Re: Broken PMTUD / ICMP blackhole?

[Celejar <celejar@gmail.com>]

On Wed, 18 Dec 2019 02:30:01 -0500
Anthony DeRobertis <anthony@derobert.net> wrote:

> On 12/17/19 11:39 AM, Celejar wrote:
> >
> > Now I just have have to figure out the best place to configure this.
> > I'm using dhcp via /etc/network/interfaces, but the 'dhcp' method
> > doesn't seem to support manual MTU setting. I could use a 'supersede
> > interface-mtu' line in dhclient.conf, but AFAICT, options there apply to
> > all dhcp connections, and I can't make out a simple way to set them on
> > a per connection basis. I suppose I could always just put 'ip link set
> > wlan0 mtu 1440' in a script and hook it in to the appropriate
> > 'iface' stanza in e/n/i with a 'post-up' line.
> 
> Personally, I'd just use the post-up line.

I'll probably do that - but I'm not sure how to restore the proper MTU
on connection down (since I'm not sure that my other connections are
setting it at all, as opposed to just leaving it as is). I suppose I can
just assume that it should always be 1500, so I can just use a
pre/post-down line to set it to 1500 on connection down, but I wonder if
there's a better way. I suppose I could also save the current MTU to
some file somewhere when I lower the MTU, and then restore it
afterward ...

> Ideally whichever device is the DHCP server would set the appropriate 
> MTU via its DHCP response, but if that's the phone, I have no idea how 
> you'd fix that easily. (That's why there is a supersede setting for it, 

Well, it's LineageOS, a fairly open OS, so I can probably figure out
how to hack it, if necessary.

> it can be part of the DHCP response — that's how different MTUs from 
> different DHCP connections should work.)
> 
> Another option might be to use Network Manager, I think its connections 
> can set a custom MTU, but I'm not 100% sure as I've never tried it.
> 
> BTW: dhclient.conf options can be per-interface (via an interface 
> section), so your wifi and wired adapters could be different.

I occasionally use wired, but I'm almost always on wifi.

Thanks,
Celejar