TCP issues with IPv6

[nibbler73]

Hi Guys,

I've configured an OPNsense router and want to add IPv6 capability to our client pcs.

Basically everything works fine. The internet connection is managed by a Fritz Box 7490 and has a static IP from the provider.

OPNsense is connected to the Fritz Box and requesting an IPv6 network via prefix delegation. Alternative configuration is to add static IPv6 routing.

(Fritz Box) → (OPNsense) → (Client Network)

Both scenarios work fine, with some slight exceptions.

Most Dual-Stack servers are reachable without any issues.

Some hosts though (like https://www.heise.de) trigger a delay around 10 seconds, some others (like https://kb.vmware.com) are not reachable via IPv6 at all.

The behaviour:

* heise.de shows an initial 10 seconds delay, after that the site responds immediately for a couple of hours
* vmware.com does not get any answer through whatsoever

It is not an issue with the server as I can reach both above URLs when I connect to Fritz Box directly or via a mobile device (also IPv6 enabled).

The TCP connection is established from what I can see.

Client side states ESTABLISHED and OPNsense lists the connection as ESTABLISHED as well, e.g. in Firewall → Diagnostics → pfTop

It also does not seem to be an MTU related issue, as I can ping heise.de with large packets.

Any ideas what I can test or how to address this issue?

Regards,
Hrvoje