[Dnsmasq-discuss] dnsmasq sending truncated DHCPv6 packets

[Derek Higgins]

Hi,

I've been trying to track down the reason for intermittent PXE
failures while using DHCPv6,
the failed PXE attempts seem to correlate with incomplete dhcp6
advertise packets.

e.g. in this sequence, the advertise packet length is 62 (all of the
failed attempts I've found happened with packets of the same length)

14:11:42.843363 00:93:12:a1:39:68 > 33:33:00:01:00:02, ethertype IPv6
(0x86dd), length 116: fe80::293:12ff:fea1:3968.546 > ff02::1:2.547:
dhcp6 solicit
14:11:42.843598 52:54:00:96:ef:51 > 00:93:12:a1:39:68, ethertype IPv6
(0x86dd), length 62: fe80::5710:879f:72e4:7b41.547 >
fe80::293:12ff:fea1:3968.546: dhcp6[|dhcp6]

Compared to a successful sequence
14:11:44.843012 00:93:12:a1:39:68 > 33:33:00:01:00:02, ethertype IPv6
(0x86dd), length 116: fe80::293:12ff:fea1:3968.546 > ff02::1:2.547:
dhcp6 solicit
14:11:44.843247 52:54:00:96:ef:51 > 00:93:12:a1:39:68, ethertype IPv6
(0x86dd), length 168: fe80::5710:879f:72e4:7b41.547 >
fe80::293:12ff:fea1:3968.546: dhcp6 advertise

I've attached 3 files, the first is my dnsmasq config file, the second
is my dnsmasq logs and the 3 is a tcpdump of the DHCPv6 traffic while
3 hosts were PXE booting one succeeded and the other 2 failed in the
way described above

I'm using dnsmasq-2.76-10.el7_7.1 on CentOS 7.7.1908 running in a container

I've also tried with the most recent version of dnsmasq
(2.80-93-g6ebdc95), and also get strange looking packets
(msgtype-134??), eg
10:40:43.204829 00:39:a6:0a:33:99 > 33:33:00:01:00:02, ethertype IPv6
(0x86dd), length 223: fe80::239:a6ff:fe0a:3399.546 > ff02::1:2.547:
dhcp6 request
10:40:43.206361 52:54:00:02:9a:11 > 00:39:a6:0a:33:99, ethertype IPv6
(0x86dd), length 126: fe80::5325:ca30:6ccd:c83.547 >
fe80::239:a6ff:fe0a:3399.546: dhcp6 msgtype-134
vs.
10:40:43.213256 00:39:a6:0a:33:95 > 33:33:00:01:00:02, ethertype IPv6
(0x86dd), length 223: fe80::239:a6ff:fe0a:3395.546 > ff02::1:2.547:
dhcp6 request
10:40:43.219308 52:54:00:02:9a:11 > 00:39:a6:0a:33:95, ethertype IPv6
(0x86dd), length 210: fe80::5325:ca30:6ccd:c83.547 >
fe80::239:a6ff:fe0a:3395.546: dhcp6 reply

but these only appear to happen after iPXE has chain loaded and also
sometimes stalls the boot process, I guess depending on when they
happen.

any help in finding the problem (or a solution) would be appreciated.

thanks,
Derek.
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