Question

我尝试将UDP数据包发送到路由器，其生存时间为1，然后接收ICMP超时回复。到目前为止，我能够发送数据包，但是当我的程序进入执行的recv部分时，它就会挂起。我有一个错误检查recvfrom，但它甚至没有达到。我的电脑正在接收请求。我知道这是因为我在运行程序时运行Wireshark并且我过滤了ICMP请求。每次我运行程序时，都会收到回复。我在recvfrom上做错了什么？

#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几乎忽略了ICMP错误，所以如果你想看到它们，你需要打开一个原始套接字来接收所有 ICMP数据包并查找与你的套接字相关的数据包。

至少在Linux上，另一种方法是设置IP_RECVERR套接字选项。如果这样做，您可以设置recvmsg并设置MSG_ERRQUEUE标志以获取与套接字关联的任何ICMP（或其他）错误。这样做的好处是不需要提升权限或第二个套接字。

Answer 2

打开插座时检查选项。

请参阅How to sniff all ICMP packets using RAW sockets。

请参阅How to receive ICMP request in C with raw sockets。

您可能还希望将套接字选项更改为非阻塞，并使用select()函数确定是否有要读取的内容。

有关使用select()功能的示例，请参阅以下内容。

Blocking recvfrom with select system call

Unexepcted results with select and recvfrom

Answer 3

在套接字的某些实现中，必须连接UDP套接字才能接收错误。

因此，您需要添加connect电话，然后使用send / recv个功能。

我在FreeBSD上证实了这一点。至少有一个消息来源明确指出：

http://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq-5.html（参见5.3。执行connect（）调用会影响套接字的接收行为吗？）

P.S。但请注意，您不会以这种方式收到确切的ICMP错误消息。你只会得到一些错误代码，没有很多细节（如果有的话）。

Answer 4

首先，您的代码具有未定义的行为，因为reply_addr未初始化。你应该先解决这个问题：

struct sockaddr_in reply_addr;

...然后：

recvfrom(recv_sock, icmp_msg, icmp_msg_len, 0, (struct sockaddr*)&reply_addr, 
         &reply_addr_len);

最后，您需要使用原始套接字而不是数据报套接字来接收ICMP数据包：

recv_sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);

C套接字发送UDP并处理来自路由器的ICMP应答

4 个答案: