[CentOS] Diagnosing IPv6 routing

Tue Apr 28 22:17:16 UTC 2020
Chris Adams <linux at cmadams.net>

Once upon a time, Kenneth Porter <shiva at sewingwitch.com> said:
> I just got 50 Mbps symmetric fiber from AT&T and it includes a /56
> of IPv6 addresses, replacing a much slower ADSL line. I never tried
> to get IPv6 working on the old connection. I'm using CentOS 7 as a
> gateway and it's worked great for several versions for IPv4.
> I'm not seeing any IPv6 default route on the WAN interface. I
> suspect I'm not getting route announcements. I think I have all the
> IPv6 variables in ifcfg-em2 set right. But I do notice that the
> accept_ra file in proc for that interface has value 1, not 2.
> Changing it to 2 doesn't change anything, though. No route appears.
> While I wait for an answer to my trouble ticket, is there some way
> to verify that I'm not receiving any RA packets? Is there a way to
> force a solicitation for one? Is there a tcpdump invocation I can
> use to watch for them? Are there log messages that will tell me when
> an RA has been seen and added to the routing table or ignored?

I haven't touched AT&T's IPv6, but the typical way WAN IPv6 works is:

- gateway sends a router solicitation and gets a router advertisement
  with "stateful config" set, which tells gateway to do DHCPv6 (but
  default route comes from RA)
- gateway does DHCPv6 to get a WAN IP
- after that completes, gateway does DHCPv6 for prefix delegation

If you are running NetworkManager, then IIRC the accept_ra flag doesn't
matter, because NM manages everything rather than have the kernel handle
autoconfiguration (because NM needs to know what's going on with IPs).

As for watching, "tcpdump -p -i <interface> -v ip6" should show
everything (and since you don't have any routing yet, you don't really
need to filter out anything else!).  You could filter "ip6 and
multicast", because RAs and DHCPv6 (and ND, neighbor discovery, the
counterpart to ARP) are all multicast.

Chris Adams <linux at cmadams.net>