//  [c]
void icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info)
{
	struct iphdr *iph;
	int room;
	struct icmp_bxm icmp_param;
	struct rtable *rt = skb_rtable(skb_in);
	struct ipcm_cookie ipc;
	__be32 saddr;
	u8  tos;
	struct net *net;
	struct sock *sk;

	if (!rt)
		goto out;
	net = dev_net(rt->dst.dev);

	/*
	 *	Find the original header. It is expected to be valid, of course.
	 *	Check this, icmp_send is called from the most obscure devices
	 *	sometimes.
	 */
	iph = ip_hdr(skb_in);

	if ((u8 *)iph < skb_in->head ||
	    (skb_in->network_header + sizeof(*iph)) > skb_in->tail)
		goto out;

	/*
	 *	No replies to physical multicast/broadcast
	 */
	if (skb_in->pkt_type != PACKET_HOST)
		goto out;

	/*
	 *	Now check at the protocol level
	 */
	if (rt->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
		goto out;

	/*
	 *	Only reply to fragment 0. We byte re-order the constant
	 *	mask for efficiency.
	 */
	if (iph->frag_off & htons(IP_OFFSET))
		goto out;

	/*
	 *	If we send an ICMP error to an ICMP error a mess would result..
	 */
	if (icmp_pointers[type].error) {
		/*
		 *	We are an error, check if we are replying to an
		 *	ICMP error
		 */
		if (iph->protocol == IPPROTO_ICMP) {
			u8 _inner_type, *itp;

			itp = skb_header_pointer(skb_in,
						 skb_network_header(skb_in) +
						 (iph->ihl << 2) +
						 offsetof(struct icmphdr,
							  type) -
						 skb_in->data,
						 sizeof(_inner_type),
						 &_inner_type);
			if (itp == NULL)
				goto out;

			/*
			 *	Assume any unknown ICMP type is an error. This
			 *	isn't specified by the RFC, but think about it..
			 */
			if (*itp > NR_ICMP_TYPES ||
			    icmp_pointers[*itp].error)
				goto out;
		}
	}

	sk = icmp_xmit_lock(net);
	if (sk == NULL)
		return;

	/*
	 *	Construct source address and options.
	 */

	saddr = iph->daddr;
	if (!(rt->rt_flags & RTCF_LOCAL)) {
		struct net_device *dev = NULL;

		rcu_read_lock();
		if (rt_is_input_route(rt) &&
		    net->ipv4.sysctl_icmp_errors_use_inbound_ifaddr)
			dev = dev_get_by_index_rcu(net, rt->rt_iif);

		if (dev)
			saddr = inet_select_addr(dev, 0, RT_SCOPE_LINK);
		else
			saddr = 0;
		rcu_read_unlock();
	}

	tos = icmp_pointers[type].error ? ((iph->tos & IPTOS_TOS_MASK) |
					   IPTOS_PREC_INTERNETCONTROL) :
					  iph->tos;

	if (ip_options_echo(&icmp_param.replyopts.opt.opt, skb_in))
		goto out_unlock;


	/*
	 *	Prepare data for ICMP header.
	 */

	icmp_param.data.icmph.type	 = type;
	icmp_param.data.icmph.code	 = code;
	icmp_param.data.icmph.un.gateway = info;
	icmp_param.data.icmph.checksum	 = 0;
	icmp_param.skb	  = skb_in;
	icmp_param.offset = skb_network_offset(skb_in);
	inet_sk(sk)->tos = tos;
	ipc.addr = iph->saddr;
	ipc.opt = &icmp_param.replyopts.opt;
	ipc.tx_flags = 0;

	rt = icmp_route_lookup(net, skb_in, iph, saddr, tos,
			       type, code, &icmp_param);
	if (IS_ERR(rt))
		goto out_unlock;

	if (!icmpv4_xrlim_allow(net, rt, type, code))
		goto ende;

	/* RFC says return as much as we can without exceeding 576 bytes. */

	room = dst_mtu(&rt->dst);
	if (room > 576)
		room = 576;
	room -= sizeof(struct iphdr) + icmp_param.replyopts.opt.opt.optlen;
	room -= sizeof(struct icmphdr);

	icmp_param.data_len = skb_in->len - icmp_param.offset;
	if (icmp_param.data_len > room)
		icmp_param.data_len = room;
	icmp_param.head_len = sizeof(struct icmphdr);

	icmp_push_reply(&icmp_param, &ipc, &rt);
ende:
	ip_rt_put(rt);
out_unlock:
	icmp_xmit_unlock(sk);
out:;
}