nginx.conf

server {
    listen 8080;
    listen [::]:8080;

    server_name ~^(?<dialtun_service>[A-Za-z]+)-(?<dialtun_host>[A-Za-z0-9\-]+)\.;

    location / {
        if ($dialtun_service = "") {
            return 400;
        }

        if ($dialtun_host = "") {
            return 400;
        }

        set_by_lua_block $backend {
            local DIGITS = {
                                  A=2,B=2,C=2,  D=3,E=3,F=3,
                G=4,H=4,I=4,      J=5,K=5,L=5,  M=6,N=6,O=6,
                P=7,Q=7,R=7,S=7,  T=8,U=8,V=8,  W=9,X=9,Y=9,Z=9
            }

            local letters = string.gsub(string.upper(ngx.var.dialtun_service), "[^A-Z]", "")
            local prefix = string.sub(letters, 1, 3)
            local numbers = string.gsub(prefix, "[A-Z]", DIGITS)

            return ngx.var.dialtun_host .. ':' .. ((os.getenv("DIALTUN_BASE_PORT") or "64000") + numbers);
        }

        # We need the final URI to be absolute, but if we do the rewrite
        # in a single pass then NGINX will see the `http://` prefix and
        # generate a (302) redirect. The fix is to build the absolute
        # URI in two `redirect` statements so that the NGINX logic will
        # not trigger.
        rewrite ^(.*)$ "://$backend$1";
        rewrite ^(.*)$ "http$1" break;

        proxy_pass http://localhost:1055;
    }
}