Linux内核Bridge模式数据处理流程

1. 前言  
 
本文简要介绍数据包在进入桥网卡后在Linux网络协议栈的处理流程，并描述netfilter的hook点的挂接处理情况，具体各部分的详细处理待后续文章中说明。  
 
以下内核代码版本为2.6.19.2.  
 
2. 函数处理流程  
bridge入口点handle_bridge()   

/* net/core/dev.c */
int netif_receive_skb(struct sk_buff *skb)
{
    //......
    if (handle_bridge(&skb, &pt_prev, &ret, orig_dev))
    //......

 
bridge基本挂接点处理函数:br_handle_frame_hook()   
    

static __inline__ int handle_bridge(struct sk_buff **pskb,
        struct packet_type **pt_prev, int *ret,
        struct net_device *orig_dev)
{
    struct net_bridge_port *port;
    if ((*pskb)->pkt_type == PACKET_LOOPBACK ||
            (port = rcu_dereference((*pskb)->dev->br_port)) == NULL)
        return 0;
    if (*pt_prev) {
        *ret = deliver_skb(*pskb, *pt_prev, orig_dev);
        *pt_prev = NULL;
    }

    return br_handle_frame_hook(port, pskb);
}

 
bridge_handle_frame_hook()的实际实现:   

/* net/bridge/br.c */
static int __init br_init(void)
{
    //......
    br_handle_frame_hook = br_handle_frame;
    //......

 
br_handle_frame: PF_BEIDGE的prerouting点  

/* net/bridge/br_input.c */
int br_handle_frame(struct net_bridge_port *p, struct sk_buff **pskb)
{
     struct sk_buff *skb = *pskb;
     const unsigned char *dest = eth_hdr(skb)->h_dest;
     if (!is_valid_ether_addr(eth_hdr(skb)->h_source))
         goto err;
     if (unlikely(is_link_local(dest))) {
         // 自身包进入PF_BEIDGE的INPUT点, 一般处理的包数不多
         skb->pkt_type = PACKET_HOST;
         // 正常是返回1的, 然后就返回1, 表示桥模块全权处理该包了

         return NF_HOOK(PF_BRIDGE, NF_BR_LOCAL_IN, skb, skb->dev,
                  NULL, br_handle_local_finish) != 0;
     }
     if (p->state == BR_STATE_FORWARDING || p->state == BR_STATE_LEARNING) {
         // br_should_route_hook函数一般没定义
            if (br_should_route_hook(pskb))
                return 0;
            skb = *pskb;
            dest = eth_hdr(skb)->h_dest;
         }
         if (!compare_ether_addr(p->br->dev->dev_addr, dest))
             skb->pkt_type = PACKET_HOST;
         // PF_BRIDGE的prerouting处理结束后进入br_handle_frame_finish

         NF_HOOK(PF_BRIDGE, NF_BR_PRE_ROUTING, skb, skb->dev, NULL,
         br_handle_frame_finish);
         // 处理后始终返回1, 表示不再进行其他协议族处理,该数据包已经完全由bridge处理完毕

         return 1;
     }
err:
    kfree_skb(skb);
    // 处理后始终返回1, 表示不再进行其他协议族处理,该数据包已经完全由bridge处理完毕

    return 1;
}

通过br_handle_frame_finish进入bridge的转发:  

/* note: already called with rcu_read_lock (preempt_disabled) */
int br_handle_frame_finish(struct sk_buff *skb)
{
     const unsigned char *dest = eth_hdr(skb)->h_dest;
     struct net_bridge_port *p = rcu_dereference(skb->dev->br_port);
     struct net_bridge *br;
     struct net_bridge_fdb_entry *dst;
     int passedup = 0;
     if (!p || p->state == BR_STATE_DISABLED)
         goto drop;
     /* insert into forwarding database after filtering to avoid spoofing */
     br = p->br;
     br_fdb_update(br, p, eth_hdr(skb)->h_source);
     if (p->state == BR_STATE_LEARNING)
         goto drop;
     if (br->dev->flags & IFF_PROMISC) {
         struct sk_buff *skb2;
         skb2 = skb_clone(skb, GFP_ATOMIC);
         if (skb2 != NULL) {
             passedup = 1;
             br_pass_frame_up(br, skb2);
         }
     }
     if (is_multicast_ether_addr(dest)) {
         // 多播转发,也是调用广播处理

         br->statistics.multicast++;
         br_flood_forward(br, skb, !passedup);
         if (!passedup)
             br_pass_frame_up(br, skb);
         goto out;
     }
     // 根据目的MAC找目的出口

     dst = __br_fdb_get(br, dest);
     if (dst != NULL && dst->is_local) {
         if (!passedup)
             br_pass_frame_up(br, skb);
         else
             kfree_skb(skb);
         goto out;
     }
     if (dst != NULL) {
         // 单播转发
          goto out;
     }
     // 广播转发

     br_flood_forward(br, skb, 0);
out:
     return 0;
drop:
     kfree_skb(skb);
     goto out;
}

广播/多播转发: br_flood_forward/br_flood   

/* called under bridge lock */
void br_flood_forward(struct net_bridge *br, struct sk_buff *skb, int clone)
{
 br_flood(br, skb, clone, __br_forward);
}
/* called under bridge lock */
static void br_flood(struct net_bridge *br, struct sk_buff *skb, int clone,
 void (*__packet_hook)(const struct net_bridge_port *p,
         struct sk_buff *skb))
{
 struct net_bridge_port *p;
 struct net_bridge_port *prev;
 if (clone) {
  struct sk_buff *skb2;
  if ((skb2 = skb_clone(skb, GFP_ATOMIC)) == NULL) {
   br->statistics.tx_dropped++;
   return;
  }
  skb = skb2;
 }
 prev = NULL;
 list_for_each_entry_rcu(p, &br->port_list, list) {
  if (should_deliver(p, skb)) {
   if (prev != NULL) {
    struct sk_buff *skb2;
    if ((skb2 = skb_clone(skb, GFP_ATOMIC)) == NULL) {
     br->statistics.tx_dropped++;
     kfree_skb(skb);
     return;
    }
// 这里实际是__br_forward

    __packet_hook(prev, skb2);
   }
   prev = p;
  }
 }
 if (prev != NULL) {
// 这里实际是__br_forward

  __packet_hook(prev, skb);
  return;
 }
 kfree_skb(skb);
}

   
单播转发: br_forward  

/* net/bridge/br_forward.c */
/* called with rcu_read_lock */
void br_forward(const struct net_bridge_port *to, struct sk_buff *skb)
{
 if (should_deliver(to, skb)) {
// 也是调用__br_forward

  __br_forward(to, skb);
  return;
 }
 kfree_skb(skb);
}

FORWARD点:   

static void __br_forward(const struct net_bridge_port *to, struct sk_buff *skb)
{
 struct net_device *indev;
 indev = skb->dev;
 skb->dev = to->dev;
 skb->ip_summed = CHECKSUM_NONE;
// 进入PF_BRIDGE的forward hook, 结束后进入br_forward_finish()

 NF_HOOK(PF_BRIDGE, NF_BR_FORWARD, skb, indev, skb->dev,
   br_forward_finish);
}

 
POSTROUTING点:  

// 从FORWARD点处理后直接进入POSTROUTING点处理

int br_forward_finish(struct sk_buff *skb)
{
// 进入PF_BRIDGE的postrouting hook, 结束后进入br_dev_queue_push_xmit()

 return NF_HOOK(PF_BRIDGE, NF_BR_POST_ROUTING, skb, NULL, skb->dev,
         br_dev_queue_push_xmit);
}

数据包发出:  

int br_dev_queue_push_xmit(struct sk_buff *skb)
{
 /* drop mtu oversized packets except gso */
 if (packet_length(skb) > skb->dev->mtu && !skb_is_gso(skb))
  kfree_skb(skb);
 else {
  /* ip_refrag calls ip_fragment, doesn't copy the MAC header. */
  if (nf_bridge_maybe_copy_header(skb))
   kfree_skb(skb);
  else {
   skb_push(skb, ETH_HLEN);
// 此处调用dev设备的hard_start_xmit()函数

   dev_queue_xmit(skb);
  }
 }
 return 0;
}

桥网卡设备的hard_start_xmit()函数定义为:   

/* net/bridge/br_device.c */
void br_dev_setup(struct net_device *dev)
{
//......

 dev->hard_start_xmit = br_dev_xmit;
//......

}
/* net device transmit always called with no BH (preempt_disabled) */
int br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
{
 struct net_bridge *br = netdev_priv(dev);
 const unsigned char *dest = skb->data;
 struct net_bridge_fdb_entry *dst;
 br->statistics.tx_packets++;
 br->statistics.tx_bytes += skb->len;
 skb->mac.raw = skb->data;
 skb_pull(skb, ETH_HLEN);
 if (dest[0] & 1)
// 多播发送

  br_flood_deliver(br, skb, 0);
 else if ((dst = __br_fdb_get(br, dest)) != NULL)
// 单播发送

  br_deliver(dst->dst, skb);
 else
// 广播发送

  br_flood_deliver(br, skb, 0);
// 这些发送函数最终都会调用__br_deliver()函数

 return 0;
}

/* net/bridge/br_forward.c */
static void __br_deliver(const struct net_bridge_port *to, struct sk_buff *skb)
{
 skb->dev = to->dev;
// 此处是PF_BRIDGE的OUTPUT点

 NF_HOOK(PF_BRIDGE, NF_BR_LOCAL_OUT, skb, NULL, skb->dev,
   br_forward_finish);
}

 
总结: PF_BRIDGE中的各个hook点和PF_INET不同, 可用下面的图表示:  
 
  PREROUTING --+--FORWARD-----POSTROUTING------+----OUTPUT  
               |                               |  
               |                               |  
              INPUT  
   
3. BF_BRIDGE的hook点  
 
在net/bridge/br_netfilter.c中定义了以下hook点,注意这些hook点主要是PF_BRIDGE协议族的。   

/* net/bridge/br_netfilter.c */
/* For br_nf_local_out we need (prio = NF_BR_PRI_FIRST), to insure that innocent
 * PF_BRIDGE/NF_BR_LOCAL_OUT functions don't get bridged traffic as input.
 * For br_nf_post_routing, we need (prio = NF_BR_PRI_LAST), because
 * ip_refrag() can return NF_STOLEN. */
static struct nf_hook_ops br_nf_ops[] = {
// PF_BRIDGE的挂接点

// PREROUTING点

 { .hook = br_nf_pre_routing,
   .owner = THIS_MODULE,
   .pf = PF_BRIDGE,
   .hooknum = NF_BR_PRE_ROUTING,
   .priority = NF_BR_PRI_BRNF, },
// INPUT点

 { .hook = br_nf_local_in,
   .owner = THIS_MODULE,
   .pf = PF_BRIDGE,
   .hooknum = NF_BR_LOCAL_IN,
   .priority = NF_BR_PRI_BRNF, },
// FORWARD点

 { .hook = br_nf_forward_ip,
   .owner = THIS_MODULE,
   .pf = PF_BRIDGE,
   .hooknum = NF_BR_FORWARD,
   .priority = NF_BR_PRI_BRNF - 1, },
// FORWARD点

 { .hook = br_nf_forward_arp,
   .owner = THIS_MODULE,
   .pf = PF_BRIDGE,
   .hooknum = NF_BR_FORWARD,
   .priority = NF_BR_PRI_BRNF, },
// OUTPUT点

 { .hook = br_nf_local_out,
   .owner = THIS_MODULE,
   .pf = PF_BRIDGE,
   .hooknum = NF_BR_LOCAL_OUT,
   .priority = NF_BR_PRI_FIRST, },
// POSTROUTING点

 { .hook = br_nf_post_routing,
   .owner = THIS_MODULE,
   .pf = PF_BRIDGE,
   .hooknum = NF_BR_POST_ROUTING,
   .priority = NF_BR_PRI_LAST, },
// 后面是PF_INET/PF_INET6的挂接点, 其实也没进行什么数据包操作,

// 就是自身的输入输出包不通过桥处理,要短路掉

 { .hook = ip_sabotage_in,
   .owner = THIS_MODULE,
   .pf = PF_INET,
   .hooknum = NF_IP_PRE_ROUTING,
   .priority = NF_IP_PRI_FIRST, },
 { .hook = ip_sabotage_in,
   .owner = THIS_MODULE,
   .pf = PF_INET6,
   .hooknum = NF_IP6_PRE_ROUTING,
   .priority = NF_IP6_PRI_FIRST, },
 { .hook = ip_sabotage_out,
   .owner = THIS_MODULE,
   .pf = PF_INET,
   .hooknum = NF_IP_FORWARD,
   .priority = NF_IP_PRI_BRIDGE_SABOTAGE_FORWARD, },
 { .hook = ip_sabotage_out,
   .owner = THIS_MODULE,
   .pf = PF_INET6,
   .hooknum = NF_IP6_FORWARD,
   .priority = NF_IP6_PRI_BRIDGE_SABOTAGE_FORWARD, },
 { .hook = ip_sabotage_out,
   .owner = THIS_MODULE,
   .pf = PF_INET,
   .hooknum = NF_IP_LOCAL_OUT,
   .priority = NF_IP_PRI_BRIDGE_SABOTAGE_LOCAL_OUT, },
 { .hook = ip_sabotage_out,
   .owner = THIS_MODULE,
   .pf = PF_INET6,
   .hooknum = NF_IP6_LOCAL_OUT,
   .priority = NF_IP6_PRI_BRIDGE_SABOTAGE_LOCAL_OUT, },
 { .hook = ip_sabotage_out,
   .owner = THIS_MODULE,
   .pf = PF_INET,
   .hooknum = NF_IP_POST_ROUTING,
   .priority = NF_IP_PRI_FIRST, },
 { .hook = ip_sabotage_out,
   .owner = THIS_MODULE,
   .pf = PF_INET6,
   .hooknum = NF_IP6_POST_ROUTING,
   .priority = NF_IP6_PRI_FIRST, },
};

// PF_BRIDGE的PRROUTING点处理函数

static unsigned int br_nf_pre_routing(unsigned int hook, struct sk_buff **pskb,
          const struct net_device *in,
          const struct net_device *out,
          int (*okfn)(struct sk_buff *))
{
......
// 此处继续调用PF_INET族的PREROUTING点的hook处理

 NF_HOOK(PF_INET, NF_IP_PRE_ROUTING, skb, skb->dev, NULL,
  br_nf_pre_routing_finish);
 return NF_STOLEN;
inhdr_error:
// IP_INC_STATS_BH(IpInHdrErrors);

out:
 return NF_DROP;
}

// PF_BRIDGE的FORWARD点处理

static unsigned int br_nf_forward_ip(unsigned int hook, struct sk_buff **pskb,
         const struct net_device *in,
         const struct net_device *out,
         int (*okfn)(struct sk_buff *))
{
......
// 此处继续调用PF_INET/PF_INET6族的FORWARD点的hook处理

 NF_HOOK(pf, NF_IP_FORWARD, skb, bridge_parent(in), parent,
  br_nf_forward_finish);
 return NF_STOLEN;
}
// PF_BRIDGE的OUTPUT点处理

static unsigned int br_nf_local_out(unsigned int hook, struct sk_buff **pskb,
        const struct net_device *in,
        const struct net_device *out,
        int (*okfn)(struct sk_buff *))
{
......
 /* IP forwarded traffic has a physindev, locally
  * generated traffic hasn't. */
 if (realindev != NULL) {
  if (!(nf_bridge->mask & BRNF_DONT_TAKE_PARENT)) {
   struct net_device *parent = bridge_parent(realindev);
   if (parent)
    realindev = parent;
  }
// 此处继续调用PF_INET/PF_INET6族的FORWARD点的hook处理, 不过优先权值要在//

NF_IP_PRI_BRIDGE_SABOTAGE_FORWARD + 1以上
  NF_HOOK_THRESH(pf, NF_IP_FORWARD, skb, realindev,
          realoutdev, br_nf_local_out_finish,
          NF_IP_PRI_BRIDGE_SABOTAGE_FORWARD + 1);
 } else {
// 此处继续调用PF_INET/PF_INET6族的FORWARD点的hook处理, 不过优先权值要在

// NF_IP_PRI_BRIDGE_SABOTAGE_LOCAL_OUT + 1以上

  NF_HOOK_THRESH(pf, NF_IP_LOCAL_OUT, skb, realindev,
          realoutdev, br_nf_local_out_finish,
          NF_IP_PRI_BRIDGE_SABOTAGE_LOCAL_OUT + 1);
 }
out:
 return NF_STOLEN;
}

// PF_BRIDGE的POSTROUTING点

static unsigned int br_nf_post_routing(unsigned int hook, struct sk_buff **pskb,
           const struct net_device *in,
           const struct net_device *out,
           int (*okfn)(struct sk_buff *))
{
......
// 此处继续调用PF_INET/PF_INET6族的POSTROUTING点的hook处理

 NF_HOOK(pf, NF_IP_POST_ROUTING, skb, NULL, realoutdev,
  br_nf_dev_queue_xmit);
 return NF_STOLEN;
#ifdef CONFIG_NETFILTER_DEBUG
print_error:
 if (skb->dev != NULL) {
  printk("[%s]", skb->dev->name);
  if (realoutdev)
   printk("[%s]", realoutdev->name);
 }
 printk(" head:%p, raw:%p, data:%p\n", skb->head, skb->mac.raw,
        skb->data);
 dump_stack();
 return NF_ACCEPT;
#endif
}

 
由此可见, PF_INET的各个hook点也被PF_BRIDGE的各个hook点调用，因此可以在桥网卡中进行过滤或NAT等操作。  
 
4. 结论  
 
BRIDGE的数据处理流程是是一个独立的处理过程, 如果处理正常的话就不再返回到其他协议处理。  
在桥的处理层次也和IP协议一样，可以挂接多个PF_BRIDGE的挂接点，这些挂接点中又调用了PF_INET族的挂接点，从而实现了桥下的过滤、NAT等功能。