Function ip_srcroute

freebsd/netinet/ip_options.c:425–477

* Retrieve incoming source route for use in replies, in the same form used * by setsockopt. The first hop is placed before the options, will be * removed later. */

 * removed later.
 */
struct mbuf *
ip_srcroute(struct mbuf *m0)
{
	struct in_addr *p, *q;
	struct mbuf *m;
	struct ipopt_tag *opts;

	opts = (struct ipopt_tag *)m_tag_find(m0, PACKET_TAG_IPOPTIONS, NULL);
	if (opts == NULL)
		return (NULL);

	if (opts->ip_nhops == 0)
		return (NULL);
	m = m_get(M_NOWAIT, MT_DATA);
	if (m == NULL)
		return (NULL);

#define OPTSIZ	(sizeof(opts->ip_srcrt.nop) + sizeof(opts->ip_srcrt.srcopt))

	/* length is (nhops+1)*sizeof(addr) + sizeof(nop + srcrt header) */
	m->m_len = opts->ip_nhops * sizeof(struct in_addr) +
	    sizeof(struct in_addr) + OPTSIZ;

	/*
	 * First, save first hop for return route.
	 */
	p = &(opts->ip_srcrt.route[opts->ip_nhops - 1]);
	*(mtod(m, struct in_addr *)) = *p--;

	/*
	 * Copy option fields and padding (nop) to mbuf.
	 */
	opts->ip_srcrt.nop = IPOPT_NOP;
	opts->ip_srcrt.srcopt[IPOPT_OFFSET] = IPOPT_MINOFF;
	(void)memcpy(mtod(m, caddr_t) + sizeof(struct in_addr),
	    &(opts->ip_srcrt.nop), OPTSIZ);
	q = (struct in_addr *)(mtod(m, caddr_t) +
	    sizeof(struct in_addr) + OPTSIZ);
#undef OPTSIZ
	/*
	 * Record return path as an IP source route, reversing the path
	 * (pointers are now aligned).
	 */
	while (p >= opts->ip_srcrt.route) {
		*q++ = *p--;
	}
	/*
	 * Last hop goes to final destination.
	 */
	*q = opts->ip_srcrt.dst;
	m_tag_delete(m0, (struct m_tag *)opts);
	return (m);
}

/*
 * Strip out IP options, at higher level protocol in the kernel.