Impossible to configure advmss on routes

[Jeremy Norris]

It does not appear to be possible to configure the `advmss` for static routes.
This is an important feature to work around PMTUD blackholes.

[Nick Tait (nick.t)]

I too would like this feature, however there also seems to be an underlying limitation that systemd-networkd doesn't have any way to set advmss, so I can't see how this can be added to netplan until systemd-networkd supports it.

[Nick Tait (nick.t)]

I raised a feature request for systemd, and it appears that the fix has now been merged into the master branch. See: https://github.com/systemd/systemd/pull/18131

So I think now is probably a good time to think about adding the feature to netplan?

[Nick Tait (nick.t)]

I understand that systemd.network now supports setting the advmss setting? From "man systemd.network":

[ROUTE] SECTION OPTIONS
...
  TCPAdvertisedMaximumSegmentSize=
    Specifies the Path MSS (in bytes) hints given on TCP layer. The usual suffixes K, M, G, are supported and are understood to the base of 1024. An unsigned integer in the range 1–4294967294. When unset, the kernel's default will be used.

I haven't tried it myself, as I am in the process of reconfiguring my network and no longer require this feature, but in theory this now means that netplan could offer this feature with networkd backend.

Thanks,
Nick.