Index: if_loop.c
===================================================================
--- if_loop.c	(revision 189825)
+++ if_loop.c	(working copy)
@@ -138,6 +138,7 @@
 	ifp->if_ioctl = loioctl;
 	ifp->if_output = looutput;
 	ifp->if_snd.ifq_maxlen = ifqmaxlen;
+	ifp->if_capabilities = ifp->if_capenable = IFCAP_RXCSUM;
 	if_attach(ifp);
 	bpfattach(ifp, DLT_NULL, sizeof(u_int32_t));
 	if (V_loif == NULL)
@@ -213,6 +214,12 @@
 	switch (dst->sa_family) {
 	case AF_INET:
 	case AF_INET6:
+		if (ifp->if_capenable & IFCAP_RXCSUM) {
+			m->m_pkthdr.csum_data = 0xffff;
+			m->m_pkthdr.csum_flags = CSUM_DATA_VALID |
+			    CSUM_PSEUDO_HDR | CSUM_IP_CHECKED |
+			    CSUM_IP_VALID;
+		}
 	case AF_IPX:
 	case AF_APPLETALK:
 		break;
@@ -348,7 +355,7 @@
 {
 	struct ifaddr *ifa;
 	struct ifreq *ifr = (struct ifreq *)data;
-	int error = 0;
+	int error = 0, mask;
 
 	switch (cmd) {
 	case SIOCSIFADDR:
@@ -391,6 +398,12 @@
 	case SIOCSIFFLAGS:
 		break;
 
+	case SIOCSIFCAP:
+		mask = ifp->if_capenable ^ ifr->ifr_reqcap;
+		if ((mask & IFCAP_RXCSUM) != 0)
+			ifp->if_capenable ^= IFCAP_RXCSUM;
+		break;
+
 	default:
 		error = EINVAL;
 	}