嵌入式Linux内核修改转发skb网络数据包

实际的项目软件开过程中，碰到一个需求，需要修改以太网接收到的数据包，主要有两点需求：

• L2层特定自定义协议包修改目的MAC地址
• 针对特定端口的UDP包，修改其目的IP后，再接交给上层（或在Bridge转发之前）处理

方案评估

第一点需求，修改L2层的MAC地址，可采用：

• 采用ebtable的DNAT修改L2层MAC地址
• 通过网络驱动传参修改

第二点需求，修改Bridge转发之前的特定端口包目的IP，可采用：

• 用iptables的INT_NET_PREROUTING Hook，结合br_netfilter模块的bridge-nf-call-iptables功能实现

  /proc/sys/net/bridge/bridge-nf-call-iptables

• 编写一个netfilter hook内核模块，通过bridge NF_BR_PRE_ROUTING hook点特殊处理

最终采用的方案

上述两点的两个方案，理论上都是第一条比较优雅，但是现实总是残酷的，在嵌入式系统上，芯片资源有限，CPU\RAM\Flash资源受限的情况下，无论是ebtables或是iptables都太重了，特别还要考虑转发执行效率尽量的减少CPU处理，最终都采用了第二种策略。

实现注意点

• 网络驱动的代码netif_rx(skb);都处于中断的下半区，代码需要高效简短，否则严重影响CPU。
• 编写netfilter hook模块时，挂载在NF_BR_PRE_ROUTING hook处理节点，处于L2层，收到的skb数据包，无论是网络层、还是传输层指针都是无效的，不能直接引用

• 挂载点协议选择NFPROTO_BRIDGE

  static struct nf_hook_ops nfho = {
    .hook = hook_func,
    .hooknum = NF_BR_PRE_ROUTING,
    .pf = NFPROTO_BRIDGE,
    .priority = NF_BR_PRI_FIRST,
  };

• 修改了UDP包的目的IP后，需要重新计算UDP及IP Header的checksum；此外，这里仅修改目的IP，不需要重新全部计算，可以使用inet_proto_csum_replace4只作替代更新，提高效率
• 目的IP如果修改为组播或广播包，则需要对应的修改eth header的目的MAC地址，否则包不会经过Bridge广播，从L2层上看还是会传给NF_BR_LOCAL_IN

参考代码及资料

1. 参考代码1

   /*
    * Copyright (c) 2007-2012 Nicira, Inc.
    *
    * This program is free software; you can redistribute it and/or
    * modify it under the terms of version 2 of the GNU General Public
    * License as published by the Free Software Foundation.
    *
    * This program is distributed in the hope that it will be useful, but
    * WITHOUT ANY WARRANTY; without even the implied warranty of
    * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
    * General Public License for more details.
    *
    * You should have received a copy of the GNU General Public License
    * along with this program; if not, write to the Free Software
    * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
    * 02110-1301, USA
    */
   
   #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
   
   #include <linux/skbuff.h>
   #include <linux/in.h>
   #include <linux/ip.h>
   #include <linux/openvswitch.h>
   #include <linux/tcp.h>
   #include <linux/udp.h>
   #include <linux/in6.h>
   #include <linux/if_arp.h>
   #include <linux/if_vlan.h>
   #include <net/ip.h>
   #include <net/ipv6.h>
   #include <net/checksum.h>
   #include <net/dsfield.h>
   
   #include "checksum.h"
   #include "datapath.h"
   #include "vlan.h"
   #include "vport.h"
   
   static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
                  const struct nlattr *attr, int len,
                  struct ovs_key_ipv4_tunnel *tun_key, bool keep_skb);
   
   static int make_writable(struct sk_buff *skb, int write_len)
   {
    if (!skb_cloned(skb) || skb_clone_writable(skb, write_len))
        return 0;
   
    return pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
   }
   
   /* remove VLAN header from packet and update csum accordingly. */
   static int __pop_vlan_tci(struct sk_buff *skb, __be16 *current_tci)
   {
    struct vlan_hdr *vhdr;
    int err;
   
    err = make_writable(skb, VLAN_ETH_HLEN);
    if (unlikely(err))
        return err;
   
    if (get_ip_summed(skb) == OVS_CSUM_COMPLETE)
        skb->csum = csum_sub(skb->csum, csum_partial(skb->data
                    + ETH_HLEN, VLAN_HLEN, 0));
   
    vhdr = (struct vlan_hdr *)(skb->data + ETH_HLEN);
    *current_tci = vhdr->h_vlan_TCI;
   
    memmove(skb->data + VLAN_HLEN, skb->data, 2 * ETH_ALEN);
    __skb_pull(skb, VLAN_HLEN);
   
    vlan_set_encap_proto(skb, vhdr);
    skb->mac_header += VLAN_HLEN;
    skb_reset_mac_len(skb);
   
    return 0;
   }
   
   static int pop_vlan(struct sk_buff *skb)
   {
    __be16 tci;
    int err;
   
    if (likely(vlan_tx_tag_present(skb))) {
        vlan_set_tci(skb, 0);
    } else {
        if (unlikely(skb->protocol != htons(ETH_P_8021Q) ||
                 skb->len < VLAN_ETH_HLEN))
            return 0;
   
        err = __pop_vlan_tci(skb, &tci);
        if (err)
            return err;
    }
    /* move next vlan tag to hw accel tag */
    if (likely(skb->protocol != htons(ETH_P_8021Q) ||
           skb->len < VLAN_ETH_HLEN))
        return 0;
   
    err = __pop_vlan_tci(skb, &tci);
    if (unlikely(err))
        return err;
   
    __vlan_hwaccel_put_tag(skb, ntohs(tci));
    return 0;
   }
   
   static int push_vlan(struct sk_buff *skb, const struct ovs_action_push_vlan *vlan)
   {
    if (unlikely(vlan_tx_tag_present(skb))) {
        u16 current_tag;
   
        /* push down current VLAN tag */
        current_tag = vlan_tx_tag_get(skb);
   
        if (!__vlan_put_tag(skb, current_tag))
            return -ENOMEM;
   
        if (get_ip_summed(skb) == OVS_CSUM_COMPLETE)
            skb->csum = csum_add(skb->csum, csum_partial(skb->data
                    + ETH_HLEN, VLAN_HLEN, 0));
   
    }
    __vlan_hwaccel_put_tag(skb, ntohs(vlan->vlan_tci) & ~VLAN_TAG_PRESENT);
    return 0;
   }
   
   static int set_eth_addr(struct sk_buff *skb,
            const struct ovs_key_ethernet *eth_key)
   {
    int err;
    err = make_writable(skb, ETH_HLEN);
    if (unlikely(err))
        return err;
   
    memcpy(eth_hdr(skb)->h_source, eth_key->eth_src, ETH_ALEN);
    memcpy(eth_hdr(skb)->h_dest, eth_key->eth_dst, ETH_ALEN);
   
    return 0;
   }
   
   static void set_ip_addr(struct sk_buff *skb, struct iphdr *nh,
                __be32 *addr, __be32 new_addr)
   {
    int transport_len = skb->len - skb_transport_offset(skb);
   
    if (nh->protocol == IPPROTO_TCP) {
        if (likely(transport_len >= sizeof(struct tcphdr)))
            inet_proto_csum_replace4(&tcp_hdr(skb)->check, skb,
                         *addr, new_addr, 1);
    } else if (nh->protocol == IPPROTO_UDP) {
        if (likely(transport_len >= sizeof(struct udphdr))) {
            struct udphdr *uh = udp_hdr(skb);
   
            if (uh->check ||
                get_ip_summed(skb) == OVS_CSUM_PARTIAL) {
                inet_proto_csum_replace4(&uh->check, skb,
                             *addr, new_addr, 1);
                if (!uh->check)
                    uh->check = CSUM_MANGLED_0;
            }
        }
    }
   
    csum_replace4(&nh->check, *addr, new_addr);
    skb_clear_rxhash(skb);
    *addr = new_addr;
   }
   
   static void update_ipv6_checksum(struct sk_buff *skb, u8 l4_proto,
                 __be32 addr[4], const __be32 new_addr[4])
   {
    int transport_len = skb->len - skb_transport_offset(skb);
   
    if (l4_proto == IPPROTO_TCP) {
        if (likely(transport_len >= sizeof(struct tcphdr)))
            inet_proto_csum_replace16(&tcp_hdr(skb)->check, skb,
                          addr, new_addr, 1);
    } else if (l4_proto == IPPROTO_UDP) {
        if (likely(transport_len >= sizeof(struct udphdr))) {
            struct udphdr *uh = udp_hdr(skb);
   
            if (uh->check ||
                get_ip_summed(skb) == OVS_CSUM_PARTIAL) {
                inet_proto_csum_replace16(&uh->check, skb,
                              addr, new_addr, 1);
                if (!uh->check)
                    uh->check = CSUM_MANGLED_0;
            }
        }
    }
   }
   
   static void set_ipv6_addr(struct sk_buff *skb, u8 l4_proto,
              __be32 addr[4], const __be32 new_addr[4],
              bool recalculate_csum)
   {
    if (recalculate_csum)
        update_ipv6_checksum(skb, l4_proto, addr, new_addr);
   
    skb_clear_rxhash(skb);
    memcpy(addr, new_addr, sizeof(__be32[4]));
   }
   
   static void set_ipv6_tc(struct ipv6hdr *nh, u8 tc)
   {
    nh->priority = tc >> 4;
    nh->flow_lbl[0] = (nh->flow_lbl[0] & 0x0F) | ((tc & 0x0F) << 4);
   }
   
   static void set_ipv6_fl(struct ipv6hdr *nh, u32 fl)
   {
    nh->flow_lbl[0] = (nh->flow_lbl[0] & 0xF0) | (fl & 0x000F0000) >> 16;
    nh->flow_lbl[1] = (fl & 0x0000FF00) >> 8;
    nh->flow_lbl[2] = fl & 0x000000FF;
   }
   
   static void set_ip_ttl(struct sk_buff *skb, struct iphdr *nh, u8 new_ttl)
   {
    csum_replace2(&nh->check, htons(nh->ttl << 8), htons(new_ttl << 8));
    nh->ttl = new_ttl;
   }
   
   static int set_ipv4(struct sk_buff *skb, const struct ovs_key_ipv4 *ipv4_key)
   {
    struct iphdr *nh;
    int err;
   
    err = make_writable(skb, skb_network_offset(skb) +
                 sizeof(struct iphdr));
    if (unlikely(err))
        return err;
   
    nh = ip_hdr(skb);
   
    if (ipv4_key->ipv4_src != nh->saddr)
        set_ip_addr(skb, nh, &nh->saddr, ipv4_key->ipv4_src);
   
    if (ipv4_key->ipv4_dst != nh->daddr)
        set_ip_addr(skb, nh, &nh->daddr, ipv4_key->ipv4_dst);
   
    if (ipv4_key->ipv4_tos != nh->tos)
        ipv4_change_dsfield(nh, 0, ipv4_key->ipv4_tos);
   
    if (ipv4_key->ipv4_ttl != nh->ttl)
        set_ip_ttl(skb, nh, ipv4_key->ipv4_ttl);
   
    return 0;
   }
   
   static int set_ipv6(struct sk_buff *skb, const struct ovs_key_ipv6 *ipv6_key)
   {
    struct ipv6hdr *nh;
    int err;
    __be32 *saddr;
    __be32 *daddr;
   
    err = make_writable(skb, skb_network_offset(skb) +
                sizeof(struct ipv6hdr));
    if (unlikely(err))
        return err;
   
    nh = ipv6_hdr(skb);
    saddr = (__be32 *)&nh->saddr;
    daddr = (__be32 *)&nh->daddr;
   
    if (memcmp(ipv6_key->ipv6_src, saddr, sizeof(ipv6_key->ipv6_src)))
        set_ipv6_addr(skb, ipv6_key->ipv6_proto, saddr,
                  ipv6_key->ipv6_src, true);
   
    if (memcmp(ipv6_key->ipv6_dst, daddr, sizeof(ipv6_key->ipv6_dst))) {
        unsigned int offset = 0;
        int flags = OVS_IP6T_FH_F_SKIP_RH;
        bool recalc_csum = true;
   
        if (ipv6_ext_hdr(nh->nexthdr))
            recalc_csum = ipv6_find_hdr(skb, &offset,
                            NEXTHDR_ROUTING, NULL,
                            &flags) != NEXTHDR_ROUTING;
   
        set_ipv6_addr(skb, ipv6_key->ipv6_proto, daddr,
                  ipv6_key->ipv6_dst, recalc_csum);
    }
   
    set_ipv6_tc(nh, ipv6_key->ipv6_tclass);
    set_ipv6_fl(nh, ntohl(ipv6_key->ipv6_label));
    nh->hop_limit = ipv6_key->ipv6_hlimit;
   
    return 0;
   }
   
   /* Must follow make_writable() since that can move the skb data. */
   static void set_tp_port(struct sk_buff *skb, __be16 *port,
             __be16 new_port, __sum16 *check)
   {
    inet_proto_csum_replace2(check, skb, *port, new_port, 0);
    *port = new_port;
    skb_clear_rxhash(skb);
   }
   
   static void set_udp_port(struct sk_buff *skb, __be16 *port, __be16 new_port)
   {
    struct udphdr *uh = udp_hdr(skb);
   
    if (uh->check && get_ip_summed(skb) != OVS_CSUM_PARTIAL) {
        set_tp_port(skb, port, new_port, &uh->check);
   
        if (!uh->check)
            uh->check = CSUM_MANGLED_0;
    } else {
        *port = new_port;
        skb_clear_rxhash(skb);
    }
   }
   
   static int set_udp(struct sk_buff *skb, const struct ovs_key_udp *udp_port_key)
   {
    struct udphdr *uh;
    int err;
   
    err = make_writable(skb, skb_transport_offset(skb) +
                 sizeof(struct udphdr));
    if (unlikely(err))
        return err;
   
    uh = udp_hdr(skb);
    if (udp_port_key->udp_src != uh->source)
        set_udp_port(skb, &uh->source, udp_port_key->udp_src);
   
    if (udp_port_key->udp_dst != uh->dest)
        set_udp_port(skb, &uh->dest, udp_port_key->udp_dst);
   
    return 0;
   }
   
   static int set_tcp(struct sk_buff *skb, const struct ovs_key_tcp *tcp_port_key)
   {
    struct tcphdr *th;
    int err;
   
    err = make_writable(skb, skb_transport_offset(skb) +
                 sizeof(struct tcphdr));
    if (unlikely(err))
        return err;
   
    th = tcp_hdr(skb);
    if (tcp_port_key->tcp_src != th->source)
        set_tp_port(skb, &th->source, tcp_port_key->tcp_src, &th->check);
   
    if (tcp_port_key->tcp_dst != th->dest)
        set_tp_port(skb, &th->dest, tcp_port_key->tcp_dst, &th->check);
   
    return 0;
   }
   
   static int do_output(struct datapath *dp, struct sk_buff *skb, int out_port)
   {
    struct vport *vport;
   
    if (unlikely(!skb))
        return -ENOMEM;
   
    vport = ovs_vport_rcu(dp, out_port);
    if (unlikely(!vport)) {
        kfree_skb(skb);
        return -ENODEV;
    }
   
    ovs_vport_send(vport, skb);
    return 0;
   }
   
   static int output_userspace(struct datapath *dp, struct sk_buff *skb,
                const struct nlattr *attr)
   {
    struct dp_upcall_info upcall;
    const struct nlattr *a;
    int rem;
   
    upcall.cmd = OVS_PACKET_CMD_ACTION;
    upcall.key = &OVS_CB(skb)->flow->key;
    upcall.userdata = NULL;
    upcall.pid = 0;
   
    for (a = nla_data(attr), rem = nla_len(attr); rem > 0;
         a = nla_next(a, &rem)) {
        switch (nla_type(a)) {
        case OVS_USERSPACE_ATTR_USERDATA:
            upcall.userdata = a;
            break;
   
        case OVS_USERSPACE_ATTR_PID:
            upcall.pid = nla_get_u32(a);
            break;
        }
    }
   
    return ovs_dp_upcall(dp, skb, &upcall);
   }
   
   static int sample(struct datapath *dp, struct sk_buff *skb,
          const struct nlattr *attr,
          struct ovs_key_ipv4_tunnel *tun_key)
   {
    const struct nlattr *acts_list = NULL;
    const struct nlattr *a;
    int rem;
   
    for (a = nla_data(attr), rem = nla_len(attr); rem > 0;
         a = nla_next(a, &rem)) {
        switch (nla_type(a)) {
        case OVS_SAMPLE_ATTR_PROBABILITY:
            if (net_random() >= nla_get_u32(a))
                return 0;
            break;
   
        case OVS_SAMPLE_ATTR_ACTIONS:
            acts_list = a;
            break;
        }
    }
   
    return do_execute_actions(dp, skb, nla_data(acts_list),
                  nla_len(acts_list), tun_key, true);
   }
   
   static int execute_set_action(struct sk_buff *skb,
                 const struct nlattr *nested_attr,
                 struct ovs_key_ipv4_tunnel *tun_key)
   {
    int err = 0;
   
    switch (nla_type(nested_attr)) {
    case OVS_KEY_ATTR_PRIORITY:
        skb->priority = nla_get_u32(nested_attr);
        break;
   
    case OVS_KEY_ATTR_TUN_ID:
        /* If we're only using the TUN_ID action, store the value in a
         * temporary instance of struct ovs_key_ipv4_tunnel on the stack.
         * If both IPV4_TUNNEL and TUN_ID are being used together we
         * can't write into the IPV4_TUNNEL action, so make a copy and
         * write into that version.
         */
        if (!OVS_CB(skb)->tun_key)
            memset(tun_key, 0, sizeof(*tun_key));
        else if (OVS_CB(skb)->tun_key != tun_key)
            memcpy(tun_key, OVS_CB(skb)->tun_key, sizeof(*tun_key));
        OVS_CB(skb)->tun_key = tun_key;
   
        OVS_CB(skb)->tun_key->tun_id = nla_get_be64(nested_attr);
        break;
   
    case OVS_KEY_ATTR_IPV4_TUNNEL:
        OVS_CB(skb)->tun_key = nla_data(nested_attr);
        break;
   
    case OVS_KEY_ATTR_ETHERNET:
        err = set_eth_addr(skb, nla_data(nested_attr));
        break;
   
    case OVS_KEY_ATTR_IPV4:
        err = set_ipv4(skb, nla_data(nested_attr));
        break;
   
    case OVS_KEY_ATTR_IPV6:
        err = set_ipv6(skb, nla_data(nested_attr));
        break;
   
    case OVS_KEY_ATTR_TCP:
        err = set_tcp(skb, nla_data(nested_attr));
        break;
   
    case OVS_KEY_ATTR_UDP:
        err = set_udp(skb, nla_data(nested_attr));
        break;
    }
   
    return err;
   }
   
   /* Execute a list of actions against 'skb'. */
   static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
            const struct nlattr *attr, int len,
            struct ovs_key_ipv4_tunnel *tun_key, bool keep_skb)
   {
    /* Every output action needs a separate clone of 'skb', but the common
     * case is just a single output action, so that doing a clone and
     * then freeing the original skbuff is wasteful.  So the following code
     * is slightly obscure just to avoid that. */
    int prev_port = -1;
    const struct nlattr *a;
    int rem;
   
    for (a = attr, rem = len; rem > 0;
         a = nla_next(a, &rem)) {
        int err = 0;
   
        if (prev_port != -1) {
            do_output(dp, skb_clone(skb, GFP_ATOMIC), prev_port);
            prev_port = -1;
        }
   
        switch (nla_type(a)) {
        case OVS_ACTION_ATTR_OUTPUT:
            prev_port = nla_get_u32(a);
            break;
   
        case OVS_ACTION_ATTR_USERSPACE:
            output_userspace(dp, skb, a);
            break;
   
        case OVS_ACTION_ATTR_PUSH_VLAN:
            err = push_vlan(skb, nla_data(a));
            if (unlikely(err)) /* skb already freed. */
                return err;
            break;
   
        case OVS_ACTION_ATTR_POP_VLAN:
            err = pop_vlan(skb);
            break;
   
        case OVS_ACTION_ATTR_SET:
            err = execute_set_action(skb, nla_data(a), tun_key);
            break;
   
        case OVS_ACTION_ATTR_SAMPLE:
            err = sample(dp, skb, a, tun_key);
            break;
        }
   
        if (unlikely(err)) {
            kfree_skb(skb);
            return err;
        }
    }
   
    if (prev_port != -1) {
        if (keep_skb)
            skb = skb_clone(skb, GFP_ATOMIC);
   
        do_output(dp, skb, prev_port);
    } else if (!keep_skb)
        consume_skb(skb);
   
    return 0;
   }
   
   /* We limit the number of times that we pass into execute_actions()
    * to avoid blowing out the stack in the event that we have a loop. */
   #define MAX_LOOPS 5
   
   struct loop_counter {
    u8 count;        /* Count. */
    bool looping;        /* Loop detected? */
   };
   
   static DEFINE_PER_CPU(struct loop_counter, loop_counters);
   
   static int loop_suppress(struct datapath *dp, struct sw_flow_actions *actions)
   {
    if (net_ratelimit())
        pr_warn("%s: flow looped %d times, dropping\n",
                ovs_dp_name(dp), MAX_LOOPS);
    actions->actions_len = 0;
    return -ELOOP;
   }
   
   /* Execute a list of actions against 'skb'. */
   int ovs_execute_actions(struct datapath *dp, struct sk_buff *skb)
   {
    struct sw_flow_actions *acts = rcu_dereference(OVS_CB(skb)->flow->sf_acts);
    struct loop_counter *loop;
    int error;
    struct ovs_key_ipv4_tunnel tun_key;
   
    /* Check whether we've looped too much. */
    loop = &__get_cpu_var(loop_counters);
    if (unlikely(++loop->count > MAX_LOOPS))
        loop->looping = true;
    if (unlikely(loop->looping)) {
        error = loop_suppress(dp, acts);
        kfree_skb(skb);
        goto out_loop;
    }
   
    OVS_CB(skb)->tun_key = NULL;
    error = do_execute_actions(dp, skb, acts->actions,
                     acts->actions_len, &tun_key, false);
   
    /* Check whether sub-actions looped too much. */
    if (unlikely(loop->looping))
        error = loop_suppress(dp, acts);
   
   out_loop:
    /* Decrement loop counter. */
    if (!--loop->count)
        loop->looping = false;
   
    return error;
   }

2. 独立内核模块示例代码

   #include <linux/module.h>
   #include <linux/netfilter.h>
   #include <linux/netfilter_ipv4.h>
   #include <linux/ip.h>
   #include <linux/udp.h>
   #include <linux/skbuff.h>
   
   #define TARGET_PORT 12345  // 设置特定的目的端口
   #define MULTICAST_IP "224.0.0.1"  // 指定的组播IP地址
   
   // Netfilter钩子函数，用于处理入站的数据包
   static unsigned int hook_func(void *priv, struct sk_buff *skb, const struct nf_hook_state *state)
   {
    struct iphdr *ip_header;
    struct udphdr *udp_header;
   
    // 检查skb包是否为空
    if (!skb)
        return NF_ACCEPT;
   
    // 检查skb包是否包含IP头部
    if (!skb_network_header(skb))
        return NF_ACCEPT;
   
    // 获取IP头部和UDP头部
    ip_header = ip_hdr(skb);
    udp_header = (struct udphdr *)(skb_transport_header(skb) + ip_hdrlen(skb));
   
    // 检查是否为UDP协议和特定目的端口
    if (ip_header->protocol == IPPROTO_UDP && ntohs(udp_header->dest) == TARGET_PORT) {
        // 修改目的IP地址为指定的组播IP
        ip_header->daddr = in_aton(MULTICAST_IP);
   
        // 重新计算校验值
        udp_header->check = 0;
        udp_header->check = csum_tcpudp_magic(ip_header->saddr, ip_header->daddr,
                                               skb->len - ip_hdrlen(skb), IPPROTO_UDP, csum_partial(udp_header, skb->len - ip_hdrlen(skb), 0));
   
        // 更新校验和
        skb->ip_summed = CHECKSUM_NONE;
        skb->pkt_type = PACKET_MULTICAST;
   
        // 让tcp/ip协议栈处理skb包
        return NF_ACCEPT;
    }
   
    return NF_ACCEPT;
   }
   
   // 初始化和清理模块
   static struct nf_hook_ops nfho = {
    .hook = hook_func,
    .hooknum = NF_BR_PRE_ROUTING,
    .pf = NFPROTO_BRIDGE,
    .priority = NF_BR_PRI_FIRST,
   };
   
   static int __init my_module_init(void)
   {
    return nf_register_hook(&nfho);
   }
   
   static void __exit my_module_exit(void)
   {
    nf_unregister_hook(&nfho);
   }
   
   module_init(my_module_init);
   module_exit(my_module_exit);
   
   MODULE_LICENSE("GPL");
   MODULE_AUTHOR("Kimson");

3. linux内核中如何修改skb报文
4. linux-2-6-network-stack-paper.pdf
5. skbuff.pdf