raw udp client port 50000 unreachable

^C[root@bogon raw-sockets-example]# cat raw_udp_client.c 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <errno.h>
#include <linux/ip.h>
#include <linux/udp.h>

struct sockaddr_in *serveraddr = NULL, *clientaddr;
int sockfd;

#define PORT 50001
#define SERVER_PORT 50000
#define SERVER_ADDR "10.10.16.82"
#define CLIENT_ADDR "10.10.16.81"
/* structure to calculate UDP checksum
* The below members are part of the IP header
* which do not change from the UDP layer and hence
* are used as a part of the UDP checksum */

struct pseudo_iphdr {
        unsigned int source_ip_addr;
        unsigned int dest_ip_addr;
        unsigned char fixed;
        unsigned char protocol;
        unsigned short udp_len;
};

/* checksum code to calculate UDP checksum
* Code taken from Unix network programming – Richard stevens*/

unsigned short in_cksum (uint16_t * addr, int len)
{
        int nleft = len;
        unsigned int sum = 0;
        unsigned short *w = addr;
        unsigned short answer = 0;

        /* Our algorithm is simple, using a 32 bit accumulator (sum), we add
        * sequential 16 bit words to it, and at the end, fold back all the
        * carry bits from the top 16 bits into the lower 16 bits.
        */
        while (nleft > 1) {
                sum += *w++;
                nleft -= 2;
        }

        /* mop up an odd byte, if necessary */
        if (nleft == 1) {
                *(unsigned char *) (&answer) = * (unsigned char *) w;
                sum += answer;
        }

        /* add back carry outs from top 16 bits to low 16 bits */
        sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
        sum += (sum >> 16); /* add carry */
        answer = (unsigned short) ~sum; /* truncate to 16 bits */
        return (answer);
}

/* Interrupt_handler – so that CTRL + C can be used to
* exit the program */
void interrupt_handler (int signum) {
        close(sockfd);
        free(clientaddr);
        exit(0);
}

#if DEBUG
/* print the IP and UDP headers */
void dumpmsg(unsigned char *recvbuffer, int length) {
        int count_per_length = 28, i = 0;
        for (i = 0; i < count_per_length; i++) {
                printf("%02x ", recvbuffer[i]);
        }
        printf("
");
}
#endif

int main () {
        char buffer[1024] = {0};
        unsigned char recvbuffer[1024] = {0};
        int length;
        char *string = "Hello UDP server";
        struct udphdr *udp_hdr = NULL;
        char *string_data = NULL;
        char *recv_string_data = NULL;
        char *csum_buffer = NULL;
        struct pseudo_iphdr csum_hdr;
        int error;

        signal (SIGINT, interrupt_handler);
        signal (SIGTERM, interrupt_handler);

        /* Part 1: create the socket */
        sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_UDP);
        if(0 > sockfd) {
                printf("unable to create socket
");
                exit(0);
        }

        clientaddr = (struct sockaddr_in *)malloc(sizeof(struct sockaddr_in));
        if (clientaddr == NULL) {
                printf("could not allocate memory
");
                goto end;
         }

        clientaddr->sin_family = AF_INET;
        clientaddr->sin_port = PORT;
        clientaddr->sin_addr.s_addr = inet_addr(CLIENT_ADDR);

        /* Part 2 – fill data structure and bind to socket */
        if (0 != (bind(sockfd, (struct sockaddr *)clientaddr, sizeof(struct sockaddr_in)))) {
                printf("could not bind server socket to address
");
                goto end1;
        }

        /* part 3: read and write data */
        serveraddr = (struct sockaddr_in *)malloc(sizeof(struct sockaddr_in));
        if (serveraddr == NULL) {
               printf("Unable to allocate memory to server address socket
");
               goto end2;
        }
        serveraddr->sin_family = AF_INET;
        serveraddr->sin_port = SERVER_PORT;
        serveraddr->sin_addr.s_addr = inet_addr(SERVER_ADDR);

        error = connect(sockfd, (struct sockaddr *)serveraddr, sizeof(struct sockaddr_in));
        if (error != 0) {
                printf("error %d", errno);
                printf("connect returned error
");
                goto end2;
       }

       /* copy the data after the UDP header */
       string_data = (char *) (buffer + sizeof(struct udphdr));
       strncpy(string_data, string, strlen(string));

       /* Modify some parameters to send to client in UDP hdr */

       udp_hdr = (struct udphdr *)buffer;
       udp_hdr->source = htons(PORT);
       udp_hdr->dest = htons(SERVER_PORT);
       udp_hdr->len = htons(sizeof(struct udphdr));

       csum_buffer = (char *)calloc((sizeof(struct pseudo_iphdr) + sizeof(struct udphdr) + strlen(string_data)), sizeof(char));
       if (csum_buffer == NULL) {
               printf("Unable to allocate csum buffer
");
               goto end1;
       }

       csum_hdr.source_ip_addr = inet_addr(CLIENT_ADDR);
       csum_hdr.dest_ip_addr = inet_addr(SERVER_ADDR);
       csum_hdr.fixed = 0;
       csum_hdr.protocol = IPPROTO_UDP; /* UDP protocol */
       csum_hdr.udp_len = htons(sizeof(struct udphdr) + strlen(string_data) + 1);

       memcpy(csum_buffer, (char *)&csum_hdr, sizeof(struct pseudo_iphdr));
       memcpy(csum_buffer + sizeof(struct pseudo_iphdr), buffer, (sizeof(struct udphdr) + strlen(string_data) + 1));

        /* since we are resending the same packet over and over again
        * free the csum buffer here */
        free (csum_buffer);

        while (1) {
                write(sockfd, buffer, sizeof(buffer));

                memset(recvbuffer, 0, sizeof(recvbuffer));
                read(sockfd, recvbuffer, sizeof(recvbuffer));

                udp_hdr = (struct udphdr *)(recvbuffer + sizeof (struct iphdr));
                recv_string_data = (char *) (recvbuffer + sizeof (struct iphdr) + sizeof (struct udphdr));
#if DEBUG
                dumpmsg((unsigned char *)&recvbuffer, (sizeof(struct iphdr) + sizeof(struct udphdr)+ strlen(string_data) + 1));
#endif
                if (PORT == ntohs(udp_hdr->dest)) {
                        printf("udp packet received from port %d
",ntohs(udp_hdr->source));
                        printf("data received from server is : %s
", recv_string_data);
                }
        }

end2:
        free(serveraddr);
end1:
        free(clientaddr);
end:
       close(sockfd);

       return 0;
}

运行

Makefile rawsockets.c raw_udp_client raw_udp_server README.md sniffer sniffer3 sniffer.c
[root@bogon raw-sockets-example]# gcc raw_udp_client.c -o raw_udp_client
[root@bogon raw-sockets-example]# ./raw_udp_client

报文发出去了

ast login: Sun May 16 16:52:01 2021 from 10.10.16.1
[root@bogon ~]# tcpdump -i enahisic2i0 udp and host 10.10.16.82 -nnvvv
tcpdump: listening on enahisic2i0, link-type EN10MB (Ethernet), capture size 262144 bytes
16:56:27.801833 IP (tos 0x0, ttl 64, id 34276, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802049 IP (tos 0x0, ttl 64, id 34277, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802145 IP (tos 0x0, ttl 64, id 34278, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802233 IP (tos 0x0, ttl 64, id 34279, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802314 IP (tos 0x0, ttl 64, id 34280, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802399 IP (tos 0x0, ttl 64, id 34281, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802490 IP (tos 0x0, ttl 64, id 34282, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0

收到端口不存在的icmp通知

tcpdump: listening on enahisic2i0, link-type EN10MB (Ethernet), capture size 262144 bytes
16:56:27.801999 IP (tos 0xc0, ttl 64, id 64114, offset 0, flags [none], proto ICMP (1), length 56)
    10.10.16.82 > 10.10.16.81: ICMP 10.10.16.82 udp port 50000 unreachable, length 36
        IP (tos 0x0, ttl 64, id 34276, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802112 IP (tos 0xc0, ttl 64, id 64115, offset 0, flags [none], proto ICMP (1), length 56)
    10.10.16.82 > 10.10.16.81: ICMP 10.10.16.82 udp port 50000 unreachable, length 36
        IP (tos 0x0, ttl 64, id 34277, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802204 IP (tos 0xc0, ttl 64, id 64116, offset 0, flags [none], proto ICMP (1), length 56)
    10.10.16.82 > 10.10.16.81: ICMP 10.10.16.82 udp port 50000 unreachable, length 36
        IP (tos 0x0, ttl 64, id 34278, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802290 IP (tos 0xc0, ttl 64, id 64117, offset 0, flags [none], proto ICMP (1), length 56)
    10.10.16.82 > 10.10.16.81: ICMP 10.10.16.82 udp port 50000 unreachable, length 36
        IP (tos 0x0, ttl 64, id 34279, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802371 IP (tos 0xc0, ttl 64, id 64118, offset 0, flags [none], proto ICMP (1), length 56)
    10.10.16.82 > 10.10.16.81: ICMP 10.10.16.82 udp port 50000 unreachable, length 36
        IP (tos 0x0, ttl 64, id 34280, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0
16:56:27.802463 IP (tos 0xc0, ttl 64, id 64119, offset 0, flags [none], proto ICMP (1), length 56)
    10.10.16.82 > 10.10.16.81: ICMP 10.10.16.82 udp port 50000 unreachable, length 36
        IP (tos 0x0, ttl 64, id 34281, offset 0, flags [DF], proto UDP (17), length 1044)
    10.10.16.81.50001 > 10.10.16.82.50000: [no cksum] UDP, length 0