On Tue, Oct 14, 2025 at 10:55:16PM -0400, Jon Maloy wrote: > ARP announcements and unsolicited NAs should be handled with caution > because of the risk of malignant users emitting them to disturb > network communication. > > There is however one case we where we know it is legitimate > and safe for us to send out such messages: The one time we switch > from using ctx->own_tap_mac to a MAC address received via the > recently added neigbour subscription function. Later changes to > the MAC address of a host in an existing entry cannot be fully > trusted, so we abstain from doing it in such cases. > > When sending this type of messages, we notice that the guest accepts > the update, but shortly later asks for a confirmation in the form of > a regular ARP/NS request. This is responded to with the new value, > and we have exactly the effect we wanted. > > This commit adds this functionality. > > Signed-off-by: Jon Maloy Reviewed-by: David Gibson > > --- > v10: -Made small changes based of feedback from David G. > v11: -Moved from 'Gratuitous ARP reply' model to 'ARP Announcement' > model. > v12: -Excluding loopback and default GW addresses from the ARP/NA > announcement to be sent to the guest > v13: -Filtering out all announcements of our_tap_mac instead of > explicitly comparing a set of IP addresses. > -Changed annc.am.tha in arp_announce() to MAC_ZERO, which is > 'unknown' according to RFC826. > -Renamed ndp_send_unsolicited_na() to ndp_unsolicited_na() > v14: -Added quotes from RFC5227 to explain the format of ARP > announce messages, as suggested by David. > --- > arp.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++ > arp.h | 2 ++ > fwd.c | 10 ++++++++++ > ndp.c | 10 ++++++++++ > ndp.h | 1 + > 5 files changed, 73 insertions(+) > > diff --git a/arp.c b/arp.c > index b4a686f..0a89ae3 100644 > --- a/arp.c > +++ b/arp.c > @@ -150,3 +150,53 @@ void arp_send_init_req(const struct ctx *c) > debug("Sending initial ARP request for guest MAC address"); > tap_send_single(c, &req, sizeof(req)); > } > + > +/** > + * arp_announce() - Send an ARP announcement for an IPv4 host > + * @c: Execution context > + * @ip: IPv4 address we announce as owned by @mac > + * @mac: MAC address to advertise for @ip > + */ > +void arp_announce(const struct ctx *c, struct in_addr *ip, > + const unsigned char *mac) > +{ > + char ip_str[INET_ADDRSTRLEN]; > + char mac_str[ETH_ADDRSTRLEN]; > + struct { > + struct ethhdr eh; > + struct arphdr ah; > + struct arpmsg am; > + } __attribute__((__packed__)) annc; > + > + /* Ethernet header */ > + annc.eh.h_proto = htons(ETH_P_ARP); > + memcpy(annc.eh.h_dest, MAC_BROADCAST, sizeof(annc.eh.h_dest)); > + memcpy(annc.eh.h_source, mac, sizeof(annc.eh.h_source)); > + > + /* ARP header */ > + annc.ah.ar_op = htons(ARPOP_REQUEST); > + annc.ah.ar_hrd = htons(ARPHRD_ETHER); > + annc.ah.ar_pro = htons(ETH_P_IP); > + annc.ah.ar_hln = ETH_ALEN; > + annc.ah.ar_pln = 4; > + > + /* RFC5227, section 2.1.1, about Probe messages: "The client MUST fill > + * in the 'sender hardware address' field of the ARP Request with the > + * hardware address of the interface through which it is sending the > + * packet. [...] The 'target hardware address' field is ignored and > + * SHOULD be set to all zeroes." > + * RFC5227, section 2.3: "An ARP Announcement is identical to the ARP > + * Probe described above, except that now the sender and target IP > + * addresses are both set to the host's newly selected IPv4 address." > + */ > + memcpy(annc.am.sha, mac, sizeof(annc.am.sha)); > + memcpy(annc.am.sip, ip, sizeof(annc.am.sip)); > + memcpy(annc.am.tha, MAC_ZERO, sizeof(annc.am.tha)); > + memcpy(annc.am.tip, ip, sizeof(annc.am.tip)); > + > + inet_ntop(AF_INET, ip, ip_str, sizeof(ip_str)); > + eth_ntop(mac, mac_str, sizeof(mac_str)); > + debug("Announcing ARP for %s / %s", ip_str, mac_str); > + > + tap_send_single(c, &annc, sizeof(annc)); > +} > diff --git a/arp.h b/arp.h > index d5ad0e1..4862e90 100644 > --- a/arp.h > +++ b/arp.h > @@ -22,5 +22,7 @@ struct arpmsg { > > int arp(const struct ctx *c, struct iov_tail *data); > void arp_send_init_req(const struct ctx *c); > +void arp_announce(const struct ctx *c, struct in_addr *ip, > + const unsigned char *mac); > > #endif /* ARP_H */ > diff --git a/fwd.c b/fwd.c > index f70e4fc..df92db8 100644 > --- a/fwd.c > +++ b/fwd.c > @@ -27,6 +27,8 @@ > #include "lineread.h" > #include "flow_table.h" > #include "netlink.h" > +#include "arp.h" > +#include "ndp.h" > > /* Empheral port range: values from RFC 6335 */ > static in_port_t fwd_ephemeral_min = (1 << 15) + (1 << 14); > @@ -140,6 +142,14 @@ void fwd_neigh_table_update(const struct ctx *c, const union inany_addr *addr, > memcpy(&e->addr, addr, sizeof(*addr)); > memcpy(e->mac, mac, ETH_ALEN); > e->permanent = permanent; > + > + if (!memcmp(mac, c->our_tap_mac, ETH_ALEN)) > + return; > + > + if (inany_v4(addr)) > + arp_announce(c, inany_v4(addr), e->mac); > + else > + ndp_unsolicited_na(c, &addr->a6); > } > > /** > diff --git a/ndp.c b/ndp.c > index 7e2ae2a..430d420 100644 > --- a/ndp.c > +++ b/ndp.c > @@ -220,6 +220,16 @@ static void ndp_na(const struct ctx *c, const struct in6_addr *dst, > ndp_send(c, dst, &na, sizeof(na)); > } > > +/** > + * ndp_unsolicited_na() - Send unsolicited NA > + * @c: Execution context > + * @addr: IPv6 address to advertise > + */ > +void ndp_unsolicited_na(const struct ctx *c, const struct in6_addr *addr) > +{ > + ndp_na(c, &in6addr_ll_all_nodes, addr); > +} > + > /** > * ndp_ra() - Send an NDP Router Advertisement (RA) message > * @c: Execution context > diff --git a/ndp.h b/ndp.h > index 781ea86..56b756d 100644 > --- a/ndp.h > +++ b/ndp.h > @@ -12,5 +12,6 @@ int ndp(const struct ctx *c, const struct in6_addr *saddr, > struct iov_tail *data); > void ndp_timer(const struct ctx *c, const struct timespec *now); > void ndp_send_init_req(const struct ctx *c); > +void ndp_unsolicited_na(const struct ctx *c, const struct in6_addr *addr); > > #endif /* NDP_H */ > -- > 2.50.1 > -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson