Re: Link MTU option ignored in IPv6 router advertisements

[Dan Williams <dcbw redhat com>]

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.
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