ip.board reverse proxy problem

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I have a reverse proxy (Apache mod_proxy) that exposes a hosted IP.Board account (e.g.
http://123456789.ipsdns.com/ ) via a URL like http://example.com/forum on my own server. The HTTP headers are rewritten like this in mod_proxy:

ProxyPass /forum/ http://123456789.ipsdns.com/
ProxyPassReverse /forum/ http://123456789.ipsdns.com/
ProxyPassReverse /forum/ /
ProxyPassReverseCookiePath / /forum/

For rewriting the HTML, I just set my conf_global.php's $INFO['board_url'] to "http://example.com/forum/".

I also have "ACP->System Settings->Security and Privacy->Enable X_FORWARDED_FOR IP" enabled and some other security things on Apache.

This all worked fine for a couple years until I recently upgraded (IP.Board 3.3.4). 3.3.4 works for the most part as well, but one problem I've found is that after the user successfully logs in, it now redirects the user to http://123456789.ipsdns.com/ rather than http://example.com/forum/ . I thought the $INFO['board_url'] setting should prevent that. I opened a ticket on this, and they said the reverse proxy was not an officially supported setup and that it would be better to post here. One workaround I've found is to add the following to reverseTemplate:

<php>
$url = str_replace($url, "http://123456789.ipsdns.com/", "http://example.com/forum/");
</php>

I post this in case anyone else also has this problem and/or has a better resolution.