在解释之前我们先来了解一下 ICMP 报文。
❔ 为什么要引入 ICMP 协议 ?
网际控制报文协议
ICMP (Internet Control Message Protocol)。ICMP 差错报告报文
共有 4 种:
ICMP 差错报告报文的数据字段
的内容:
❔既然是差错报告报文,那么他肯定是有差错的时候才发送的吧 ?
下面我们来看一下它什么时候不需要发送:
询问报文
有两种:
说了那么多好像和标题咩有任何关系额?,下面我们就来回答标题的问题。
其实有一个最常见的例子,就是我们常用的 ping
操作,我们常常使用 ping
来看一下网络连接是否畅通?。
PING
(Packet InterNet Groper)也就是说直接从应用层跳到网络层?。
再来看一个Traceroute 的应用举例:
? 最后附上使用 C# 模拟 ping
指令的部分源码:
namespace SaurabhPing
{
using System;
using System.Net;
using System.Net.Sockets;
/// <summary>
/// The Main Ping Class
/// </summary>
class Ping
{
//声明常量
const int SOCKET_ERROR = -1;
const int ICMP_ECHO = 8;
public static void Main(string[] argv)
{
if(argv.Length==0)
{
//如果用户没有输入任何参数则给出提示
Console.WriteLine("Usage:Ping <hostname> /r") ;
Console.WriteLine("<hostname> The name of the Host who you want to ping");
Console.WriteLine("/r Optional Switch to Ping the host continuously") ;
}
else if(argv.Length==1)
{
//即为用户提供的主机名
//调用 PingHost 方法并将主机名作为参数传递给它
PingHost(argv[0]) ;
}
else if(argv.Length==2)
{
//用户提供了主机名和循环参数(开关,“/r”)
if(argv[1]=="/r")
{
//无限重复下去
while(true)
{
//调用"PingHost"方法并把主机名作为参数传递过去
PingHost(argv[0]) ;
}
}
else
{
//如果用户还提供了其他的参数则忽略
PingHost(argv[0]) ;
}
}
else
{
//出现错误
Console.WriteLine("Error in Arguments") ;
}
}
/// <summary>
/// 这个方法以主机名作为参数ping远程主机,并显示回复时间
/// </summary>
public static void PingHost(string host)
{
//声明IPHostEntry
IPHostEntry serverHE, fromHE;
int nBytes = 0;
int dwStart = 0, dwStop = 0;
//初始化一个ICMP类型的Socket
Socket socket = new Socket(AddressFamily.InterNetwork , SocketType.Raw, ProtocolType.Icmp);
// 取得目标主机的主机名
try
{
serverHE = Dns.GetHostByName(host);
}
catch(Exception)
{
Console.WriteLine("目标主机不存在"); // 失败
return ;
}
IPEndPoint ipepServer = new IPEndPoint(serverHE.AddressList[0], 0);
EndPoint epServer = (ipepServer);
fromHE = Dns.GetHostByName(Dns.GetHostName());
IPEndPoint ipEndPointFrom = new IPEndPoint(fromHE.AddressList[0], 0); EndPoint EndPointFrom = (ipEndPointFrom);
int PacketSize = 0;
IcmpPacket packet = new IcmpPacket();
// 构造数据报
packet.Type = ICMP_ECHO; //8
packet.SubCode = 0;
packet.CheckSum = UInt16.Parse("0");
packet.Identifier = UInt16.Parse("45");
packet.SequenceNumber = UInt16.Parse("0");
int PingData = 32; // sizeof(IcmpPacket) - 8;
packet.Data = new Byte[PingData];
//初始化 Packet.Data
for (int i = 0; i < PingData; i++)
{
packet.Data[i] = (byte)'#';
}
//保存数据报的长度
PacketSize = PingData + 8;
Byte [] icmp_pkt_buffer = new Byte[ PacketSize ];
Int32 Index = 0;
//调用Serialize方法
//报文总共的字节数
Index = Serialize(
packet,
icmp_pkt_buffer,
PacketSize,
PingData );
//报文大小有错
if( Index == -1 )
{
Console.WriteLine("Error in Making Packet");
return ;
}
// 转化为Uint16类型的数组
//取得数据报长度的一半
Double double_length = Convert.ToDouble(Index);
Double dtemp = Math.Ceiling( double_length / 2);
int cksum_buffer_length = Convert.ToInt32(dtemp);
//生成一个字节数组
UInt16 [] cksum_buffer = new UInt16[cksum_buffer_length];
//初始化 Uint16类型 array
int icmp_header_buffer_index = 0;
for( int i = 0; i < cksum_buffer_length; i++ )
{
cksum_buffer[i] =
BitConverter.ToUInt16(icmp_pkt_buffer,icmp_header_buffer_index);
icmp_header_buffer_index += 2;
}
//调用checksum,返回检查和
UInt16 u_cksum = checksum(cksum_buffer, cksum_buffer_length);
//检查和存在报文中
packet.CheckSum = u_cksum;
// Now that we have the checksum, serialize the packet again
Byte [] sendbuf = new Byte[ PacketSize ];
//再次检查报文大小
Index = Serialize(
packet,
sendbuf,
PacketSize,
PingData );
//如果有错,则报告错误
if( Index == -1 )
{
Console.WriteLine("Error in Making Packet");
return ;
}
dwStart = System.Environment.TickCount; // 开始时间
//用socket发送数据报
if ((nBytes = socket.SendTo(sendbuf, PacketSize, 0, epServer)) == SOCKET_ERROR)
{
Console.WriteLine("Socket Error cannot Send Packet");
}
//初始化缓冲区.接受缓冲区 Initialize the buffers. The receive buffer is the size of the
// ICMP 头 +IP 头 (20 字节)
Byte [] ReceiveBuffer = new Byte[256];
nBytes = 0;
//接受字节流
bool recd =false ;
int timeout=0 ;
//循环检查目标主机相应时间
while(!recd)
{
nBytes = socket.ReceiveFrom(ReceiveBuffer, 256, 0, ref EndPointFrom);
if (nBytes == SOCKET_ERROR)
{
Console.WriteLine("Host not Responding") ;
recd=true ;
break;
}
else if(nBytes>0)
{
dwStop = System.Environment.TickCount - dwStart;
// 停止计时
Console.WriteLine("Reply from "+epServer.ToString()+": bytes=" + nBytes + " time="+dwStop + "ms");
recd=true;
break;
}
timeout=System.Environment.TickCount - dwStart;
if(timeout>1000)
{
Console.WriteLine("Time Out") ;
recd=true;
}
}
//关闭socket
socket.Close();
}
/// <summary>
/// 取得报文内容,转化为字节数组,然后计算报文的长度
/// </summary>
public static Int32 Serialize( IcmpPacket packet, Byte [] Buffer, Int32 PacketSize, Int32 PingData )
{
Int32 cbReturn = 0;
// 数据报结构转化为数组
int Index=0;
Byte [] b_type = new Byte[1];
b_type[0] = (packet.Type);
Byte [] b_code = new Byte[1];
b_code[0] = (packet.SubCode);
Byte [] b_cksum = BitConverter.GetBytes(packet.CheckSum);
Byte [] b_id = BitConverter.GetBytes(packet.Identifier);
Byte [] b_seq = BitConverter.GetBytes(packet.SequenceNumber);
Array.Copy( b_type, 0, Buffer, Index, b_type.Length );
Index += b_type.Length;
Array.Copy( b_code, 0, Buffer, Index, b_code.Length );
Index += b_code.Length;
Array.Copy( b_cksum, 0, Buffer, Index, b_cksum.Length );
Index += b_cksum.Length;
Array.Copy( b_id, 0, Buffer, Index, b_id.Length );
Index += b_id.Length;
Array.Copy( b_seq, 0, Buffer, Index, b_seq.Length );
Index += b_seq.Length;
// 复制数据
Array.Copy( packet.Data, 0, Buffer, Index, PingData );
Index += PingData;
if( Index != PacketSize/* sizeof(IcmpPacket) */)
{
cbReturn = -1;
return cbReturn;
}
cbReturn = Index;
return cbReturn;
}
/// <summary>
/// 校验和算法
/// </summary>
public static UInt16 checksum( UInt16[] buffer, int size )
{
Int32 cksum = 0;
int counter;
counter = 0;
/*把ICMP报头二进制数据以2字节为单位累加起来*/
while ( size > 0 )
{
UInt16 val = buffer[counter];
cksum += Convert.ToInt32( buffer[counter] );
counter += 1;
size -= 1;
}
/* 若ICMP报头为奇数个字节,会剩下最后一字节。把最后一个字节视为一个
* 2字节数据的高字节,这个2字节数据的低字节为0,继续累加*/
cksum = (cksum >> 16) + (cksum & 0xffff);
cksum += (cksum >> 16);
return (UInt16)(~cksum);
}
} // class ping
/// <summary>
/// IcmpPacket类,存储报文内容
/// </summary>
public class IcmpPacket
{
public Byte Type; // 消息类型
public Byte SubCode; // 子码类型
public UInt16 CheckSum; // 校检和
public UInt16 Identifier; // 标志符
public UInt16 SequenceNumber; // 顺序号
public Byte [] Data; // 数据
} // ICMP包
}