feat: add VRRP unicast peer support for macvlan and restricted network environments

- Add unicast_src_ip and unicast_peers config fields
- NewUnicastSocket binds to source IP for proper packet reception
- SendTo() sends VRRP advertisements directly to peer IPs
- Receive loop filters packets from non-peer sources in unicast mode
- Backward compatible: multicast mode used when unicast_peers is empty
- Add unicast config example (etc/config.unicast.yaml)
- Update README with unicast documentation and examples

README.md

@@ -7,6 +7,7 @@ A lightweight VRRP (Virtual Router Redundancy Protocol) implementation in Go, de
 - **VRRP Protocol**: RFC 3768/5798 compliant implementation
 - **High Availability**: Automatic failover with priority-based master election
 - **Health Checking**: TCP, HTTP/HTTPS, ICMP ping, and script-based checks
+- **Unicast Support**: Direct peer communication for restricted network environments (macvlan, cloud)
 - **Easy Deployment**: Built-in install command with systemd/init.d support
 - **Hot Reload**: Configuration reload via SIGHUP without service restart
 - **Zero Dependencies**: Single static binary, no runtime dependencies
@@ -145,6 +146,54 @@ vrrp_instances:
       - "192.168.1.100/24"     # Must match
 ```
 
+### Unicast Configuration (for macvlan / cloud environments)
+
+In environments where multicast is not available (macvlan networks, public clouds, etc.), use unicast to send VRRP advertisements directly to peer IPs:
+
+**Node 1 (MASTER)**:
+```yaml
+global:
+  router_id: "node1"
+
+vrrp_instances:
+  - name: "VI_1"
+    interface: "eth0"
+    state: "MASTER"
+    virtual_router_id: 51
+    priority: 100
+    advert_interval: 1
+    auth_type: "PASS"
+    auth_pass: "secret123"
+    virtual_ips:
+      - "192.168.1.100/24"
+    unicast_src_ip: "192.168.1.10"
+    unicast_peers:
+      - "192.168.1.11"
+```
+
+**Node 2 (BACKUP)**:
+```yaml
+global:
+  router_id: "node2"
+
+vrrp_instances:
+  - name: "VI_1"
+    interface: "eth0"
+    state: "BACKUP"
+    virtual_router_id: 51
+    priority: 50
+    advert_interval: 1
+    auth_type: "PASS"
+    auth_pass: "secret123"
+    virtual_ips:
+      - "192.168.1.100/24"
+    unicast_src_ip: "192.168.1.11"
+    unicast_peers:
+      - "192.168.1.10"
+```
+
+See `etc/config.unicast.yaml` for a complete unicast example.
+
 ### Health Checking
 
 ```yaml
@@ -245,9 +294,17 @@ sudo kill -HUP $(pgrep go-alived)
 | VirtualBox | Full | Bridged network + promiscuous mode |
 | Docker | Limited | Requires `--privileged --net=host` |
 | OpenWrt/iStoreOS | Full | Use `--method service` for install |
-| AWS/Aliyun/Azure | None | Multicast disabled |
+| macvlan networks | Full | Use unicast mode |
+| AWS/Aliyun/Azure/GCP | Limited | Use unicast mode |
 
-> **Note**: VRRP requires multicast support (224.0.0.18). Most public clouds disable multicast at the network layer. Use cloud-native HA solutions instead.
+> **Note**: VRRP defaults to multicast (224.0.0.18). In environments where multicast is disabled or unavailable (public clouds, macvlan networks), use **unicast mode** by configuring `unicast_src_ip` and `unicast_peers`.
+
+## Requirements
+
+- Go 1.21+ (for building)
+- Linux/macOS with root privileges (for raw sockets and interface management)
+- Network interface with IPv4 address
+- Multicast support (default mode) **or** unicast configuration (for restricted environments)
 
 ## Troubleshooting
 
@@ -266,7 +323,8 @@ sudo go-alived run -c /etc/go-alived/config.yaml
 **3. Both nodes become MASTER (split-brain)**
 - Check network connectivity between nodes
 - Verify `virtual_router_id` matches
-- Ensure multicast traffic is allowed
+- In multicast mode: ensure multicast traffic is allowed
+- In unicast mode: verify `unicast_peers` and `unicast_src_ip` are correct
 
 **4. VIP not pingable after failover**
 - Gratuitous ARP may be blocked