Ping in C

Difficulty Level : Hard
Last Updated : 07 Jun, 2018

Prerequisites : ICMP | Raw Socket | Internet Checksum | DNS

Ping is a necessity for debugging of the Internet.
Ping is a basic Internet tool that allows a user to verify that a particular IP address exists and can accept requests., with other facilities.

Ping sends out ICMP packets by opening a RAW socket, which is separate from TCP and UDP.

Since IP does not have any inbuilt mechanism for sending error and control messages. It depends on Internet Control Message Protocol (ICMP) to provide an error control. It is used for reporting errors and management queries.

Example of Ubuntu Ping

ping www.google.com
PING www.google.com (172.217.194.105) 56(84) bytes of data.
64 bytes from 172.217.194.105 (172.217.194.105): icmp_seq=1 ttl=46 time=116 ms
64 bytes from 172.217.194.105 (172.217.194.105): icmp_seq=2 ttl=46 time=102 ms
64 bytes from 172.217.194.105 (172.217.194.105): icmp_seq=3 ttl=46 time=119 ms
^C
--- www.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 3110ms

Working Mechanism
The Internet Ping program works much like a sonar echo-location, sending a small packet of information containing an ICMP ECHO_REQUEST to a specified computer, which then sends an ECHO_REPLY packet in return.
The packet has a TTL (time-to-live) value determining max number of router hops.

If the packet does not reach, then the sender is noted back with the error.

Errors are of following types:

TTL Expired in Transit
Destination Host Unreachable
Request Timed Out i.e. no reply
Unknown Host

Implementation
The steps followed by a simple ping program are:

Take a hostname as input
Do a DNS lookup
DNS lookup can be done using gethostbyname(). The gethostbyname() function converts a normal human readable website and returns a structure of type hostent which contains IP address in form of binary dot notation and also address type.
Some ping programs like the one given with ubuntu support reverse DNS lookup.
Reverse DNS lookup is performed using getnameinfo(), and it converts dot notation IP address to hostname.

for example, the pinging of google.com frequently gives a strange address:
bom07s18-in-f14.1e100.net
This is as a result of a reverse DNS lookup.
Open a Raw socket using SOCK_RAW with protocol as IPPROTO_ICMP.
Note: raw socket requires superuser rights so you have to run this code using sudo
When crtl + C is pressed, ping gives a report. This interrupt is caught by an interrupt handler
which just sets our pinging looping condition to false.

Here comes the main ping sending loop.
We have to:

Set the ttl option to a value in the socket
TTL value is set to limit the number of hops a packet can make.
Set the timeout of the recv function
If timeout is not set, recv will wait forever, halting the loop.
Fill up the icmp packet
As follows:
1. Set packet header type to ICMP_ECHO.
2. Set id to pid of process
3. Fill msg part randomly.
4. Calculate checksum and fill it in checksum field.
Send the packet

Wait for it to be received.
The main problem here is that the packet received does not mean that that the destination is working.
Echo reply means destination is OK. Echo reply is sent from destination OS kernel.

This is the list of all types and codes.
A issue here is that the program shows type 69 and code 0 if all goes correct instead of 0 which stands for echo_reply.

// C program to Implement Ping
  
// compile as -o ping
// run as sudo ./ping <hostname>
  
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <netinet/ip_icmp.h>
#include <time.h>
#include <fcntl.h>
#include <signal.h>
#include <time.h>
  
// Define the Packet Constants
// ping packet size
#define PING_PKT_S 64
   
// Automatic port number
#define PORT_NO 0 
  
// Automatic port number
#define PING_SLEEP_RATE 1000000 x
  
// Gives the timeout delay for receiving packets
// in seconds
#define RECV_TIMEOUT 1 
  
// Define the Ping Loop
int pingloop=1;
  
  
// ping packet structure
struct ping_pkt
{
    struct icmphdr hdr;
    char msg[PING_PKT_S-sizeof(struct icmphdr)];
};
  
// Calculating the Check Sum
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;
}
  
  
// Interrupt handler
void intHandler(int dummy)
{
    pingloop=0;
}
  
// Performs a DNS lookup 
char *dns_lookup(char *addr_host, struct sockaddr_in *addr_con)
{
    printf("\nResolving DNS..\n");
    struct hostent *host_entity;
    char *ip=(char*)malloc(NI_MAXHOST*sizeof(char));
    int i;
  
    if ((host_entity = gethostbyname(addr_host)) == NULL)
    {
        // No ip found for hostname
        return NULL;
    }
      
    //filling up address structure
    strcpy(ip, inet_ntoa(*(struct in_addr *)
                          host_entity->h_addr));
  
    (*addr_con).sin_family = host_entity->h_addrtype;
    (*addr_con).sin_port = htons (PORT_NO);
    (*addr_con).sin_addr.s_addr  = *(long*)host_entity->h_addr;
  
    return ip;
      
}
  
// Resolves the reverse lookup of the hostname
char* reverse_dns_lookup(char *ip_addr)
{
    struct sockaddr_in temp_addr;    
    socklen_t len;
    char buf[NI_MAXHOST], *ret_buf;
  
    temp_addr.sin_family = AF_INET;
    temp_addr.sin_addr.s_addr = inet_addr(ip_addr);
    len = sizeof(struct sockaddr_in);
  
    if (getnameinfo((struct sockaddr *) &temp_addr, len, buf, 
                    sizeof(buf), NULL, 0, NI_NAMEREQD)) 
    {
        printf("Could not resolve reverse lookup of hostname\n");
        return NULL;
    }
    ret_buf = (char*)malloc((strlen(buf) +1)*sizeof(char) );
    strcpy(ret_buf, buf);
    return ret_buf;
}
  
// make a ping request
void send_ping(int ping_sockfd, struct sockaddr_in *ping_addr,
                char *ping_dom, char *ping_ip, char *rev_host)
{
    int ttl_val=64, msg_count=0, i, addr_len, flag=1,
               msg_received_count=0;
      
    struct ping_pkt pckt;
    struct sockaddr_in r_addr;
    struct timespec time_start, time_end, tfs, tfe;
    long double rtt_msec=0, total_msec=0;
    struct timeval tv_out;
    tv_out.tv_sec = RECV_TIMEOUT;
    tv_out.tv_usec = 0;
  
    clock_gettime(CLOCK_MONOTONIC, &tfs);
  
      
    // set socket options at ip to TTL and value to 64,
    // change to what you want by setting ttl_val
    if (setsockopt(ping_sockfd, SOL_IP, IP_TTL, 
               &ttl_val, sizeof(ttl_val)) != 0)
    {
        printf("\nSetting socket options 
                 to TTL failed!\n");
        return;
    }
  
    else
    {
        printf("\nSocket set to TTL..\n");
    }
  
    // setting timeout of recv setting
    setsockopt(ping_sockfd, SOL_SOCKET, SO_RCVTIMEO,
                   (const char*)&tv_out, sizeof tv_out);
  
    // send icmp packet in an infinite loop
    while(pingloop)
    {
        // flag is whether packet was sent or not
        flag=1;
       
        //filling packet
        bzero(&pckt, sizeof(pckt));
          
        pckt.hdr.type = ICMP_ECHO;
        pckt.hdr.un.echo.id = getpid();
          
        for ( i = 0; i < sizeof(pckt.msg)-1; i++ )
            pckt.msg[i] = i+'0';
          
        pckt.msg[i] = 0;
        pckt.hdr.un.echo.sequence = msg_count++;
        pckt.hdr.checksum = checksum(&pckt, sizeof(pckt));
  
  
        usleep(PING_SLEEP_RATE);
  
        //send packet
        clock_gettime(CLOCK_MONOTONIC, &time_start);
        if ( sendto(ping_sockfd, &pckt, sizeof(pckt), 0, 
           (struct sockaddr*) ping_addr, 
            sizeof(*ping_addr)) <= 0)
        {
            printf("\nPacket Sending Failed!\n");
            flag=0;
        }
  
        //receive packet
        addr_len=sizeof(r_addr);
  
        if ( recvfrom(ping_sockfd, &pckt, sizeof(pckt), 0, 
             (struct sockaddr*)&r_addr, &addr_len) <= 0
              && msg_count>1) 
        {
            printf("\nPacket receive failed!\n");
        }
  
        else
        {
            clock_gettime(CLOCK_MONOTONIC, &time_end);
              
            double timeElapsed = ((double)(time_end.tv_nsec - 
                                 time_start.tv_nsec))/1000000.0
            rtt_msec = (time_end.tv_sec-
                          time_start.tv_sec) * 1000.0
                        + timeElapsed;
              
            // if packet was not sent, don't receive
            if(flag)
            {
                if(!(pckt.hdr.type ==69 && pckt.hdr.code==0)) 
                {
                    printf("Error..Packet received with ICMP 
                           type %d code %d\n", 
                           pckt.hdr.type, pckt.hdr.code);
                }
                else
                {
                    printf("%d bytes from %s (h: %s) 
                          (%s) msg_seq=%d ttl=%d 
                          rtt = %Lf ms.\n", 
                          PING_PKT_S, ping_dom, rev_host, 
                          ping_ip, msg_count,
                          ttl_val, rtt_msec);
  
                    msg_received_count++;
                }
            }
        }    
    }
    clock_gettime(CLOCK_MONOTONIC, &tfe);
    double timeElapsed = ((double)(tfe.tv_nsec - 
                          tfs.tv_nsec))/1000000.0;
      
    total_msec = (tfe.tv_sec-tfs.tv_sec)*1000.0+ 
                          timeElapsed
                     
    printf("\n===%s ping statistics===\n", ping_ip);
    printf("\n%d packets sent, %d packets received, %f percent
           packet loss. Total time: %Lf ms.\n\n", 
           msg_count, msg_received_count,
           ((msg_count - msg_received_count)/msg_count) * 100.0,
          total_msec); 
}
  
// Driver Code
int main(int argc, char *argv[])
{
    int sockfd;
    char *ip_addr, *reverse_hostname;
    struct sockaddr_in addr_con;
    int addrlen = sizeof(addr_con);
    char net_buf[NI_MAXHOST];
  
    if(argc!=2)
    {
        printf("\nFormat %s <address>\n", argv[0]);
        return 0;
    }
  
    ip_addr = dns_lookup(argv[1], &addr_con);
    if(ip_addr==NULL)
    {
        printf("\nDNS lookup failed! Could 
                   not resolve hostname!\n");
        return 0;
    }
  
    reverse_hostname = reverse_dns_lookup(ip_addr);
    printf("\nTrying to connect to '%s' IP: %s\n",
                                       argv[1], ip_addr);
    printf("\nReverse Lookup domain: %s", 
                           reverse_hostname);
  
    //socket()
    sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
    if(sockfd<0)
    {
        printf("\nSocket file descriptor not received!!\n");
        return 0;
    }
    else
        printf("\nSocket file descriptor %d received\n", sockfd);
  
    signal(SIGINT, intHandler);//catching interrupt
  
    //send pings continuously
    send_ping(sockfd, &addr_con, reverse_hostname, 
                                 ip_addr, argv[1]);
      
    return 0;
}

An example output:
Run sudo ./ping google.com

Resolving DNS..

Trying to connect to 'google.com' IP: 172.217.27.206

Reverse Lookup domain: bom07s15-in-f14.1e100.net
Socket file descriptor 3 received

Socket set to TTL..
64 bytes from bom07s15-in-f14.1e100.net (h: google.com) (172.217.27.206)
                                msg_seq=1 ttl=64 rtt = 57.320584 ms.

64 bytes from bom07s15-in-f14.1e100.net (h: google.com) (172.217.27.206)
                                msg_seq=2 ttl=64 rtt = 58.666775 ms.

64 bytes from bom07s15-in-f14.1e100.net (h: google.com) (172.217.27.206)
                                msg_seq=3 ttl=64 rtt = 58.081148 ms.

64 bytes from bom07s15-in-f14.1e100.net (h: google.com) (172.217.27.206) 
                                msg_seq=4 ttl=64 rtt = 58.700630 ms.

64 bytes from bom07s15-in-f14.1e100.net (h: google.com) (172.217.27.206) 
                                msg_seq=5 ttl=64 rtt = 58.281802 ms.

64 bytes from bom07s15-in-f14.1e100.net (h: google.com) (172.217.27.206) 
                                msg_seq=6 ttl=64 rtt = 58.360916 ms.

===172.217.27.206 ping statistics===

6 packets sent, 6 packets received, 0.000000 percent packet loss. 
Total time: 6295.187804 ms.

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