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扯谈网络编程之自己实现ping

ping是基于ICMP(Internet Control Message Protocol)协议实现的,而ICMP协议是在IP层实现的。

ping实际上是发起者发送一个Echo Request(type = 8)的,远程主机回应一个Echo Reply(type = 0)的过程。

为什么用ping不能测试某一个端口

刚开始接触网络的时候,可能很多人都有疑问,怎么用ping来测试远程主机的某个特定端口?

其实如果看下ICMP协议,就可以发现ICMP里根本没有端口这个概念,也就根本无法实现测试某一个端口了。

ICMP协议的包格式(来自wiki):

 Bits 0–7Bits 8–15Bits 16–23Bits 24–31
IP Header
(20 bytes)
Version/IHLType of serviceLength
Identificationflags and offset
Time To Live (TTL)ProtocolChecksum
Source IP address
Destination IP address
ICMP Header
(8 bytes)
Type of messageCodeChecksum
Header Data
ICMP Payload
(optional)
Payload Data
Echo Request的ICMP包格式(from wiki):

0001020304050607080910111213141516171819202122232425262728293031
Type = 8Code = 0Header Checksum
IdentifierSequence Number
Data

Ping如何计算请问耗时

在ping命令的输出上,可以看到有显示请求的耗时,那么这个耗时是怎么得到的呢?

64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=6.28 ms

从Echo Request的格式里,看到不时间相关的东东,但是因为是Echo,即远程主机会原样返回Data数据,所以Ping的发起方把时间放到了Data数据里,当得到Echo Reply里,取到发送时间,再和当前时间比较,就可以得到耗时了。当然,还有其它的思路,比如记录每一个包的发送时间,当得到返回时,再计算得到时间差,但显然这样的实现太复杂了。

Ping如何区分不同的进程?

我们都知道本机IP,远程IP,本机端口,远程端口,四个元素才可以确定唯的一个信道。而ICMP里没有端口,那么一个ping程序如何知道哪些包才是发给自己的?或者说操作系统如何区别哪个Echo Reply是要发给哪个进程的?

实际上操作系统不能区别,所有的本机IP,远程IP相同的ICMP程序都可以接收到同一份数据。

程序自己要根据Identifier来区分到底一个ICMP包是不是发给自己的。在Linux下,Ping发出去的Echo Request包里Identifier就是进程pid,远程主机会返回一个Identifier相同的Echo Reply包。

可以接下面的方法简单验证:

启动系统自带的ping程序,查看其pid。

设定自己实现的ping程序的identifier为上面得到的pid,然后发Echo Request包。

可以发现系统ping程序会接收到远程主机的回应。

自己实现ping

自己实现ping要用到rawsocket,在linux下需要root权限。网上有很多实现的程序,但是有很多地方不太对的。自己总结实现了一个(最好用g++编绎):

#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <errno.h>
#include <netinet/ip.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>

unsigned short csum(unsigned short *ptr, int nbytes) {
	register long sum;
	unsigned short oddbyte;
	register short answer;

	sum = 0;
	while (nbytes > 1) {
		sum += *ptr++;
		nbytes -= 2;
	}
	if (nbytes == 1) {
		oddbyte = 0;
		*((u_char*) &oddbyte) = *(u_char*) ptr;
		sum += oddbyte;
	}

	sum = (sum >> 16) + (sum & 0xffff);
	sum = sum + (sum >> 16);
	answer = (short) ~sum;

	return (answer);
}

inline double countMs(timeval before, timeval after){
	return (after.tv_sec - before.tv_sec)*1000 + (after.tv_usec - before.tv_usec)/1000.0;
}

#pragma pack(1)
struct EchoPacket {
	u_int8_t type;
	u_int8_t code;
	u_int16_t checksum;
	u_int16_t identifier;
	u_int16_t sequence;
	timeval timestamp;
	char data[40];   //sizeof(EchoPacket) == 64
};
#pragma pack()

void ping(in_addr_t source, in_addr_t destination) {
	static int sequence = 1;
	static int pid = getpid();
	static int ipId = 0;

	char sendBuf[sizeof(iphdr) + sizeof(EchoPacket)] = { 0 };

	struct iphdr* ipHeader = (iphdr*)sendBuf;
	ipHeader->version = 4;
	ipHeader->ihl = 5;

	ipHeader->tos = 0;
	ipHeader->tot_len = htons(sizeof(sendBuf));

	ipHeader->id = htons(ipId++);
	ipHeader->frag_off = htons(0x4000);  //set Flags: don‘t fragment

	ipHeader->ttl = 64;
	ipHeader->protocol = IPPROTO_ICMP;
	ipHeader->check = 0;
	ipHeader->saddr = source;
	ipHeader->daddr = destination;

	ipHeader->check = csum((unsigned short*)ipHeader, ipHeader->ihl * 2);

	EchoPacket* echoRequest = (EchoPacket*)(sendBuf + sizeof(iphdr));
	echoRequest->type = 8;
	echoRequest->code = 0;
	echoRequest->checksum = 0;
	echoRequest->identifier = htons(pid);
	echoRequest->sequence = htons(sequence++);
	gettimeofday(&(echoRequest->timestamp), NULL);
	u_int16_t ccsum = csum((unsigned short*)echoRequest, sizeof(sendBuf) - sizeof(iphdr));

	echoRequest->checksum = ccsum;

	struct sockaddr_in sin;
	sin.sin_family = AF_INET;
	sin.sin_port = htons(0);
	sin.sin_addr.s_addr = destination;

	int s = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
	if (s == -1) {
		perror("socket");
		return;
	}

	//IP_HDRINCL to tell the kernel that headers are included in the packet
	if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, "1",sizeof("1")) < 0) {
		perror("Error setting IP_HDRINCL");
		exit(0);
	}

	while(true){
		sendto(s, sendBuf, sizeof(sendBuf), 0, (struct sockaddr *) &sin, sizeof(sin));
		usleep(100);
	}


	char responseBuf[sizeof(iphdr) + sizeof(EchoPacket)] = {0};

	struct sockaddr_in receiveAddress;
	socklen_t len = sizeof(receiveAddress);
	int reveiveSize = recvfrom(s, (void*)responseBuf, sizeof(responseBuf), 0, (struct sockaddr *) &receiveAddress, &len);

	if(reveiveSize == sizeof(responseBuf)){
		EchoPacket* echoResponse = (EchoPacket*) (responseBuf + sizeof(iphdr));
		//TODO check identifier == pid ?
		if(echoResponse->type == 0){
			struct timeval tv;
			gettimeofday(&tv, NULL);

			in_addr tempAddr;
			tempAddr.s_addr = destination;
			printf("%d bytes from %s : icmp_seq=%d ttl=%d time=%.2f ms\n",
					sizeof(EchoPacket),
					inet_ntoa(tempAddr),
					ntohs(echoResponse->sequence),
					((iphdr*)responseBuf)->ttl,
					countMs(echoResponse->timestamp, tv));
		}else{
			printf("response error, type:%d\n", echoResponse->type);
		}
	}else{
		printf("error, response size != request size.\n");
	}

	close(s);
}

int main(void) {
	in_addr_t source = inet_addr("192.168.1.100");
	in_addr_t destination = inet_addr("192.168.1.1");
	for(;;){
		ping(source, destination);
		sleep(1);
	}

	return 0;
}

安全相关的一些东东:

死亡之Ping  http://zh.wikipedia.org/wiki/%E6%AD%BB%E4%BA%A1%E4%B9%8BPing

尽管是很老的漏洞,但是也可以看出协议栈的实现也不是那么的靠谱。

Ping flood   http://en.wikipedia.org/wiki/Ping_flood

总结:

在自己实现的过程中,发现有一些蛋疼的地方,如

协议文档不够清晰,得反复对照;

有时候一个小地方处理不对,很难查bug,即使程序能正常工作,但也并不代表它是正确的;

用wireshark可以很方便验证自己写的程序有没有问题。

参考:

http://en.wikipedia.org/wiki/Ping_(networking_utility)

http://en.wikipedia.org/wiki/ICMP_Destination_Unreachable

http://tools.ietf.org/pdf/rfc792.pdf