语法:
cksum [--help] [--version] [file...]选项介绍:
--help: 在线帮助
--version: 显示版本信息
执行范例:
1 $ cksum text
2 2828606745 24 text #2828606745为CRC的值，24为text文件的大小(单位:字节)
扩展阅读:
cksum命令是确保文件从一个系统传输到另一个系统地过程中没有被损坏。这个测试要求校验和在源系统中被计算出来，在目的系统中又被计算一次，两个数字比较，如果校验和相等，则该文件被认为是被正确传输了。
当需要检查文件或目录是否被改动过时就要用到cksum命令。通过将一个目录或文件的校验和与它以前的校验和相比较，就能判断该文件是否被改动过。
myping.c#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <sys/time.h>
#include <netinet/ip_icmp.h>
#include <unistd.h>
#include <signal.h>
#define MAX_SIZE 1024
char send_buf[MAX_SIZE]
char recv_buf[MAX_SIZE]
int nsend = 0,nrecv = 0
int datalen = 56
//统计结果
void statistics(int signum)
{
printf("\n----------------PING statistics---------------\n")
printf("%d packets transmitted,%d recevid,%%%d lost\n",nsend,nrecv,(nsend - nrecv)/nsend * 100)
exit(EXIT_SUCCESS)
}
//校验和算法
int calc_chsum(unsigned short *addr,int len)
{
int sum = 0,n = len
unsigned short answer = 0
unsigned short *p = addr
//每两个字节相加
while(n >1)
{
sum += *p ++
n -= 2
}
//处理数据大小是奇数，在最后一个字节后面补0
if(n == 1)
{
*((unsigned char *)&answer) = *(unsigned char *)p
sum += answer
}
//将得到的sum值的高2字节和低2字节相加
sum = (sum >>16) + (sum &0xffff)
//处理溢出的情况
sum += sum >>16
answer = ~sum
return answer
}
int pack(int pack_num)
{
int packsize
struct icmp *icmp
struct timeval *tv
icmp = (struct icmp *)send_buf
icmp->icmp_type = ICMP_ECHO
icmp->icmp_code = 0
icmp->icmp_cksum = 0
icmp->icmp_id = htons(getpid())
icmp->icmp_seq = htons(pack_num)
tv = (struct timeval *)icmp->icmp_data
//记录发送时间
if(gettimeofday(tv,NULL) <0)
{
perror("Fail to gettimeofday")
return -1
}
packsize = 8 + datalen
icmp->icmp_cksum = calc_chsum((unsigned short *)icmp,packsize)
return packsize
}
int send_packet(int sockfd,struct sockaddr *paddr)
{
int packsize
//将send_buf填上a
memset(send_buf,'a',sizeof(send_buf))
nsend ++
//打icmp包
packsize = pack(nsend)
if(sendto(sockfd,send_buf,packsize,0,paddr,sizeof(struct sockaddr)) <0)
{
perror("Fail to sendto")
return -1
}
return 0
}
struct timeval time_sub(struct timeval *tv_send,struct timeval *tv_recv)
{
struct timeval ts
if(tv_recv->tv_usec - tv_send->tv_usec <0)
{
tv_recv->tv_sec --
tv_recv->tv_usec += 1000000
}
ts.tv_sec = tv_recv->tv_sec - tv_send->tv_sec
ts.tv_usec = tv_recv->tv_usec - tv_send->tv_usec
return ts
}
int unpack(int len,struct timeval *tv_recv,struct sockaddr *paddr,char *ipname)
{
struct ip *ip
struct icmp *icmp
struct timeval *tv_send,ts
int ip_head_len
float rtt
ip = (struct ip *)recv_buf
ip_head_len = ip->ip_hl <<2
icmp = (struct icmp *)(recv_buf + ip_head_len)
len -= ip_head_len
if(len <8)
{
printf("ICMP packets\'s is less than 8.\n")
return -1
}
if(ntohs(icmp->icmp_id) == getpid() &&icmp->icmp_type == ICMP_ECHOREPLY)
{
nrecv ++
tv_send = (struct timeval *)icmp->icmp_data
ts = time_sub(tv_send,tv_recv)
rtt = ts.tv_sec * 1000 + (float)ts.tv_usec/1000//以毫秒为单位
printf("%d bytes from %s (%s):icmp_req = %d ttl=%d time=%.3fms.\n",
len,ipname,inet_ntoa(((struct sockaddr_in *)paddr)->sin_addr),ntohs(icmp->icmp_seq),ip->ip_ttl,rtt)
}
return 0
}
int recv_packet(int sockfd,char *ipname)
{
int addr_len ,n
struct timeval tv
struct sockaddr from_addr
addr_len = sizeof(struct sockaddr)
if((n = recvfrom(sockfd,recv_buf,sizeof(recv_buf),0,&from_addr,&addr_len)) <0)
{
perror("Fail to recvfrom")
return -1
}
if(gettimeofday(&tv,NULL) <0)
{
perror("Fail to gettimeofday")
return -1
}
unpack(n,&tv,&from_addr,ipname)
return 0
}
int main(int argc,char *argv[])
{
int size = 50 * 1024
int sockfd,netaddr
struct protoent *protocol
struct hostent *host
struct sockaddr_in peer_addr
if(argc <2)
{
fprintf(stderr,"usage : %s ip.\n",argv[0])
exit(EXIT_FAILURE)
}
//获取icmp的信息
if((protocol = getprotobyname("icmp")) == NULL)
{
perror("Fail to getprotobyname")
exit(EXIT_FAILURE)
}
//创建原始套接字
if((sockfd = socket(AF_INET,SOCK_RAW,protocol->p_proto)) <0)
{
perror("Fail to socket")
exit(EXIT_FAILURE)
}
//回收root权限,设置当前用户权限
setuid(getuid())
/*
扩大套接子接收缓冲区到50k，这样做主要为了减少接收缓冲区溢出的可能性
若无影中ping一个广播地址或多播地址，将会引来大量应答
*/
if(setsockopt(sockfd,SOL_SOCKET,SO_RCVBUF,&size,sizeof(size)) <0)
{
perror("Fail to setsockopt")
exit(EXIT_FAILURE)
}
//填充对方的地址
bzero(&peer_addr,sizeof(peer_addr))
peer_addr.sin_family = AF_INET
//判断是主机名(域名)还是ip
if((netaddr = inet_addr(argv[1])) == INADDR_NONE)
{
//是主机名（域名）
if((host = gethostbyname(argv[1])) == NULL)
{
fprintf(stderr,"%s unknown host : %s.\n",argv[0],argv[1])
exit(EXIT_FAILURE)
}
memcpy((char *)&peer_addr.sin_addr,host->h_addr,host->h_length)
}else{//ip地址
peer_addr.sin_addr.s_addr = netaddr
}
//注册信号处理函数
signal(SIGALRM,statistics)
signal(SIGINT,statistics)
alarm(5)
//开始信息
printf("PING %s(%s) %d bytes of data.\n",argv[1],inet_ntoa(peer_addr.sin_addr),datalen)
//发送包文和接收报文
while(1)
{
send_packet(sockfd,(struct sockaddr *)&peer_addr)
recv_packet(sockfd,argv[1])
alarm(5)
sleep(1)
}
exit(EXIT_SUCCESS)
}
gcc -o myping myping.c
./myping 10.1.1.1
PING 10.1.1.1(10.1.1.1) 56 bytes of data.
64 bytes from 10.1.1.1 (10.1.1.1):icmp_req = 1 ttl=253 time=10.573ms.
64 bytes from 10.1.1.1 (10.1.1.1):icmp_req = 2 ttl=253 time=12.585ms.
64 bytes from 10.1.1.1 (10.1.1.1):icmp_req = 3 ttl=253 time=9.440ms.
64 bytes from 10.1.1.1 (10.1.1.1):icmp_req = 4 ttl=253 time=12.923ms.
64 bytes from 10.1.1.1 (10.1.1.1):icmp_req = 5 ttl=253 time=11.937ms.
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