[theomega]

Thanks, that was of great help!

Just for reference, the ping command is a little different

  sudo ping6 -D -G 1500,1400 fly.io

I set an MSS to 1492 which pfsense (my router) translates to an MSS clamp of 1492-60 for IPv6 and 1492-40 for IPv4. This is a German Deutsche Telekom Fiber connection. Now everything works fine, I can request fly.io (and also discovered that https://ipv6-test.com was not working before and now does with the MSS clamping)

Does MSS clamping have any disadvantages? Are there any alternatives in my case?

[toast0]

Excellent, happy to help, glad you figured out the right command!. 1492 MTU is consistent with PPPoE, or a network where they didn't want to run a separate MTU for PPPoE and straight ethernet.

The only downside to MSS clamping is the computational expense of inspecting and modifying the packets. On a residential connection, where you're running pfsense already, it's probably not even noticeable; but your ISP wouldn't be able to do clamping for you, because large scale routers don't have the processing budget to inspect packets at that level. I've seen some MSS clamping implementations that only clamp packets going out to the internet, and not the return packets... that can lead to problems sending large packets (which isn't always very noticeable, actually; a lot of basic browsing doesn't send packets large enough to hit this, unless you go to a site that sets huge cookies or do some real uploading)

The alternative would be to run a 1492 MTU on your LAN, but that has the marginal negative of reducing your maximum packet size for LAN to LAN transfers.