[GLLUG] basic IPv6 questions

[Tim Woodall]

On Mon, 4 Oct 2021, Andy Smith via GLLUG wrote:

> Hi,
>
> On Sun, Oct 03, 2021 at 09:12:46PM +0200, Carles Pina i Estany wrote:
>> If the Raspberry pi + BT router used SLAAC is this more or less what
>> happens?
>> -Raspberry pi sends a broadcast using NDP probably type "Router
>> Solicitation (Type 133)"
>> (https://www.rfc-editor.org/rfc/rfc4861.html#section-4.1)
>> -Router probably answers with a "Router Advertisement (Type 134)"
>> (https://www.rfc-editor.org/rfc/rfc4861.html#section-4.2)
>>
>> The Router Advertisement includes the IP of the router (in the "Source
>> Address"?)
>
> On your network where the raspberry pi is located, if you do a
> tcpdump you should see the periodic router advertisements from the
> BT router:
>
> # tcpdump -vni eth0 'icmp6 and ip6[40] == 134'
>
> (change "eth0" for whatever interface name is the one that's on the
> same network as the BT router)
>
> All RA packets are ICMP (v6) and the type byte is at position 40
> which is 134 as you mentioned above.
>
> Use more -v or a -X to see full packet contents.
>

is there any advantage to doing this over running radvdump?

If you change radvd to transmit frequently (say every 10s) for debugging
purposes and you include DNS entries in the broadcasts and you have
redundant servers broadcasting then you might have performance issues on
the machines consuming the packets and rewriting resolv.conf.

(it took me a long time to track that one down as the problems were
intermittent...)