家庭宽带Full Cone NAT网络环境下使用Classicstun协议获取UdpClient的公网IPEndPoint
背景
由于IPv4地址数量稀缺，家庭用户宽带普遍为NAT共享IP地址。这种情况下两台在不同区域的设备无法直接通信，如果数据全部通过服务器的话会造成服务器的负担过大，同时受到服务器带宽限制。
因此Classicstun协议诞生。通过Classicstun协议可获取双方的IP地址以及端口号从而实现两台计算机直接通信，不经过服务器。
协议介绍
Classicstun RFC3489，是一种基于UDP的协议，端口为3478，广泛用于UDP P2P通信的建立。
当本地设备向服务器发送请求时，NAT设备会为本地设备打开一个端口，使外网设备可以通过打开的端口和本地设备通信（当NAT类型为全锥型时）
它由20字节的头部，以及可选的参数组成。
目前可用的Classicstun服务器有：
stun.syncthing.net
stun.hot-chilli.net
stun.fitauto.ru
stun.miwifi.com
stun.qq.com
这是Classicstun的头部以C#语法表示：
[StructLayout(LayoutKind.Sequential,Pack = 1)]
unsafe struct ClassicstunHeader
{
public ushort MessageType; //Big endian
public ushort MessageLength; //Big endian（不包括头部长度）
public fixed byte ID[16];
}
Classicstun参数结构：
[StructLayout(LayoutKind.Sequential,Pack = 1)]
unsafe struct ClassicstunAttribute
{
public ushort AttributeType; //Big endian
public ushort AttributeLength; //Big endian（不包括头部长度）
//...可变长度内容
}
以下是获取当前UDPClient的公网IPEndPoint的函数（仅支持Full Cone NAT）：
using System.Net;
using System.Net.Sockets;
UdpClient uc = new UdpClient(0, AddressFamily.InterNetwork);
if (GetNATPublicEndPointIfFullCone(ref uc, out var mapped))
{
Console.WriteLine($"UdpClient的公网IPEndPoint是：{mapped}");
}
else
{
throw new Exception("NAT类型不是Full Cone");
}
unsafe bool GetNATPublicEndPointIfFullCone(ref UdpClient uc, out IPEndPoint mappedAddress)
{
ushort ntoh(ushort Value)
{
return ((ushort)((((Value) & 0xff) << 8) | (((Value) & 0xff00) >> 8)));
}
mappedAddress = new IPEndPoint(IPAddress.None, 0);
IPEndPoint stun_server = new IPEndPoint(Dns.GetHostEntry("stun.syncthing.net", AddressFamily.InterNetwork).AddressList.First(), 3478);
Guid requestGUID = Guid.NewGuid();
{
MemoryStream ms = new MemoryStream();
ms.Write(BitConverter.GetBytes((ushort)ntoh(1))); //MessageType
ms.Write(BitConverter.GetBytes((ushort)ntoh(0))); //MessageLength
ms.Write(requestGUID.ToByteArray());
byte[] req = ms.ToArray();
uc.Send(req, stun_server);
ms.Close();
}
IPEndPoint changedAddress = null;
Task<UdpReceiveResult> responseAsync = uc.ReceiveAsync();
if (!responseAsync.Wait(1000))
{
return false;
}
if (!(responseAsync.Result.RemoteEndPoint.IPEndPointEquals(stun_server))) throw new InvalidOperationException("Packet received from a bad remote end point");
byte[] buffer = responseAsync.Result.Buffer;
{
if (buffer.Length <= (sizeof(ushort) + sizeof(ushort) + sizeof(Guid))) throw new InvalidDataException();
MemoryStream ms = new MemoryStream(buffer);
ms.Seek(sizeof(ushort) + sizeof(ushort), SeekOrigin.Begin);
Guid responseGUID = new Guid(
BitConverter.ToUInt32([(byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte()]),
BitConverter.ToUInt16([(byte)ms.ReadByte(), (byte)ms.ReadByte()]),
BitConverter.ToUInt16([(byte)ms.ReadByte(), (byte)ms.ReadByte()]),
(byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte()
);
if (requestGUID != responseGUID) throw new InvalidDataException("Wrong packet");
for (; ; )
{
ushort attr = ntoh(BitConverter.ToUInt16([
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
]));
if (attr == 0xffff) break;
ushort length = ntoh(BitConverter.ToUInt16([
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
]));
if (attr == 1)
{
ushort prot_fam = ntoh(BitConverter.ToUInt16([
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
]));
if (prot_fam == 1)
{
ushort port = ntoh(BitConverter.ToUInt16([(byte)ms.ReadByte(), (byte)ms.ReadByte()]));
IPAddress ip = new IPAddress([
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
(byte)ms.ReadByte()
]);
mappedAddress = new IPEndPoint(ip, port);
}
}
else if (attr == 5)
{
ushort prot_fam = ntoh(BitConverter.ToUInt16([
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
]));
if (prot_fam == 1)
{
ushort port = ntoh(BitConverter.ToUInt16([(byte)ms.ReadByte(), (byte)ms.ReadByte()]));
IPAddress ip = new IPAddress([
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
(byte)ms.ReadByte(),
(byte)ms.ReadByte()
]);
changedAddress = new IPEndPoint(ip, port);
}
}
else
{
ms.Seek(length, SeekOrigin.Current);
}
}
}
if (changedAddress == null)
{
throw new InvalidOperationException("Bad stun server or stun server does not support change address");
}
Guid changeRequestGUID = Guid.NewGuid();
{
MemoryStream ms = new MemoryStream();
ms.Write(BitConverter.GetBytes((ushort)ntoh(1))); //MessageType
ms.Write(BitConverter.GetBytes((ushort)ntoh(8))); //MessageLength
ms.Write(changeRequestGUID.ToByteArray());
ms.Write(new byte[]
{
0x00,0x03,
0x00,0x04,
0x00,0x00,0x00,0b00000110
//ChangeIP 0b00000100
//ChangePort 0b00000010
});
byte[] req = ms.ToArray();
uc.Send(req, stun_server);
ms.Close();
}
Task<UdpReceiveResult> changeResponseAsync = uc.ReceiveAsync();
if (!changeResponseAsync.Wait(1000))
{
return false;
}
if (!(changeResponseAsync.Result.RemoteEndPoint.IPEndPointEquals(changedAddress))) throw new InvalidOperationException("Packet received from a bad remote end point");
byte[] resp = changeResponseAsync.Result.Buffer;
{
if (resp.Length <= (sizeof(ushort) + sizeof(ushort) + sizeof(Guid))) throw new InvalidDataException();
MemoryStream ms = new MemoryStream(resp);
ms.Seek(sizeof(ushort) + sizeof(ushort), SeekOrigin.Begin);
Guid changeResponseGUID = new Guid(
BitConverter.ToUInt32([(byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte()]),
BitConverter.ToUInt16([(byte)ms.ReadByte(), (byte)ms.ReadByte()]),
BitConverter.ToUInt16([(byte)ms.ReadByte(), (byte)ms.ReadByte()]),
(byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte(), (byte)ms.ReadByte()
);
if (changeRequestGUID != changeResponseGUID) throw new InvalidDataException("Wrong packet");
}
return true;
}
2024年7月3日