Bug#892794: systemd-networkd fails to configure IPv6 without MTU from RA

[Florent Fourcot]

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