Regularly I seem to be part of a conversation that goes something like "lets have multiple URLS in a given Jira/Confluence environment", and the answer is always "there can be only one", ie each instance can only have one base url. In the community, there are lots of comments saying "there can only be one" but don't elaborate into why. I'd like to get deeper understanding about this, and hence this discussion post.
Lets get a discussion going around answering the question "what specifically breaks"and "why things break" when the URL you use for your instance doesn't match the base url configured in the application.
Something to note, it's enviable that reverse proxys become part of the discussion at some point when talking base url. My understanding is that reverse proxies are "out of scope" for Atlassian support. https://confluence.atlassian.com/jirakb/jira-displays-an-error-banner-about-the-base-url-314447143.html). So keeping that in mind, I'll kick things off.
1) For Confluence, there seems to be some good documentation about what breaks (which includes configuration loss) - https://confluence.atlassian.com/confkb/various-issues-caused-when-server-base-url-does-not-match-the-url-used-to-access-confluence-215485002.html
2) For Jira it seems less curated;
2a) Gadgets are effected -"any mismatch between this Base URL setting and the URL requested by your JIRA users will cause problems with dashboard gadgets."- https://community.atlassian.com/t5/Answers-Developer-Questions/JIRA-Base-URL-Issue/qaq-p/507905
2b) And "If you configure a different base URL or if visitors use some other URL to access Jira, it is possible that you may encounter errors while viewing some pages." https://confluence.atlassian.com/adminjiraserver/configuring-the-base-url-938847830.html.
This one is pretty general. It would be great to know what pages are impacted.