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Can the fe80 link-local address be determined by other means? Just thinking about anyone not using pfSense/OPNSense routers (me as an example - I'm using a UDM-Pro)
Lit Fibre
- Thread starter bimbleuk
- Start date
I'm not too familar with Ubiquity equipment (although I'd very much like to get the chance to). I'm sure you'll have the ability on your router to perform a network capture on a particular network interface. If you turn off DHCPv6 on your WAN interface, start the network capture and then turn DHCPv6 back on again you should be able to capture the detail.
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Thanks @pingloss - I'll leave this to someone who knows how. I wouldn't know where to start! 
Is the method of picking up the IPv6 gateway based on where the DHCP response came from even an RFC? I'm wondering how the Lit-supplied router is doing it and whether it's a DHCPv6 fallback method or whether they've just lucked out with having two non-standard ways of doing something work together.
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Thanks! I'll take a look
I've been reading here: https://www.rfc-editor.org/rfc/rfc4861#section-6 and can't find anything that references a DHCPv6 fallback method
@Caged I wonder if they're doing something similar to this
DHCPv6 client and default gateway - MikroTik
forum.mikrotik.com
Edit: it seems it's exactly what they are doing. A quick and dirty out of RFC spec workaround
Odhcp6c installs bogus default route via DHCPv6 server
When booting OpenWrt 23.05.2 using a DHCPv6 server on the network segment, it appears that odhcp6c installs a default route via the interface on the DHCPv6 server (ending :f20a) when the address first eth0.2 bound (as per a dump using /etc/odhcp6c.user)...
forum.openwrt.org
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If the Lit-supplied router uses odhcp6c then I think we have the answer - presumably someone can packet capture the WAN port and see if it mentions that in the DHCPv6 request. The maintainers of that project are clear that it's a workaround for broken implementations.
There's nothing specific in the request and response but that's not to say these MitraStar routers aren't doing something similar where the logic is
If ICMPv6 RA exists then use this for default gateway
else
use DHCPv6 server address for default gateway
I've sent my email off to LitFibre asking why ICMPv6 isn't seemingly being used correctly.
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