C套接字发送UDP并处理来自路由器的ICMP答复

Question

我正在尝试将UDP数据包发送到生存时间为1的路由器，然后收到ICMP时间超限答复。到目前为止，我已经能够发送数据包，但是当我的程序进入执行的recv部分时，它只是挂起。我对recvfrom进行了错误检查，但甚至没有做到这一点。我的计算机正在接收请求。我知道这是因为我在运行程序时运行Wireshark，并为ICMP请求筛选。每次我运行程序时，都会收到答复。recvfrom我在做什么错？

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <errno.h>

#define UNSPEC_PROTO 0

int main(int argc, const char *argv[])
{ 
if (argc != 2) {
    printf("usage: routetracer <ip address or hostname>\n");
    return -1;
}

struct addrinfo hints; //params for ret val of getaddrinfo
struct addrinfo* ret; //return value of getaddrinfo
struct sockaddr* reply_addr;
char ipv4[INET_ADDRSTRLEN];
char* msg = "THE PORT IS OVER 9000!!!!";
int status = 0;
int ttl = 0;
int src_sock = 0;
int recv_sock = 0;
socklen_t reply_addr_len = sizeof(struct sockaddr);
const char* dest_port = "9001";
int icmp_msg_len = 100;
char icmp_msg[icmp_msg_len];

//define what we want from getaddrinfo
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET; //IPv4
hints.ai_socktype = SOCK_DGRAM; //UDP packets

//call getaddrinfo to fill ret, w/ error chk
if ((status = getaddrinfo(argv[1], dest_port, &hints, &ret)) != 0) {
    printf("getaddrinfo: %s\n", gai_strerror(status));
    return -1;
}

//extract IPv4 address from ret
struct sockaddr_in* ip = (struct sockaddr_in *)ret->ai_addr;

//convert address from pure numbers to something easier to read
inet_ntop(ret->ai_family, &(ip->sin_addr), ipv4, INET_ADDRSTRLEN);

//kindly inform the user of which hostname they are connecting to
printf("Route for: %s\n", ipv4);

//create a socket for our machine
if ((src_sock = socket(ret->ai_family, ret->ai_socktype, 
                ret->ai_protocol)) < 0) {
    fprintf(stderr, "Error creating host socket: %s\n", strerror(errno));
    return -1;
}

//create a socket to recv icmp packet from hops 
if ((recv_sock = socket(AF_INET, SOCK_DGRAM, UNSPEC_PROTO)) < 0){
    fprintf(stderr, "Error creating recv socket: %s\n", strerror(errno));
}

/*
 * We go from hop to hop by incrementing the time to live in the IP header
 * for each hop we visit until we reach the destination IP address (which we
 * already have). Time to live decrements for every hop, so once it reaches
 * zero we report the IP address of the node we are connected to.
 */

//while(test_ip != dest_ip)
//time_to_live++
//send_to(dest_addr)
//receive icmp error message
//get src addr of error msg from ip header of icmp
//test_ip = src addr
/*
while (last_hop == 0) {
    ttl++;
    setsockopt(sock, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
    sendto(sock, msg, strlen(msg), 0, (struct sockaddr *)ip, sizeof(ip));
}
*/

ttl = 1;
if (!(setsockopt(src_sock, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl)))) {
    printf("TTL set successfully\n");
} else {
    printf("Error setting TTL: %s\n", strerror(errno));
}

if ((sendto(src_sock, msg, strlen(msg), 0, ret->ai_addr, 
                ret->ai_addrlen)) > 0) {
    printf("msg sent successfully\n");
} else {
    fprintf(stderr, "Error sending msg: %s\n", strerror(errno));
}

if ((recvfrom(recv_sock, icmp_msg, icmp_msg_len, 0, reply_addr, 
                &reply_addr_len)) != -1) {
    /* PROCESS THE INFORMATION */
    printf("Packet received\n");
} else {
    fprintf(stderr, "Error receiving packet: %s\n", strerror(errno));
}

return 0;
}

Answer 1

通常，UDP几乎会忽略ICMP错误，因此，如果要查看它们，则需要打开一个原始套接字以接收所有 ICMP数据包并查找与该套接字相关的数据包。

至少在Linux上，一种替代方法是设置IP_RECVERR套接字选项。如果这样做，则可以recvmsg对MSG_ERRQUEUE标志进行设置以获取与套接字相关的任何ICMP（或其他）错误。这样的优点是不需要提升的特权或第二个套接字。