关于ip_finish_output2的一点分析

时间：2009-04-19 来源：ggyyunix

首先声明，因为刚开始看linux内核协议栈，所以肯定有许多不正确的地方，如果大家发现不对的地方请帮我指正。在此先谢过了。

ip_finish_output2函数在文件“net/ipv4/ip_output.c”中定义，如下：

kfree_skb(skb);
return -EINVAL;
}

对于“hh->hh_output(skb);”，这里hh_output的赋值，在google上没找到，粗略看了一下代码，估计是在函数neigh_hh_init中赋值的（我也不能确定，还需要以后更熟悉协议栈代码后，如有错再更正），此函数在文件“net/core/neighbour.c”中定义，如下

   The only thing, that I can say now:
   It is very sad that we need to keep ugly obsolete
   code to make them happy.

   They should be moved to more reasonable state, now
   they use rebuild_header INSTEAD OF
   Besides that, they are sort of out of date
   (a lot of redundant clones/copies, useless in 2.1),
   I wonder why people believe that they work.
   */
switch (dev->type) {
default:
break;
case ARPHRD_ROSE:
#if defined(CONFIG_AX25) || defined(CONFIG_AX25_MODULE)
case ARPHRD_AX25:
#if defined(CONFIG_NETROM) || defined(CONFIG_NETROM_MODULE)
case ARPHRD_NETROM:
#endif
neigh->ops = &arp_broken_ops;
neigh->output = neigh->ops->output;
return 0;
#endif
;}
#endif
if (neigh->type == RTN_MULTICAST) {
neigh->nud_state = NUD_NOARP;
arp_mc_map(addr, neigh->ha, dev, 1);
} else if (dev->flags&(IFF_NOARP|IFF_LOOPBACK)) {
neigh->nud_state = NUD_NOARP;
memcpy(neigh->ha, dev->dev_addr, dev->addr_len);
} else if (neigh->type == RTN_BROADCAST || dev->flags&IFF_POINTOPOINT) {
neigh->nud_state = NUD_NOARP;
memcpy(neigh->ha, dev->broadcast, dev->addr_len);
}
if (dev->hard_header_cache)
neigh->ops = &arp_hh_ops;
else
neigh->ops = &arp_generic_ops;
if (neigh->nud_state&NUD_VALID)
neigh->output = neigh->ops->connected_output;
else
neigh->output = neigh->ops->output;
}
return 0;
}