在前面已經分析過，查找到路由後，會調用arp_bind_neighbour綁定一個鄰居項
int arp_bind_neighbour(struct dst_entry *dst)
{
struct net_device *dev = dst->dev;
struct neighbour *n = dst->neighbour;
if (dev == NULL)
return -EINVAL;
//如果鄰居項不存在
if (n == NULL) {
u32 nexthop = ((struct rtable*)dst)->rt_gateway;
if (dev->flags&(IFF_LOOPBACK|IFF_POINTOPOINT))
nexthop = 0;
n = __neigh_lookup_errno(
#if defined(CONFIG_ATM_CLIP) || defined(CONFIG_ATM_CLIP_MODULE)
dev->type == ARPHRD_ATM ? clip_tbl_hook :
#endif
&arp_tbl, &nexthop, dev);
if (IS_ERR(n))
return PTR_ERR(n);
dst->neighbour = n;
}
return 0;
}
如果鄰居項不存同，則執行__neigh_lookup_errno（）
__neigh_lookup_errno(struct neigh_table *tbl, const void *pkey,
struct net_device *dev)
{
//在鄰居表中查找鄰居項
struct neighbour *n = neigh_lookup(tbl, pkey, dev);
if (n)
return n;
//新建鄰居項
return neigh_create(tbl, pkey, dev);
}
從上面可以看到，它會先到鄰居表中尋找對應的鄰居項，如果不存在，則新建一項。繼續跟進
struct neighbour *neigh_create(struct neigh_table *tbl, const void *pkey,
struct net_device *dev)
{
u32 hash_val;
int key_len = tbl->key_len;
int error;
struct neighbour *n1, *rc, *n = neigh_alloc(tbl);
if (!n) {
rc = ERR_PTR(-ENOBUFS);
goto out;
}
//從此可以看到，哈希鍵值就是目的IP
memcpy(n->primary_key, pkey, key_len);
n->dev = dev;
dev_hold(dev);
/* Protocol specific setup. */
//初始化函數
if (tbl->constructor && (error = tbl->constructor(n)) < 0) {
rc = ERR_PTR(error);
goto out_neigh_release;
}
/* Device specific setup. */
if (n->parms->neigh_setup &&
(error = n->parms->neigh_setup(n)) < 0) {
rc = ERR_PTR(error);
goto out_neigh_release;
}
n->confirmed = jiffies - (n->parms->base_reachable_time << 1);
write_lock_bh(&tbl->lock);
//如果總數超過了hash_mask +1，則增長哈希表
if (tbl->entries > (tbl->hash_mask + 1))
neigh_hash_grow(tbl, (tbl->hash_mask + 1) << 1);
hash_val = tbl->hash(pkey, dev) & tbl->hash_mask;
// 如果鄰居表項為刪除項
if (n->parms->dead) {
rc = ERR_PTR(-EINVAL);
goto out_tbl_unlock;
}
//遍歷對應的哈希數組項。如果已經存在，則更新引用計數
for (n1 = tbl->hash_buckets[hash_val]; n1; n1 = n1->next) {
if (dev == n1->dev && !memcmp(n1->primary_key, pkey, key_len)) {
neigh_hold(n1);
rc = n1;
goto out_tbl_unlock;
}
}
// 如果不存在，把插入項加到哈希數組項的頭部
n->next = tbl->hash_buckets[hash_val];
tbl->hash_buckets[hash_val] = n;
n->dead = 0;
neigh_hold(n);
write_unlock_bh(&tbl->lock);
NEIGH_PRINTK2("neigh %p is created.\n", n);
rc = n;
out:
return rc;
out_tbl_unlock:
write_unlock_bh(&tbl->lock);
out_neigh_release:
neigh_release(n);
goto out;
}
在函數裡，會調用tbl->constructor（）進行初始化。在arp_tbl結構中，為constructor賦值為arp_constructor。
static int arp_constructor(struct neighbour *neigh)
{
u32 addr = *(u32*)neigh->primary_key;
struct net_device *dev = neigh->dev;
struct in_device *in_dev;
struct neigh_parms *parms;
neigh->type = inet_addr_type(addr);
rcu_read_lock();
in_dev = rcu_dereference(__in_dev_get(dev));
if (in_dev == NULL) {
rcu_read_unlock();
return -EINVAL;
}
parms = in_dev->arp_parms;
__neigh_parms_put(neigh->parms);