:  fec2 0000 0000 0000 aa60 b6ff fe1d 8dbc
	0x0040:  0e01 c983 4557 f1d3

The first packet is the host sending a router solicitation.  The second =
is the router responding with the advertisement with the various routes =
it has.  The third and fourth packets appear to be the Duplicate Address =
Detection packets for the two addresses on the host.  There is no =
response to either.  After this, a number of other packets continue on =
and on.

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?

-- Doug




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