我将ICMP重定向数据包发送到我的IP地址，但我的IP路由表没有更改

Question

#include<stdlib.h>
#include<sys/socket.h>
#include<netinet/ip_icmp.h>
#include<netinet/in.h>
#include<netinet/udp.h>
#include<netinet/ip.h>
void send_icmptime(int sockfd, struct sockaddr *sa, socklen_t len);
uint16_t in_cksum(uint16_t *addr, int len);
#define MAXLINE 1024
void main()
{   
    int sockfd;
    struct sockaddr_in sin;
    sin.sin_family = AF_INET;
    struct addrinfo *ai;
    int on =1;
    inet_pton(AF_INET, "186.9.9.9", &sin.sin_addr);
    if( (sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0)
        perror("socket");
    if( setsockopt(sockfd, IPPROTO_IP, IP_HDRINCL, &on, sizeof(int)) < 0)
        perror("setsockopt");
    send_icmptime(sockfd, (struct sockaddr *)&sin, sizeof(sin));
}
void send_icmptime(int sockfd, struct sockaddr *s, socklen_t len)
{


    struct in_addr myaddr;
    inet_pton(AF_INET, "172.21.203.91", &myaddr);
    struct icmp *icmp;
    struct timeval val;
    struct ip *ip1;

    ip1 = (struct ip *)malloc(56);
    ip1->ip_v = 4;
    ip1->ip_hl = 5;
    ip1->ip_tos = 0;
    ip1->ip_len = 56;   
    ip1->ip_id = 0;
    ip1->ip_off = 0;
    ip1->ip_ttl = 64;
    ip1->ip_p = IPPROTO_ICMP;
    ip1->ip_sum = 0;
    inet_pton(AF_INET, "172.21.203.91", &ip1->ip_src);
    inet_pton(AF_INET, "61.135.169.125", &ip1->ip_dst);


    icmp = (struct icmp *)((char *)ip1 + 20);
    icmp->icmp_type = ICMP_REDIRECT;
    icmp->icmp_code = ICMP_REDIRECT_HOST;
    icmp->icmp_cksum = 0;
    icmp->icmp_gwaddr = myaddr;

    struct ip *ip;
    ip = (struct ip *)((char *)icmp + 8);
    ip->ip_v = 4;
    ip->ip_hl = 5;
    ip->ip_tos = 0;
    ip->ip_len = htons(55); 
    ip->ip_id = 0;
    ip->ip_off = 0;
    ip->ip_ttl = 54;
    ip->ip_p = IPPROTO_UDP;
    ip->ip_sum = 0;
    inet_pton(AF_INET, "172.31.0.1", &ip->ip_src);
    inet_pton(AF_INET, "172.21.202.91", &ip->ip_dst);
    ip->ip_sum = in_cksum((u_short *)ip, 20);

    struct udphdr *udp;
    udp = (struct udphdr*)((char *)ip + 20);
    udp->uh_sport =  6666;
    udp->uh_dport =  6666;  
    udp->uh_ulen = htons(55);
    udp->uh_sum = in_cksum((u_short *)udp, 8);
    icmp->icmp_cksum = in_cksum((u_short *)icmp, 36);
    if(sendto(sockfd, ip1, 56, 0 ,s, len) < 0)
        perror("sendto");
}
uint16_t in_cksum(uint16_t *addr, int len)
{
    int nleft = len; 
    uint32_t sum = 0;
    uint16_t *w = addr;
    uint16_t answer = 0;
    while(nleft > 1)
    {
        sum += *w++;
        nleft -= 2;
    }
    if(nleft == 1)
    {
        *(unsigned char *)(&answer) = *(unsigned char *)w;
        sum += answer;
    }
    sum = ( sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    answer =~sum;
    return answer;

}

root@zz-E431:/home/zz# tcpdump -i ppp0 icmp tcpdump:已抑制详细输出，使用-v或-vv进行完整协议解码侦听ppp0，链路类型LINUX_SLL (Linux已完成)，捕获大小65535字节13:01:46.891807 IP 172.31.0.1 > 172.21.202.91: ICMP重定向61.135.169.125到主机172.21.203.91，长度36

Answer 1

您可能需要配置ICMP参数。您是否更改了处理ICMP的任何内核默认值？正如http://man7.org/linux/man-pages/man7/icmp.7.html所说：

   ICMP_REDIRECT packets are not sent when Linux is not acting as a
   router.  They are also accepted only from the old gateway defined in
   the routing table and the redirect routes are expired after some
   time.

可以在/proc下更改ICMP设置请阅读https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt，特别是accept_redirects和secure_redirects。后者在默认情况下禁止从除网关(在路由表中)以外的任何位置进行重定向。

Answer 2

Hi @ZhangZhao:上面列出的代码是否正在向您子网中的主机发送icmp重定向报文？您是否在linux内核文件系统中的文件"proc/sys/net/ipv4/conf/all/accept_redirects = 1“下启用了icmp重定向。您可以将ipv4/conf/all文件夹中所有文件的accept_redirects设置为1，然后检查