为什么我的ICMP Ping TTL参数在我的C代码中不起作用

Question

我写了一个C程序来实现Traceroute，因为我需要有效地获取ip路径数据。我希望当我ping IP时，我可以得到src和dst IP之间的所有Ips。因此我使用TTL来获取中间IP，我想每次增加TTL时，我就可以逐渐获取中间IP。该代码段如下：

for(;ttl_val<20;ttl_val++)
  setsockopt(ping_sockfd, SOL_IP, IP_TTL,&ttl_val, sizeof(ttl_val)

但是当我使用recvfrom获取中间Ips时，我发现它是不正确的，中间Ips总是在变化，并且与我用linux traceroute命令获得的Ips不同。

摘要代码如下：

struct sockaddr_in r_addr;
addr_len=sizeof(r_addr);
if ( recvfrom(ping_sockfd, &pckt, sizeof(pckt), 0,(struct sockaddr*)&r_addr, &addr_len) <= 0)
    printf("\nPacket receive failed!\n");
else
    if(!(pckt.hdr.type ==69 && pckt.hdr.code==0))
        printf("Error..Packet \n");
    else
        //THE MIDDLE IPs were not correct!!!!!!
        printf("=%s\n", inet_ntoa(r_addr.sin_addr));

例如，我的IP为1.1.1.1，我要对目标IP 5.5.5.5进行ping操作，中间IP为：2.2.2.2、3.3.3.3、4.4.4.4，即这些是IP中的路由IP路径。我希望当我设置TTL = 1时我可以得到2.2.2.2，当我设置TTL = 2时我可以得到3.3.3.3，以此类推。我不知道为什么上面的方法不起作用，当我设置TTL时，代码返回的是随机IP，而不是真正的中间IP，因为它们与linux traceroute命令获得的Ips不同。我的整个代码如下：

#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

// 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;



int atoint(char s[])
    int i,n=0;
    for(i=0;s[i]>='0' && s[i]<='9';i++)
        n=10*n+(s[i]-'0');
    
    return n;



// 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)

    int msg_count=0, i, addr_len, flag=1,msg_received_count=0;

    struct ping_pkt pckt;

    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;
        

        struct sockaddr_in r_addr;
        //receive packet
        addr_len=sizeof(r_addr);
//        struct sockaddr_in from;


        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);
                    printf("saddr = %d, %s: %d\n", r_addr.sin_family, inet_ntoa(r_addr.sin_addr), r_addr.sin_port);
                    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!=3)
        printf("\nFormat %s <address> ttl \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

    int ttl = atoint(argv[2]);
    //send pings continuously
    send_ping(sockfd, &addr_con, reverse_hostname,ip_addr, argv[1],ttl);

    return 0;