Re: Link MTU option ignored in IPv6 router advertisements

[Chuck Anderson <cra WPI EDU>]

Thanks, bug filed:

https://bugzilla.gnome.org/show_bug.cgi?id=738104

On Fri, Oct 03, 2014 at 12:01:38PM -0500, 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.
>
> Dan
>
> > On my Fedora 20 system, the Link MTU option recevied from IPv6 router
> > advertisements is not being applied to the interface configuration:
> >
> > kernel-3.16.2-200.fc20.x86_64
> > NetworkManager-0.9.9.0-44.git20131003.fc20.x86_64
> >
> > > cat /proc/sys/net/ipv6/conf/wlp2s0/mtu 
> > 1500
> >
> > On my Fedora 19 system, this is working:
> >
> > kernel-3.14.18-100.fc19.x86_64
> > NetworkManager-0.9.8.8-2.fc19.x86_64
> >
> > > cat /proc/sys/net/ipv6/conf/p4p1/mtu
> > 1480
> >
> > In both cases, the RA has the Link MTU option:
> >
> > > sudo rdisc6 -1 wlp2s0
> > Soliciting ff02::2 (ff02::2) on wlp2s0...
> >
> > Hop limit                 :           64 (      0x40)
> > Stateful address conf.    :           No
> > Stateful other conf.      :           No
> > Mobile home agent         :           No
> > Router preference         :       medium
> > Neighbor discovery proxy  :           No
> > Router lifetime           :          300 (0x0000012c) seconds
> > Reachable time            :  unspecified (0x00000000)
> > Retransmit time           :  unspecified (0x00000000)
> >  Prefix                   : 2001:xxxx:xxxx:1::1/64
> >   On-link                 :          Yes
> >   Autonomous address conf.:          Yes
> >   Valid time              :          300 (0x0000012c) seconds
> >   Pref. time              :          300 (0x0000012c) seconds
> >  MTU                      :         1480 bytes (valid)
> >  Source link-layer address: 68:7F:74:xx:xx:xx
> >  from fe80::6a7f:74ff:fexx:xxxx
> >
> > I need this option because my IPv6 router uses a HE.net tunnel which
> > adds 20 bytes of overhead for the 6in4 encapsulation, and many sites
> > break PMTUD which causes those sites to hang.