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 jira.external-share.com.
We should discourage usage of A record. Technically jira.external-share.com points to 22.214.171.124 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.