Function udp_ipsec_adjust_cksum

freebsd/netipsec/udpencap.c:245–293

}

void
udp_ipsec_adjust_cksum(struct mbuf *m, struct secasvar *sav, int proto,
    int skip)
{
	struct ip *ip;
	uint16_t cksum, off;

	IPSEC_ASSERT(sav->natt != NULL, ("NAT-T isn't required"));
	IPSEC_ASSERT(proto == IPPROTO_UDP || proto == IPPROTO_TCP,
	    ("unexpected protocol %u", proto));

	if (proto == IPPROTO_UDP)
		off = offsetof(struct udphdr, uh_sum);
	else
		off = offsetof(struct tcphdr, th_sum);

	if (V_natt_cksum_policy == 0) {	/* auto */
		if (sav->natt->cksum != 0) {
			/* Incrementally recompute. */
			m_copydata(m, skip + off, sizeof(cksum),
			    (caddr_t)&cksum);
			/* Do not adjust UDP checksum if it is zero. */
			if (proto == IPPROTO_UDP && cksum == 0)
				return;
			cksum = in_addword(cksum, sav->natt->cksum);
		} else {
			/* No OA from IKEd. */
			if (proto == IPPROTO_TCP) {
				/* Ignore for TCP. */
				m->m_pkthdr.csum_data = 0xffff;
				m->m_pkthdr.csum_flags |= (CSUM_DATA_VALID |
				    CSUM_PSEUDO_HDR);
				return;
			}
			cksum = 0; /* Reset for UDP. */
		}
		m_copyback(m, skip + off, sizeof(cksum), (caddr_t)&cksum);
	} else { /* Fully recompute */
		ip = mtod(m, struct ip *);
		cksum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
		    htons(m->m_pkthdr.len - skip + proto));
		m_copyback(m, skip + off, sizeof(cksum), (caddr_t)&cksum);
		m->m_pkthdr.csum_flags =
		    (proto == IPPROTO_UDP) ? CSUM_UDP: CSUM_TCP;
		m->m_pkthdr.csum_data = off;
		in_delayed_cksum(m);
		m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA;
	}
}