ICMP recvfrom function always receive data even though the destination was shutdown - ping

Question

I wrote a program in C to ping a machine and receive the echo from that machine to detect if the machine is still alive. My code as following please:
#define PING_PKT_S 64
#define PORT_NO 0
#define PING_SLEEP_RATE 1000000
#define RECV_TIMEOUT 1
int pingloop=1;
struct ping_pkt {
struct icmphdr hdr;
char msg[PING_PKT_S-sizeof(struct icmphdr)];
};
unsigned short checksum(void *b, int len) {
unsigned short *buf = b;
unsigned int sum=0;
unsigned short result;
for ( sum = 0; len > 1; len -= 2 )
sum += *buf++;
if ( len == 1 )
sum += *(unsigned char*)buf;
sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);
result = ~sum;
return result;
}
//to check if this machine is actively working
int isActiveNow(){
const char *pre = "161.";
char *prefixArr[] = {pre};
char myIp[64];
memset(myIp, 0, sizeof(myIp));
get_local_ip(myIp, prefixArr);
printf("---isActiveNow:%s\n",myIp);
if(myIp[0]=='\0'){
printf("Not Active Now!\n");
return 0;
}
return 1;
}
void intHandler(int dummy) {
pingloop=0;
}
long current_timestamp() {
struct timeval te;
gettimeofday(&te, NULL);
long milliseconds = te.tv_sec*1000LL + te.tv_usec/1000;
return milliseconds;
}
void display(void *buf, int bytes){
int i;
struct iphdr *ip = buf;
struct icmphdr *icmp = buf+ip->ihl*4;
printf("---------------------Start Display\n");
for ( i = 0; i < bytes; i++ ){
printf("%c", ((unsigned char*)buf)[i]);
}
printf("---------------------End Display\n");
}
int failedNum = 0;
// make a ping request
void send_ping(int ping_sockfd, char *ping_ip) {
int ttl_val=64, msg_count=0, i, addr_len, msg_received_count=0;
struct ping_pkt pckt;
struct sockaddr_in r_addr;
struct timespec time_start, time_end;
// set socket options at ip to TTL and value to 64,
if (setsockopt(ping_sockfd, SOL_IP, IP_TTL, &ttl_val, sizeof(ttl_val)) != 0) {
printf("Setting socket options to TTL failed!\n");
return;
}else{
printf("\nSocket set to TTL..\n");
}
// send icmp packet in an infinite loop
while(pingloop) {
sleep(2);
if(isActiveNow()){
printf("--current is active!");
continue;
}
//filling packet
bzero(&pckt, sizeof(pckt));
pckt.hdr.type = ICMP_ECHO;
pckt.hdr.un.echo.id = getpid();
for ( i = 0; i < 10; i++ ){
pckt.msg[i] = 'w';
}
pckt.msg[i] = 0;
pckt.hdr.un.echo.sequence = msg_count++;
pckt.hdr.checksum = checksum(&pckt, sizeof(pckt));
long s1 = current_timestamp();
struct hostent *hostentObj;
hostentObj = gethostbyname(ping_ip);
struct sockaddr_in ping_addr;
ping_addr.sin_port = 0;
ping_addr.sin_family = hostentObj->h_addrtype;
ping_addr.sin_addr.s_addr = *(long*) hostentObj->h_addr;
//Using synchronize
if ( sendto(ping_sockfd, &pckt, sizeof(pckt), 0, (struct sockaddr*) &ping_addr, sizeof(ping_addr)) <= 0) {
printf("\nPacket Sending Failed!\n");
}
struct timeval tv_out;
tv_out.tv_sec = 10;
tv_out.tv_usec = 0;
// setsockopt(ping_sockfd, SOL_SOCKET, SO_RCVTIMEO, (const char*)&tv_out, sizeof tv_out);
struct sockaddr_in myAddr;
unsigned char buf[1024];
int len=sizeof(myAddr);
bzero(buf, sizeof(buf));
struct ping_pkt pckt2;
int bytes = recvfrom(ping_sockfd, &pckt2, sizeof(pckt2), 0, (struct sockaddr*)&myAddr, &len);
printf("---------------------bytes len:%d\n",bytes);
if ( bytes > 0 ){
display(&pckt2, bytes);
long s2 = current_timestamp();
printf("from %s msg_seq=%d ttl=%d rtt = %ld ms. pckt.hdr.type=%d \n", ping_ip, msg_count,ttl_val, (s2-s1), pckt.hdr.type);
}else{
printf("-------NO RESPONSE!!!!!!\n");
failedNum++;
//maximum detected failed number is 10
if(failedNum>3){
printf("--The other server dead!!!\n");
}
}
}
}
bool prefix(const char *pre, const char *str){
return strncmp(pre, str, strlen(pre)) == 0;
}
void get_local_ip(char *pubIp, char *prefixArr[]){
char *ip;
int fd, intrface, retn = 0;
struct ifreq buf[INET_ADDRSTRLEN];
struct ifconf ifc;
if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) >= 0){
ifc.ifc_len = sizeof(buf);
ifc.ifc_buf = (caddr_t)buf;
if (!ioctl(fd, SIOCGIFCONF, (char *)&ifc)){
intrface = ifc.ifc_len/sizeof(struct ifreq);
while (intrface-- > 0){
if (!(ioctl(fd, SIOCGIFADDR, (char *)&buf[intrface]))){
ip=(inet_ntoa(((struct sockaddr_in*)(&buf[intrface].ifr_addr))->sin_addr));
int len = 0,i = 0;
len = sizeof(prefixArr) / sizeof(*prefixArr);
for (i = 0; i < len; i++) {
if(prefix(prefixArr[i],ip)){
strcpy(pubIp,ip);
return;
}
}
}
}
}
close(fd);
}
}
// Driver Code
int main(int argc, char *argv[]){
int sockfd;
char *pingIp;
char *prefixArr[] = { "11.","10.","172."};
char myIp[64];
memset(myIp, 0, sizeof(myIp));
get_local_ip(myIp, prefixArr);
//the other server's ip for detecting it's alive or dead
pingIp = argv[1];
printf("my ip:%s-----ping ip:%s\n", myIp, pingIp);
//socket()
sockfd = socket(PF_INET, SOCK_RAW, IPPROTO_ICMP);
struct sockaddr_in r_addr;
r_addr.sin_family = AF_INET;
r_addr.sin_addr.s_addr = inet_addr(myIp);
r_addr.sin_port = 0;
bind(sockfd, (struct sockaddr *) &r_addr, sizeof(r_addr));
if(sockfd<0){
printf("\nSocket file descriptor not received!!\n");
return 0;
}else{
printf("\nSocket file descriptor %d received\n", sockfd);
}
//once error seen, it will stop the loop
signal(SIGINT, intHandler);//catching interrupt
//send pings continuously
send_ping(sockfd, pingIp);
return 0;
}
The problem is that the value of myReceiveBytes is alway 64, even though the destination server has already been shutdown.

Answer 1

Your code is not actually decoding the received packet. In all likelihood, what you are receiving is an ICMP Destination Unreachable packet either from your own host or from your gateway.
Something along these lines:
#include <netinet/ip.h> /* struct iphdr */
...
...
struct sockaddr_in peerAddr;
unsigned char buf[1024];
unsigned int len = sizeof(peerAddr);
int bytes_received = recvfrom(sockfd, buf, sizeof(buf), 0,
(struct sockaddr *) &peerAddr, &len);
if (bytes_received > 0) {
printf("ICMP packet received from %s\n", inet_ntoa(peerAddr.sin_addr));
struct icmphdr *icmp = (struct icmphdr *) (buf + sizeof(struct iphdr));
switch (icmp->type) {
case ICMP_ECHOREPLY: printf("Type: ICMP_ECHOREPLY\n"); break;
case ICMP_DEST_UNREACH: printf("Type: ICMP_DEST_UNREACH\n"); break;
case ICMP_SOURCE_QUENCH: printf("Type: ICMP_SOURCE_QUENCH\n"); break;
case ICMP_REDIRECT: printf("Type: ICMP_REDIRECT\n"); break;
case ICMP_ECHO: printf("Type: ICMP_ECHO\n"); break;
case ICMP_TIME_EXCEEDED: printf("Type: ICMP_TIME_EXCEEDED\n"); break;
case ICMP_PARAMETERPROB: printf("Type: ICMP_PARAMETERPROB\n"); break;
case ICMP_TIMESTAMP: printf("Type: ICMP_TIMESTAMP\n"); break;
case ICMP_TIMESTAMPREPLY: printf("Type: ICMP_TIMESTAMPREPLY\n"); break;
case ICMP_INFO_REQUEST: printf("Type: ICMP_INFO_REQUEST\n"); break;
case ICMP_INFO_REPLY: printf("Type: ICMP_INFO_REPLY\n"); break;
case ICMP_ADDRESS: printf("Type: ICMP_ADDRESS\n"); break;
case ICMP_ADDRESSREPLY: printf("Type: ICMP_ADDRESSREPLY\n"); break;
default: printf("Type: <0x%02x>\n", icmp->type); break;
}
}