support configuration of fragmentation in vxlan(4)

[David Gwynne]

this is a simple port of the tunneldf stuff to vxlan.

ok?

Index: if_vxlan.c
===================================================================
RCS file: /cvs/src/sys/net/if_vxlan.c,v
retrieving revision 1.66
diff -u -p -r1.66 if_vxlan.c
--- if_vxlan.c  22 Jan 2018 09:05:06 -0000      1.66
+++ if_vxlan.c  19 Feb 2018 00:51:00 -0000
@@ -71,6 +71,7 @@ struct vxlan_softc {
        in_port_t                sc_dstport;
        u_int                    sc_rdomain;
        int64_t                  sc_vnetid;
+       uint16_t                 sc_df;
        u_int8_t                 sc_ttl;
 
        struct task              sc_sendtask;
@@ -135,6 +136,7 @@ vxlan_clone_create(struct if_clone *ifc,
        sc->sc_imo.imo_max_memberships = IP_MIN_MEMBERSHIPS;
        sc->sc_dstport = htons(VXLAN_PORT);
        sc->sc_vnetid = VXLAN_VNI_UNSET;
+       sc->sc_df = htons(0);
        task_set(&sc->sc_sendtask, vxlan_send_dispatch, sc);
 
        ifp = &sc->sc_ac.ac_if;
@@ -496,6 +498,14 @@ vxlanioctl(struct ifnet *ifp, u_long cmd
                ifr->ifr_ttl = (int)sc->sc_ttl;
                break;
 
+       case SIOCSLIFPHYDF:
+               /* commit */
+               sc->sc_df = ifr->ifr_df ? htons(IP_DF) : htons(0);
+               break;
+       case SIOCGLIFPHYDF:
+               ifr->ifr_df = sc->sc_df ? 1 : 0;
+               break;
+
        case SIOCSVNETID:
                if (sc->sc_vnetid == ifr->ifr_vnetid)
                        break;
@@ -744,7 +754,7 @@ vxlan_encap4(struct ifnet *ifp, struct m
        ip->ip_v = IPVERSION;
        ip->ip_hl = sizeof(struct ip) >> 2;
        ip->ip_id = htons(ip_randomid());
-       ip->ip_off = 0; /* htons(IP_DF); XXX should we disallow IP fragments? */
+       ip->ip_off = sc->sc_df;
        ip->ip_p = IPPROTO_UDP;
        ip->ip_tos = IPTOS_LOWDELAY;
        ip->ip_len = htons(m->m_pkthdr.len);
@@ -802,6 +812,9 @@ vxlan_encap6(struct ifnet *ifp, struct m
 
                ip6->ip6_src = *in6a;
        }
+
+       if (sc->sc_df)
+               SET(m->m_pkthdr.csum_flags, M_IPV6_DF_OUT);
 
        /*
         * The UDP checksum of VXLAN packets should be set to zero,