Custom domain for shared pages
In global settings administrator should be capable of adding custom domain.
All shared pages should be available under defined custom domain.
Our user should point DNS CNAME to confluence.external-share.com.
We should discourage usage of A record. Technically confluence.external-share.com points to 188.8.131.52 but we do not guarantees that this might change in future.
if user would like to point root domain then they should use dns flattening.