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My ISP provides native IPv6 with DHCP6-PD; my router is a Netgear router running OpenWrt. Everything works fine, but I want to understand how exactly, because there are no RAs whatsoever from the ISP's routers.

The traffic dump shows two RS messages from my router, followed by DHCP6 solicit. The ISP replies with DHCP6 advertisement, and DHCP6 request/reply follows. After this the ISP's host that replied to my DHCP6 requests sends an NS message for my router's link-local address, my router sends an NA with rtr,sol flags set; my router then sends an NS for the aforementioned host which responds with an NA with the same flags. Neither of the NS/NA messages contain any route/prefix information. And at this point a default route appears and traffic starts flowing. DHCP6, obviously, does not provide routing information.

How exactly does this work and how is the default route established? I tried to research but nothing came up so far.

Router advertisements appear on the external interface sometimes (maybe once in 2-4 hours) but they do not seem to be the source for the default route in my table as they happen well after the entry appears.

Xiatian
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1 Answers1

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Disclaimer: I don't use OpenWRT so I'm taking some stabs here...

Looks like OpenWRT uses odhcp6c as the client. Also looks like they might "fake" a default route to the DHCP interface ?

https://openwrt.org/docs/guide-user/network/ipv6/start

(Note the section Protocol dhcpv6 options concerning default route)

https://gitlab.labs.nic.cz/turris/openwrt/commit/28c47a66d9b988e242bbba36883f639c592941a5

Brandon Applegate
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  • Regular SOHO routers do not use odhcpd, yet they still work fine under these conditions (judging by the local forums), which is why I am wondering how exactly the gateway address is derived. – Xiatian Jul 13 '19 at 18:43