Optimize sysctl for your TOR relay system for network.

The setting `sysctl -w net.ipv4.ip_local_port_range="15000 64000"` configures the range of local ports that can be used by a Linux system when initiating outgoing connections. In the context of a Tor relay, this setting has specific effects and advantages:

### Detailed effects in the context of a Tor relay:

1. **Expanding the available port range**:
   - A Tor relay initiates many outgoing connections to other Tor nodes (relays, bridges) as well as to Tor clients.
   - The setting extends the range of available local ports, which means that the Tor relay has more ports available to initiate outgoing connections. This is especially important for a busy relay that needs to handle a high number of simultaneous connections.

2 **Avoidance of port collisions**:
   - A tightly restricted port range can result in available ports being exhausted quickly, especially when connection traffic is high.
   - Extending the port range to 15000 to 64000 reduces the risk of the system not being able to find free ports, which could lead to connection errors.

3 **Stability and performance**:
   - A larger pool of available ports can increase the stability of the Tor relay as there will be fewer connection drops due to exhausted port resources.
   - This can also improve performance as the system will be able to initiate and handle new connections more efficiently.

4 **Security considerations**:
   - A non-standard port range can help make certain types of network scans or attacks more difficult, as it becomes harder for attackers to predict which ports the system is using.

### Specific functionality:

- **Source port selection on outbound connections**:
  - When Tor Relay initiates an outbound connection to another node on the Tor network, the operating system selects a free local port from the configured range (15000 to 64000).
  - Example: If the relay initiates a connection to another relay on port 9001, the local address and port could be e.g. `192.168.1.10:15000`, where 15000 is from the configured range.

- System-wide configuration**:
  - This setting affects all outgoing connections on the system, not just those of the Tor relay. However, in the case of a Tor relay, it has a particularly notable effect due to the high number of simultaneous connections.

### Example scenarios:

1. **High connection volume**:
   - Suppose the Tor relay needs to handle 5000 outgoing connections at the same time. With a smaller port range, the ports may run out quickly. With the extended range of 15000 to 64000, there are enough ports available.

2 **Avoidance of system overload**:
   - If the port range is too small and many connections are established and closed in quick succession, the system can become overloaded, resulting in delayed connections or even disconnections. The larger port range helps to prevent this.

### Summary:

The setting `sysctl -w net.ipv4.ip_local_port_range="15000 64000"` ensures that the Tor relay has enough local ports available to efficiently handle a large number of simultaneous outgoing connections. This contributes to the stability and performance of the relay and can also increase security by using a non-standard port range.