[CentOS] Diagnosing IPv6 routing

Wed Apr 29 03:21:30 UTC 2020
Kenneth Porter <shiva at sewingwitch.com>

--On Tuesday, April 28, 2020 10:16 PM -0500 Chris Adams <linux at cmadams.net> 

> I didn't get that you have a static assignment (presumably a business
> connection) - they may not do RAs on that (I don't at my ISP job).
> Business connections (or at least, connections with static assignments)
> tend to operate differently.  For that, they should have given you a
> static v6 address and gateway, just like they did for v4.

I didn't think to ask when we were turning up the v4 and phones. I'm 
betting there's a setting in their gateway box but I'm waiting for them to 
give me the credentials to log into it.

> So... there's one thing you could try (but probably won't work to a
> regular router interface) - see if there's a MAC-derived fe80::/64
> link-local address on their end.  Get the MAC of the gateway from the v4
> ARP entry and expand it to a LL v6 address as fe80::xxxx:xxff:fexx:xxxx
> (split the MAC, put ff:fe in the middle).  Try ping6 that address with
> %em2 appended (have to append the interface when using link-local
> addresses).  I doubt it'll work, since I know Juniper (which IIRC AT&T
> likes) doesn't assign those (I can't remember for sure about Cisco and
> don't have a handy test target).

Good idea but alas it's not routing. I can ping6 their gateway but it won't 
route after I add it to the route table for the WAN interface. While 
pinging a remote server, in a second shell I can tcpdump icmp6 packets and 
I see the packets going out but no replies coming back.

> And frankly, giving you a /56 is pretty crappy, since ARIN rules say to
> give every site a /48.  I'd only do a /56 for a home connection prefix
> delegation.  But, that's AT&T! :)

I'd just read about that when researching this. Maybe they decided that 
since we only have about a dozen people at our site, we won't have a lot of 
subnets. What do small offices DO with 256 public subnets, anyway? I 
suppose eventually we'll have an IoT subnet on every person.