Hello,
I confirm that patched package works as well for networks with MTU
options enabled in RA.
I used nldecap tool (https://github.com/etene/nldecap) to check systemd
behavior for IPv6 configuration.
232-25+deb9u1 => not reading MTU at all, and it breaks partially IPv6
connectivity for networks with lower MTU (regression between jessie and
stretch, since systemd was not configuring IPv6 before and that kernel
was reading MTU values).
nldecap output (for a link without MTU enabled, but same trace for a
link with MTU):
[packet 528] route, message 1 (unknown type)
├─header
│ ├─pid : 1
│ ├─length : 92
[...]
│ │ ├─flags : 0
│ │ ├─attrs
│ │ │ ├[0] RTA_GATEWAY : 'fe80::50:43ff:fef5:1c47'
│ │ │ ├[1] RTA_PRIORITY : 0
│ │ │ ├[2] RTA_PREF : '00'
│ │ │ └[3] RTA_OIF : 2
│ │ ├─table : 254
232-25+deb9u2 => working MTU, but no default route anymore for networks
without MTU option provided. nldecap reports only route provided with
MTU attribute (not a surprise, consistent with the current bug report),
routes without MTU are not configured at all.
nldecap output with MTU enabled (see new RTA_METRICS field):
[packet 2] route, message 1 (unknown type)
├─header
│ ├─pid : 1
│ ├─length : 104
[...]
│ │ ├─flags : 0
│ │ ├─attrs
│ │ │ ├[0] RTA_GATEWAY : 'fe80::224:d4ff:fea7:108'
│ │ │ ├[1] RTA_PRIORITY : 0
│ │ │ ├[2] RTA_PREF : '00'
│ │ │ ├[3] RTA_OIF : 2
│ │ │ └[4] RTA_METRICS
│ │ │ └─attrs
│ │ │ └[0] RTAX_MTU : 1480
│ │ ├─table : 254
With version compiled by Cyril Brulebois, both version are working, but
there is a small difference for link without MTU (RTA_metrics is set,
even if this is empty):
├─header
│ ├─pid : 1
│ ├─length : 96
[...]
│ │ ├─flags : 0
│ │ ├─attrs
│ │ │ ├[0] RTA_GATEWAY : 'fe80::50:43ff:fef5:1c47'
│ │ │ ├[1] RTA_PRIORITY : 0
│ │ │ ├[2] RTA_PREF : '00'
│ │ │ ├[3] RTA_OIF : 2
│ │ │ └[4] RTA_METRICS <= this is new
│ │ │ └─attrs : []
│ │ ├─table : 254
For the best of my knowledge, it's safe and should not have any side
effect. But it's not exactly the same netlink packet than with version
232-25+deb9u1
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