On Sun, Jan 03, 2016 at 04:14:41PM +0100, Laurens Blankers wrote: > >Hm, this is the first time I've heard about a separate MTU setting for > >IPv6. > > Didn't know it either until I read it on radvd dev list [1]. Apparently > this is for IPv6 only, there is no protocol specific MTU setting for IPv4. [...] > Before using static I was using SLAAC, I think it was taking the MTU from > the RA broadcast then, at least [1] suggests that that is what is is > supposed to do.
Ok, then I'll try that out myself as well and see if I can get auto and
static do the same thing.
> >If you have a host on a LAN with a gateway machine that has a SixXS
> >tunnel, then you shouldn't have to lower the MTU on that host, since the
> >gateway machine should send the appropriate ICMPv6 packets to the host
> >if the host sends packets that are too large. If that doesn't work,
> >check if you have firewall rules on either machine blocking ICMPv6
> >packets.
>
> In theory yes. Unfortunately not every application works well with path
> discovery. In my case the problem is with OpenVPN, it uses its own MTU
> values and doesn't do any path discovery [2]. I worked around this by
> setting the mssfix option [3], however I would like to fix it properly
> hence my attempt at setting the MTU on the interface.
Shameless plug: tinc is another VPN daemon that does do proper MTU
discovery (but it's not compatible with OpenVPN).
> For IPv4 it gets even more interesting. When configuring the interface
> to a MTU of 1480 this should work fine for IPv4. Unfortunately it seems
> that the SMTP servers of least one large bank and one large ISP can't
> handle any MTU below 1500, causing incoming mail to drop with symptoms
> similar to MTU issues [4] for some, but not all connections. So I was
> forced to set the MTU on the interface back to 1500.
Ah, they probably have firewalls of their own that are misconfigured and
are blocking the ICMP Fragmentation Needed packets, or perhaps are
blocking fragments themselves :( I see your problem.
--
Met vriendelijke groet / with kind regards,
Guus Sliepen <g...@debian.org>
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