Some sites inaccessible with IPv6 with 23.05
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After updating to 23.05 (on an SG-1100), the Signal app on my Mac was unable to connect. There had been occasional disconnections before, but after the update it wouldn't connect at all. Disabling IPv6 on the Mac fixes it.
I saw a suggestion elsewhere of using "curl k https://textsecure-service.whispersystems.org/" to test Signal connectivity. This returns 404 (as expected) if I use "curl -4", but hangs indefinitely with "curl -6".
Most other sites such as google.com work perfectly well with IPv6, but at least one other site has the same problem (https://cemantle.certitudes.org/).
And running the curl commands on the router itself works fine for both IPv4 and IPv6. Which suggests the problem is on the LAN side, but why then would it work for other sites? It's all very mysterious.
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I have found that reducing the MTU on my Mac to 1492 fixes the problem - though only on that Mac of course. Does this suggest an IPv6 MTU discovery problem?
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If so, only with the Mac, as PMTUD is only performed by the end device, not any router.. Are you on ADSL? That's where you normally use the smaller MTU. What's happening with IPv4? Is it using PMTUD or fragmenting? You can use packet capture or Wireshark to capture some packets and examine the do not fragment flag.
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It now seems likely that this wasn't related to the 23.05, but to a change in the Signal app around the same time which causes it to start preferring IPv6 over IPv4. There's a thread about it at https://github.com/signalapp/Signal-Desktop/issues/6393
It's still mysterious what the actual problem is.
