IPv6 and MSS clamping on native PPPoE
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For people having MTU issues or questions, I was looking into this a while ago, trying to troubleshoot some connectivity problems. I found some useful info.
There is "MTU Path Maximum network path size scan utility", which can be downloaded here: https://www.iea-software.com/products/mtupath/. It supports both ipv4 and ipv6. It uses both icmp and udp. You can sort of do this manually using ping, but this runs more quickly.
There are some more powerful but less flexible utilities available from wand.net.nz. There are two utilities and you can download them at https://wand.net.nz/pmtud/. They are based on open-source software. The outgoing test could probably be built-into pfsense. It would make a good addition to the diagnostics. The incoming test requires an external host to run the query on.