nginx reverse proxy depends on requested port - http

Question

I want to listen port a range, and bind it with reverse proxy with incremental value(10000).
In example, i want to listen and bind it to this values:
example.com:20000 -> http://0.0.0.0:30000
example.com:20010 -> http://0.0.0.0:30010
example.com:20200 -> http://0.0.0.0:30200
my nxing conf:
server {
listen 20000-20200;
server_name example.com;
location / {
proxy_pass http://0.0.0.0:$server_port; ## << I want increment this port with 10000
}
}
How can i do this?

Answer 1

Wow, I didn't know that nginx allows to listen on port range. I didn't find it in documentation, but when I checked it myself, it is really working as expected.
Well, back to the question. Without additional modules nginx doesn't have any built-in mathematics. However, since all you need is the only one digit replacing, you can do it via regex capture group and concatenation of strings:
map $server_port $port {
"~\d(\d{4})" 3$1;
}
server {
listen 20000-20200;
server_name example.com;
location / {
proxy_pass http://0.0.0.0:$port;
}
}
If you're using OpenResty (or build nginx yourself with lua-nginx-module), you can use real mathematics with LUA code within the nginx config:
server {
listen 20000-20200;
server_name example.com;
location / {
set_by_lua_block $port { return ngx.var.server_port + 10000 }
proxy_pass http://0.0.0.0:$port;
}
}