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IPv6 radvd questions

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  IPv6 radvd questions Richard Hector <richard@walnut.gen.nz> - 2018-03-26 09:30 +0200
    Re: IPv6 radvd questions Dan Ritter <dsr@randomstring.org> - 2018-03-26 14:50 +0200
      Re: IPv6 radvd questions Richard Hector <richard@walnut.gen.nz> - 2018-03-27 00:50 +0200

#194172 — IPv6 radvd questions

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Hi all,

I'm getting a little confused by the radvd docs, and possibly by IPv6
concepts in general ...

The router I'm configuring isn't the default gateway of the LAN; it's an
openvpn endpoint (server), and I just want to advertise the routes
available via tunnels to the LAN.

radvd's prefix blocks, if I understand correctly, are for telling other
machines on the local network what range to use for SLAAC, right? Which
is a job already being done by (multiple) other routers.

Given that, can I ignore the prefix blocks, and just include route blocks?

Will that be enough?

I'd rather not have to add static routes all over the place.

Thanks,
Richard

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#194177

On Mon, Mar 26, 2018 at 08:25:56PM +1300, Richard Hector wrote:
> Hi all,
> 
> I'm getting a little confused by the radvd docs, and possibly by IPv6
> concepts in general ...
> 
> The router I'm configuring isn't the default gateway of the LAN; it's an
> openvpn endpoint (server), and I just want to advertise the routes
> available via tunnels to the LAN.
> 
> radvd's prefix blocks, if I understand correctly, are for telling other
> machines on the local network what range to use for SLAAC, right? Which
> is a job already being done by (multiple) other routers.
> 
> Given that, can I ignore the prefix blocks, and just include route blocks?

Yes.


> Will that be enough?

Almost. The problem is that Linux makes a preventative
assumption that only default routes will be advertised.
(At least, this used to be the case. I don't have a
currently running example.)

On each client, you will need to set a sysctl:
net.ipv6.conf.all.accept_ra_rt_info_max_plen = 128

to enable accepting those non-default routes.

Also note that your openvpn server will probably stop
listening to route advertisements once it is an IPv6 router,
and so you will either need to set the default route there
statically, or run a routing protocol like RIPng between
them. If this is your only internal router, I would recommend
static.

-dsr-

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#194190

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On 27/03/18 01:45, Dan Ritter wrote:
> On Mon, Mar 26, 2018 at 08:25:56PM +1300, Richard Hector wrote:
>> Hi all,
>>
>> I'm getting a little confused by the radvd docs, and possibly by IPv6
>> concepts in general ...
>>
>> The router I'm configuring isn't the default gateway of the LAN; it's an
>> openvpn endpoint (server), and I just want to advertise the routes
>> available via tunnels to the LAN.
>>
>> radvd's prefix blocks, if I understand correctly, are for telling other
>> machines on the local network what range to use for SLAAC, right? Which
>> is a job already being done by (multiple) other routers.
>>
>> Given that, can I ignore the prefix blocks, and just include route blocks?
> 
> Yes.
> 
> 
>> Will that be enough?
> 
> Almost. The problem is that Linux makes a preventative
> assumption that only default routes will be advertised.
> (At least, this used to be the case. I don't have a
> currently running example.)
> 
> On each client, you will need to set a sysctl:
> net.ipv6.conf.all.accept_ra_rt_info_max_plen = 128
> 
> to enable accepting those non-default routes.

I read about that, and tried it - didn't seem to help.

> Also note that your openvpn server will probably stop
> listening to route advertisements once it is an IPv6 router,
> and so you will either need to set the default route there
> statically, or run a routing protocol like RIPng between
> them. If this is your only internal router, I would recommend
> static.

The openvpn server is the one (that was) _sending_ the RAs - though it
also receives them from the internet router (telco-branded Huawei), so I
changed accept_ra to get it to keep receiving those, which it does.

I was hoping RAs would allow me to avoid using a conventional routing
protocol, but it seems it doesn't work that way. If I have to update
configuration on everywhere that should see the routes, I might as well
add the routes instead - which is what I did, in the end.

Oddly, I also have an OpenWRT box (not really doing anything; I just
needed its switch) which also sends out RAs (odhcpd not radvd) - and
those _don't_ cause other machines to add a default route. I tried
making my RAs as similar as possible (by watching radvdump), but I
couldn't make my desktop respond in the same way. No idea what was going
on there.

Cheers,

Richard

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