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.netThis 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:- Set packet header type to ICMP_ECHO.
- Set id to pid of process
- Fill msg part randomly.
- 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;
}
chevron_rightfilter_noneAn example output:
Run sudo ./ping google.comResolving 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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- Set the ttl option to a value in the socket