Simple ping routine, not receiving anything.

Hi,

I'm trying to create a simple ping routine, that can be used in other programs to test internet connectivity. I'm stuck, as the following code seems to send the request, but `recvfrom` blocks forever.

Since I don't normally deal with sockets like this, I'm completely lost as to what is wrong. And I don't think I can look at more code that doesn't satisfy my need.

Here's what I have:

Code:

---

// Declare includes.
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <libgen.h>
#include <netinet/in.h>
#include <netinet/ip_icmp.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

// Declare defines.
#define BUFFER_SIZE      1024
#define PING_ADDRESS        "8.8.8.8"
#define RETURN_FAILURE      0
#define RETURN_SUCCESS      1

// Declare function prototypes.
int pinger (char *address);

int main (int argc, __attribute__ ((unused)) char *argv[])
{
// Check number of arguments.
    if(argc != 1)
    {
        fprintf(stderr, "Usage: %s", program_invocation_short_name);
        exit(EXIT_FAILURE);
    }

// Check that user is root or that sudo is used.
    if(geteuid() != 0)
    {
        fprintf(stderr, "%s: must be run as root or with 'sudo'", program_invocation_short_name);
        exit(EXIT_FAILURE);
    }

// Run process.
    printf("Pinging %s was %s\n", PING_ADDRESS, pinger(PING_ADDRESS) == RETURN_SUCCESS ? "successful." : "unsuccessful.");

// Exit cleanly.
    exit(EXIT_SUCCESS);
}

int pinger (char *address)
{
// Declare variables.
    char buffer[BUFFER_SIZE] = {0};
    struct icmp icmp_hdr = {0};
    struct sockaddr_in dest_addr = {0};
    struct sockaddr_in from_addr = {0};
    socklen_t fromlen = {0};
    int sockfd = {0};

// Populate the icmp structure.
    icmp_hdr.icmp_type = ICMP_ECHO;
    icmp_hdr.icmp_code = 0;
    icmp_hdr.icmp_cksum = 0;
    icmp_hdr.icmp_id = getpid();
    icmp_hdr.icmp_seq = 1;

// Populate the socket address structure.
    dest_addr.sin_family = AF_INET;
    dest_addr.sin_addr.s_addr = inet_addr(address);
    memset(&dest_addr.sin_zero, 0, sizeof(dest_addr.sin_zero));

// Create a raw socket.
    if((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) == -1)
    {
        fprintf(stderr, "%s: %s error: socket failed (%s)", program_invocation_short_name, __func__, strerror(errno));
        return(RETURN_FAILURE);
    }

// Send icmp request.
    if(sendto(sockfd, &icmp_hdr, sizeof(struct icmp), 0, (struct sockaddr *) &dest_addr, sizeof(struct sockaddr_in)) == -1)
    {
        fprintf(stderr, "%s: %s error: sendto failed (%s)", program_invocation_short_name, __func__, strerror(errno));
        return(RETURN_FAILURE);
    }

// Receive icmp response.
    if(recvfrom(sockfd, buffer, BUFFER_SIZE, 0, (struct sockaddr *) &from_addr, &fromlen) == -1)
    {
        fprintf(stderr, "%s: %s error: recvfrom failed (%s)", program_invocation_short_name, __func__, strerror(errno));
        return(RETURN_FAILURE);
    }

// Return success.
    return(RETURN_SUCCESS);
}

---

Any help or insight is greatly appreciated. Ty.

When compiling with gcc on Linux, I get the following errors:

Code:

---

com bar.c
bar.c: In function ‘main’:
bar.c:28:38: error: ‘program_invocation_short_name’ undeclared (first use in this function)
  28 |        fprintf(stderr, "Usage: %s", program_invocation_short_name);
      |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bar.c:28:38: note: each undeclared identifier is reported only once for each function it appears in
bar.c: In function ‘pinger’:
bar.c:50:12: error: variable ‘icmp_hdr’ has initializer but incomplete type
  50 |    struct icmp icmp_hdr = {0};
      |            ^~~~
bar.c:50:29: warning: excess elements in struct initializer
  50 |    struct icmp icmp_hdr = {0};
      |                            ^
bar.c:50:29: note: (near initialization for ‘icmp_hdr’)
bar.c:50:17: error: storage size of ‘icmp_hdr’ isn’t known
  50 |    struct icmp icmp_hdr = {0};
      |                ^~~~~~~~
bar.c:71:61: error: ‘program_invocation_short_name’ undeclared (first use in this function)
  71 |        fprintf(stderr, "%s: %s error: socket failed (%s)", program_invocation_short_name, __func__, strerror(errno));
      |                                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bar.c:76:41: error: invalid application of ‘sizeof’ to incomplete type ‘struct icmp’
  76 |    if(sendto(sockfd, &icmp_hdr, sizeof(struct icmp), 0, (struct sockaddr *) &dest_addr, sizeof(struct sockaddr_in)) == -1)
      |                                        ^~~~~~
bar.c:50:17: warning: unused variable ‘icmp_hdr’ [-Wunused-variable]
  50 |    struct icmp icmp_hdr = {0};
      |                ^~~~~~~~

---

Clear these warnings/errors first. Turn on warnings options and turn up to the highest level in whatever O/S & Compiler you are using.

Quote:

---

Originally Posted by rstanley

When compiling with gcc on Linux, I get the following errors:
Clear these warnings/errors first. Turn on warnings options and turn up to the highest level in whatever O/S & Compiler you are using.

---

Sorry, I don't get those errors, but it's because of my gcc compile options. I'm compiling with:

Code:

---

gcc -Wall -Werror -Wextra -Wfatal-errors -D_GNU_SOURCE -O3 -o ./pinger pinger.c

---

I just noticed something strange. I ran the program, and while it was hanging and not responding, I opened a terminal and ran `ping 8.8.8.8`. While ping ran, my program actually returned 'successful.'

So I'm thinking that my code might be correct, it's just missing something. But what?

Back to view more code... :(

I think you need to set icmp_cksum properly.
Ping in C - GeeksforGeeks

Quote:

---

Originally Posted by john.c

I think you need to set icmp_cksum properly.
Ping in C - GeeksforGeeks

---

I did come across that link, but the site clearly doesn't check it's work once posted. I found a bunch of problems with this code. For example, the following function:

Code:

---

// 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 & gt; 1; len -= 2)

        sum += *buf++;

    if (len == 1)

        sum += *(unsigned char*)buf;

    sum = (sum & gt; > 16) + (sum & amp; 0xFFFF);

    sum += (sum & gt; > 16);

    result = ~sum;

    return result;

 }

---

I'm not great with `ones' compliments`, so when I come across `sum += (sum & gt; > 16);` or this `sum = (sum & gt; > 16) + (sum & amp; 0xFFFF);`, it kills my spirit and I must stop looking at it.

I'll try to decipher this, but I'm not holding hope. Thanks for your reply.

I hadn't looked at it closely, but it clearly has some issues (> is how you write > in html, as you probably know). It does make me wonder if it's even a (robustly) correct calculation. Hers's the actual RFC: https://www.ietf.org/rfc/rfc1071.txt

I have seen the following in some code, but it doesn't seem to change/fix anything.

Code:

---

icmp_hdr.icmp_cksum = htons(~(ICMP_ECHO << 8));

---

I'll give the RFC a look.

Quote:

---

Originally Posted by Yonut

I just noticed something strange. I ran the program, and while it was hanging and not responding, I opened a terminal and ran `ping 8.8.8.8`. While ping ran, my program actually returned 'successful.'

---

Why would the program only work once a proper ping is issued? Why would the second ping correct the checksum of the original ping? I'm thinking this might have something to do with bind.

Quote:

---

Originally Posted by Yonut

Why would the program only work once a proper ping is issued? Why would the second ping correct the checksum of the original ping? I'm thinking this might have something to do with bind.

---

Perhaps your program was waiting for a response and stole the response from the other ping program (if that's possible).
Here's an implementation based on the RFC example C code that might work:

Code:

---

#include <stdio.h>
#include <inttypes.h>

// Compute Internet Checksum for "count" bytes
// beginning at location "addr".
uint16_t checksum(void* addr, int count)
{
    uint16_t* buf = addr;
    uint32_t sum = 0;

    for ( ; count > 1; count -= 2)
        sum += *(uint16_t*)buf++;

    //  Add left-over byte, if any
    if (count > 0)
        sum += *(unsigned char*)addr;

    //  Fold 32-bit sum to 16 bits
    while (sum >> 16)
        sum = (sum & 0xffff) + (sum >> 16);

    return (uint16_t)~sum;
}

int main() {
    // ICMP_ECHO == 8
    char dat[] = "/x08/x00/x00/x00/x12/x34/x00/x00";
    uint16_t cs = checksum(dat, 8);
    printf("%04" PRIX16 "\n", cs); // yields A740
    return 0;
}

---

Looking at this again, I see that I accidentally used / instead of \ for my hex characters in the string (!), and also since my machine is little-endian I need to swap the bytes at the end of the calculation. This gets the correct result for the first example in the RFC.

Code:

---

#include <stdio.h>
#include <inttypes.h>

// Compute Internet Checksum for "count" bytes
// beginning at location "addr".
uint16_t checksum(void* addr, unsigned count)
{
    uint16_t* buf = addr;
    uint32_t sum = 0;

    for ( ; count > 1; count -= 2)
        sum += *buf++;

    //  Add left-over byte, if any
    if (count > 0)
        sum += *(unsigned char*)addr;

    //  Fold 32-bit sum to 16 bits
    while (sum >> 16)
        sum = (sum & 0xffff) + (sum >> 16);

    // swap bytes since my machine is little endian
    uint16_t result = (uint16_t)~sum;
    result = (result << 8) | ((result >> 8) & 0xFF);

    return result;
}

int main() {
    char dat[] = "\x45\x00\x00\x73\x00\x00\x40\x00\x40\x11"
                "\x00\x00\xc0\xa8\x00\x01\xc0\xa8\x00\xc7";
    int len = sizeof dat - 1; // -1 for \0 byte added to end of string

    uint16_t cs = checksum(dat, len);
    printf("%04" PRIX16 "\n", cs);  // yields B861

    return 0;
}

---

The first function you posted didn't seem to help. So I threw in the towel.

During my searches the following became clear, regarding what I was trying to do...
1) ICMP request are the lowest priority, has to wait for everything else.
2) ICMP requires root access.
3) Just too many variables as to what I m supposed to do.

So I copped out and just wrote a simple http tester that checks for access to my service providers DNS server. Super quick, and it solves the 3 problems listed above.

Code:

---

int test_internet_conectivity (void)
{
// Declare variables.
    struct addrinfo hints = {0};
    struct addrinfo *res = NULL;
    int ret = {0};
    int sockfd = {0};

// Populate the structure.
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;

// Get network address translation.
    if((ret = getaddrinfo(IP_ADDRESS, "http", &hints, &res)) != 0)
    {
        fprintf(stderr, "%s: %s error: getaddrinfo failed (%s) (%s)", program_invocation_short_name, __func__, gai_strerror(ret), IP_ADDRESS);
        return(RETURN_FAILURE);
    }

// Open the socket.
    if((sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol)) == -1)
    {
        fprintf(stderr, "%s: %s error: socket failed (%s)", program_invocation_short_name, __func__, strerror(errno));
// Free allocated memory.
        freeaddrinfo(res);
        return(RETURN_FAILURE);
    }

// Connect to web server.
    if(connect(sockfd, res->ai_addr, res->ai_addrlen) == -1)
    {
        fprintf(stderr, "%s: %s error: connect failed (%s) (%s)", program_invocation_short_name, __func__, strerror(errno), IP_ADDRESS);
// Free allocated memory and close the socket.
        freeaddrinfo(res);
        close(sockfd);
        return(RETURN_FAILURE);
    }

// Free allocated memory and close the socket.
    freeaddrinfo(res);
    close(sockfd);
// Alert of success.
    return(RETURN_SUCCESS);
}

---

John, I will try your new function a little later, but I need let my brain decompress from today's events.

I see you may not want this anymore, but had a little time to look at it today and I think the following works.

The dump of the returned message looks like this:

Code:

---

45 00 00 30 00 00 00 00 3C 01
AC D1 08 08 08 08 C0 A8 01 44
00 00 B8 F0 46 0F 01 00 00 00
00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

I believe it is interpreted like this:

Version/IHL:    45          IPv4; length 5 * 32-bits = 20 bytes
ToS:            00          Type of Service (low priority)
Length:        00 30        Total length: 48 bytes (big endian)
Identification: 00 00
flags & offset: 00 00
TTL:            3C          Time To Live (max hops before discarding)
Protocol:      01
Checksum:      AC D1
Source IP:      08 08 08 08
Destination IP: 12 34 56 78  my address, which I've changed

Type:          00          ICMP_ECHOREPLY
Code:          00
Checksum:      B8 F0
Data:
  ID:          46 0F        this is the PID we sent
  Sequence:    01 00        the sequence number we sent
Payload:        00 00 00 00 00 00 00 00 00 00
                00 00 00 00 00 00 00 00 00 00  dummy 20-byte payload

---

Code:

---

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <fcntl.h>
#include <libgen.h>
#include <netinet/in.h>
#include <netinet/ip_icmp.h>
#include <sys/socket.h>
 
#define BUFFER_SIZE  1024
#define PING_ADDRESS  "8.8.8.8"
 
bool pinger (char *address);
 
int main()
{
    // Check that user has root access.
    if (geteuid() != 0) {
        fprintf(stderr, "%s: must be run as root or with 'sudo'\n",
            program_invocation_short_name);
        exit(EXIT_FAILURE);
    }
 
    printf("Pinging %s was %ssuccessful.\n", PING_ADDRESS,
        pinger(PING_ADDRESS) ? "" : "un");

    return 0;
}

uint16_t checksum(void* addr, unsigned count)
{
    uint16_t* buf = addr;
    uint32_t sum = 0;

    for ( ; count > 1; count -= 2)
        sum += *buf++;

    //  Add left-over byte, if any
    if (count > 0)
        sum += *(unsigned char*)addr;

    //  Fold 32-bit sum to 16 bits
    while (sum >> 16)
        sum = (sum & 0xffff) + (sum >> 16);

    return (uint16_t)~sum;
}

bool do_error(const char *msg) {
    fprintf(stderr, "%s: %s error: %s (%s)",
        program_invocation_short_name, __func__, msg, strerror(errno));
    return false;
}

bool pinger (char *address)
{
    char buffer[BUFFER_SIZE] = {0};
    struct icmp icmp_hdr = {0};
    struct sockaddr_in dest_addr = {0};
    struct sockaddr_in from_addr = {0};
    socklen_t fromlen = {0};
    int sockfd = {0};
 
    icmp_hdr.icmp_type = ICMP_ECHO;
    icmp_hdr.icmp_code = 0;
    icmp_hdr.icmp_cksum = 0;
    icmp_hdr.icmp_id = getpid();
    icmp_hdr.icmp_seq = 1;

    printf("PID: %04X\n", (unsigned)icmp_hdr.icmp_id);

    icmp_hdr.icmp_cksum = checksum(&icmp_hdr, sizeof icmp_hdr);
 
    dest_addr.sin_family = AF_INET;
    dest_addr.sin_addr.s_addr = inet_addr(address);
    memset(&dest_addr.sin_zero, 0, sizeof(dest_addr.sin_zero));
 
    // Create a raw socket.
    if ((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) == -1)
        return do_error("socket failed");

    // Send icmp request.
    if (sendto(sockfd, &icmp_hdr, sizeof(struct icmp), 0,
        (struct sockaddr *) &dest_addr, sizeof(struct sockaddr_in)) == -1)
        return do_error("sendto failed");

    // Receive icmp response.
    ssize_t len = recvfrom(sockfd, buffer, BUFFER_SIZE, 0,
                          (struct sockaddr *) &from_addr, &fromlen);
    if (len == -1)
        return do_error("recvfrom failed");
 
    for (ssize_t i = 0; i < len; ++i)
        printf("%02X ", (unsigned char)buffer[i]);
    printf("\n");

    return true;
}

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