HAProxy to use DNS from operating system?
Seems to be correct that HAProxy does cache the resolved IP unless you tell it otherwise.
As you already found the configuration using a resolver and a custom check interval should do the trick (resolvers dns check inter 1000 and hold valid), but you are also right that this requires a resolvers section as well. Since HAProxy 1.9 you can use parse-resolv-conf to use the local resolver:
resolvers mydns parse-resolv-conf hold valid 10sbackend site-backend balance leastconn server site server.example.com:80 resolvers mydns check inter 1000The HAProxy documentation can help you with further configuration: https://cbonte.github.io/haproxy-dconv/1.9/configuration.html#5.3.2-parse-resolv-conf