Building a VPN with Go requires handling networking, encryption, and tunneling. Here’s a structured approach to get started:
---
### **1. Understand VPN Basics**
Before jumping into code, make sure you have a strong grasp of:
- **Tunneling:** Encapsulating IP packets over a different transport protocol.
IPX NetBEUI from layer 2 tunneling - passenger protocols
IPSec for layer 3 (IP only)
- Encryption: Using TLS, AES, or ChaCha20-Poly1305 for securing data.
- **Authentication:** Handling users securely with passwords, keys, or certificates.
- **Networking Concepts:** NAT traversal, routing tables, and packet forwarding.
**Recommended reading:**
- [WireGuard paper](https://www.wireguard.com/papers/wireguard.pdf)
- Linux networking (`iptables`, `iproute2`)
- Go’s `net` and `x/net` libraries.
---
### **2. Choose Your Approach**
There are multiple ways to implement a VPN:
- **Layer 2 (Ethernet-based, TAP interface)**
- **Layer 3 (IP-based, TUN interface)** - This is more common.
- **User-space forwarding (Go handles packets manually)**
- **Kernel-space forwarding (More efficient but complex, requires `XDP` or `ebpf`)**
For simplicity, start with a **TUN-based Layer 3 VPN**.
---
### **3. Set Up a Basic TUN Interface**
You'll need to use `golang.org/x/net/tun` (or `github.com/songgao/water` for cross-platform support) to create a TUN interface.
Example:
```go
package main
import (
"fmt"
"log"
"os"
"golang.org/x/net/tun"
)
func main() {
tunDevice, err := tun.Create("mytun0", 1500)
if err != nil {
log.Fatalf("Failed to create TUN interface: %v", err)
}
defer tunDevice.Close()
fmt.Println("TUN interface created:", tunDevice.Name())
// Read from the TUN interface
packet := make([]byte, 1500)
for {
n, err := tunDevice.Read(packet)
if err != nil {
log.Fatalf("Error reading from TUN device: %v", err)
}
fmt.Printf("Received packet: %x\n", packet[:n])
}
}
```
- This sets up a **TUN** interface.
- You can check it with:
```sh
ip link show mytun0
```
---
### **4. Handle Packet Encryption**
Use `golang.org/x/crypto/chacha20poly1305` (WireGuard's choice) or `crypto/tls`.
**Example of encrypting/decrypting packets with ChaCha20-Poly1305:**
```go
package main
import (
"crypto/rand"
"golang.org/x/crypto/chacha20poly1305"
"log"
)
func main() {
key := make([]byte, chacha20poly1305.KeySize)
rand.Read(key)
aead, err := chacha20poly1305.New(key)
if err != nil {
log.Fatalf("Failed to create cipher: %v", err)
}
nonce := make([]byte, aead.NonceSize())
rand.Read(nonce)
plaintext := []byte("Hello, VPN!")
ciphertext := aead.Seal(nil, nonce, plaintext, nil)
log.Printf("Encrypted: %x", ciphertext)
}
```
---
### **5. Implement a Secure Transport (TCP/UDP)**
You’ll need to send encrypted packets between VPN peers.
- **UDP** (like WireGuard): lower latency, better performance.
- **TCP** (like OpenVPN in TCP mode): easier to traverse firewalls.
Example UDP transport:
```go
package main
import (
"fmt"
"net"
)
func main() {
conn, err := net.ListenPacket("udp", ":51820")
if err != nil {
panic(err)
}
defer conn.Close()
fmt.Println("VPN Server listening on UDP 51820")
buf := make([]byte, 1500)
for {
n, addr, err := conn.ReadFrom(buf)
if err != nil {
fmt.Println("Read error:", err)
continue
}
fmt.Printf("Received %d bytes from %s\n", n, addr)
}
}
```
---
### **6. Implement Routing and NAT Handling**
You need to:
- **Set up IP forwarding:**
```sh
sysctl -w net.ipv4.ip_forward=1
```
- **Configure NAT:**
```sh
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
```
---
### **7. Add Authentication (Optional)**
- Simple pre-shared keys (like WireGuard)
- TLS-based authentication (like OpenVPN)
- JWT or OAuth-based authentication
---
### **8. Test Your VPN**
- Run your VPN client and server.
- Set up routing rules:
```sh
ip route add 10.8.0.0/24 via <vpn-server-ip> dev mytun0
```
- Ping across VPN.
---
### **9. Optimize and Expand**
- Support IPv6.
- Improve performance (eBPF, multithreading).
- Add multi-client support.
- Consider STUN/TURN for NAT traversal.
- GUI (even a simple web UI).
---
## **Final Thoughts**
Start simple:
✅ Get a basic **TUN interface working**.
✅ Encrypt packets with **ChaCha20-Poly1305**.
✅ Send data over **UDP**.
✅ Add **routing** and **authentication**.
Would you like to see a full working prototype or focus on a specific feature?