[Cerowrt-devel] possible ipv6 path mtu issue, address assignment, and dhcpv6 renews

Dave Taht dave.taht at gmail.com
Thu May 29 19:36:33 EDT 2014

On Thu, May 29, 2014 at 1:54 PM, Dave Taht <dave.taht at gmail.com> wrote:
> I've had a couple reports of "slowness" on native ipv6, and possibly
> there is some sort of path mtu issue being forced, by something.
> What I am seeing is ipv4 "bytes on the wire" is 1514, but
> native ipv6 "bytes on the wire" is 1494, according to wireshark.
> Any attempt to send a "native" 1514 size tcp packet gets an ETOOBIG
> icmp response. (In an age of short tcp transactions this is bad)
> The topology is host - cero1 - cero2 - internet
> There is a he tunnel (mtu 1480) and comcast on the link I'm testing,
> and the ETOOBIG is coming from the internal interface of cero2. I will
> disable the tunnel there... but arguably, using source specific
> routing, it should only be stuff being forwarded through the tunnel
> that would give that error... and this happens before it gets to the
> tunnel.

I disabled the tunnel and now 1514 byte ipv6 packets do get through.
It's rather unclear why the lower tunnel mtu was causing problems

> The second thing I'm seeing periodically, for no reason I can tell, is that
> I'm getting an address on a network that doesn't exist on that host.
> d at nuc:~$ ifconfig eth0
> eth0      Link encap:Ethernet  HWaddr ec:a8:6b:fe:09:a2
>           inet addr:  Bcast:  Mask:
>           inet6 addr: fe80::eea8:6bff:fefe:9a2/64 Scope:Link
>           inet6 addr: 2601:9:6680:4aa::3ce/64 Scope:Global
>                              exists
>           inet6 addr: 2601:9:6680:56a::3ce/64 Scope:Global
>                              doesn't exist
> Could be some other device on the link supplying it, but doesn't seem so.
> I will keep monitoring radvdump...

It looks like dhclient6 on the host gets wedged somehow and this
isn't a cerowrt problem.

I filed a bug on it here.


If your underlying ipv6 address changes and you get stuck with
an old one, or an old one keeps coming back, well...

> The third, is like others here, I'm not seeing dhcpv6 renews work.

by this, I meant, not seeing them in the web interface.

Another problem is I'm not getting a default route from ras for some reason.

> --
> Dave Täht
> NSFW: https://w2.eff.org/Censorship/Internet_censorship_bills/russell_0296_indecent.article

Dave Täht

NSFW: https://w2.eff.org/Censorship/Internet_censorship_bills/russell_0296_indecent.article

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