Re: Link MTU option ignored in IPv6 router advertisements

[Lubomir Rintel <lrintel redhat com>]

Hello Glen,

On Tue, 2014-10-28 at 11:26 +1030, Glen Turner wrote:
> On 04/10/2014, at 2:31 AM, Dan Williams wrote:
>
> > On Fri, 2014-10-03 at 11:36 -0400, Chuck Anderson wrote:
> > > I'm not sure if this is a kernel thing or a NetworkManager thing.  Did
> > > something change in how IPv6 router advertisements are handled by
> > > NetworkManager in Fedora 20?
> >
> > I think it's a NetworkManager thing.  We'll fix it.
>
> Hi Dan,
>
> I've had a lot of trouble in IPv4 with DHCP in hotel systems setting the MTU to less than the 576 minimum 
> (and sometimes less than the size of the IPv4 header). I believe that this was a misguided effort to 
> implement fair queuing.
>
> It might be worthwhile anticipating such issues for IPv6, roughly:
>
> /* IPv6 minimum link MTU specified on page 24 of RFC 2460. */
> #define IPV6_MINIMUM_MTU 1280
>
> unsigned int enforce_link_mtu_bounds(unsigned int offered_link_mtu,
>                                      char *interface_name)
> {
>   if (offered_link_mtu < IPV6_MINIMUM_MTU) {
>     syslog("Offered link MTU for %s is %u, increased to " IPV6_MINIMUM_MTU "\n",
>            (interface_name) ? interface_name : "an interface",
>            offered_link_mtu);
>     return IPV6_MINIMUM_MTU;
>   }
>   return offered_link_mtu;
> }

We now ignore bad IPv6 MTUs:

src/rdisc/nm-lndp-rdisc.c:
627                 if (mtu >= 1280) {
628                         rdisc->mtu = mtu;
629                         changed |= NM_RDISC_CONFIG_MTU;
630                 } else {
631                         /* All sorts of bad things would happen if we accepted this.
632                          * Kernel would set it, but would flush out all IPv6 addresses away
633                          * from the link, even the link-local, and we wouldn't be able to
634                          * listen for further RAs that could fix the MTU. */
635                         warning ("(%s): MTU too small for IPv6 ignored: %d", rdisc->ifname, mtu);
636                 }

> -glen

Lubo