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<h1>Boulder<br>
<small>The Let's Encrypt CA</small></h1>
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<p>This is an <a href="https://github.com/ietf-wg-acme/acme/">ACME</a> Certificate Authority running <a href="https://github.com/letsencrypt/boulder">Boulder</a>.</p>
<p>This is a <em>programmatic</em> endpoint, an API for a computer to talk to. You should probably be using a specialized client to utilize the service, and not your web browser. See <a href="https://letsencrypt.org/docs"><tt>https://letsencrypt.org/docs</tt></a> for help.</p>
<p>If you're trying to use this service, note that the starting point, <em>the directory</em>, is available at this URL: <a href="https://acme-v02.api.letsencrypt.org/directory"><tt>https://acme-v02.api.letsencrypt.org/directory</a></tt>.</p>
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So your IPv6 connectivity is broken it seems. This has nothing to do with Let's Encrypts ACME API though, as ifconfig.co is also unreachable through IPv6.
Is IPv6 connectivity necessary?
I never really used IPv6 on any of my machines.
There are no logs in /var/log/letsencrypt/letsencrypt.log.
I checked the network configuration again i noticed that your cloud provider pushed a IPv6 gateway which doesn't seem to work.
After removing the entry I can run dehydrated again without errors.
But curl -6 ifconfig.co now returns curl: (7) Couldn't connect to server
which makes sense because there is no default route.