Implementing Dynamic Port Exposure for Inter-Subnet Communication using Lucky and FRPC

## Preceding Content

To expose port 7000 to the public internet without relying on a public server, we can use Lucky, a STUN-based NAT traversal tool. This allows us to access various network services provided by one subnet from another without the need for a public server.

We can configure FRPC to connect to the dynamic server port and address by editing the `frpc.toml` file. Lucky can also be configured to update the STUN port via WebHook to Cloudflare.

There are two schemes to achieve this:

### DNS SRV Record

We can use the DNS SRV record to query the dynamic server port and address. The format is `_service._protocol.domain`.

For example, `_sip._tcp.example.com`.

Here is a Python script to query the SRV record:

```python
import dns.resolver

def query_srv_record(service, protocol, domain):