Function icmp_error

freebsd/netinet/ip_icmp.c:203–397

* Generate an error packet of type error * in response to bad packet ip. */

 * in response to bad packet ip.
 */
void
icmp_error(struct mbuf *n, int type, int code, uint32_t dest, int mtu)
{
	struct ip *oip, *nip;
	struct icmp *icp;
	struct mbuf *m;
	unsigned icmplen, icmpelen, nlen, oiphlen;

	KASSERT((u_int)type <= ICMP_MAXTYPE, ("%s: illegal ICMP type",
	    __func__));

	if (type != ICMP_REDIRECT)
		ICMPSTAT_INC(icps_error);
	/*
	 * Don't send error:
	 *  if the original packet was encrypted.
	 *  if not the first fragment of message.
	 *  in response to a multicast or broadcast packet.
	 *  if the old packet protocol was an ICMP error message.
	 */
	if (n->m_flags & M_DECRYPTED)
		goto freeit;
	if (n->m_flags & (M_BCAST|M_MCAST))
		goto freeit;

	/* Drop if IP header plus 8 bytes is not contiguous in first mbuf. */
	if (n->m_len < sizeof(struct ip) + ICMP_MINLEN)
		goto freeit;
	oip = mtod(n, struct ip *);
	oiphlen = oip->ip_hl << 2;
	if (n->m_len < oiphlen + ICMP_MINLEN)
		goto freeit;
#ifdef ICMPPRINTFS
	if (icmpprintfs)
		printf("icmp_error(%p, %x, %d)\n", oip, type, code);
#endif
	if (oip->ip_off & htons(~(IP_MF|IP_DF)))
		goto freeit;
	if (oip->ip_p == IPPROTO_ICMP && type != ICMP_REDIRECT &&
	    !ICMP_INFOTYPE(((struct icmp *)((caddr_t)oip +
		oiphlen))->icmp_type)) {
		ICMPSTAT_INC(icps_oldicmp);
		goto freeit;
	}
	/*
	 * Calculate length to quote from original packet and
	 * prevent the ICMP mbuf from overflowing.
	 * Unfortunately this is non-trivial since ip_forward()
	 * sends us truncated packets.
	 */
	nlen = m_length(n, NULL);
	if (oip->ip_p == IPPROTO_TCP) {
		struct tcphdr *th;
		int tcphlen;

		if (oiphlen + sizeof(struct tcphdr) > n->m_len &&
		    n->m_next == NULL)
			goto stdreply;