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
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