Function LibAliasUnaliasOut

freebsd/netinet/libalias/alias.c:1552–1657

}

int
LibAliasUnaliasOut(struct libalias *la, void *ptr,	/* valid IP packet */
    int maxpacketsize		/* for error checking */
)
{
	struct ip *pip;
	struct icmp *ic;
	struct udphdr *ud;
	struct tcphdr *tc;
	struct alias_link *lnk;
	int iresult = PKT_ALIAS_IGNORED;

	LIBALIAS_LOCK(la);
	pip = (struct ip *)ptr;

	/* Defense against mangled packets */
	if (ntohs(pip->ip_len) > maxpacketsize
	    || (pip->ip_hl << 2) > maxpacketsize)
		goto getout;

	ud = (struct udphdr *)ip_next(pip);
	tc = (struct tcphdr *)ip_next(pip);
	ic = (struct icmp *)ip_next(pip);

	/* Find a link */
	if (pip->ip_p == IPPROTO_UDP)
		lnk = FindUdpTcpIn(la, pip->ip_dst, pip->ip_src,
		    ud->uh_dport, ud->uh_sport,
		    IPPROTO_UDP, 0);
	else if (pip->ip_p == IPPROTO_TCP)
		lnk = FindUdpTcpIn(la, pip->ip_dst, pip->ip_src,
		    tc->th_dport, tc->th_sport,
		    IPPROTO_TCP, 0);
	else if (pip->ip_p == IPPROTO_ICMP)
		lnk = FindIcmpIn(la, pip->ip_dst, pip->ip_src, ic->icmp_id, 0);
	else
		lnk = NULL;

	/* Change it from an aliased packet to an unaliased packet */
	if (lnk != NULL) {
		if (pip->ip_p == IPPROTO_UDP || pip->ip_p == IPPROTO_TCP) {
			int accumulate;
			struct in_addr original_address;
			u_short original_port;

			original_address = GetOriginalAddress(lnk);
			original_port = GetOriginalPort(lnk);

			/* Adjust TCP/UDP checksum */
			accumulate = twowords(&pip->ip_src);
			accumulate -= twowords(&original_address);

			if (pip->ip_p == IPPROTO_UDP) {
				accumulate += ud->uh_sport;
				accumulate -= original_port;
				ADJUST_CHECKSUM(accumulate, ud->uh_sum);
			} else {
				accumulate += tc->th_sport;