c - SOCK_RAW 套接字中缺少 ARP 数据包

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <linux/if_ether.h>
#include <net/if.h>
#include <netpacket/packet.h>

struct ethernet {
    unsigned char dest[6];
    unsigned char source[6];
    uint16_t eth_type;
};

struct arp {
    uint16_t htype;
    uint16_t ptype;
    unsigned char hlen;
    unsigned char plen;
    uint16_t oper;
    /* addresses */
    unsigned char sender_ha[6];
    unsigned char sender_pa[4];
    unsigned char target_ha[6];
    unsigned char target_pa[4];
};

#define ETH_HDR_LEN 14
#define BUFF_SIZE 2048

#define ARP_PROTO 0x0806

static void dump_arp(struct arp *arp_hdr);

int main(void)
{
    int sock, err;
    void *buffer = NULL;
    ssize_t recvd_size;
    struct sockaddr_ll s_ll;
    struct ethernet *eth_hdr = NULL;
    struct arp *arp_hdr = NULL;

    if( (sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ARP))) == -1)
    // if( (sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))) == -1)
    {
        perror("socket(): ");
        exit(-1);
    }
    s_ll.sll_family = AF_PACKET;
    s_ll.sll_protocol = htons(ETH_P_ARP); 
    //s_ll.sll_protocol = htons(ETH_P_ARP);
    s_ll.sll_ifindex = 0; // all ifaces 
    //s_ll.sll_ifindex = 2 // eth0
    if( (err = bind(sock, (struct sockaddr *)&s_ll, sizeof(s_ll))) == -1)
    {
        perror("bind(): ");
        exit(-1);
    }

    buffer = malloc(BUFF_SIZE);
    while(1)
    {
        if( (recvd_size = recv(sock, buffer, BUFF_SIZE, 0)) == -1)
        {
            perror("recv(): ");
            free(buffer);
            close(sock);
            exit(-1);
        }
        if(recvd_size <= (sizeof(struct ethernet) + sizeof(struct arp)))
        {
            printf("Short packet. Packet len: %d\n", recvd_size);
            continue;
        }
        eth_hdr = (struct ethernet *)buffer;
        if(ntohs(eth_hdr->eth_type) != ARP_PROTO)
            continue;
        arp_hdr = (struct arp *)(buffer+ETH_HDR_LEN);
        dump_arp(arp_hdr);
    }
    free(buffer);
    close(sock);
}

static void
dump_arp(struct arp *arp_hdr)
{
    uint16_t htype = ntohs(arp_hdr->htype);
    uint16_t ptype = ntohs(arp_hdr->ptype);
    uint16_t oper = ntohs(arp_hdr->oper);
    switch(htype)
    {
        case 0x0001:
            printf("ARP HTYPE: Ethernet(0x%04X)\n", htype);
            break;
        default:
            printf("ARP HYPE: 0x%04X\n", htype);
            break;
    }
    switch(ptype)
    {
        case 0x0800:
            printf("ARP PTYPE: IPv4(0x%04X)\n", ptype);
            break;
        default:
            printf("ARP PTYPE: 0x%04X\n", ptype);
            break;
    }
    printf("ARP HLEN: %d\n", arp_hdr->hlen);
    printf("ARP PLEN: %d\n", arp_hdr->plen);
    switch(oper)
    {
        case 0x0001:
            printf("ARP OPER: Request(0x%04X)\n", oper);
            break;
        case 0x0002:
            printf("ARP OPER: Response(0x%04X)\n", oper);
            break;
        default:
            printf("ARP OPER: 0x%04X\n", oper);
            break;
    }
    printf("ARP Sender HA: %02X:%02X:%02X:%02X:%02X:%02X\n",
           arp_hdr->sender_ha[0],arp_hdr->sender_ha[1],arp_hdr->sender_ha[2],
           arp_hdr->sender_ha[3], arp_hdr->sender_ha[4], arp_hdr->sender_ha[5]);
    printf("ARP Sender PA: %d.%d.%d.%d\n", arp_hdr->sender_pa[0],
           arp_hdr->sender_pa[1], arp_hdr->sender_pa[2], arp_hdr->sender_pa[3]);
    printf("ARP Target HA: %02X:%02X:%02X:%02X:%02X:%02X\n",
           arp_hdr->target_ha[0],arp_hdr->target_ha[1],arp_hdr->target_ha[2],
           arp_hdr->target_ha[3], arp_hdr->target_ha[4], arp_hdr->target_ha[5]);
    printf("ARP Target PA: %d.%d.%d.%d\n", arp_hdr->target_pa[0],
           arp_hdr->target_pa[1], arp_hdr->target_pa[2], arp_hdr->target_pa[3]);
    printf("ARP DONE =====================\n");
}

我使用 AF_PACKET、SOCK_RAW、ETH_P_ARP 参数创建 socket()。并在所有接口(interface)上使用 args(AF_PACKET, ETH_P_ARP) bind() 它(无关紧要)。因此，所有 ARP 数据包都必须流经此套接字。

我的主机:192.168.1.2 远程主机:192.168.1.7，主机不包含192.167.1.7的ARP记录。

程序输出，当我 ping 192.168.1.7 时:

...ARP OPER: Response(0x0002)ARP Sender HA: 50:67:F0:94:70:F5ARP Sender PA: 192.168.1.7ARP Target HA: 00:22:15:A2:D0:C5ARP Target PA: 192.168.1.2ARP DONE =====================...ARP OPER: Request(0x0001)ARP Sender HA: 50:67:F0:94:70:F5ARP Sender PA: 192.168.1.7ARP Target HA: 00:00:00:00:00:00ARP Target PA: 192.168.1.2ARP DONE =====================

我的套接字只有 2 个数据包，共 4 个(my_host 请求和 my_host 响应丢失)。 tcpdump -n -p -i eth0 arp 显示所有 4 个数据包。

如果我在 socket() 和 bind() 中将 ETH_P_ARP 更改为 ETH_P_ALL，那么所有 4 个数据包都会进入套接字(使用 IP 和其他)。

为什么？如何解决？

附言。请告诉我一些邮件列表的地址，我可以在那里询问这种行为。

Answer 1

有点迟到的答案，但我为了好玩而尝试了这个。也许一些 googler/duckduckgoer 可以受益。

我的建议是使用 ETH_P_ALL 接收所有数据包，然后使用 Linux 套接字过滤器对其进行过滤，以便应用程序仅接收请求的 ARP 数据包。

这是我的代码。较大的更改标有 CHANGE 注释

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <linux/if_ether.h>
#include <net/if.h>
#include <netpacket/packet.h>
#include <linux/filter.h> // CHANGE: include lsf

struct ethernet {
    unsigned char dest[6];
    unsigned char source[6];
    uint16_t eth_type;
};

struct arp {
    uint16_t htype;
    uint16_t ptype;
    unsigned char hlen;
    unsigned char plen;
    uint16_t oper;
    /* addresses */
    unsigned char sender_ha[6];
    unsigned char sender_pa[4];
    unsigned char target_ha[6];
    unsigned char target_pa[4];
};

#define ETH_HDR_LEN 14
#define BUFF_SIZE 2048

/* CHANGE
   Linux socket filters use the Berkeley packet filter syntax. 
   This was adapted from BSDs "man 4 bpf" example for RARP.
*/
struct sock_filter arpfilter[] = {
    BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12), /* Skip 12 bytes */
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETH_P_ARP, 0, 1), /* if eth type != ARP
                                                         skip next instr. */
    BPF_STMT(BPF_RET+BPF_K, sizeof(struct arp) +
                 sizeof(struct ethernet)),
    BPF_STMT(BPF_RET+BPF_K, 0), /* Return, either the ARP packet or nil */
};

static void dump_arp(struct arp *arp_hdr);

int main(void)
{
    int sock;
    void *buffer = NULL;
    ssize_t recvd_size;
    struct ethernet *eth_hdr = NULL;
    struct arp *arp_hdr = NULL;
    struct sock_filter *filter;
    struct sock_fprog  fprog;

    if( (sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))) == -1)
    {
        perror("socket(): ");
        exit(-1);
    }

    /* CHANGE prepare linux packet filter */
    if ((filter = malloc(sizeof(arpfilter))) == NULL) {
        perror("malloc");
        close(sock);
        exit(1);
    }
    memcpy(filter, &arpfilter, sizeof(arpfilter));
    fprog.filter = filter;
    fprog.len = sizeof(arpfilter)/sizeof(struct sock_filter);

    /* CHANGE add filter */
    if (setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &fprog, sizeof(fprog)) == -1) {
        perror("setsockopt");
        close(sock);
        exit(1);
    }

    buffer = malloc(BUFF_SIZE);
    while(1)
    {
        if( (recvd_size = recv(sock, buffer, BUFF_SIZE, 0)) < 0)
        {
            perror("recv(): ");
            free(buffer);
            close(sock);
            exit(-1);
        }
        if((size_t)recvd_size < (sizeof(struct ethernet) + sizeof(struct arp)))
        {
            printf("Short packet. Packet len: %ld\n", recvd_size);
            continue;
        }
        eth_hdr = (struct ethernet *)buffer;
        if(ntohs(eth_hdr->eth_type) != ETH_P_ARP) {
            printf("Received wrong ethernet type: %X\n", eth_hdr->eth_type);
            exit(1);
        }          
        arp_hdr = (struct arp *)(buffer+ETH_HDR_LEN);
        dump_arp(arp_hdr);
    }
    free(buffer);
    close(sock);
}

static void
dump_arp(struct arp *arp_hdr)
{
    uint16_t htype = ntohs(arp_hdr->htype);
    uint16_t ptype = ntohs(arp_hdr->ptype);
    uint16_t oper = ntohs(arp_hdr->oper);
    switch(htype)
    {
        case 0x0001:
            printf("ARP HTYPE: Ethernet(0x%04X)\n", htype);
            break;
        default:
            printf("ARP HYPE: 0x%04X\n", htype);
            break;
    }
    switch(ptype)
    {
        case 0x0800:
            printf("ARP PTYPE: IPv4(0x%04X)\n", ptype);
            break;
        default:
            printf("ARP PTYPE: 0x%04X\n", ptype);
            break;
    }
    printf("ARP HLEN: %d\n", arp_hdr->hlen);
    printf("ARP PLEN: %d\n", arp_hdr->plen);
    switch(oper)
    {
        case 0x0001:
            printf("ARP OPER: Request(0x%04X)\n", oper);
            break;
        case 0x0002:
            printf("ARP OPER: Response(0x%04X)\n", oper);
            break;
        default:
            printf("ARP OPER: 0x%04X\n", oper);
            break;
    }
    printf("ARP Sender HA: %02X:%02X:%02X:%02X:%02X:%02X\n",
           arp_hdr->sender_ha[0],arp_hdr->sender_ha[1],arp_hdr->sender_ha[2],
           arp_hdr->sender_ha[3], arp_hdr->sender_ha[4], arp_hdr->sender_ha[5]);
    printf("ARP Sender PA: %d.%d.%d.%d\n", arp_hdr->sender_pa[0],
           arp_hdr->sender_pa[1], arp_hdr->sender_pa[2], arp_hdr->sender_pa[3]);
    printf("ARP Target HA: %02X:%02X:%02X:%02X:%02X:%02X\n",
           arp_hdr->target_ha[0],arp_hdr->target_ha[1],arp_hdr->target_ha[2],
           arp_hdr->target_ha[3], arp_hdr->target_ha[4], arp_hdr->target_ha[5]);
    printf("ARP Target PA: %d.%d.%d.%d\n", arp_hdr->target_pa[0],
           arp_hdr->target_pa[1], arp_hdr->target_pa[2], arp_hdr->target_pa[3]);
    printf("ARP DONE =====================\n");
}

我还删除了 bind()作为不必要的并在捕获的数据包大小的比较中纠正了一个。比较是<= sizeof(struct ethernet) + sizeof(struct arp)什么时候应该是<

由于我暂时没有阅读数据包套接字内核源代码，所以我没有找到很好的解释来解释为什么示例代码只接收寻址到主机 IP 的数据包。正如 OP 和互联网上的许多示例所证实的那样，当级别为 ETH_P_ALL 时，套接字也会接收传出数据包。我想这只是一个实现选择。这可能是大多数应用程序的首选行为，例如那些实现协议(protocol)，而不是窥探现有协议(protocol)的人。

注意 说到内核源代码，我完全不确定为什么 lsf/bpf 过滤器在我给它两个字节的 ETH_P_ARP 而没有字节顺序修改时起作用。我认为这可能是因为内核中的这些行:

case BPF_S_ANC_PROTOCOL:
A = ntohs(skb->protocol);

来自 http://lxr.linux.no/#linux+v3.11/net/core/filter.c#L317