第3章 IP路由与三层转发

struct fib_table {
    struct hlist_node tb_hlist;
    u32 tb_id;
    int tb_default;
    int tb_num_default;
    unsigned long tb_data[0];
};

struct fib_info {
    struct hlist_node fib_hash;
    struct hlist_node fib_lhash;
    int fib_treeref;
    atomic_t fib_clntref;
    unsigned int fib_flags;
    unsigned char fib_dead;
    unsigned char fib_protocol;
    unsigned char fib_scope;
    unsigned char fib_type;
    __be32 fib_prefsrc;
    u32 fib_priority;
    u32 fib_metrics[RTAX_MAX];
    struct fib_nh fib_nh[0];
};

static int fib_table_lookup(struct fib_table *tb, const struct flowi4 *flp,
                           struct fib_result *res, int fib_flags)
{
    struct trie *t = (struct trie *) tb->tb_data;
    struct tnode *n;
    struct fib_result res;
    
    // 最长前缀匹配算法
    n = trie_lookup(t, flp->daddr);
    if (n) {
        res = n->res;
        return 0;
    }
    
    return -ENOENT;
}

struct fib_rule {
    struct list_head list;
    atomic_t refcnt;
    int ifindex;
    char ifname[IFNAMSIZ];
    u32 mark;
    u32 mark_mask;
    u32 priority;
    u32 flags;
    u32 table;
    u8 action;
    u8 l3mdev;
    struct fib_rule __rcu *next;
    struct net *fr_net;
};

// route_table.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <net/route.h>
#include <netinet/in.h>

int main() {
    int sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock < 0) {
        perror("socket");
        return 1;
    }
    
    char buffer[8192];
    struct rt_msghdr *rtm;
    struct sockaddr_in *sin;
    
    // 读取路由表
    int len = read(sock, buffer, sizeof(buffer));
    if (len < 0) {
        perror("read");
        close(sock);
        return 1;
    }
    
    rtm = (struct rt_msghdr *)buffer;
    
    printf("Destination     Gateway         Flags    Refs    Use    Interface\n");
    printf("-------------   -------------   ------   ----    ---    ---------\n");
    
    // 解析路由条目
    while (len > 0) {
        if (rtm->rtm_type == RTM_GET) {
            sin = (struct sockaddr_in *)(rtm + 1);
            char dst[INET_ADDRSTRLEN];
            char gw[INET_ADDRSTRLEN];
            
            inet_ntop(AF_INET, &sin->sin_addr, dst, INET_ADDRSTRLEN);
            
            if (rtm->rtm_flags & RTF_GATEWAY) {
                sin = (struct sockaddr_in *)((char *)sin + 
                       ((sin->sin_len + 3) & ~3));
                inet_ntop(AF_INET, &sin->sin_addr, gw, INET_ADDRSTRLEN);
            } else {
                strcpy(gw, "*");
            }
            
            printf("%-15s %-15s %-6x %-4d %-6d %s\n",
                   dst, gw, rtm->rtm_flags, rtm->rtm_refcnt, 
                   rtm->rtm_use, "eth0");
        }
        
        len -= rtm->rtm_msglen;
        rtm = (struct rt_msghdr *)((char *)rtm + rtm->rtm_msglen);
    }
    
    close(sock);
    return 0;
}

// simple_ping.c
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>

unsigned short checksum(void *b, int len) {
    unsigned short *buf = b;
    unsigned int sum = 0;
    unsigned short result;
    
    while (len > 1) {
        sum += *buf++;
        len -= 2;
    }
    
    if (len == 1) {
        sum += *(unsigned char*)buf << 8;
    }
    
    while (sum >> 16) {
        sum = (sum & 0xFFFF) + (sum >> 16);
    }
    
    result = ~sum;
    return result;
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: %s <host>\n", argv[0]);
        return 1;
    }
    
    int sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
    if (sock < 0) {
        perror("socket");
        return 1;
    }
    
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    inet_pton(AF_INET, argv[1], &addr.sin_addr);
    
    struct icmp icmp_pkt;
    icmp_pkt.icmp_type = ICMP_ECHO;
    icmp_pkt.icmp_code = 0;
    icmp_pkt.icmp_cksum = 0;
    icmp_pkt.icmp_id = getpid();
    icmp_pkt.icmp_seq = 1;
    
    icmp_pkt.icmp_cksum = checksum(&icmp_pkt, sizeof(icmp_pkt));
    
    if (sendto(sock, &icmp_pkt, sizeof(icmp_pkt), 0, 
               (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        perror("sendto");
        close(sock);
        return 1;
    }
    
    printf("PING %s: %d data bytes\n", argv[1], sizeof(icmp_pkt));
    
    char buffer[1024];
    struct sockaddr_in from;
    socklen_t fromlen = sizeof(from);
    
    if (recvfrom(sock, buffer, sizeof(buffer), 0, 
                 (struct sockaddr*)&from, &fromlen) < 0) {
        perror("recvfrom");
        close(sock);
        return 1;
    }
    
    struct iphdr *ip = (struct iphdr*)buffer;
    struct icmp *icmp = (struct icmp*)(buffer + (ip->ihl << 2));
    
    if (icmp->icmp_type == ICMP_ECHOREPLY) {
        printf("64 bytes from %s: icmp_seq=1 time=0.001 ms\n", argv[1]);
    }
    
    close(sock);
    return 0;
}