Re: [PATCH v11 3/9] arp/ndp: send gratuitous ARP / unsolicitated NA when MAC cache entry added

On Tue, Sep 30, 2025 at 01:58:56AM +0200, Stefano Brivio wrote:

On Sat, 27 Sep 2025 15:25:16 -0400 Jon Maloy wrote:

...

Gratuitious ARP and unsolicitated NA 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 also 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

--- v10: -Made small changes based of feedback from David G. v11: -Moved from 'Gratuitous ARP reply' model to 'ARP Announcement' model. --- arp.c | 41 +++++++++++++++++++++++++++++++++++++++++ arp.h | 2 ++ fwd.c | 8 ++++++++ ndp.c | 10 ++++++++++ ndp.h | 1 + 5 files changed, 62 insertions(+)

diff --git a/arp.c b/arp.c index ad088b1..57e7b59 100644 --- a/arp.c +++ b/arp.c @@ -146,3 +146,44 @@ 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_send_gratuitous() - Send a gratuitous 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_send_gratuitous(const struct ctx *c, struct in_addr *ip, + const unsigned char *mac) +{ + char ip_str[INET_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; + + /* ARP message */ + memcpy(annc.am.sha, mac, sizeof(annc.am.sha)); + memcpy(annc.am.sip, ip, sizeof(annc.am.sip)); + memcpy(annc.am.tha, MAC_BROADCAST, sizeof(annc.am.tha)); + memcpy(annc.am.tip, ip, sizeof(annc.am.tip)); + + inet_ntop(AF_INET, ip, ip_str, sizeof(ip_str)); + debug("Sending ARP announcement for %s", ip_str); + + tap_send_single(c, &annc, sizeof(annc)); +} +

Nit: git show / log highlights this excess newline in red, and I merely relay the pedantry.

...

diff --git a/arp.h b/arp.h index d5ad0e1..2cf1326 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_send_gratuitous(const struct ctx *c, struct in_addr *ip, + const unsigned char *mac);

#endif /* ARP_H */ diff --git a/fwd.c b/fwd.c index 2fd6cee..7f38b40 100644 --- a/fwd.c +++ b/fwd.c @@ -26,6 +26,8 @@ #include "passt.h" #include "lineread.h" #include "flow_table.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); @@ -131,6 +133,12 @@ void fwd_neigh_table_update(const struct ctx *c,

memcpy(&e->addr, addr, sizeof(*addr)); memcpy(e->mac, mac, ETH_ALEN); + + /* Send gratuitous ARP / unsolicited NA for the new mapping */ + if (inany_v4(addr)) + arp_send_gratuitous(c, inany_v4(addr), e->mac); + else + ndp_send_unsolicited_na(c, &addr->a6); }

/** diff --git a/ndp.c b/ndp.c index 588b48f..d7f64a3 100644 --- a/ndp.c +++ b/ndp.c @@ -218,6 +218,16 @@ static void ndp_na(const struct ctx *c, const struct in6_addr *dst, ndp_send(c, dst, &na, sizeof(na)); }

+/** + * ndp_send_unsolicited_na() - Send unsolicited NA

This comment, strictly speaking, should be related to the notification mechanism, but the usage here makes me wonder: do we want to announce link-layer addresses for link-local (IPv6) addresses as well?

Those are not reachable from the guest. I'm not sure if we want to filter IPv6 addresses a bit, in general. I didn't really think it through.

I think (host) link-local addresses are actually reachable from the guest. I think the semantics of that are kind of confusing (at least with multiple host links), and I've talked about disallowing that when we don't have an explicitly selected host interface we're dealing with. But I haven't done that yet. -- 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