How to hide port number from URL

I've just set up a new Vanilla forum (latest 2.1.8, no addon, no theme) on a Centos/Nginx server. The forum is running at port 8085. Everything work fine except now all discussions URLs have port number 8085 in it as can be seen below
http://forum.giamcankhoahoc.vn:8085/discussion/1/bam-you-ve-got-a-sweet-forum
How can I hide this port number from the URL. The URL still work if I manually remove the 8085 port from the URL but I want it to be automatically removed by the software. I also run other webs at different ports and they all don't have this port issue.
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Comments
http://vanillaforums.org/discussion/comment/202051#Comment_202051
http://vanillaforums.org/discussion/comment/209901/#Comment_209901
grep is your friend.