With IPv6 cluster setup it's kind of expected that kubelet will expose healthcheck on IPv6 address. Te default value for HOST is right now 127.0.0.1 which requires setting the HOST_ADDRESS env.

This still might be problematic when cluster is dual stack IPv4/IPv6 and some monitored resources expose healthcheck on IPv4 and some IPv6. The workaround right now is to use HOST_ADDRESS=localhost which then use resolver to translate localhost to both 127.0.0.1 and ::1 and check will succeed regardless if target service is listening on ipv4 or ipv6 local address.

The proposal is to change the current defaultHostAddress from 127.0.0.1 to localhost.