Redirect localhost port to external ip and port

Question

I am running macOS Mojave 10.14.6.

I have an app that is looking for a service hosted on localhost:9999, but I am hosting this service on a different server at 12.3.4.5:8000. How can I setup firewall rules, dns rules, or some other option, in order to redirect all requests to localhost:9999 on my machine to go to 12.3.4.5:8000?

I found many questions asking how to redirect requests to a domain to localhost, but that is the opposite of what I am looking for. I want to redirect requests from localhost to a domain (or ip):

I found this question when searching but it is not the same question as I am asking: Can I specify a port in an entry in my /etc/hosts on OS X? I also tried to use ifconfig and pfctl as in this answer to fit my case but I was unable to make it work: iptables equivalent for mac os x

Answer 1

Assuming that you want to route TCP traffic and that your server has SSH access, you can use an SSH tunnel:

ssh -L 9999:12.3.4.5:8000 remote-server-user@12.3.4.5

Then all traffic to localhost:9999 is tunneled to 12.3.4.5:8000